Directly downwind faster than the wind - part 3

Monday's post about a propellor-driven wind cart designed to travel directly downwind faster than the wind (DWFTTW) has generated an emotionally-charged discussion about the feasibility of such a vehicle.

There are three camps -- the people who think it's possible, the people who think it isn't, and the people who don't know. All three camps have members claiming to have degrees in physics, engineering, and aeronautics, and members from each camp are guilty of name-calling, insults, and cheerleading for their "side."

One fellow, a proponent of the idea that DWFTTW is possible, even told me that I should "prepare to be disappointed" because I have my doubts about DWFTTW! I would actually be delighted to learn the truth about this, whatever it is.

In MAKE Vol. 11, Charles Platt made a miniature model of the vehicle and came to the conclusion that there is no such thing as a wind-powered vehicle that can travel downwind faster than the speed of the wind. Last year, while Charles was working on the MAKE piece, he emailed me this charming sketch and description:

Lack of imagination among wind-cart enthusiasts has prevented them from realizing that a simple modern invention can solve the problem of net forward air flow trying to stop the cart. That invention is--the air duct!
A swivelling duct would be able to take advantage of wind coming from any direction. A vane at the rear of the duct would automatically turn it into the wind. Even on a windless day, the lucky owner of this windmobile would only have to give it a push before leaping aboard, to create some relative air flow that would power up the fan and accelerate the cart. Who could have imagined that the answer to the problem of non-renewable resources could be so simple?

Of course, he is being facetious. This morning, Charles emailed me the following, along with permission to post it:

Cart

I have browsed the huge discussion in response to your cart posting. Amazingly, so far as I can see, no one has addressed the fundamental problem that if the cart transitions from moving slower than the wind to faster than the wind, the reversal of air flow will try to turn the propeller backward, thus tending to stop the cart. It bothers me that so many people are conned by this idea (or con themselves).
--
Three questions for cart enthusiasts:
1. When the cart begins running slower than a tail wind, does the air move through the propeller from the back toward the front?
2. If the cart can somehow accelerate faster than the tail wind (as its proponents claim), does this means that air will now move through the propeller from the front toward the back?
3. If the flow of air through the propeller reverses in this way, will it tend to reverse the rotation of the propeller?
Answers to (1) and (2) are clearly "yes." Answer to (3) can be determined empirically by blowing air at a small fan, first from the front, then from the back, and watching which way it turns. Answer to (3) will also be "yes."
Therefore, the reversed air flow will retard forward motion, the speed of the cart is self-limiting, and the claim is false.

If you have something to contribute in the discussion boards about this, please refrain from insults and name-calling.

Side note: I emailed Adam Savage about this, and he said it's "in the hopper" for a Mythbuster's experiment! Im considering running another article about this in a future issue of MAKE, as well.

Previously:

Mark Frauenfelder is the founder of Boing Boing and the editor-in-chief of MAKE and Cool Tools. Twitter: @frauenfelder. Come and hear Mark speak at the ALA conference in Chicago on July 1.

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spork

I see a lot of insults Greg, but I don’t see a video of your cart working. And I don’t see you taking me up on my bet. Let’s make it easier – you name the amount – any amount. I accept – in advance.
Anonymous

It’s not a perpetual motion machine, it’s supposedly extracting energy from the relative motion between the air and ground, which isn’t affected by the reference frame.
But I’ve yet to see a convincing explanation of how it does it.
shMerker

I see people contending that that the treadmill test is different from the ground test because on the treadmill the cart is already “moving”. That only applies if we are worried about whether the cart can accelerate to windspeed under wind power. That’s not really at issue here. If you want to make the treadmill test equivalent to the open road test you simply push or drag the cart until it reaches wind speed and then let go.
dejanigma

I know I am late to the game and probably wont see any responses to this, but this is what I have been thinking.

Let’s say for sake of argument that the speed of the wind and the force it imparts are not one and the same.. IE, a 6mph wind can provide 7mph speed for the cart. This could make some sense, the blades are getting more energy from the wind than is required to push the cart.

Now, yes the cart does exceed the speed of the wind, but it could still continue to travel faster as long as the wind maintained. It would oscillate somewhere between 6+7 mph as it reaches its peak, lessens and picks up the full force of the wind again.

One other thing I’d like to mention that I thought about. In both famous videos I don’t see anyone measuring the wind. And given my previous comment, how does wind speed relate to force on the gearbox and how does that relate to the speed of the cart?
All Jelly No Toast

None of these imaginary contraptions people are describing to make this ‘easier’ (skateboard, generator-fans, etc.) make this any easier.

In the end, I see no reasonable explanation of how this thing can go any faster than the wind. Or a balloon traveling in said wind. Maybe a little bit in oscillations, but definitely not steady state.

It just can’t be done. it’s a mechanical device with wind energy acting upon it. there’s nothing else to take advantage of. The ‘differential between wind and ground’ that was being bandied about in the other thread isn’t doing a goddamn thing-there’s no way to harness it. The wheels are traveling across the ground–they can’t take energy from that travel and put it into the fan, unless that kinetic energy is already being taken from the things acceleration.

It don’t work. And I’ve read a ton of posts here, and no one even makes anything close to an attempt to refute that. A bet and some shady videos are not a refutation.

If the stated goal of the contraption is ‘waste the time of heaps of curious nerds,’ however, it is a smashing success.
Mender

“Go back to the treadmill test. Pick apart my analysis.”

Your analysis claims that the energy flow through the device changes when the cart is on the treadmill as opposed to the outdoor test. That is not true. The direction of the energy flow and the loading of the drive system of the cart in both cases is the same. The energy is from a difference in the speed of the air and the speed of the ground (air to ground interface). The same amount of energy is captured and used by the cart in both cases.

The source of the energy however is quite different. Outside, the relative movement is provided by a huge mass of air. Inside, the relative movement is provided by a small strip of rubber. In both cases, when the cart is moving forward at wind speed, the air around it is stationary relative to the cart and the ground is moving back under the wheels. The cart doesn’t know the difference.

Don’t confuse the energy source with the energy flow.

JB’s question about the moving town is perfectly valid. Maybe I can offer another one that might make sense to you, borrowing/plagiarizing from several of the many explanations of inertial frames of reference that spork and JB have offered in the countless replies they have patiently been supplying.

You wake up on an aircraft carrier far out at sea. You walk out onto the deck where you find JB and spork testing the cart in a 10 mph tailwind. JB asks you what is providing the 10 mph speed difference between the deck and the air.

Choices:
A) The ship is stationary and there is a 10 mph tailwind.

B) The ship is moving backward at 10 mph and the air is stationary.

C) The ship is moving forward at 10 mph and there is a 20 mph tailwind.

There is no easy way for you to distinguish between the three choices when the carrier is in the middle of the ocean. The cart can’t tell either. All it knows (and needs to know) is that when it is at 10 mph (windspeed), the air around is at the same speed as the cart, and the ground under the wheels is moving back at 10 mph. The treadmill “feels’ exactly the same to the cart as an outdoor test.

This is fortunate because very few people can afford the aircraft carrier.
dny

Re: Three questions for cart enthusiasts:

Couldn’t you just block the wind coming from the front and making the fan turn backwards? Then it would cease to be a factor.

Here’s an amateurish diagram I drew of what I’m talking about.

The wind shield would have to be bigger, so it could cover the entire front of the fan and direct the air from the back down. This would have the added advantage of acting as both a sail from the back and a wing/needle from the front.
dustbuster7000

@ shMerker post # 158 – Agreed.
GregLondon

I see a lot of insults Greg,

yeah, me too:

Your tumbleweed design can’t and won’t go faster than the wind (I just hope no one dies in a horrible ball of flame as a result).

I truly want to thank you for this document Greg. It now has the prize position in my “Museum of DDWFTTW FAIL”

Again, please tell me what engineering designs I should avoid if I don’t want to die in a fiery death at your hands.

Fail.

And that’s just from your two latests posts. So, you’re telling me I should get used to lots more insults from you?
GregLondon

Count,

When the wheels turn, it spins the prop. That, by itself doesn’t do anything but move energy around, and churn up the air.

If you push the cart in an enclosed room with no wind, it’ll come to a halt eventually.

But pushing the wheels to spin the prop in a room with wind allows the wind to engage the prop when it couldn’t before. Think of spinning the prop as hoisting a sail. If it isn’t spinning, the wind can’t affect the prop.

Once it’s spinning, the tailwind can push back on the prop which can then push back on teh wheel and speed it up.

Maybe if you imagine using the wheels to generate electricity, and then using that electricity to spin the propeller. And then the wind pushes on the prop like a normal tailwind and adds to the total forward velocity. That might help you picture what this thing is trying to do. Sort of.

If you have an electro-prop cart in a room with no wind, the wheels make electricity and the electricity spins the prop. But the wind is still, so the cart goes forward until friction makes it stop. Have the wind push on the cart, and you could use that wind to turn the wheel and generate electricity. Then use that electricity to turn the prop and push off the wind.

The cart is sort of like that, but rather than using electricity in the middle, it uses gears.
jackofsometrades

I posted this earlier on “part-2″, not realizing we had moved on. Please forgive the repost, but I think it answers some questions about the difference between force and energy.

Consider the cart at zero relative wind speed (either stationary on the treadmill or moving downwind at the same speed as the wind). In order for the cart to go faster than the wind, it must have a positive (forward) acceleration in this state. That is, the forward force (in pounds or grams or whatever) on the propeller must exceed the horizontal, backward force acting on the wheels.

As the pulley examples demonstrate, there is no such thing as a Conservation of Force Law, only a Conservation of Energy Law. (See Post #341: “to lift a load with pulleys you must pull in more rope than the actual lift, trading distance for force.”)

Energy is force times distance, and its derivative, Power, is force times velocity (assuming constant force). So, what the propeller is doing is trading velocity for force (restated, the wheel is using less force but more velocity to power the propeller). The propeller can have a greater force than the force on the wheels exactly because it has a lower velocity. The air coming off the propeller is not actually moving backwards (relative to the ground, that is), in fact it is still moving in the same direction as the cart, only slower.

The cart will go faster than the wind as long as the propeller generates more thrust than the load on the wheels. This can be guaranteed with the right combination of gear ratios and propeller pitch. Of course, you also have to overcome friction losses, but if you follow this argument, you should agree at least in principle that DWFTTW is theoretically possible. But then, seeing is believing.
Brian

The transition between going downwind slower than the wind the then transitioning to faster than the wind may rely on wind gusts in order for it to happen (if it happens at all).

Not that I think about it, I’m not sure about this statement.
GregLondon

Well, I spent 8 hours of the last 24 driving on the highway. Lots of time to think. First I figured out a way to make the flapping anemometer possibly work.Instead of mounting the hinge to the axle, I make the plates flat and mount the hinge to the spokes. The flaps then fold flat and get completely out of the way from the airstreem when open, and when the plates swing shut, it’s like saloon doors. a lever on the rim under each door could provide the mechanism to shut the doors when that side of the wheel is touching the ground.

THen I got an image of a weird rube-goldberg machine involving chutes and ropes and pulleys and came up with a way to have the rope pulling the tumbleweed forward be something that has infinite rope. Basically the rope wraps around pulleys and there are chutes attched all along the rope.

A couple tweaks and it looks a lot like a propeller driven cart.

Everything in the document from page 14 forward is new. There’s like 14 new pages. A lot of it explaining how to imagine building the parachute-on-a-rope-turned-into-a-ever looping cart.

I think I”m getting closer.

I also win the prize for most rube goldbergish design I think. (not a lot to do on 8 hours of driving the highway) It’s a hypothetical machine, but it got me a lot closer to understanding the prop-carts.
ThinAirDesigns

Dust:

The only time energy from the ground can enter the wheels is when the wheels are driven by the treadmill. This not the case outdoor, since the ground cannot add energy to the wheels

This is just soooo incredibly and demonstrably wrong, but you have a lot of company I admit. Sad but true.

Best wishes Dust. I know how this story ends — it works like we say it does and between Mythbusters and MAKE and others it will be shown to be so — and contrary to your position, it won’t be particularly useful.

Hope you will return when BoingBoing posts the final word. Perhaps then you will be willing to understand the flaw you cling so tightly to.

JB
spork

I’m telling you you seem pretty sure you’re right, and you’ve been absolutely certain I don’t know what I’m talking about from the very start. So I’m wondering why you don’t liberate some money from me.
ratwerks

Wow, go away for a few hours and see what happens.

Nelson C and Antonius (various comments):

It seems to me that the people who built this thing and shared it with the internet are not the ones who started this thread here. BB posted a link to the their foo with a note “This can’t work” or “Some people we like say this can’t work.” The builders show up and provide tons of evidence that it does in fact work and that they are in fact not cheating. Then you guys whinge about their tone? They don’t explain clearly enough? They don’t sound humble enough?

Well, it was clear enough for me starting from a point of near complete ignorance. I guess I never took offense at their tone because I never shot my mouth off and said “perpetual motion” or “charlatan” or “hoax.” I never did see either of them fail to answer and actual question with an actual answer. I did see them answer various charges and slurs and demands with snark. Personally, I think snark is the right answer for that sort of rudeness. Especially clever snark that makes me laugh.

Greg:

You are acting a bellicose git. You continually demonstrate both your inability to understand the principles in play here and your inability to understand polite disagreement. You don’t really get to shift your burden of understanding to others. You absolutely do not get to interpret your failure of understanding as a failure of others to understand. Oh, and your thing won’t work. If you want something that does work without moving parts think “venturi” but good luck working up a demo that won’t earn you pages of derision from all points of the internet wind.
spork

I’m having a hard time following that, but the cart can definitely travel directly downwind faster than the wind indefinitely. And it doesn’t require any change in prop-pitch or gearing at any time.
dustbuster7000

Mender

Outdoors – energy flows into the ground out of the cart as the cart moves forward. No energy goes into the cart.

Indoor – energy goes into the cart via the wheels and out of the propeller.

Energy flow is in reverse, which it should be since the ground is now moving not the cart, but there is no energy entering the cart in the outdoor case. None. Or the thing doesn’t work as Spork and JB claim it does.
SubductionZone

I just drove by the world’s largest building tonight on my way to dinner. Have you ever been to Kosta’s?
GregLondon

And I don’t see you taking me up on my bet.

God your a pain.
GregLondon

Yah, I was wrong, the fixed vane version doesn’t work.

If you change the timelapse on page 5 and replace the guy pulling the rope with parachute at the end of a rope, it still works. So, the rope/pulley diagram is still good. I just have to figure out a way to translate it into air/paddles, since parachute on a rope doesn’t work for very long.

some quick reading around says anemometers are only 30% efficient. so even if replacing the parachute with flapping the anemometer plates works technically, it might not work well enough to justify the extra problems of moving parts.

some more quick reading says props are up to 80% efficient, so that more than anything explains why a prop is the way to go.

Now I just need to figure out how a prop looks like on a rope/pulley diagram.
Michael C

Here’s a new demo of a mechanical cart that goes faster than the thing that’s pushing it:

http://www.youtube.com/watch?v=k-trDF8Yldc
ThinAirDesigns

IDONTCOMMIT:

you guys think the greg cart won’t work?

It can never work, it has a simple but fatal flaw.

it looks like a version of airshowfan’s ferris wheel, which seems like it could work in theory at least.

There is one very important difference between Greg’s and Airshowfan’s devices. Greg’s vanes present a symetrical projection to the relative airflow. Airshowfans’ does not.

Can the “Ferris Wheel” be done in practice? Perhaps. Greg’s can’t even equal the wind in a theoretical no rolling friction environment.

In fact, due to the churning of the fore and aft vanes (the ones not projecting to the horizontal airflow), I present that this device will accelerate slower and top out lower than a simple bluff sail on wheels.

Here is a drawing that demonstrates the fatal flaw of any design with symetrical vanes around a hub: it simply doesn’t matter what shape you make them — as the device speeds up they present the same shape to the relative wind.

http://www.mediafire.com/imageview.php?quickkey=ndm4ymzmund&thumb=5

When you look at the device at the speed of the wind it’s easy to see that all you can do is churn air with the vanes — no way whatsoever to get thrust.

JB
spork

I want to check out these new designs. Where do I find the document?

And I’m dying to see someone make a non-prop design that can work. Are you going to give it a go?
Count One

Greg,

If you take energy from the forward motion of the cart (ie using the wheels to generate energy) to power the propellers then you will lose more energy than you gain.

A cart with fixed propellers that took no energy from the wheels (ie disengaging the gears) would accelerate faster than a cart that powered propellers from the wheels when placed in a wind.

All the energy that you take from the wheels comes directly from the kinetic energy of the cart itself.
airshowfan

I would love to believe that the decrease in traffic necessarily means that “DWFTTW is possible” has won (as some “The tide is turning!”-type comments announce). It may just be that the skeptics gave up getting us to explain things to their satisfaction. Who knows.

I, for one, have a hard time giving up when I have a model of something that is happening physically, and someone else has a different model that leads to a different conclusion, and I really think my model is better and that my conclusion is correct. When this happens, and I try to explain things in terms of my model for long enough, either I figure out that I was wrong and I learn something, or I end up making people (sometimes more experienced or knowledgeable than I) go “Hmm, I guess you’re right”, which I must admit is satisfying. Just to make clear what my motivations are here. And it’s fun to think about these things :]
spork

Well, if you think a half-assed apology is your free ticket to insult me forever more – think again.
HarveyBoing

Note that everyone knows that perpetual motion machines can’t exist, even those who propose them. Those that propose them always include some justification for why their perpetual motion machines aren’t (e.g. “we’re extracting the energy from the relative motion between the air and the ground”…well, duh…sailboats, kites, etc. do that all the time, but they don’t violate the laws of themodynamics in the process).

So, the presence of such a justification doesn’t mean that the proponents aren’t arguing in favor of a perpetual motion machine. It just could mean that they are following the standard template for proposing perpetual motion machines, in which one of the very first steps is to try to explain away the “something for nothing” aspect of their design.
GregLondon

Why not give him the credit for it as well?

Given how much JB has insulted that document, I doubt he’d want to be associated with it in any way.

I don’t hide the fact that my first design was a fixed vane anemometer. At least, I thought I mention that in my document somewhere that it was my first design idea, and that it didn’t work.

If JB wouldn’t mind having his name (or initials or pseudonym or whatever) associated with the document, I can put it in. But last I heard, that wasn’t the case.

Jack Goodman gives all the necessary details about his cart

That looks good. I might end my document by putting a link to Goodman’s plans so people can build a specific implementation if they want.

Hopefully by the time most people finish reading the explanation, they don’t need to build it to prove it to themselves. But it would be nice to have plans in case people want to build one to play around with it. I’d like to tinker around with one myself, after I figure out how it works.
GregLondon

Once you actually make it to the top , of course :p

:)

I’m getting there. slightly bruised and a couple of scrapes, but I’m close.
kerinin

How is this at all controversial? In order for a vehicle to go from being at rest to going faster than the wind, it must at some point be moving at the SAME speed as the wind. At this point in time there will be NO force acting on the vehicle from the wind, but there WILL be force acting on the vehicle from the ground (friction on the wheels), which means the vehicle will slow down.
spork

For those that want a copy of the original Greg London write-up, it can be downloaded here:

http://www.filesavr.com/tumbleweed

Definitely a classic.
Anonymous

I think i may have an idea.

If you think about how a freestanding propeller spins in the wind it will always go at some speed slower than the wind speed and will try to maintain that speed. Slower, and it will gain energy, Faster and it will lose it.

Now we add greater momentum to it by attaching mass. It will spin at the same speed eventually.

Just because there is an object(the ground) giving some slight drag to it only affects this by means of friction, which in this case is pretty low. (Wheel rolling along the ground.)

Because the speed of the device relative to the ground is greater than the speed of the wind relative ground doesn’t mean that anything weird is happening.

The device tries to match the speed it ought to considering the wind speed RELATIVE TO THE DEVICE, which it naturally will always be slower than.

This “slower than the relative wind speed” just happens to be faster than the wind speed relative to the ground.
dustbuster7000

Jack, the propeller is either a source of energy or a sink. JB and Spork say its a sink, since it providing thrust. Energy leaves the cart via the propeller. Where does that energy come from? From the wheels, by slowing them down. Assuming perfect conversion and no net gain of energy (at least everyone agrees that is true), at best the thrust energy can equal the loss of wheel rotation energy. And the cart stays at the same speed. Wind does not enter into it, since we’ve been discussing a ‘no apparent wind’ scenario. And in any case, the propeller can’t output energy to the air and draw energy from the air at the same time. They would cancel each other out.
airshowfan

I, for one, think that Greg’s Tumbleweeb (or something like it) can go DWFTTW, at least in theory. It would come down to the ratio between the drag of the surfaces on the lower half and the ones on the top half. If the bottom surfaces are super draggy and the topside ones aren’t very draggy, then why wouldn’t it work?

But I also think that the treadmill test and the outdoors wind test are equivalent. So Greg, why don’t you build one that is now as wide, and put it on a treadmill?
highlyverbal

I am confused by the analogy to iceboats tacking to go faster than the wind. There are persons in the vessel ADDING WORK. Pulling the boom around takes effort.

The wind doesn’t power the tacking.
ratwerks

I have a couple of observations on the social side of how this seems to play out in its many internet incarnations. Please feel free to read no further if your interest here is in physics.

The law has different rules for using deadly force. You may use deadly force against a person whom you reasonably fear threatens you with death or serious injury. You may use deadly force against a person in defense of others whom actually do threaten those others with death or serious injury. The delta is in defense of self you must reasonably believe but in defense of others you must be right.

What I think that should mean here is that if you are actually correct you have a lot more latitude than if you “reasonably” thought you were correct.

My unicycle club had a rule: You get to laugh at people who fail at doing something that you yourself can do, you may not laugh at people who fail at doing things that you yourself cannot do. Now, personally, I try to far toward the side of not laughing at anybody, unless those people have been particularly egregious in laughing at their betters.

/me points at GregLandon and cracks up.
GregLondon

I want to check out these new designs. Where do I find the document?

Same place. I just change the revision number on the document every time I make a change (rev number is the date I made the change).

http://www.greglondon.com/tumbleweed/

And I’m dying to see someone make a non-prop design that can work.

It’s really nothing more than the tumbleweed with a rope coming off the bottom and a parachute at the end. with some weirdness to make sure you don’t run out of rope.

I’m pretty sure it will work. But I trust you to be rigorous in making sure I didn’t make any mistakes.

:)

Are you going to give it a go?

When you see how complicated it is, you might see why I haven’t tried it. The thing has to open and close the parachutes, which would mean I’d have to make sure the chutes don’t get torn and the lines from the chute don’t get tangled.

Like I said, I had to drive 8 hours on the highway yesterday, and I got pretty bored.

I don’t expect it to be something anyone would construct, but it is a mental experiment that shows you can go faster than the wind.

The last leap to converting it to a prop is fairly small for me now.

So, here’s a dumb question I’ve been trying to figure out. When you spin a prop over different RPM’s, is the force it exerts linear to RPM? I’m trying to imagine converting torque to linear momentum through a propeller, and I can’t quite figure out if I get better “traction” at certain RPM’s or if the conversion is linear regardless of the prop RPM. If linear, then why spin the prop so fast? Which makes me assume it isn’t linear, but I’m not sure.

I keep thinking about constant-rpm props and how they change the angle of the blades, and I think that was to get better efficiency. I know you change the angle of the rotors in a chopper with cyclic and collective and use throttle to maintain constant RPM, but I assumed that was more about keeping the rotors from going over a certain RPM and tearing themselves apart. I distinctly remember having to train with the throttle govornor off and how the instructor impressed upon me that we would all die if I let the needle get above a certain point. But flying an airplane or helo is a whole lot more intuitive than designing one. power out was “stick forward” for a plane or “left hand backward, right hand down” for a chopper. You’re not thinking, you’re doing.
ThinAirDesigns

AIRSOWFAN: @262.

Depends on your definition of “(or something like it). Please tell me if you agree with my assesment in #261.

I again believe my drawing clearly shows that unless the vane move relative to each other, it matters not what shape they are because they eventually present the same face to the same velocity of relative wind.

JB
John H.

I don’t know myself, but in the example you gave, wouldn’t it be possible to have a gear that prevents backward rotation from the propeller from being transmitted to the wheels?
jackofsometrades

Indoors or Outdoors – energy goes into the cart via the wheels and out of the propeller.

THERE IS NO DIFFERENCE! If you are on the cart, you cannot tell whether you are moving with the wind, or the ground is moving beneath you. The treadmill is an exact model of moving with the wind.
Takuan

(Homer Simpson voice) Dumb that down a little more.
ThinAirDesigns

Dust, thanks for your response. I propose that your test will not produce the results you think they will.

Because you’ll still think I’m wrong, tell me what instruments you need on the cart and where they need to be placed to prove you’re right.

If the ‘energy’ travels differently through the device when it’s on a treadmill vs when it’s on the street, this difference can be instrumented and displayed.

Test please. Tell me what instruments, where to put them, what test to run and what difference the instruments will display in the two scenarios.

>Please meet mine and provide a critique
>of my analysis of the treadmill test in
>post #144.

This *is* the critique. You are making a claim regarding different ‘energy paths’ between the scnarios — I claim there is no difference in any of it and am asking you to support your assertion by telling me what instruments mounted to the cart will prove you right and me wrong.

I’m very willing to listen, and if you can actually define a test and sufficiently support your claim I might even perform the test for you.

JB
GregLondon

Mark, did the rope and pulley drawing on page 7 help maybe? Just wondering. Some people can see it as a block and tackle easier than seeing it as a yoyo.

Oh, and thanks for the feedback.
- Mark Frauenfelder
  
  I understand the rope and pulley drawing, but not how it relates to the yo-yo. I’m going to dig around the house for one of my kids’ yo-yos and see for myself. Thanks!
weeble

I agree with the others: the machine in those diagrams could never work, but it doesn’t sound like that’s what has been proposed. It’s clearly connected up the wrong way round. I reserve a little scepticism, but I must say so far (some of) the arguments for sound plausible and none of the arguments against have sounded convincing. It seems the kind of idea that, if impossible, should be easily proven to be, such as by reduction to perpetual motion.
GregLondon

Thinair, that’s probably the first helpful thing I’ve read on this thread in a while.

so, the idea is to get the wind to only push on the bottom vanes. When I was drawing it up, I had the idea of having the vanes on hinges so they drop down when each vane rotates towards the bottom. and then fall back in when at the top. But I was trying to avoid moving parts.

I just read that an anometer is only thiry percent efficient, so I was thinking I’d have to rebuild it with flaps anyway.

something that could move some flaps the same way the Voith Schneider blades move (something I just discovered looking up aeronautical stuff)

The rope pulley drawing adds up, so it proves it you can go faster than the wind. I just need to find a way to implement the rope/pulley with air to blade or something.
deejayqueue

Ok, I think I get where the idea comes from. I still think it’s bogus, but here goes my best guess:

The wind pushes on the prop and the whole craft, making it go.
(this is the bogus part, because I don’t think the wind could provide enough push without a sail to start the thing moving or get it up to speed).
Where people are getting caught up is by thinking the wind pushes on the prop, making it spin, which drives the wheels. This is not how it works.

Imagine the prop was fixed and not geared to the wheels, just the wind pushing on it and the craft. This is what makes it go, the prop spinning because the wheels are turning is “bonus energy” That’s the force they’re talking about.

They’re saying that if you had a craft that the wind could push up to its own speed, gearing the wheels to a propeller would make the craft go faster than that.

The thrust wind from the propeller would push against the tailwind that’s already pushing the craft, and it would go faster than the wind that’s pushing it.

I don’t think it works that way though, and their tests have failed to illustrate it working in the way that they mean, and they seem to get defensive and dodgy when asked for more direct proof, so I’m still leaning on the “bunk” side of the argument. I see where the idea comes from but it’s still crap.
idontcomment

It does seem to be pretty over here, but as you guys have said its not clear everyone is convinced. And scienceblogs is getting a bit over-heated. This has been amazing, I haven’t seen science so beaten down in a while. (though i do live in sheltered boston)

It’s nice people aren’t into magic anymore, but a little bit of science and a lot of confidence can be a pretty bad combo as well (not at all pointing this at the many skeptics here who have been genuinely going back and forth trying to understand with an open mind)

You guys have been keeping it surprisingly cool – also I can’t imagine you’ve gotten much done other than responding to comments these last days ; )

Can’t figure out if there is some way to contact people on the comment system here, but if anyone still remaining (spork, jb, mender, airshow, …) wants to stay in touch email vnk256@yahoo.com (temp email), because A) I’d be down to get a celebratory victory drink with anyone local (if there is an eventual victory!) and B) I’m hoping to show an “impossible” project of my own (if i ever get it finished..) that might interest you guys (and it won’t be so controversial, but if it is I want you on my side!)
airshowfan

Posts 346 and 378 in the previous discussion address this. You’d need a transmission with a Reverse gear, or variable-incidence prop blades, in order to have a prop that is spun by the wind and powers the wheels AND that is later spun by the wheels to generate thrust.

I don’t think anyone claims that the DWFTTW cart is self-starting. I think the claim is; A cart that can be brought up to wind speed with some kind of assistance (like, in the Youtube video, a hand holding it), then that assistance is removed, and the cart can keep up the speed or accelerate.

The spinny thing is a propeller, not a turbine. (Search for that phrase in the previous thread and you’ll see that many people seem to ignore this, but I’m getting tired of writing it over and over ;] )
SubductionZone

Highly Verbal, yes it does take work to change the sails of an iceboat, but that work does not go directly into the boats speed. And this “work” is dependent on how long the run is. If the run was arbitrarily long the work done by the crew would not amount to anything compared to the actual work done by the ice boat. This is called grasping at straws, when you know that you are losing your argument you tend to look anywhere for salvation.
ratwerks

#264 posted by ThinAirDesigns

Not to pick nits, but your are incorrect. The top vanes are progressing into the air faster than the bottom vanes. Remember that a rolling wheel pivots around its contact point, not its axle. This, of course, does not help GregL’s device in its vain (or vane) attempt to beat the wind.
ztoop

The reason why this all works is because the propellor is not a sail (the wind applies negligible force on the propellor). It works because the propellor spins _much faster_ than the wind.

So, question 1 is not obviously a yes.

On to the physics of the situation: Many people keep talking about energy here, and asking where it is coming from. Once the craft reaches a steady speed, the wind adds very little energy to the craft,because all it has to do is provide enough energy to overcome the friction in the wheels and the drag on the craft.
Count One

Aaargh!

The topic has risen from the grave (once more).

I must slough off to slumber once more until other posters have had their fill.

Then I too shall return.

Bwa ha ha!
Takuan

what if there were a clutch that sensed relative speed that disengaged when the cart speed transitions?
airshowfan

No, I just moved into town 5 days ago. But if you want to grab some pizza, let me know ;)
ThinAirDesigns

Airshowfan:

So Greg, why don’t you build one that is now as wide, and put it on a treadmill?

Been there done that, or more aptly titled:

“Doing what Greg won’t do because he already knows the answer — seeing how ropes and pulleys are a mature technology and all”.

Greg: ropes pull, pulleys spin, and your particular device can never equal the speed of the wind. It’s the law

http://www.youtube.com/watch?v=axoflkkwwyI

JB
GregLondon

I just uploaded the first version of the document that goes all the way from zero to a propeller driven cart.

http://www.greglondon.com/tumbleweed/

It is a total of 30 pages long. most of it is drawings.

At the very least, I think that most people unfamiliar with the cart could read it and get that the cart works and doesn’t violate any laws of physics.

Hopefully, folks will read it and actually understand it on some level. I believe it explains everything all the way from zero to the prop-cart at least in principle.

I didn’t include plans. Instead I linked to Jack Goodman’s plans which seems like a really nice description with lots of details and his plans all fits on one page.

Enjoy.
- Mark Frauenfelder
  
  Thanks, Greg. I’m having trouble visualizing how pulling the string of a yo-yo on the ground will cause the yo-yo to move faster than the string. It seems to me that the yo-yo will just unwind as the string is pulled.
icky2000

Interesting stuff. I really don’t get the heated debate and anger, though. People are odd. Lately when I see a post that has 50+ comments I usually just move along because that usually means it has turned into a pissing match over nothing.
ThinAirDesigns

Rat @264:

I believe if you take the average height position of the total vane, (as I did) you will find I am right. If you take the *tip*, then you would be correct.

I chose the center of the vane as an approximation to the center of drag.

JB
spork

Greg says:
“Why in the world do all the designs out there right now use propellers? … the people who built the propeller driven machines didn’t understand what was going on exactly and so just tinkered with something until they got it to work.”

Now Greg says:
“some quick reading around says anemometers are only 30% efficient”

Interesting. Maybe another hint that the folks that built the prop cart actually do understand how it works?

Here’s another hint for you Greg – what you call an anemometer is going to be less that 0 percent efficient in this application. There is no way around it.

“Yah, I was wrong, the fixed vane version doesn’t work.”

Fair enough – but to be more accurate, you were wrong about *everything* – whether we understand how our cart works, whether there’s any place for experiments in actual engineering, and whether my degree came from a diploma mill or one of the finest engineering schools in the U.S. – to name a *few*.

“Trying to get the builders to explain how said machines actually work resulted in an amazing amount of unobtainium, handwavium, and complicated invented phrases that don’t mean anything.”

Oh, and you were wrong about this. You should have tried asking us – as many others did. And then listening, and trying to understand the answers, as many others did.

Above quotes are from Greg’s “rigid” treatment of this topic: http://www.filesavr.com/tumbleweed
agraham999

“I have browsed the huge discussion in response to your cart posting. Amazingly, so far as I can see, no one has addressed the fundamental problem that if the cart transitions from moving slower than the wind to faster than the wind, the reversal of air flow will try to turn the propeller backward, thus tending to stop the cart. It bothers me that so many people are conned by this idea (or con themselves).”

Okay…what about a helicopter? In autorotation the “wind” reversal does not cause the blades to turn the opposite direction.

“For a helicopter, “autorotation” refers to the descending maneuver where the engine is disengaged from the main rotor system and the rotor blades are driven solely by the upward flow of air through the rotor. The freewheeling unit is a special clutch mechanism that disengages anytime the engine rpm is less than the rotor rpm. If the engine fails, the freewheeling unit automatically disengages the engine from the main rotor allowing the main rotor to rotate freely.”
John Markos O’Neill

Actually, they were merely proving that they could travel downwind faster than the wino. http://search.twitter.com/search?q=wino
joelphillips

#11,

Does that mean if you put two propellers on it, it will accelerate twice as fast?

No, because you’ve increased the mass of the cart.

In spite of the facetious comment, there’s an interesting question here. I wonder if there’s a easily calculable theoretical limit to how much faster than the wind you can travel. For example, in an instantaneous collision between a moving massive object and a stationary smaller object, the resulting speed of the smaller object is at most twice that of the larger object.
MORAY

This whole discussion is pretty funny.

You have a wind blowing left over the ground, and some vehicle that wants to go faster left than the wind is blowing.

This is the same situation as the ground moving right in still air, and the vehicle wanting to go faster right than the ground is moving. (If you can’t see this, now’s a good time to go looking for funny pictures of cats :)

To restate the same thing, it’s the same as a wind powered vehicle wanting to move directly upwind.

Hopefully, it should be fairly clear that the last one is easy.

Take a huge, honking great propeller. Let the wind drive it. The power you get from this is roughly proportional to the wind speed multiplied by the square of the prop radius.

Now gear down the prop and have it drive the wheels forward (moving upwind).

The energy this takes is proportional to the drag multiplied by the distance moved.

“Wait!” you cry, “The drag is higher than the power you get out of the propeller”. Sure! Why not. Let’s assume that. The ‘trick’ is we’re using time on the prop side to get energy, and distance on the wheels side to use energy.

Now that the energy from the prop is proportional to time, but the energy used in moving forward is proportional to distance, we get move forward just by going slow enough.

So now we have a vehicle going directly upwind.
(and yes, this is exactly the analogous thing to the yo-yo moving forward faster than the string is being pulled).

Which with a bit of flipping of the reference frames is identical to going downwind faster than the wind.

Note that the upper limit here is that you can’t move downwind faster than twice the ground wind speed. i.e. The upper limit on the ground speed of the vehicle will be ground wind speed * 2 * prop efficiency. (This follows directly from power from the prop = drag * prop efficiency, substitute in, and you can ground speed in terms of wind speed).

There’s no free lunch here: It taking energy from the wind moving relative to the ground, _NOT_ from the wind relative to the vehicle. The vehicle won’t keep accelerating, it will speed up to near the upper limit and then stay at that speed.

(Energy is Power multiplied by time. Energy is Work is Force multiplied by distance).

And Charles needn’t feel too silly. I agree that it’s really non-obvious in the original reference frame.
dustbuster7000

Jack,

You say, outdoors, the energy goes into the wheels. From where? When something rolls forward on wheels it is pushing against the ground. It is applying force to the ground. It is losing energy to the ground. Even if that were not true, where does this energy going into the wheels from the ground to drive the propeller, where does that come from? If the ground is putting energy into things that move forward against it, why do I get tired when I run?
bbblaker

I agree with #14. I’m unsure whether the DWFTTW claims are true, but I think the refutations are attacking a straw man. The wheels are meant to drive the propeller, not the other way around.
spork

“what would be the very best basic physics primer?”

Shoot – that’s a tougher question than all the others on the thread so far. I haven’t verified this first hand, but this series is quite good in my opinion. I’d seriously consider “Physics for Dummies”
http://www.amazon.com/Physics-Dummies-Math-Science/dp/0764554336

And I wouldn’t let the name bug me. In my opinion these are generally good books that explain the basics clearly without making assumptions about your background in the subject.
misterfricative

Regarding my earlier post #142 –

One clarification: I do not mean to imply that there’s a ‘switchover’ point at 9 knots, where the pushing wind suddenly stops working and the prop thrust effect abruptly switches on. What happens is that the wind’s pushing effect *gradually* drops to zero; it reaches zero when cart’s groundspeed equals tailwind speed. Conversely, the prop thrust effect is initially zero (cart at rest; prop not turning) and it increases gradually to reach a maximum when the cart is at terminal velocity.

And a correction: The major factor that will determine the upper limit on the system’s airspeed is going to be wind resistance (which increases with the square of velocity). Unless the system is using an unusually fragile prop, then this limit will be reached well before the prop reaches its maximum rpm. Having the prop rotate at its ‘sweet spot’ at terminal velocity is still an elegant and desirable design criterion though.
airshowfan

JB,

I do agree. When the wheel reaches wind speed, the drag coefficients of the top and bottom surfaces will match (since both are moving through the air at the same speed and in the same way), unless the surfaces are movable and orient themselves in a more streamlined way on top than they do on the bottom. (And my Ferris Wheel idea did include this observation).
ztoop

Here are some physical answers to the questions posed:

1. When the cart begins running slower than a tail wind, does the air move through the propeller from the back toward the front?

If the cart is not moving, than this is true. However, once it begins to move the propellor will force more air past it (pushing against the wind) because they propellors can actually move much faster than the wind is moving relative to the cart.

2. If the cart can somehow accelerate faster than the tail wind (as its proponents claim), does this means that air will now move through the propeller from the front toward the back?

The air will almost always be moving through the propellor from the front toward the back.

3. If the flow of air through the propeller reverses in this way, will it tend to reverse the rotation of the propeller?

The flow doesn’t actually reverse (see 1), however, the wind from the front will actually help the propellor to a certain extant as it is moving in the direction the propellor wants to make it move in. The only thing the wind in the front doesn’t help with is drag on the rest of the cart, which is why the best carts seem to have very little surface area.

Now there is some resistance caused by the propellor moving through the wind, however this is typically very small compared to the amount of lift the propellors can provide. This is why airplanes use large wings with small engines facing forward, and not no wings with engines facing down.

A good way to think about the above is to think about what would happen to a cart with a propellor that was wielded in place (and disconnected from the wheels). Here the propellors would act as a sail and the cart could only move (ideally) up to the speed of the wind.
spork

“The top vanes are progressing into the air faster than the bottom vanes.”

I don’t think so. It’s true that the wheel pivots about its contact point, but if we assume it’s going downwind at wind speed that would mean the hub is stationary while it rolls on the treadmill. In this state you can see the wheel is fully symmetric.

Greg says:
“Thinair, that’s probably the first helpful thing I’ve read on this thread in a while.”

Now THAT’s what I call a gracious apology. Are we still completely wrong? Can you see any reason at all now for testing your theories? Can you understand why we might be just a tad tweaked when you tell us that our lax sense of engineering doesn’t measure up to your rigid life and death standards?

Don’t you think it’s just possible that we might actually know what we’re talking about when we can take one glance at your design and give you an immediate guarantee that it can never work?

Nope – I didn’t think so.
arkizzle

MichaelC, that is an excellent video.
Unusually done, but perfectly explained.
GregLondon

spork: I particularly enjoyed the fact that he made sure to copyright it so he got due credit for it

I’ve been using open source licenses for years to give away the rights to stuff I’ve written. I wrote a book on how to program in perl, a couple books about copyright law, and a whole bunch of open source software stuff that’s all out there on the net being used by people, and I licensed it all under open source licenses so that people can use it however they want and improve upon it if they can make it better.

I’m not the one trying to get on Mythbusters or bet people a hundred thousand dollars over this gizmo. I gave it away for free.

If you want to take someone putting something under an open source license and try to say they did it for some nefarious purpose, well, you just insulted the whole open source community.

And you’re insulting anyone who contributed to open source, just so you can try and club me with “Look! Look! He made sure to copyright it!”

Except, the way the law works, aything you write is copywritten as soon as you put it in some fixed medium. The only question is whether you license any of those copyrights away.

I wrote my perl book years ago and I still get emails from people who say they learned a lot from it. I put stuff under an open source license because I’m more interested in learning something and putting it into some format so that others can learn it too than I am interested in making money over it.

Can you claim that? You seem more interested in getting to meet Jamie and Adam and working with them somehow, and more interested in shouting down anyone who doesn’t understand you with “Bet me!”.

I’m trying to understand how this works and put it into some format that can quickly and easily explain it to others. I think the rope/pulley diagram is an easy way to show to people that this doesn’t violate any basic laws of conservation of energy. Converting that to a vehicle with fixed vanes was a mistake. I screwed up.

Thing is, I don’t mind admitting that. I tried to explain it and I failed. was wrong about how fixed vanes work.

I was wrong about using fixed vanes. But once I figure that out, I hope to have a short little paper that I can give away that explains this to anyone who wants to learn it. I really don’t want anything else out of this other than to maybe have someone tell me “I understand it now because of reading that”.

On the other hand, would you be involved in this project if there was never any hope of this getting on mythbusters? You were right about this vehicle working. But that doesn’t mean you’ve explained it to anyone else. To you, it seems a video and a wager is an explanation. So far, the biggest breakthroughs I’ve had in understanding this came from people who weren’t involved with the project at all or people who don’t care about whether they end up meeting Jamie and Adam or making a hundred thousand dollars out of this.
JakeTheSnake

Why is everybody focused on downwind faster than the wind, can they already go upwind?
Michael C

Greg, now you’ve found out that propellers are way more efficient than anemometers, you can start to think of the best way to connect the movement of the propeller with the movement of the machine along the ground.

Once you’ve done that, you may well find that your machine is “evolving” towards something similar to the other DDWFTTW designs. Food for thought?
Mender

That’s the interesting thing about the car: spork and a very few others didn’t “discover” the cart. They came up with it on their own, without help, based on what they knew and putting two and another distant two together. That takes a pretty sharp mind.

Reverse engineering is simple compared to that. Climbing the mountain just takes time and effort.
spork

“I think more skeptics could be won over if you provided this explanation up front. A lot of people think not that the cart specifically is impossible, but that all dwfttw travel is impossible.”

No excuses – but maybe an explanation or two…

I’ve been arguing with countless people on countless forums (and even aero and physics professors) for 3 years that this thing is possible. I’ve used dozens of proofs, examples, analyses, over and over again. Some people click on one, some on another, and some don’t click on anything.

I often forget what I’ve said where, and I’ve developed a hair trigger when people jump right in and tell me I’m a fool, I don’t understand how it works, I’m explaining it wrong, etc.

I’m trying to start fresh on this thread like I’ve never explained it before. In fact over the past hour or two I’ve been starting my draft of a build document. I’ll post a link shortly.
misterfricative

Hey, three in a row! I must be the King of Sad!

Anyway I just want to say that I’m encouraged that my understanding of this thing is correct: Hudman has posted an explanation that’s similar to mine (except his is clearer and better written) over on Mark C’s blog http://scienceblogs.com/goodmath/2008/12/windpowered_perpetual_motion.php#comment-1242905
GregLondon

Now Greg says: “some quick reading around says anemometers are only 30% efficient”

Interesting. Maybe another hint that the folks that built the prop cart actually do understand how it works?

spork, you were right. the thing goes faster than the wind. I’m sorry.

I was wrong.

You were right.

I’m sorry.
spork

It seems your “first version” of the document, differs from the copy of the document I downloaded earlier. It seems the NEW “first version” has lost some of the insults. In fact in the new “first version” your unworkable design has been redesigned so it’s only impractical (rather than 100% impossible). If I recall correctly, this was the version that didn’t even need to be tested because your rigid engineering analysis proved it’s superiority over our prop design.

I still like the first “first version”:

http://www.filesavr.com/tumbleweed
GregLondon

If you take energy from the forward motion of the cart (ie using the wheels to generate energy) to power the propellers then you will lose more energy than you gain.

Yep. But once the propeller starts spinning, the tailwind can start pushing on the prop and speed you up.

If the prop doesn’t spin, the wind can’t push on it.

If it takes X number of (energy units) to spin the prop, but that then allows the wind to push you and introduce (Y) energy units into the sytem, and Y greater than X, then you’ll go faster than you would have with no wind.

A cart with fixed propellers that took no energy from the wheels (ie disengaging the gears) would accelerate faster than a cart that powered propellers from the wheels when placed in a wind.

I don’t know how it would accelerate at all, the wind can’t push on anything then. You could give it a shove, but it would coast to a stop.

All the energy that you take from the wheels comes directly from the kinetic energy of the cart itself.

But it’s not just that the energy adds up, it’s that it is leveraged.

If you look at it as a yoyo with a rope, and the rope has a parachute on the end, then all the energy is coming from the wind pushing on the parachute.

Once the yoyo starts moving, it has whatever kinetic energy it has becuase it’s moving at whatever speed its moving at. I admit, it isn’t magical energy anywhere.

But there is a leveraging going on between the wind pushign on teh parachute, and the ground, and it’s the yoyo that acts as the lever.

The point is that the wind moving at 5 fps against the parachute injects energy into the system, but the system is designed to leverage that energy so it can move up at 10 fps. If the wind pushes with a force of 10 pounds, the yoyo might only accelerate as if it had a 5 pound force pushing on it because the rest of that force is leveraged into making the cart go faster than the wind.

If the parachute moves 5 feet, the cart leverages that into moving 10 feet. It just has to accelerate slower than it would have.

When you pedal your bike, you’re feet might move at 3 mph, but gears in your bike let your bike move at 15 mph. Your bike moves faster than your feet because the gears translate the energy you put into your feet and convert it into the same amount of energy into the bike, just different velocity.

That’s all this really does. The wind is pushing on the “pedals” at 3 mph, but it pushes the cart up to 6 mph.
Anonymous

“Of course he’s being facetious”? Not necessarily. Most wind-farm propellers have blades that can feather themselves — adjust their blade pitch between ‘spinning in response to wind’ and not — so the extra mechanical linkages between the blade surfaces and the spindle are already a solved problem from an engineering standpoint. All you need now is to adjust those linkages from the current state of work/stall to work/work-the-other-way.

So, looking at the two diagrams that illustrate differential wind one way, rotation one way, wind the other way, rotation the other way, it’s possible to envision a machine that could transition from one to the other. …except for one thing: at that transition point, what accelerates you from one state to the other?

Whups. Damn. Back to the drawing board.
bonafidebob

I only got a couple of pages into it, but the “wheels drive the propeller which provides thrust” seems obviously a perpetual motion machine in the making… stunned the comments continue.

This comment is prompted by comment 25: “For a helicopter, “autorotation” refers to the descending maneuver where the engine is disengaged from the main rotor system and the rotor blades are driven solely by the upward flow of air through the rotor.”

Um, no. The helicopter blades don’t start turning the other way, and there is no upward flow of air through the rotor. Autorotation works by using the *rearward* (horizontal) flow of air to keep the blades rotating and thus keep pushing the air down. For gliding helicopters it’s gravity and angle of descent that provides the flow, for autogyros it’s a regular prop.
spork

Your new cart is reminicent of a design I use strictly as illustration:

http://www.putfile.com/pic/2794071

I’m pretty sure that both the torque and thrust increase roughly with the square of the RPM on a prop. But generally the efficiency also increases – up to a point.

Constant RPM props are a compromise that act a bit like a typical mechanical transmission. The can change the angle of attack of the blades to achieve higher or lower pitch for climb vs. speed (for example). But they can’t chnage the twist along the blade, so there is only one setting at which they have “true pitch”. You want this to be your typical operational setting.

Heli blades are another matter. They rarely have any twist at all. This is partly because twist isn’t necessarily desired during hover (even during forward flight the flow passes through the disk at an extremely oblique angle), partly because of ease of manufacturing, partly because only the last 1/3 of the blades are doing much good anyway…

“I distinctly remember having to train with the throttle govornor off…”

You had a throttle governer!!? That’s cheating. Although I guess these days even R-22′s have them.

“…and how the instructor impressed upon me that we would all die if I let the needle get above a certain point.”

Instructors like to say that sort of thing. But you probably don’t want to throw a blade.

“But flying an airplane or helo is a whole lot more intuitive than designing one.”

Rotor head dynamics are extremely interesting – and plenty complicated. I suspect most people don’t realize you increase lift on the BACK of the disk to turn to the side (because of gyroscopic precession). That’s just where the whole thing begins.
Marcelo

You can just email Adam Savage? That’s pretty cool.

Tell him to get a haircut. He looked much better without the shag.
spork

“What happens is that the wind’s pushing effect *gradually* drops to zero; it reaches zero when cart’s groundspeed equals tailwind speed.”

I don’t think it’s accurate to look at it that way. If you look at the relative wind seen by the prop, it’s simply a vector that starts as a direct tailwind, and rotates further around as the cart’s speed increases. It’s true that the relative wind hitting the prop blade is a direct cross-wind when the cart reaches wind speed, and even becomes a bit of a head-wind as it passes wind-speed (but still hits the back of the prop-blade). But… the prop ceases to act as a bluff-body on which the wind simply “pushes” well before reaching wind speed. As soon as laminar flow is achieved, the prop is acting as a lifting surface.

“The major factor that will determine the upper limit on the system’s airspeed is going to be wind resistance (which increases with the square of velocity).”

A prop-cart with a fixed advance ratio has a maximum theoretical velocity relative to wind speed even with no drag or losses. This is a simple geometric relation. The wind resistance may or may not have a very significant effect on this. With very low advance ratios, the cart will only pass the wind speed by a small margin, and therefore never feel much in the way of wind resistance even as it nears its theoretical speed limit.
spork

“I, for one, think that Greg’s Tumbleweeb (or something like it) can go DWFTTW, at least in theory. It would come down to the ratio between the drag of the surfaces on the lower half and the ones on the top half. If the bottom surfaces are super draggy and the topside ones aren’t very draggy, then why wouldn’t it work?”

Airshowfan, you’re clearly a pretty sharp guy, so I feel like I’m missing something. But I just can’t see how the bottom vanes can be any more or less draggy than the top ones unless they feather or are otherwise faired. Are you thinking about the aerodynamic interference with the ground? I’m pretty sure that would be more than offset by the gradient.
John Markos O’Neill

Now, if only we could somehow harness some of the relative motion between Earth and space, that would give us some serious energy. Also, it would slow aging slightly.
ratwerks

Me @ 264, JB @ 268, spork @ 270

Hmm, what path to follow?

1> Perhaps I could complain how the poor understanding evinced by JB and spork have led to my assertion that the top vanes ‘see’ more wind than the bottom vanes of the TumbleWeed. Maybe embellish that with some assertion about how my various important duties imbue me with a deeper understanding of such things. Finally, demand to see it built and instrumented and run in a wind tunnel (because treadmills are ENTIRELY IRRELEVANT) and vow to keep the internet safe from charlatans by stamping my foot until I am persuaded.

2> Ponder, ponder, ponder, doh! My bad. Thanks for the help guys.
yesno

The refutations are not attacking a straw man, and the anger comes from people willfully disregarding basic physics. The refutations are trying to get people to think about how first principles rule this out. Getting into the details of propellers and wheels is a waste of time, because they cannot make a difference.

Travelling 11 mph from a 10 mph wind is exactly the same as travelling 1 mph from a 0 mph wind. If it is impossible to use the non-existent wind to move 1 mph, it is impossible to get the extra 1 mph.

If the wind is blowing 5 mph east relative to the ground, and you are travelling 4 mph east relative the the ground, you only have a 1 mph wind to work with. If you are travelling at 6 mph, then the wind relative to you is -1 mph, and it can only slow you down. The only thing that matters is the speed of the wind relative to the cart.
idontcomment

#122 – yes i think the tide is turning! Maybe I can go to sleep!

yay iceboats! some good descriptions here, eg 80, spork, or alpine for bring back the goodies from last time ; )

Also #69 is great! That’s similar to the way I first convinced myself this is possible : )

Paper faster than the Paper is a great visualization, and #128 moving directly upwind I think it pretty nice vis. also (though its not obvious to me that that’s exactly the same – i think moving upwind you have reversed the air and ground also?)

some specific comments:

#81 – what’s magic about the propeller is that its pushing the air backwards, not the ground. You’re harnessing power from the air/ground speed difference, so you must slow the air or speed up the ground. Pushing with a wheel slows the ground (wrong direction) whereas generating power from a wheel on the ground speeds the ground up (reduces wind/ground delta). If you could increase speed in some non-propeller way, it wouldn’t be good because now you’d be outpacing the wind and no longer feel any force there – the propeller surface allows the wind to act on you even though your body may be going too fast.

#83 – in part B, it’s not the case that the cart with the generator would slow to a stop. The generator will apply some breaking force, and the cart will come to an equilibrium speed. You can even increase your sail size, and either raise the power you can generate at that speed or the speed at which that power is generated. IE, you can get arbitrarily close to wind speed generating arbitrary power. If you use that power to pull yourself forward through the wind, you can increase your speed.

#68 (and others) – there’s this idea that the breaking force on the wheels used to generate the fan power will be equal to the extra speed the fan can produce. Pretty reasonable on the surface, but these are not zero sum. For any given breaking force we want, we can just increase the wind cross section to get our un-powered speed arbitrarily close to wind speed. now if we actually use the power we’re getting from the wheels, we can pull our way forward through the wind exceeding its speed. The power we need to move the cart faster by a given amount is governed only by the drag of the passive components and wheel friction. The power we get from the wind is based on the wind cross section (well probably more complicated than that, but certainly bigger props means more wind force). It happens both things affect cart speed, but they are not zero sum! If they were (say we decrease our speed by 1mph through our wheel breaking force, but that is used to power the prop which increases our speed 1pmh) we could just increase prop size, then suddenly the same amount of breaking is only slowing us down by 0.5mph, but we are still producing enough power to increase our speed by 1 mph! You’re thinking, but couldn’t we do that all day and have infinite energy? Probably, but only in the same way you could make a windfarm bigger all day.

#91 – The propeller is being spun by the wheels as you say – every last bit of extra oomph it can add to the speed is coming from those wheels. That doesn’t mean that every bit of speed added to the cart comes from speed removed from the cart. The wheels transfer energy from wind pushing the cart into fan motion – an arbitrary amount of wind. want more power, build bigger propellers. to use that energy to increase speed, you must only overcome frictional losses. i think #128 covers this well.
Chrs

Of course it’s a pissing match!

On the simplest point to debunk: If any sort of clutch were involved to prevent the propeller from working against the motion of the cart when moving faster than the airspeed, how, exactly, do you plan on getting faster than the air in the first place?

Another simple explanation (avoiding the energy math):

In order to reach a speed higher than windspeed, it is necessary to pass through a transition when the air around your cart is, relative to you, motionless. If you can extract mechanical energy from still air, congratulations!
spork

My buddy JB and I made the cart in the video in question. We have posted the complete parts list and the key build notes so that anyone can build one for themselves and test it under whatever conditions they like. I have agreed to build about 12 of these for people both skeptics and believers that want to test them.

The cart is real and there is no trickery in the videos.

The cart does not represent perpetual motion. It is a simple novelty, but something of a mind-bender. It is wind powered, but through a somewhat clever mechanism is able to outpace (but not outrun) the wind. This mechanism, like a mechanical gear, allows the cart to trade force for speed.

The energy is exploited from the ground/air interface. The cart leaves in its wake a “tube” of air that is now moving downwind more slowly than the free-stream wind, it’s energy having been used to propel the cart.

If the wind stops, the cart stops.

I’ve explained this MANY times on many forums, and I regret that my patience runs out when I’m called a fool or a scam artist. I’m neither – and I have nothing to gain by scamming anyone. So, I will do my best to explain this here and now, and will try my best to answer any questions – I just ask that I not be attacked for my effort.

First question: can ANY wind powered vehicle go directly downwind faster than the wind (never mind our little cart)? Ans: YES.

Let me explain… We know that ice boats can follow a downwind course – say 45 degrees off the direct downwind vector – at several times the wind’s speed. This means they can actually take two such tacks and arrive at a point well downwind of their starting point before a leaf floating in the wind that started at the same point.

This is true. I do NOT mean that they can simply go cross-wind faster than the wind. They can and do maintain a steady-state course whose downwind velocity vector is 3X to 4X the wind speed. We know this from measurements (GPS and anemometers), as well as from simple vector analysis. I won’t clutter this post with that, but I will be happy to provide such evidence upon request. You can also look at the response when the question was posed on a landsailing forum.

Now, let’s imagine I have two of these ice-boats that can tack their way downwind faster than the wind. I can start them side by side and keep them on alternate tacks as they do their thing. I could in theory connect the two with a telescoping pole with a seat right in the middle. I could then sit in that seat and I would travel downwind, faster than the wind. And my vehicle (which admittedtly has some significant moving parts) would have it’s center of gravity also traveling downwind faster than the wind.

Does this in and of itself prove our cart can or does do as we claim? No. But it does, and I’m confident I can convince most anyone with an open mind (as I have with many already).

And I ENCOURAGE people to try this for themselves. I have nothing to hide. I’ve posted the complete parts list in many places, and I’ll be happy to post it here. I will answer any serious question to the best of my ability, and do my best to insure anyone who attempts to duplicate our results succeeds. With the right parts it’s really not very difficult.

I am aware this is only a little teeny part of the story, but it would be very easy to write a chapter on this silly thing. As I said, I have nothing to hide, but let’s start with this, and I’ll add to it as we go. Fair enough?
ztoop

To elaborate on my last two points, I would like to encourage everybody to think of forces as opposed to energy (which is a force over a distance).

Here are the two situations, one everybody agrees with (a sailcart) and one there is some confusion about (the cart)

If we have a sailcart, there are two main forces on the cart, the push from the wind (which is essentially a drag force in the direction of travel) and the friction of the wheels. We all agree that when the vehicle is going at a steady speed (which is less than the wind) the force of the wind on the cart equals that of the friction of the wheels. We have equal and opposite forces, and the cart stays in uniform motion. If the cart were to travel the same speed as the wind, it would only have the resistance from the wheels, which would cause it to slow down. This we all agree with. It can’t go faster than the wind, because then all the forces would resist its motion.

Now the cart with the propellor which is attached to the wheels. In this case, if the cart is moving slower than the wind speed, it will also feel the push from the wind, the friction from the wheels, two additional forces: The propellor is pushing air backwards, applying a force in the direction of travel. At the same time, the propellor blades are going very quickly through the air, this is effectively a drag force on the blade, which will resist the motion of the wheels. (* The resistance on the wheels however, is almost always less than the force of propulsion… here we again can compare it to a plane where the wings provide lift, and the drag on the wings is less than this lift and is offset by the relatively week engines). So the net result of the propellor is always a force forward if the cart is moving forward. In this case, we see that where the sail cart would stop, this cart should move faster since it has an additional forward force.

Now, if the cart is moving the same speed of the wind, there is effectively no wind, and it feels only the drag of the wheels and the propulsion of the propellors (which we said before is always postive). If the wheels have only a small amount of friction, this means the cart _must_ move faster. The forces aren’t balanced until after the cart goes faster than the wind and the drag on the front of the cart resists the additional force of the propellor.

However, with the propellor (which is attached to the front wheels) also applies a force. Pressure gradients across the blades cause a net force to be applied to the blades in the direction of travel. There is some friction on the blades which introduce more drag in the situation ( but think of airplanes: the engines face forward because that friction is less the amount of lift you can obtain). The net result is that the craft feels a net force forward if the propellors provide more force than all the drag combined. This net force results in an acceleration forward, causing it to go faster than the wind, until the force of the blades is offset by the drag on the blades, wheels, and headwind. This objects isn’t going to oscillate significantly like some propose because friction is only a dissapative force.
SubductionZone

Pizza, there is one decent place I know of in Everett. Maybe my girlfriend will be amenable to a night out, though I know she would be bored by tech talk.
HarveyBoing

Here’s the thing: there’s nothing magical about a propeller in how it converts energy into thrust. It is not any different from a driven wheel in contact with the ground, other than that it accelerates air to create an opposite reaction, rather than pushing directly on something.

In fact, because of the mechanism used, it’s actually not as efficient as a driven wheel in contact with the ground. There is some “slippage” in a prop, with relates to its efficiency, whereas a driven wheel can (as long as it has enough traction) transmit nearly 100% of the input force as thrust (rolling resistance saps a tiny amount of this).

Now, I think everyone can agree that if you took the wheel(s) used to drive the propeller, and instead connected them with a direct driveshaft to another wheel in contact with the ground, that this would not in fact provide any net positive thrust to the vehicle. So, why are there people so willing to believe that simply by using a different mechanism to create the thrust, this basic fact can be ignored?

If there were something unique about the use of a propeller to convert mechanical energy into thrust, we’d have all sorts of vehicles that use propellers for their locomotive power. And guess what…we don’t!
SubductionZone

For those who argue that the wind cannot give any force to the propeller once the cart is moving at wind speed then how do you explain the well documented fact that ice yachts can go several times the speed of the wind with a slight downwind tack. It is not directly downwind but its downwind portion of its velocity is much greater than the wind’s.
ThinAirDesigns

Kerinin:
>How is this at all controversial?

In case you missed it, here’s the controversial part:

It doesn’t do what you say it will do. Instead of “slowing down”, it powers right on past the speed of the wind and in this race leaves the wind cryin’ for it’s mommy.

JB
rasz

All we need now is Charles Platt to write MAKE piece about him eating a sock.
yesno

As many on this thread know, Mark at “Good Math, Bad Math” is convinced that it’s possible and has changed his blog.
spork

Ooh! Ooh! I know this one. It’s number 1 – right?
ThinAirDesigns

Greg:
>people who don’t care about whether
>they end up meeting Jamie and Adam

If you think this about meeting ‘famous’ people, you haven’t been noticing the genuine Emmy statuettes on the mantle in the background of various treadmill video shots. We interact with those sort of people on a daily basis in our work and it’s not the thrill people might think it to be.

If you think this partly about meeting and networking with bright, talanted, people who love a mental and mechanical challenge, you would be right.

Our list of people with whom interactions fall into the “time well spent” catagory, has grown very nicely through this affair.

JB
spork

>>
Um, no. The helicopter blades don’t start turning the other way, and there is no upward flow of air through the rotor. Autorotation works by using the *rearward* (horizontal) flow of air to keep the blades rotating and thus keep pushing the air down. For gliding helicopters it’s gravity and angle of descent that provides the flow, for autogyros it’s a regular prop.
<<

With all due respect, I’ve done one heck of a lot of autorotations with R/C helis, been in the left seat for a few in real helis (with instructor in the right seat), and studied the heck out of it. The collective is set to negative pitch for R/C helis (anywhere from -3 to -15 degrees) and anywhere from about +0.5 to -1.5 or so on full-scale helis. The rotors continue rotating in the same direction, but the flow does in fact move through the disk in the opposite direction as the heli glides down in a very controlled fashion (with complete cyclic and collective).
Anonymous

Dust:

I think you need to be careful about reasoning with your “energy flow” concept. The idea seems to be that energy “flows” out of objects which are moving, and into ones which are still. But motion is a property of the selected reference frame, so the “direction of energy flow” is not Galilean invariant. Forces are invariants, so work with them.
spork

Well I just slammed together a quick draft of build notes for the DWFTTW cart. I plan to flesh them out significantly – soon.

http://www.hanggliding.org/viewtopic.php?t=9340&postdays=0&postorder=asc&start=480
Anonymous

Hey Greg, since you went to the trouble of building the tumbleweed, you might as well try running it on a treadmill just for the sake of amusement. Go to the gym and find two side-by-side treadmills, and set them to run at the same speed, so that you can put each wheel on one of them. The speeds won’t match perfectly, so it will drift, and the whole thing won’t be able to advance because the vanes don’t get feathered, but you might as well do something with it.

jjcote
spork

See post #485 for those build notes.
Chrs

Ah, the point of extracting energy from wheels:

Say you’re travelling ten miles an hour, with a ten mile an hour wind. You have zero energy being added to your system by the wind. Any energy you extract from relative motion with the ground (i.e. spinning wheels) must be taken from your kinetic energy, and by reducing your kinetic energy, you slow down.

There is no free lunch.
davefoc

Hi folks, there has been a long discussion going on at the JREF forum about this.

Personally, I have almost zero doubt that it is possible and that it has been demonstrated.

There is a fairly simple plausible explanation available. My cut at that:

Energy can be recovered by extracting it from the rotation of the axle driven by the wheels.

In the normal situation this will slow the card down to something less than the wind speed when energy is extracted from the rotation of the axle.

However that available energy can be used to spin a propeller to provide thrust.

At first it might seem that the energy available will be less than the energy needed to propel the cart above wind speed, but that energy is actually very small because the wind drag on a vehicle is extremely small at very low velocities and the card just has to creep above the wind speed to be going faster than the wind.

Whether this thing works depends on whether the enough thrust can be produced to overcome the losses to forward motion caused by the resistance to rotation transferred back to the wheels by the propeller and the aerodynamic drag of the propeller and the cart as the cart goes past the downwind speed. The mathematics to analyze that are beyond me but the empirical evidence is moving in the direction of overwhelming that enough thrust can be generated despite the various losses to make this thing possible.
Takuan

we have to set up a demonstration contest with fatal results for the loser.
Scuba SM

Several commentators have claimed that the situation described in the post is not how the vehicle operates at all. I wanted to think through how a device like they described would operate.

Problem Statement:

Cart is accelerated to wind speed NOT by the propeller, or by a power source to the wheels. It is accelerated by the force of the wind on all the components of the cart, much like spinaker sail or a sea anchor. You arrive at the condition where the airspeed around the cart is at zero, but the wheels are spinning.

Givens:
1. “An object at rest tends to stay at rest, and an object in motion tends to stay in motion, unless acted upon by an outside force.”
2. “For every action, there is an equal and opposite reaction.”
3. The cart is constructed such that when the cart moves forward, and the propeller turns, the air is pushed from the front of the cart to the rear of the cart.

Assumptions:
1. All internal bearings and gears are assumed frictionless.
2. The wheels are assumed to roll without slipping.
3. The propeller is 100% efficient, meaning all work put into spinning the propeller is used to move air straight backwards.
4. The propeller, wheels, axles, and gears are all massless. The cart, however, does have mass.

Solution, Part A.

First, consider a cart that has no propeller attached. It is traveling at same speed as the wind, and it has no friction on any of its axles or bearings. “An object in motion tends to stay in motion unless acted upon by an outside force.” This cart, with the assumptions stated, would glide at a constant velocity forever.

Solution, Part B.

We will take the cart explained in Part A, and attach the propeller mechanism to it. The cart is moving at the same speed as the air around it. It will continue at this speed unless a force is applied to it. So, we will use the propeller to apply this force. When the propeller turns, it pushes air; it applies a force to the air for some distance (The definition of Work). A force has been applied to the air, an equal and opposite force must be applied to the propeller (the same amount of work). This work is supplied by the wheels. In order to supply this work, however, the wheels must have a force applied to them over some distance.

This is where it gets a little tricky. In our solution to Part A, the cart continued to roll forever because there was no force acting on the cart to slow it down, and there was no force acting on the wheels to slow down their rotation. Now you are demanding the wheels provide some amount of work for the propeller. The work must be provided by a force applied to the rim of the wheel. The only place that this force can be applied is where the wheel touches the ground. In order to produce this force where the wheel touches the ground, you must have an equal and opposite force. This force resists the rotation of the wheel. If you took our cart from Part A, and instead of putting a propeller on it, you connected a perfectly efficient electric motor/alternator and battery on it, and demanded the same Work (force times distance) out of the wheels, the cart would slow down to a stop. Using your perfectly efficient motor and battery, you could then take the same amount of work, and propel the cart back up to the original steady state speed. In our wind cart, however, we’re using the work produced by the wheels to do work on the air, and push the cart. However, the work being done by the wheels (which is slowing down the cart) exactly equals the work by the propeller (which is speeding up the cart). These two cancel each other out exactly; therefore this cart will also continue at constant velocity forever. You will note that this cart cannot go faster. To go faster, your propeller needs to produce more work than the wheels put in, which is impossible without some other source of work (like an engine).

Solution, Part C.

If you examine the assumptions, you’ll find that several are not reasonable in the real world. Propellers, for one, are no where near 100% efficient. They impart a lot of turbulence to the air they move, which results in some motion of air in directions other than straight backwards. Your bearings and gears will also have friction in them. These will all conspire to slow the cart down, so it will not glide forever.

Conclusion:
The cart cannot sail downwind faster than the wind speed. The best it can ever hope for, under ideal conditions, with idealized components, is to sail at the same speed as the wind when traveling directly down wind.
airshowfan

Ok, I have gone into ScienceBlogs to carry on this battle ;)

http://scienceblogs.com/goodmath/2008/12/windpowered_perpetual_motion.php#comment-1244678

What I am going to do is create a webpage where I lay out the thought experiments, analogies, math, and Youtube videos, that show that this should be possible. I will the just start linking to that.

And my website is AirShowFan.com. (Sorry, I have not updated it in over a year). Anyone can feel free to email me from there. But I will probably ignore any emails that say that DWFTTW is impossible…
ratwerks

Well, I was leaning toward number 2, but now I’m not so sure. Has anybody eaten that worm so they can regurgitate it into my open screaming mouth?
All Jelly No Toast

Ok. I think, finally, I see where you folks are coming from.

So, essentially, if there is no prop, this thing would act analogous to a balloon in the wind, moving nearly at wind speed.

Now, with the prop, and the gear, the gear/prop essentially adds extra resistance for the wind to push on (similar to if we add a sail: more for the wind to push.) Bigger sail=more force captured from the wind.

I think the idea is that the gear captures energy from the wind. I.e., even though it’s extra resistance on the car, it can still ideally get near wind speed, it just might take longer to accelerate.

The ingenious part seems to be transferring this energy captured by adding in the gears, and transferring it to a propeller (better thought of as a fan, in my mind,) on back of the thing which then pushes backwards giving it thrust.

The way I’m envisioning it is like this: imagine the same cart had an obscene gear ratio, like 100-to-1, so that if you pushed it by hand at say a steady 1 mph, it would be hard to do because so much of the energy is transfered to making the fan spin.

What these folks are proposing is that if you push it at a steady 1 mph, you’re capturing a lot more energy than if you push it at the same speed with a smaller ratio.

Where they seem to lose me is that this will ever make the thing go faster than the 1mph we’re pushing it at. What it would likely do is just make it a lot easier to push, as it goes and the wind keeps pushing it, making the prop (but not the wheels! They’re going to stay at 1 mph) go faster, until you don’t need to apply any force (this is where it reaches wind speed, or hand speed in my mind here.) However, all that this seems to do, and I cannot get past this, is make it really easy to push at a steady speed after you expend a lot of energy getting it up to speed.

I can’t buy it that this works to do what they want it to do.
spork

“Why is everybody focused on downwind faster than the wind, can they already go upwind?”

Going directly upwind is a piece of cake. In fact this very cart will do it nicely. All we have to do is put smaller wheels on it.
joelphillips

#17

… the presence of such a justification doesn’t mean that the proponents aren’t arguing in favor of a perpetual motion machine. It just could mean that they are following the standard template for proposing perpetual motion machines …

The flip side of this is that there’s an onus on the people on the “you’re creating a perpetual motion machine and therefore what you’re doing is impossible” side of the argument to demonstrate how to create a perpetual motion machine out of the device.

Here’s one way you might try to do that, and the reason why it wouldn’t work. We need a closed system, so lets build a circular track. We need wind relative to the ground, so lets enclose the track in a toroidal bubble and build the track and the bubble out of a frictionless material and establish an non-turbulent air flow around the torus. Lets assume that the spinning propeller don’t create any turbulence (maybe rather than a cart, we mount lots of propellers on a second circular platform that sits on top of the circular track with lots of frictionless wheels the whole way around). You still don’t get a perpetual motion machine, because the propeller pushes air backwards out of the cart – in the opposite direction to the air flow. Eventually the velocity of the air in the torus will go to zero.

I think that the same principle will break any perpetual motion construct. The cart slows air down as it passes. In a closed system, you’ll eventually have to revisit the same places and every time you visit it, the wind will be less. Eventually you stop.
shMerker

Everybody knows that a tailwind helps a plane move faster even if it is already moving downwind faster than the wind right?
airshowfan

Can they go upwind? I think so:

http://www.youtube.com/watch?v=XRYkIzjmCR4

http://www.youtube.com/watch?v=23yfmslvpzY

http://www.youtube.com/watch?v=0lP3f4d8rdc

http://www.youtube.com/watch?v=NOSFpnbnGjY
Takuan

got it: zero point energy!
ictharus

I have an idea that I’m pretty sure would work (but of course isn’t as simple as just attaching a propeller to a pair of wheels).

If the contraption brakes, it can use the propeller to charge up some sort of energy storage (i.e. rubber band, spring, battery). Once it’s stored up a bit, it can release the brakes and the energy will push the machine forward a bit, brake and start storing up energy again.
ratwerks

Spork, JB, drop me a line at jr@ratwerks.com or find me as ratwerks on the major IM services. Maybe we can grab some beers. Also, I have contacts at Fort Funston.

JR
JakeTheSnake

@197

Huh, well, there goes intuition as an argument …

Ok, why don’t these little windcars accelerate on a still day if you just give them a push?
spork

Greg, you have to decide if you’re an open minded guy willing to learn something, or a guy that knows everything and you’re just here to insult others. If you don’t come back with insults I don’t come back with your old quotes. It’s pretty simple.

“And I’m done apologizing to someone who has no interest in accepting my apology.”

I accept your apology. And if you start it up again, we’ll go through it again.
GregLondon

It will work great as long as no actual testing is required.

I guess I’m just lucky I figured it out before I actually built it. Weird.
Takuan

a cart rolling down an inclined plane that uses the Bernoulli Effect to create pressure differential from the “rammed air” that is used to further accelerate the cart faster than gravity alone would pull it down the ramp?
Anonymous

“It’s not a perpetual energy pyramid; it’s a perpetual energy trapezoid!”
ztoop

@#83,

You made a critical error in your part B. The opposite force of the propellor is on the entire cart, not the wheels, which is why it accelerates forward. The propellors also feels friction from all the air it is cutting through, but this is typically much less than teh amount of ‘lift’ the propellor gives. Look at airplanes flying with tiny engines and big wings. There reason the engines face forward is to resist this ‘drag’.
spork

By the way, do you see me going at it with Mark C? No. He admitted he made a mistake, I accepted his apology, and we left it at that.

Now I’ll admit, Mark’s was a world-class apology. I don’t intend to ever hold anyone to that standard. But you can’t apologize and then start right back in with the insults.
spork

Mark, we commonly think of a wheel as rotating about it’s axis. But in reality the wheel rotates about the point in contact with the ground (which is constantly moving of course). So if you think about the torque placed on the yo-yo when you realize the yo-yo pivots about the ground contact point, you’ll see the torque is actually in the direction of rolling the string up.

The greater the diameter of the hub, the greater the speed of the yo-yo relative to the object pulling it. This is true up to the point where the yo-yo hub is exactly equal to the yo-yo diameter. This ratio is directly analagous to the “advance ratio” of the prop-cart. The closer the advance ratio gets to 1.0, the faster the cart wants to go relative to the wind speed, until all of a sudden… if you use an advance ratio greater than 1.0, the cart will now go directly upwind rather than downwind.

This is the same for the yo-yo. If the hub were bigger than the diameter of the yo-yo (which could only happen if it were straddling a trough) then pulling the string would make it go away from you. But approaching 1.0 from either above or below, will increase the theoretical speed toward infinity (with both the yo-yo or the cart).

This is the best example I’ve seen to demonstrate this effect:
http://www.youtube.com/watch?v=E7vcQcIaWSQ
Bill Beaty

Here’s a major issue: Perpetual Motion Machines (PMMs) are described wrong, since a PMM is supposedly an energy source and not a low-friction “coasting” effect. A PMM is a perpetual acceleration machine. For example, a frictionless flywheel is not a PMM no matter how many centuries it can coast. A true PMM flywheel would speed up until it explodes.

Knowing that, look at the FTTW device. Suppose it’s already moving FTTW. Does it supposedly extract energy from the relative motion, and therefore move faster and faster? Yes, since if it didn’t, it couldn’t exceed wind speed in the first place.

From where does the energy come? Nowhere: the device just accelerates. What keeps it from accelerating to destruction?
Anonymous

This is a classic example of a perpetual motion machine of the second kind.

http://en.wikipedia.org/wiki/Perpetual_motion

As they say, those who fail to learn the laws of thermodynamics are doomed to make perpetual motion machines.

The first law of thermodynamics basically states that you can’t get something for nothing.
ie. the change in energy equals the work done plus heat added or removed.

The second law states that you can’t break even.
ie. there is always non-conservative forces such as friction that cause energy losses.

The third law states that no matter how hard you try you can’t win. ie. you always have entropy.
dustbuster7000

The treadmill. I’ve been giving this part a little thought as I could muster, since I wanted to concentrate on the device in the situation that it was designed to work in, i.e. outside in a tailwind. Now that I have JB’s (but not Spork’s) confirmation of how they say it works, I started thinking about the treadmill. Since looking at the video, it clear that the cart keeps pace with the treadmill by spinning its wheels and then that is converted into thrust from the propeller, thus the cart advances up the treadmill. The prop thrust is greater than the friction and mechanical losses between wheel-treadmill interface and prop-air interface, cart goes forward. No problem.

So all that remains is to check if the treadmill is equivalent to the claimed case, outside on a flat surface, travelling the same speed as a tailwind of some velocity.

Outside case: energy leaves the cart via the wheels, since its moving forward. Prop only provides thrust, per JB confirmation, thus no energy is entering the cart. Even if JB was wrong, the prop can’t draw energy from the air, since the air is still relative to the prop.

Inside case: energy enters the cart via the wheels and leaves the cart via the prop, which causes the observed forward motion of the cart.

If the two cases were equivalent, energy flows would be the same, but in opposite directions (since the object which is moving has changed). In this case the energy flows are not the equal but opposite, thus the cases are not equivalent.
Alex_M

Seems to me most of the skeptics here are basing their arguments on fairly simplistic models. (As in #1 – the energy should be expected to come from a pressure differential, not the momentum of the air) And sometimes incorrect assumptions. (in #1 – the second law of thermodynamics applies only to heat transfer)

I’m one of those folks with a physics degree and I can’t say whether I think this is possible or not. I do say I know enough to know that I don’t know enough. Aerodynamics and fluid dynamics are very unpredictable things. It all boils down to simple but unsolvable differential equations. Which is why we still use wind tunnels, although we can now do a lot of the math with FEM methods on high-powered computers.

Or in short: Stick it in a wind tunnel and find out. Speculating on aerodynamic effects is very risky.
spork

“Some people get to teh top and look at that as the final measure of their accomplishment. “Look what I did.” That’s fine. On this thing, my measurement isn’t whether I made it to the top, but how many people I can help get there too.”

If all I wanted to do was build the cart, I wouldn’t spend a ludicrous number of hours trying to help others understand how it works. Nor would I publish the build docs, and even build a number of these things for people I’ve never met.

“If when I finish this paper, we take it to some place where there is an ongoing argument about the propeller cart, and drop it in there, and the argument ends, I’ll assume people used the map to get to the top and now understand it. And then I’ll have accomplished what I wanted.”

That is admirable – honestly. If I may offer a word of advice… prepare yourself for continuous merciless personal attacks from people that don’t have a clue what you’ve done or what you’re talking about. People will CERTAINLY tell you that it’s a hoax while others will claim you don’t have any idea how the thing works.
Alpinwolf

*PAUSE*

Oh, for the love of FSM…

Ur doin it rong!!!!

I’m one of the proponents from “Part 2″

I know how this works, there is no free lunch, if the wind dies it would stop, and it gets old repeating arguments, especially to what seems to be an entirely different crowd. Very little overlap here from the Monday crowd.

IDONTCOMMENT ans SHMERKER are doing it right.

Most everyone else is doin it rong, either confusing units, like velocity and force and momentum and energy, or maybe misunderstanding first principals or reference frames, or starting with bad assumptions…

Charles Platt, unfortunately, (and I’m talking to our original poster Mark too), he is doing it very wrong by having the prop reversed. Or the drive belt; switching either would fix it.

The drawings at the top of this page have the blades set as a turbine, and behaving as a turbine, which is backwards. IGNORE THEM!

I don’t intend to be curt, but I spent some time crafting what I thought were good descriptions of the operation in Monday’s thread.

See post #115 and #239 from myself (and #289 if you need to), and anything from AirShowFan. Mine are lengthy, but some readers found them helpful, so give ‘em a try. There were other good points as well. ThinAirDesigns and Spork are the builders, so obviously they have points, but they kinda got singed in the debate (which seems to be active on multiple disparate forums) so they responded less. I’m beginning to know how they feel.
Monday’s post:
http://www.boingboing.net/2008/12/01/downwind-faster-than.html

I don’t think it would be good form to cut and paste 2 to 3 forum-novels just to repeat myself here.

Also, browse YouTube for stuff about “DDWFTTW”, as it shows the builders’ cart self-starting outdoors and indoors, a pretty good view of how it’s made, and some other folks’ vids on related concepts.

Let me know that you’ve read the posts mentioned, and if that helps. If you think I’m full of it as well as the builders, I’d be happy to answer specific questions, but please: have a somewhat open mind and don’t come in shouting “you can’t do that!” or “prove it!”, because these are not constructive.

*PLAY*
ThinAirDesigns

As much as I would like to, I don’t subscribe to the “diminishing traffic = truth won out” theory.

If I were over on some perpetual motion thread arguing with them, I would just eventually leave out of exhaustion and they would still be there spouting.

For the most part, the people who have left this thread did so for the exact reason above. We are nothing but the PM crowd to them.

The nice thing of course it that we know how the story ends and the humble pie delivery has their name on it and not ours.

JB
GregLondon

Greg: ropes pull, pulleys spin, and your particular device can never equal the speed of the wind. It’s the law

So, the rope/pulley diagram on page 7 is wrong?

or the use of fixed blades to implement the rope/pulley is wrong?

Cause I still don’t see anything wrong with the rope diagram. The thing I see I did wrong was picking fixed blades to try and implement the rope on the inner pulley piece of the diagram.

But if the plates flap, that should fix it so it behaves similar to the rope/pulley.
spork

“If we’re handing out advice to one another, I’d say it’s all about the narrative. It’s not about whether or not you understand how it works. It’s about whether or not you can understand where the other person is stuck.”

In this particular case the other person was “stuck” calling me a fool, telling me the I didn’t understand, that he did, and that he could give me a 100% guarantee that his design would work and didn’t even need to be tested.

“And how do you think they take it when you ask them to bet you 100k? Are you helping them get unstuck? Or are you taking advantage of the fact that they are stuck? Just because someone doubts the car doesn’t mean its a personal attack on your integrity.”

I consider that taking advantage of someone that’s attacking me. Do I need to actually quote all your personal attacks on me?

“I swear, if you’d discovered this thing a couple hundred years ago, you’d be shouting “Pistols at dawn!” to doubters”

I can easily point you to hundreds of very civil posts where we debate this with doubters. And I can find you just as many where we respond in kind to fools that attack us.
winterwyn

Okay okay, against my better judgment, here’s my understanding of the problem with this device that prevents it from working as claimed.

First, it’s supposed to be being pushed by the wind. Assuming everything’s 100% efficient with no friction, it will approach the speed of the wind ever more narrowly but not reach it. No problem here.

Next, add gears and a propeller attached to the wheels. These are supposed to push it even faster. But since it’s basically done being pushed by the wind, the only source of energy to drive the propeller is the momentum of the device.

The propeller is geared up really fast, which makes it more efficient, but not more than 100%. More distance in turns means harder to drive – the same force is required either way. We can assume the propeller is doing fine at pushing, but even if it’s nearly 100% efficient, all of its force comes directly from the momentum of the device via the wheels. Any inefficiency means the propeller pushes with less energy than it drains from the wheels. The net speed is negative. It’s essentially a very bad brake.

The twist which is probably causing all this ruckus is that having it geared up fast like that makes it a good flywheel, which stores energy between fast and slow input. A gust would fairly easily push it above the regular wind speed, but a regular flywheel would be better at it and you would get a higher average speed.

I hope this additional explanation on top of all the other ones adds some light. It was fun, anyway. :)
spork

>>
So, the rope/pulley diagram on page 7 is wrong?

or the use of fixed blades to implement the rope/pulley is wrong?
<<

I win. Looks like we’re going with option 1.
spork

“If there were something unique about the use of a propeller to convert mechanical energy into thrust, we’d have all sorts of vehicles that use propellers for their locomotive power. And guess what…we don’t!”

You’re absolutely right that propellers have no intrinsic magic. We simply use a propeller because it’s what’s needed to propel our cart directly downwind faster than the wind. We could use a sail to tack our way downwind faster than the wind, but that’s less of a mind-bender (which was the reason I originally posed this question 3 years ago).

But a propeller is nothing more than 2 sails attached at the base and spinning in a circle. Part of the “secret” to this cart is that the tips of the propeller are on a continuous downwind tack. They move as far around the circle as they move forward in any given chunk of time. This means they will always feel a relative wind even when the cart is moving at exactly the speed of the wind.

In fact the tips of our prop behave exactly as the sails of the ice-boats on a 45 degree downwind tack. The wheels and transmission simply exist to provide the same kinematic constraint as the blades of the ice-boat – namely to force the blade tips to rotate an equal amount to their downwind travel.
spork

“Ok, why don’t these little windcars accelerate on a still day if you just give them a push?”

They get their energy from the wind over the ground – not the wind over the cart. They trade force for speed. So if you gear them low enough to work against a head-wind, they’ll do just that.
Alex_M

Spork @ #139: I wasn’t arguing against the treadmill. What was first in my mind when I said “stick it in a wind-tunnel” is that it’s an environment with a variable, well-measured and well-defined wind speed and flow.

I do believe that there are wind tunnels that allow for moving objects. But I don’t see any problems with putting it on a treadmill in a wind-tunnel either.
spork

“isn’t there a type of propeller that has non-tilted blades that are airfoils, and rotates in the same direction even if the airflow is reversed? The propeller is symmetrical when viewed edgewise.”

I don’t know if there are propellers intended to operate that way, but a propeller with 0 twist and symmetric airfoil does operate that way. One example is a helicopter rotor during autorotation. Some helicopters actually autorotate with slightly positive pitch. Full scale helis rarely have symmetric airfoils, but it’s the norm on R/C helis.

“I would love to believe that the decrease in traffic necessarily means that “DWFTTW is possible” has won…”

I think it does mean that to an extent. I think those that were absolutely certain it can’t work may only be saying “maybe I’m not so sure anymore”. But I’ve been through this process far too many times. There’s typically one guy left with absolute conviction that just won’t be swayed. I guess we’ll see if we have that guy here.

I’m extremely encouraged that Mark F. seems to have an open mind on the topic, and I sincerely hope he’ll allow me to do a build article on our little cart. The latter is seeming like a remote possibility to me at this point.
winterwyn

Sigh, and of course I said ‘force’ a couple times when I meant ‘energy’. Maybe I was using it colloquially, like ‘shove’ or ‘oomph’. Yes, that must be it, cough.
spork

For the record, I would love to meet Adam, Jamie, and the rest of the crew. And for the record, it never occured to me to try to get Mythbusters to do a segment on this until after I had already been debating DDWFTTW on forums like this for quite some time.

Finally, I’ve gotten just as much of a kick out of meeting (by phone) people like Jack Goodman, Andrew Bauer’s wife, and several of Bauer’s colleagues. It’s amazing to talk to them about the things they did during their career in and out of aero. And the most surprising thing – they seem to feel lucky to have people like JB and I call to ask them about their work.
GregLondon

Mark,

I uploaded a new revision

http://www.greglondon.com/tumbleweed/

I tweaked the yoyo-with-a-parachute-on-the-end-of-a-rope diagram so that the wheels have numbers all the way around them, so you can see it turning.

I also added the new diagram which took teh rope and pulley diagram and turned it sideways. It might help get how the rope/pulley system works.

Hope this helps,
Greg
Mender

“…why do I get tired when I run?”

Why do you get tired running on a treadmill? Can your muscles tell the difference?

The cart does not have an internal energy source. The energy needed for motion is provided by the speed difference between the air and the ground. Effectively, the wind is pushing against the ground through the cart, or the ground is pushing against the air through the cart. The forces are the same, the direction of the forces is the same, and the result is the same.

Just like squeezing a watermelon seed between your fingers – which finger provided the energy needed?
ThinAirDesigns

Among other things, I’ve got a ride-along version of the DWFTTW cart to build and test. I’m going to end (or at least suspend) my participation in this thread with the following taken from the top of the thread:

Mr. Charles Platt:

It bothers me that so many people are conned by this idea

The “con” is gaining steam.

It happens thread after thread — smart, careful, thoughtful and curious people are won over by the intriguing physics of this device and the reality of it’s performance. Others *belligerently* hold to their misconceptions, calling those who do grasp the concept every name in the book rather than listen to the answers given.

The second group usually doesn’t get it until making huge fools of themselves.

This “con” isn’t going anywhere but up Charles … it’s not too late for you to leave the second group and join the first — but the story of the principles and the device ends the same way in either case.

It’s real and it does what we say it does for the reasons we say it does. This fact is becoming more established with time and independent confirmation and that trend is accelerating.

Over on the Randi forum, a non-believer in New Zealand built a round ‘treadmill’ (turntable) because be believed it wouldn’t run longer than our tests show — of course he is now firmly in the believer camp.

One by one.

Best wishes all.

JB
Anonymous

#37: I disagree. The suction would only be directly towards the ramp, which is parallel to the car. In order for that suction to accelerate the car, it would need to be angled forward. If anything, it probably adds a tiny bit of weight to the car, causing increased friction, slowing it down.

Not only that, but the induced drag cause by the “lift” (directed down) would slow the car down as well: http://en.wikipedia.org/wiki/Lift-induced_drag
Bloodboiler

Here’s an easy way to think about this:
A 100% efficient propeller would be able to create a forward force equal to the backward push of the airflow to the propeller. Those conditions would allow the propeller to maintain constant velocity in any direction (including 0 m/s).
Giving the propeller a push against the air flow would increase the force the propeller generates exactly as much as force from airflow would increase, so the propeller would maintain its forward velocity in frictionless environment.
-> Perfect propeller in perfect environment would have no effect to its own velocity, let alone a car it is attached. Any wind resistance from the car, and the airflow pushes the car backwards.

If Mythbusters bust this, they should also take that gravity powered lamp “concept” from a while back to the same show.
Inkstain

Once people believe they have discovered a hidden truth, no amount of evidence will dissuade them. It’s like that XKCD.com comic.
ztoop

I encourage everyone to glance at this site on airplanes: http://www.livescience.com/technology/060828_how_planes_fly.html

The point is that the drag on the wing of a plane (or propellor) typically is much less than the amount of lift it can provide. In the case of the cart, the lift of the propellors pushes the _cart_, and the drag of the propellors resists the motion of the wheels. Because the two are not always equal, the cart can accelerate from the motion of the propellor _as long as_ the wheels are moving. This is why you need a net motion between the ground and air, to get those wheels moving.
weeble

Okay, here’s my guess at how it works, if it works.

We have wind-velocity (which is fixed). Call it W.
We have the cart-velocity. Call it C.
We have the rate of spin of the propeller. Call it S. It is directly related to cart-speed based on the wheel radius and the gearing:

S = k * C (for some constant k)

The rate of spin of the propeller, along with its pitch, determines another velocity, which is the speed at which air passing through the propeller would neither speed it up nor slow it down. Let’s call this P.

P = j * S (for some constant j)
So
P = j * k * C

Let’s call j * k our gearing constant. Name it G:
G = j * k

Relative wind is the speed of the wind relative to the cart. Call it R:

R = W – C

Now, when P < R, the wind pushes the propeller (and the cart) forward/faster. When P > R, the wind pushes the cart backward/slower.

I’m going to guess that the cart is designed such that -1 < G < 0. That is, the propeller turns at a speed that “blows” air backwards (relative to the propellor) more slowly than the cart. Suppose that G = 0.5:

When the cart is at rest:
C=0
P=-0.5 * C
=-0.5 * 0
=0
R=W-C
=W-0
=W
P
When the cart is moving at half the speed of the wind:
C=0.5 * W
P=-0.5 * C
=-0.5 * 0.5 W
=-0.25 * W
R=W-C
=W – 0.5 * W
=0.5 * W
P
When the cart is moving at the speed of the wind:
C=W
P=-0.5 * C
=-0.5 * W
R=W-C
=W – W
=0 (Yup, zero relative wind)
P
When the cart is moving at twice the speed of the wind:
C=2*W
P=-0.5 * C
=-0.5 * 2 * W
=-W
R=W-C
=W-2*W
=-W
P=R, so the cart experiences no force from the wind. It can’t get any faster.

If our gearing constant were greater than zero the cart would come to its maximum speed *slower* than the wind. At and beyond negative one, I believe the force of the wind pushing the propellor is no longer sufficient to turn the wheels.

Does that make sense? Is that what proponents are claiming?
misterfricative

@ Spork #207

‘[T]he prop ceases to act as a bluff-body on which the wind simply “pushes” well before reaching wind speed. As soon as laminar flow is achieved, the prop is acting as a lifting surface.’

Yes, I agree. As soon as the prop starts generating thrust, then it’s no longer just a passive ‘bluff body’. And with the gear ratios shown in the videos, this is already happening — ie the prop is already providing much of the thrust a long time before the cart system reaches wind speed. What I was trying to say was that the last bit of ‘passive help’ that the cart gets (ie just from being passively pushed by the tailwind) falls to zero when the cart’s speed matches the tailwind speed.

Re low gear ratios: yes, the gear ratio is absolutely critical. I was assuming that the cart would have as high a gear ratio as practical. But sure, it would be possible to set the gear ratio low enough so that it limited the system to a very low airspeed, where wind resistance would be negligible. Although I’m not sure why you might want to do that? I mean, sure, DDFTTW is the goal, and that would still be achieved, but hey, I’d like to see this thing hurtle along as fast as possible! Of course if I was riding it, or trying to get it to do some useful transportation work, then the design priorities would change — but at the moment I’m thinking F1, not station wagon :-)
Scuba SM

@85 ZTOOP,

I agree that the propeller generates a force, which affects the whole cart. However, it takes a certain amount of Work to spin that propeller to generate a certain amount of force. That Work is being supplied by the wheels. A component of that Work is force. Like the propeller, that force doesn’t just affect the wheels, it affects the whole cart. The forces generated are equal and opposite (assuming idealized components).

I do not claim that the cart would not be self starting, nor do I claim that it would not self-sustain. I state that the cart will not travel directly downwind faster than the wind.

@Spork

I would appreciate it if you could provide links to the data that demonstrates a downwind vector speed greater than the wind speed. I have been unable to find any data in my searching that supports that claim, and I’d be really interested in seeing it (if for no other reason than I’m a numbers/data geek at times).
spork

Some of my favorite Greg London quotes (these mostly just explain that he was “stuck” and asking kindly for answers):

Now that I’ve written it up, I don’t know why there is any controversy around this other than no one understood it well enough to explain it in plain english.

I don’t need experiements to prove the theory.
Since I don’t rely on intuition, I don’t need experiments and trial and error to figure out whether the concept will work or not.

Was this a mail order degree?

As an engineer, I’ve spent years working on projects where a screw up on my part means someone (or a lot of someones) could literally die a horrible ball of flame kind of death.
…
I’m sorry if applying that level of engineering rigidity pissed off a lot of people, but it seems fairly clear that a number of people who have “intuited” how this thing works, really have no farking clue how it works.
…
Really though, I don’t need experiements to prove the theory.
andyduncan

Yeah yeah yeah, but would it work if it was on a conveyor belt?
idontcomment

mender – how do propose to make one without a propeller – I’m curious!!
SubductionZone

Because they work off of the difference between the speed of the wind and the speed of the ground. Now it may seem silly to list it that way but due to the concept of equivalence of inertial frames of reference it is actually quite a handy tool to use. For example if you were riding on this vehicle with a 10 mph tail wind, but your ground speed was 11 mph DDW as you were riding the cart you would notice a 1 mph headwind, but there still would be a 10 mph difference between the windspeed and the groundspeed. On a still day the difference between ground speed and windspeed would be zero mph. So since you are working off of the difference in speed between road and air there would be no difference therefore no energy and you would sit there.
airshowfan

NelsonC,

I have done my best to consolidate the information…

http://dwfttw.blogspot.com

GregLondon,

Your Tumbleweed idea is similar to the very first DWFTTW vehicle I proposed, back when BoingBoeing didn’t have comments and the discussion was posted on Quicktopic. I explained it again here, comment #175, do a Ctrl+F for “ferris wheel”.

Spork,

I’m not 100% sure that that video shows a self-start. I mean, the cart gets blown by the wind, sure, but who is to say whether it will keep accelerating all the way up to, and through, wind speed. I’m still skeptical on that aspect. But, of course, you do deserve congratulations not only for your experiments but also for your patience on all these blogs and forums.
jeffareid

> 1. When the cart begins running slower than a tail wind, does the air move through the propeller from the back toward the front?

Yes, but the advance ratio (gearing, prop pitch) is less than 1, say 1:2 (prop effective pitch speed : cart speed), and friction in the tires will result in the prop turing against the wind, and so the prop acts as a combination bluff body (sail) and as a prop (some thrust generated opposing the tailwind).

> 2. If the cart can somehow accelerate faster than the tail wind (as its proponents claim), does this means that air will now move through the propeller from the front toward the back?

It’s a smooth transition. The prop is generating some amount of thrust regardless if the wind is flowing relatively forwards or backwards through the prop. Because of induced wash, the relative air speed through the prop is zero at some speed a bit below the tail wind. Once above this speed, the induced wash through the prop is upwind.

> 3. If the flow of air through the propeller reverses in this way, will it tend to reverse the rotation of the propeller?

Yes, but as stated in my response for 1., the advance ratio is setup so that the wheels have “leverage” over the prop, and they will drive the prop “forwards”, assuming the wheels aren’t sliding.

The prop can only turn “backwards” by spinning the wheels in reverse (which is what happens in one of the videos when a gust of wind hits a cart before it recovers).

> how it works

Perhaps an example with guestimate numbers. A tailwind of 10 mph, and the DDWFTTW cart moving at 12 mph. Assume advance ratio is 1:2 and that there are 16.7% losses. The exit velocity of the prop wash would be 5 mph (83.3% x 1/2 x 12 mph), in an apparent headwind of 2 mph, with the excess 3 mph being the result of thrust from the prop. The drag on the cart is 12 mph of rolling related drag, and 2 mph of aerodynamic drag. If 5 mph of acceleration of air times the effective area displaced by the prop produces enough thrust to overcome or equal the drag factors then the cart doesn’t decelerate.

The effective advance ratio multiplies the force (minus losses) and divides the speed from the wheel to ground interface to the prop to air interface. In a no wind situation, this doesn’t accomplish anything. In a tailwind situation, the tailwind compensates for the reduction of speed at the prop, so that the thrust at the prop is higher than the opposing force from the wheels driving the prop, and at sufficient speed because of the difference between the wind speed and ground speed.

The limiting factor is the advance ratio and the overall losses in the system. The same force used to drive the prop is also opposing the forward motion of the cart, but the advance ratio multiples the force (minus losses) at the prop, which is operating in a tailwind, so the cart can go faster than the wind.

The maximum speed of a DDWFTTW cart is

1 / ( 1 – (advance ratio))

advance ratio of 1:2 => 2, 2:3 => 3, 3:4 => 4, …
ThinAirDesigns

Dust, you’re “energy flow” post is fundamentally flawed. Rather than get into the specific flaws at the moment I would ask that you answer my below question. Thanks.

Again, for all the folks out there who are claiming that the treadmill in a still air room is not a valid testing environment for a DDWFTTW vehicle, please address the following:

——————————–
You wake up suddenly at my house — a place you’ve never visited and with no idea how you got there. You look out the window and see me standing in the middle of the street with this silly little DDWFTTW cart in my hand.

You walk out the door, issue the appropriate “WTF?” and notice a 10mph wind blowing down the street from the south to the north. You comment on it, and I respond by saying “the air is actually quite still … it’s the ground that’s moving” I continue with the explanation: “this entire town is on a giant treadmill — houses, trees, streets, everything as far as the eye can see and beyond is moving at 10mph”

Now, let’s pause the above for a moment and get to the heart of the treadmill equivilancy matter here –

Can you devise a test, or do you believe one can be devised that without outside reference can be executed in the middle of the street to determine whether I am telling the truth as to whether we are on a treadmill not?

You are allowed to enlist the services of anyone and any organization you wish, from Steven Hawking on down. I will also provide you with absolutely endless funding in your quest.

A: Do you believe such a test exists or can be devised?

B: Can you please describe it for me.

Thanks

JB
airshowfan

There are lots of ways to answer that.

– Because that would be perpetual motion.

– Because DWFTTW carts exploit the energy inherent in the difference in velocity between the air and the ground. On a still day, there is none.

– Because the prop would not make as much thrust when it feels a significant headwind (i.e. you pushing it into still air) as when it is moving forward slowly relative to the air (i.e. going downwind barely faster than the wind).

– Because it’s easier to push against the air at a slow relative speed when the ground is going by at a faster relative speed (so the air must be going by more slowly than the ground), as in the conveyor belt example.

– Because power = force x speed, so in order for the power into the wheels to be greater than the power out the prop but the prop force still being greater than the wheel force, the speed of the air through the prop must be lower than the speed of the ground past the wheels.
Anonymous

If the machine in question accelerated to the point where it was equaling the speed of the wind, it would be unable to continue to accelerate because there would no longer be an aerodynamic affect on the propeller.
cweiss

1. When the cart begins running slower than a tail wind, does the air move through the propeller from the back toward the front?

Yes, however the propeller does not windmill. It will begin to turn against the wind. Wind resistance pushes the cart forward, turning the wheels which set the propeller spinning against the wind.

2. If the cart can somehow accelerate faster than the tail wind (as its proponents claim), does this means that air will now move through the propeller from the front toward the back?

Yes, obviously.

3. If the flow of air through the propeller reverses in this way, will it tend to reverse the rotation of the propeller?

No, the propeller continues spinning against the wind. The important thing to keep in mind is that the propeller is not windmilling. It does the opposite, i.e. forward rotation of the wheels causes the propeller to spin to provide forward thrust.

It is not a perpetual motion machine. The cart can only go faster by a multiple of the wind speed. When the wind speed is zero the multiple is zero.

Also, the wind has many orders of magnitude more energy than the moving cart even when the cart is moving faster than the wind.
airshowfan

“The conveyor belt example” being Comment #67.
airshowfan

Indeed. Reason IS winning after all.

And I have finally put the time into making one comprehensive page that gathers all the points made to show that DWFTTW is possible:

http://dwfttw.blogspot.com

Please please please check it out, and let me know if I missed anything so that I can keep it complete.

And feel free to link to it (or link straight to one of its sections) when debating DWFTTW with skeptics.

I will add some diagrams and animated GIFs soon, but for now I need sleep…
HarveyBoing

You’re absolutely right that propellers have no intrinsic magic. We simply use a propeller because it’s what’s needed to propel our cart directly downwind faster than the wind.

Those two statements contradict each other. If there’s no intrinsic magic in the propeller, then you wouldn’t need to use a propeller to accomplish the goal. You could use any form of propulsion. Conversely, if you claim that you must use a propeller, that is the same thing as saying there’s some magic intrinsic in the propeller.

This means they will always feel a relative wind even when the cart is moving at exactly the speed of the wind

Propellers make their own relative wind, by spinning. It’s no surprise that they “always feel a relative wind”, but there’s nothing about this process that would allow you to get more thrust out of the propeller than you created in drag by turning it.
Jewels Vern

This is a thought experiment. The fallacy of thought experiments is that the universe often doesn’t work the way you think it should.

You can easily demonstrate this for yourself. Get two fans and turn them face to face. Only plug in one fan. Measure the RPM any way you can. The powered fan will turn at a constant speed, usually 3450 RPM. The other fan will turn FASTER than that!

Think some more and get back to us.
spork

Well, I stepped away for 5 minutes so I’m about 90 posts behind. But I do want to continue this treadmill vs. “real world” issue with Dustbuster7000.

Dustbuster says:
“My analysis shows you a non-equivalence in the two cases. Energy flow is not equivalent in both cases, thus the cases are not equivalent”

But there are two problems with this.

First, the principle of equivalence of inertial frames states that no experiment can show different results based on being performed in a different inertial frame. But energy it not an experimental result. Energy is not an intrinsic property something has. Both kinetic and potential energy are like velocity in that there is no such thing as how much something “really” has. It’s strictly related to the frame you measure in. Even the question of whether A does work on B or B does work on A depends on the frame you choose for your analysis. But it is just bookkeeping. The experimental results won’t change. There is no such thing as a sensor that can measure the true amount of kinetic or potential energy an object has.

Secondly, the proposal that the two are different is an extraordinarily bold one. It means by definition that you’ve found an experiment that violates a principle that has been accepted and proven for over three centuries. To make such an assertion requires that you offer some pretty astounding evidence. This would be FAR bigger news than our little DWFTTW cart.
weeble

Oops, my post used some less-than signs that got eaten up.

“Now, when P R, the wind pushes the cart backward/slower.”

Should have been:

“When P *is less than* R, the wind pushes the propeller forward/faster. When P *is greater than* R, the wind pushes it backward/slower.”

“I’m going to guess that the cart is designed such that -1″

Should have been:

“I’m going to guess that the cart is designed such that -1 *is less than* G *is less than* 0.”

Each of the lines that reads “P” should have read:
“P *is less than* R, so the cart experiences an accelerating force from the wind.”
spork

“I’m not 100% sure that that video shows a self-start. I mean, the cart gets blown by the wind, sure, but who is to say whether it will keep accelerating all the way up to, and through, wind speed. I’m still skeptical on that aspect.”

I’ve shown analytically that the transition from stand-still to greater than wind speed is a smooth transition throughout. There is no point at which we have to change the direction of prop spin or pitch, the prop acts as a propeller at all times.

Adittionally we’ve demonstrated self start in two of our videos. Most of our videos also show the cart transitioning from slower than the wind to faster than the wind.

If you have any reasonable test to demonstrate what you’d like to see I’ll be happy to consider it – and post it if we do it.

spork
GregLondon

Oh crap. There are two typos in the new drawing. i say “counter clockwise” but should be “clockwise” and somehow I typed “degrees per second” instead of “feet per second”. That’s what I get for trying to do this at 2 am in the morning. I’ll fix it in the next rev.

I read through the whole thing again and realized that I show a gear version of the prop-cart, but don’t show an actual prop-cart type drawing. I should probably add that at the very end.

Oh, and lastly, to the aero types out there: what happens when you take a fixed vane tumbleweed and put a fixed plate over the top of the blades and leave the bottom vanes exposed?

I think it would work. And the cool thing would be it could be built with only two moving parts.

You’d have the fixed vane anemometer, tumbleweed thing as the first part.

The second part would look sort of like a snow-shovel. You’d start with a cylinder, cut it in half down it’s axis, so you’ve got a half pipe. Then just stick some kind of handle on it that drags on the ground.

Assembly would be to put the tumbleweed on the ground, then simply take the snowshovel thingy and lay the shovel/pipe part on top of the vanes to cover the top half and let the shovel handle drag on the ground behind the tumbleweed (if forward is to the right, the handle would be on the left)

Anyway, I think it would work if you could do it with minimum drag. ANd it would be pretty cool to have something that only has two moving parts.
Anonymous

How about using a propeller with variable pitched blades? The pitch could go negative once the air speed is slower than the car.
spork

“I would appreciate it if you could provide links to the data that demonstrates a downwind vector speed greater than the wind speed.”

You got it. This article shows both a plot of GPS data that just happens to include ice-boats beating the wind by a large margin, as well as a vector diagram that shows the same:

http://www.nalsa.org/Articles/Cetus/Iceboat%20Sailing%20Performance-Cetus.pdf

Here is the vector analysis I originally did to convince myself it’s possible. This diagram probably needs a few words of description, but for now the important thing is that the resultant force vector must have a projection on the positive velocity vector in order to cause acceleration from the assumed condition (which is already a downwind VMG faster than the wind):

http://www.putfile.com/pic/8419299

I’ll be happy to provide additional description of the basic vector analysis as desired.
Antinous

ThinAirDesigns,

When your project is posted here, people come to discuss it. That’s the point of the comment thread. If you find that unbearably offensive, just stop reading it. If you can’t answer questions to the satisfaction of the other commenters, they’re going to assume that your conclusions are unsupportable.

You and Spork are the strangers here. You might want to consider that when adopting a tone.
remi

It seems pretty easy to debunk this particular instrument and there is no need to know any complicated physics:

1. At any point in time, this cart is being bombarded by some air molecules from behind and some air molecules from the back.

2. When there is no wind OR when you are moving at the speed and in the direction of the wind, the number of molecules hitting it from the front statistically averages to the number hitting it from the back and so there is no net force. Meaning that as far as cart is concerned, those two cases are equivalent.

3. If you are in either of the two situations above, and you want to go faster, what you have to do is take some air moving at speed ‘v’ from your front and push it towards your back at speed ‘v+x’. The energy to do this has to come from somewhere; it can’t come from the wind itself because it is only moving at speed ‘v’.

The whole thing basically boils down to this: when you are moving at exactly the same speed as the wind; what is the next step? Where are you going to get the energy to move you that infinitesimal increment above the speed of the wind because that energy will surely not come from the wind itself.
Anonymous

The question can be restated with all quantities conserved as follows:

“Can a device that is pointed directly into the wind, and placed on a treadmill moving backwards make progress towards the wind?”

The thing is that the YouTube videos prove the opposite of this. They prove that under this exact situation the contraption flees the headwind. The restated question is identical because if the contraption moves downwind faster than the tail wind, it will be moving over ground that is receding (treadmill), towards a headwind.

Seems pretty simple to me, so I’ll not discuss it further.
John Markos O’Neill

#86 Will a propeller make my bicycle go faster, too?
Anonymous

A specific “energy flow” example, demonstrating that the direction of “flow” depends on the reference frame selected: two equal mass billiard balls, called A and B, collide head on in an otherwise empty universe. In a frame where A is motionless before the collision, I see ball B come in from the right, hits A and stops, and transfer its kinetic energy 1/2mv2 to A, which departs to the left. The energy “flows” from B to A. In the frame where B is fixed before the collision, I see ball A come in from the left, hits B and stops, transferring its energy to B, which departs to the right. The direction of “energy flow” has switched! Just by changing frames. Either every physicist since Galileo is wrong, or this “energy flow” is not an observable.
rcgldr

The diagrams are a bit misleading. Once moving, the wheels are always driving the prop, and the prop always opposes the movement of the wheels related to the torque it take to spin the prop, regardless if the car is moving slower or faster than the wind.

The prop and it’s induced wash act as a bluff body, slowing down the tail wind. Slowing down the wind is how any wind powered device extracts energy from that wind. The induced wash from the wheel driven propeller, with the proper effective gearing, allows the wind to be slowed down even when the cart itself is moving DDWFTTW.

Assume a 5 m/s tailwind, and the cart moving at 5 m/s, so it sees 0 m/s relative wind. Ignoring rolling resistance, the torque at the wheels and propeller are equal and opposing asssuming no physical gearing involved. Assume the cart is prop’s pitch and diameter produce +2 newtons of force and 2 m/s of induced wash at this speed, and that the torque required to drive the propeller requires -1 newton of force at -5 m/s at the driven wheels. Assume that rolling resitance of the cart consume .2 newtons of force. The power input is -1 newtons x -5 m/s = 5 watts. The power output is 2 newtons x 2 m/s = 4 watts, less than the power input. The net force = 2 – 1 – .2 = +.8 netwons, so the cart accelerates into a DDWFTTW situation.

Perhaps the terminal velocity will be 6 m/s. Assuming linear increases to simplify this example: power input would be -1.2 newtons x -6 m/s = 7.2 watts, power output would be 2.4 newtons x 2.4 m/s = 5.76 watts, less than the power input. Assume that rollling resistance and aerodynamic drag (cart’s speed is +2 m/s relative to wind) is 1.2 netwons. The net force is 2.4 -1.2 – 1.2 = 0 newtons, so no acceleration and the cart has reached it’s terminal velocity.

If the rolling resistance and/or aerodynamic drag factors were reduced, then the cart’s terminal velocity would increase.

As mentioned before, these videos offer proof that DDWFTTW cart can be constructed:

http://www.youtube.com/watch?v=MCB1Jczysrk&fmt=18

http://www.youtube.com/watch?v=oTMRviy-5zY&fmt=18
- rcgldr
  
  >(cart’s speed is +2 m/s relative to wind)
  
  That should be
  
  (cart’s speed is +1 m/s relative to wind)
spork

“Or in short: Stick it in a wind tunnel and find out. Speculating on aerodynamic effects is very risky.”

A wind tunnel is just a tool for investigating the properties of something that moves very fast while it sits still so you can instrument it. It’s intended for things that move into the wind rapidly, and it takes advantage of the equivalence of inertial frames.

Our vehicle moves downwind – faster than the wind. In a traditional wind tunnel it would be out of the test section in a couple of seconds. The treadmill is the tool for testing such a vehicle – a vehicle that goes downwind. This lets us test a vehicle that goes downwind at exactly the speed of the wind – while it remains stationary relative to the earth. If it advances on the treadmill, it’s going downwind faster than the wind. If it retreats, it’s going downwind slower than the wind. Our video shows that it can go from slower than the wind, transition through wind speed, and end up faster than the wind.

The principle of equivalence of inertial frames tells us that the treadmill is more than a good “model” of the situation we desire – the treadmill literally IS the situation in question. We simply want a situation in which there is a specified velocity differential between the surface and air. Because velocity is strictly relative, this frame is as good as (and indistinguishable) from any other.
GregLondon

So, after finally reverse engineering this thing down to the bones, I wrote up a spec that describes how this bloody thing actually works, and how to build a device that will go directly downwind faster than the wind without any moving parts.

Now that I’ve written it up, I don’t know why there is any controversy around this other than no one understood it well enough to explain it in plain english.

Spec is here:
http://www.greglondon.com/tumbleweed/

FoetusNail and/or Codesuidae, I’d appreciate it if you could check my work. Either it works, you don’t even need a prop, and the whole thing is about as interesting as a block and tackle, or I’ve been up way too long… And I trust you guys to be rigorous.
GregLondon

This “con” isn’t going anywhere but up Charles … it’s not too late for you to leave the second group and join the first

This whole attitude is just weird. I don’t look at this as “us” versus “them” (or “them” versus “us”).

I look at it as “explained” versus “not explained”.

Over on the Randi forum, a non-believer in New Zealand built a round ‘treadmill’ (turntable) because be believed it wouldn’t run longer than our tests show — of course he is now firmly in the believer camp. One by one.

And then you will be vindicated? Because you knew something they didnt’ and then they had to go out and figure it out for themselves?

We’re definitely operating towards different goals.

Maybe it goes back to ratwerks arbitrary rule from his unicycle club: you get to laugh at poeple who can’t do something you can do.

Its like a person struggled at some point to learn something and now enjoys watching other people going through the exact same struggle.

Me, I’d rather learn it and figure out a way to make it easier for everyone else to learn.
HarveyBoing

Okay…what about a helicopter? In autorotation the “wind” reversal does not cause the blades to turn the opposite direction.

Granted, this is completely irrelevant to the proposed vehicle, because they aren’t suggesting the wind ever drives the propeller.

But, a helicopter autorotates because it has variable-pitch blades. And if you lose power, you have only a very short period of time in which to push the collective control all the way down, to get the blades into the proper position, otherwise the up-flow through the rotor does slow the rotor, eventually to a point where there’s not any way to get the rotor back up to a high enough speed to arrest the descent at the end of the auto-rotation.

In other words, helicopter autorotation would be exactly an example of the kind of design this vehicle would require, if the proponents were arguing that the wind ever drives the prop (as near as I can tell, they aren’t).
CS Loser

The funny thing is, the people arguing the correct position come across almost as dumb as the people arguing the incorrect position. That’s the definition of a really good flame war.
Anonymous

These videos should convince any doubters:

http://www.youtube.com/watch?v=MCB1Jczysrk&fmt=18

http://www.youtube.com/watch?v=oTMRviy-5zY&fmt=18
Mike Estee

Hmm, I wonder if this works in the videos because none of the demonstrated vehicles are actually traveling in a straight line relative to the wind? Wind is finicky stuff at those scales, and that tack angle is a very important variable.

It’s quite easy to excede wind speed at a tack angle, as any sailor, kitesurfer, or kite buggier can tell you. For direct downwind tacks you can excede wind speed only by oscillating back and forth across the average center wind direction. On a kite buggy, If you don’t oscillate you will rapidly excede wind speed and overrun your kite.

Much like a small resonance bringing down a large building, it only takes small oscillations off of center to keep adding energy back into the system. Ground friction and drag then become the limiting factors which govern maximum speed.

It’s a fun problem :)
ThinAirDesigns

CMPalmer:
>Can Spork33 or whoever from the other
>discussion at least chime in and confirm
>that this drawing is completely backwards?
>If the drawing is correct and that is the
>kind of machine they’re claiming, it’s
>obviously BS. However, I don’t think that
>mechanism is drawn right (the prop should
>rotate the other way).

Hi, I’m JB and I have been partners with spork in building several of the carts in question.

The drawings provided by Mr. Platt do not accurately reflect any working version of the DDWFTTW carts that I am familiar with.

It does not reflect the cart that started it all — that of Mr Bauer from the ~’50s. Nor does it reflect the cart of Mr. Goodman — the cart that started it all on Boing Boing. And it certainly doesn’t reflect any of the carts build by Rick and I which were featured in the OP video on this site a few days ago.

I have no idea where those drawings came from, but one of several things would have to happen for the cart to have any chance.

A: The belt would need it’s twist reversed
or
B: the prop would need it’s twist reversed
or
C: the descriptions would need to be switched and then altered to reflect the new CW/CCW conditions.

The cart as drawn has no chance.

JB
spork

“Those two statements contradict each other. If there’s no intrinsic magic in the propeller, then you wouldn’t need to use a propeller to accomplish the goal. You could use any form of propulsion. Conversely, if you claim that you must use a propeller, that is the same thing as saying there’s some magic intrinsic in the propeller.”

They may seem to contradict each other. That’s the brainteaser aspect of this thing. The prop is not magic, and it doesn’t create its own energy. It’s simply the right tool for the job. Let me give you an example. If I weren’t making a cart to go downwind faster than the wind, but rather a cart that goes faster than something mechanical that’s pushing it, I would use something like this:

http://www.youtube.com/watch?v=E7vcQcIaWSQ

Very simple and clever device.

The prop and the wheels are simply being “squeezed” between the ground and the moving air, and converting that available energy to forward motion through a sort of aerodynamic lever. The trick is to have the prop blow aft with less velocity than the wheels roll forward. But that works fine for two reasons. I don’t have to blow air aft at the speed of the cart, because we operating in a tailwind. The prop operates in a completely different medium than the wheels. Secondly, that’s how I can have enough energy available to the prop to do its job. If it had to blow as fast as the cart is going it would never work.
spork

“Will a propeller make my bicycle go faster, too?”

If you use the right prop and gear it to the wheels at the right gear ratio, and point yourself downwind it will. With any luck you’ll get to see something like this on an upcoming Mythbusters episode.

Now I know some question the Mythbuster rigor, but I don’t think anyone questions their integrity. This is one of the primary reasons we want to see them demonstrate this – plus it’s friggin’ cool!
dustbuster7000

Mender, if the energy comes from the ‘wind pushing against the ground via the cart”, then the energy is flowing from the air into the ground, via as I stated before. Since JB and Spork’s device puts energy into the air via the propeller (their claim), then the energy must be coming from the ground into the wheels and then to the propeller and then to the air. Since this is not true (the ground is not pushing against the air), there is something wrong with their explanation.

As for my muscles, no they can’t tell the different, but then they have their own energy source (gylcogen), which the cart does not, so these cases are not equivalent either.
GregLondon

Ah, maybe not. I think I’m trying to build a centrifugal fan instead of a prop, but the fan still has the problem of stalling out once it goes exactly the speed of the wind.

That’s a bit of a bummer. 2 parts would have been cool.

ah well.
Anonymous

It does seem strange that while a wind-powered cart seemingly cannot travel faster than the wind, SAILBOATS can sail forward into the wind, thereby moving in a direction faster than the wind blows with regard to the direction of the sailboat’s movement.

What does this mean? Has god allowed a contradiction to the venerable thought experiment? Will the world come to an end if it is true that a (wind-powered) vehicle can move faster in a direction than the wind moves in that direction?

Argh, I can’t see. what is happening to us now? alasdml;inr;wjena oops, I’ve destroyed the world by indulging in a false thought experiment, Ahh!

Einstein, save us!?!?!
davidglover

Actually at first outside in the wind the wind back-winds the prop and the wheels turn backwards for a moment until the cart starts moving forward. It is a strange little device, I live in Oklahoma, they live in California, I have known spork/rc and thinair/jb from the hang gliding community for about 10 years, they are the real deal – they are 2 of the smartest people I know, I think most of their fun is watching people having a brain/freeze/meltdown in discussions, partly because they know it is real and works, because they built it. One test is worth a thousand theories and reams of formulas.
spork

“You and Spork are the strangers here. You might want to consider that when adopting a tone.”

I think the tone was set when others exclaimed “They’re witches! Burn them!”

But then again I may be wrong.

Spork
- Antinous
  
  People are going to pick your experiment apart. That’s sort of the fundamental notion in science. All you can really do is answer their questions. If someone is specifically rude, click the eyeball on that comment and we’ll take a look at it.
Anonymous

The energy conservation analysis is pretty simple, and something I haven’t seen done in this thread. The following is not a proof that the cart will work; it is a demonstration that, assuming it *does* work as advertised, it does not violate conservation of energy.

Say there are two net forces on the cart: the traction of the wheels (F_t) and the force of the prop (F_p). Also, there are two relevant velocities: the speed of the cart (v_c), and the speed of the wind (v_w). All of the above variables are treated as positive vectors pointing in the appropriate direction (see below).

Energy conservation (plus friction) says that the power being extracted from the wheels must be more than that applied to the prop. Power = Force * velocity, so:

F_t * v_c >= F_p * (v_c – v_w)

Note that v_c and v_w are in the same direction, so the airspeed (v_c – v_w) is less than the rolling speed (v_c).

Note also that F_t and F_p are pointing in opposite directions (this is necessary for the traction to be the source of energy, but for the prop to be an energy sink). For a steady state, the net force must be zero — i.e., F_t = F_p.

Thus, you can see that the above conservation condition is satisfied: no free energy, no free lunch. As has been repeatedly mentioned in these threads, you can look at the whole contraption as a device for grabbing hold of the ground and the air, and extracting the energy from the wind’s velocity v_w.
John Markos O’Neill

#86 So basically, what you’re saying is that it’s like a propeller plane, but the “engine” is the wheels themselves.

I’m thinking propeller bikes and scooters here. Forget the wind — let the energy going to the wheels come from the rider, then supplement that with the propeller. This would be a great product for the new Sharper Image. Also, how about propeller attachments for cars? When the price of oil goes back up, it will be a great way to save a little money on gasoline.
spork

These threads grow faster than I can type (and I have to go stick a burrito down my neck right now). If you ask a serious question and I fail to answer it, please ask again. My aim is to help people understand that this thing is just a novelty – but a darn cool one.

Some of us have spoken to Jack Goodman a number of times, and he’s a really great guy. I hate to see his integrity challenged when he never asked to be a part of this party (he actually didn’t post that video – he made it his group of friends, and one of them posted it).

By the way, we’ve talked (and argued) with some extremely educated people that also refuse to believe this (including aero and physics professors). But like anyone else you don’t have to take our word, you can build one for yourself.
ThinAirDesigns

Antinous:
>When your project is posted here, people
>come to discuss it. That’s the point of
>the comment thread.

Of course that’s the point and I enjoy the discussion as much as anyone else.

>If you find that unbearably offensive,
>just stop reading it.

You won’t find any evidence for an assertion that I find the discussion offensive, unbearably or otherwise. What I do find entertaining however is people *telling* us to do things for them as if there is some obligation beyond the discussion.

>If you can’t answer questions to the
>satisfaction of the other commenters,
>they’re going to assume that your
>conclusions are unsupportable.

Of course some will assume that and I take no offense from it. Everyone doesn’t get it. Some get it from different explanations than the ones we give, but there are quite a number of people on this thread and elsewere who get it just fine from our demonstrations and explanations.

If someone doesn’t like our explanations, so be it — still doesn’t change the reality. I’ve got one, it does what I say it does and I’m willing to donate my time to help people build their own to try it themselves — but I donate my time my way, not on demand.

I’ve offered Nelson C to help him personally build one — and yet he demands something else.

I’m here for discussion — not demands.

JB
Michael C

Greg, I see that you have integrated JB’s explanation of why the fixed-vane tumbleweed won’t go faster than the wind into your PDF. Why not give him the credit for it as well?

I’m also surprised at what you say at the beginning of the PDF: Trying to get any kind of details about the method of building this gizmo was grasping smoke. Questions about the construction of the design, operation of the design, and plans for the design were generally lacking in sufficient technical detail to reproduce the machine in question.
No one had any plans. No one could point to one document that could explain from start to finish how this thing work.ed.

I first stumbled upon the DDWFTTW cart in the Randi forums thread, and within minutes I had a wealth of information. What about http://www.ayrs.org/DWFTTW_from_Catalyst_N23_Jan_2006.pdf where Jack Goodman gives all the necessary details about his cart? That document should be useful to you now, since it gives details about the pitch of propeller and the gearing ratio between propeller and wheels.
Anonymous

How about this:
Device sits still, big propeller or turbine catches the wind, stores up the energy; then everything folds into a streamlines bundle and leaps forward, using up the stored energy; repeat until satisfied.
With appropriate and judicious engineering, it should be possible to average a speed greater than the wind.
shMerker

Because you process corn to generate energy to move your feet forward, rather than letting the wind push you, dragging your legs to generate energy. The wheels on this vehicle only collect energy, they never expend energy to drive forward. If you drag a toy car across a table the wheels gain energy and continue to spin once you remove the car from the table. If the wheels were geared to a propeller they would convert some of that energy into forward motion. Although once you let go the car would inevitably coast to a stop unless something else began pushing it.

The treadmill is the same inertial frame as the open road. Assuming that the cart rolling forward on against a treadmill is possible, consider what the air is doing as the vehicle rolls forward. If we were to place a measurement device, such as an anemometer on the car, what would it detect? Since the vehicle is moving forward in still air, it should detect a headwind. The case in open ground when moving faster than the wind should be the same.

Alternatively consider a simpler setup, a cart that has free spinning wheels and no propeller. If you were to push it by hand it would travel forward until you stoped pushing it and then coast to a stop. Conversely you could set it on a treadmill and hold your hand stationary behind it and it would remain stationary until you let go, at which point it would accelerate backwards until it was off the treadmill. Now lets add some gearing back in and say that the wheels are connected to a dynamo. Is there a difference in how much power it will generate when it is being held in place on the treadmill versus being pushed along stationary ground?

As I understand it this is the objection presented by many of the detractors of the whole proposal: that there is no such thing as moving faster than a tailwind under wind power, as it would mean entering a relative, measurable headwind that would cause drag, preventing you from continuing at the same pace.
Michael C

Anybody who thinks that this is perpetual motion needs to think again. As a friend of mine remarked, the relevant law is “Conservation of Energy”. There is no law of “Conservation of Velocity”!.
Michael C

Several posters have already pointed out that Charles Platt’s pictures of the cart show that he has the gearing the wrong way round. This is an effective way of preventing the cart from going faster than the wind.
spork

“Its like a person struggled at some point to learn something and now enjoys watching other people going through the exact same struggle.”

Actually, it’s more like we’ve been working on an intriguing little problem, and we enjoy discussing and describing it to anyone that wants to discuss and learn – but no so much with those that just seek to insult us.
spork

“So basically, what you’re saying is that it’s like a propeller plane, but the “engine” is the wheels themselves”

Well, you could sort of say the engine is the wheels (actually it’s the wheels, prop, and drive-train). That’s the good news. The bad news… the fuel is the wind – or more accurately the relative motion of the air over the ground.

Still no free lunch.
spork

Well, a lot has happened since this topic was started. JB and have since built a full-scale, manned DDWFTTW cart. We took it to the NALSA (North American Land Sailing Association) event in Ivanpah, NV a couple weeks ago for it’s first shakeout runs. While we did not attempt an official record at this event, we did achieve better than 2X windspeed DIRECTLY downwind.

Here are some videos from the event:

First run: http://www.vimeo.com/10476453
Second run: http://www.vimeo.com/10477373
Last run: http://www.vimeo.com/10476216
shMerker

I’ve been thinking about this some more and people talk about the air behind the cart. Has anyone considered the air in front? The propeller is grabbing onto that too, and that air is already moving. Last I checked when you latch onto a massive object, like a particle of air, you get some of it’s momentum.

A thought experiment for those who think the cart should be able to work on the treadmill with still air, but not on the open road with moving air: What happens if you put the cart on a treadmill in an air-tight box which is itself mounted on a sail-car? Will the cart continue to advance on the treadmill, thus moving faster than the box, thus, if efficient enough, moving faster than the wind?
SubductionZone

Well I read part of Greg’s pdf. His argument seems to be I don’t understand how this could work so it must be fake, a strange twist on the logical fallacy of arguing from ignorance. Since he cannot disprove the obvious, that the frame of reference is identical to a tailwind on a treadmill, and he cannot figure it out from the arguments given here, it must be a hoax! Huh??? Greg if they are faking it how are they doing so? I have seen bad youtube video’s of “over unity” machines and there are usually a dozen places where the power sources could be. They have explained this many times, I cannot do the math myself so I won’t try, but engineering wise this is no more spectacular than a boat tacking with or against the wind. Tacking with the wind has been shown to enable a boat to go downwind faster than the wind, tacking against the wind lets a boat run against the winds direction. Both of these are everyday occurrences. I cannot describe the math for these either, but you are not going to dispute these I hope.
dustbuster7000

So you’re going to address the validity of a model (the treadmill video)of the normal operation of the cart by proposing another model, which is even more confusing and has even less clear boundaries? That doesn’t seem like a very good idea.

The treadmill test (can we call it that?) is great in that it has very clear boundaries and changes as little as possible about conditions: same cart, same velocity, same masses, similar friction co-efficients (comparing tire-treadmill and tire-road). The only shift is the thing which is moving, the cart moving forward is transposed with the ‘road’ moving backwards. So you can very quickly compare and control all the relevant factors. The stated purpose of the treadmill test was to answer the critics of the outdoor test by removing extra variables and thus eliminating sources of error. Which I agree is a good idea. And external test under normal operating conditions would be better, since it would test the cart directly, but less controllable. An indoor test which more exactly replicated normal operating conditions would be better still. Empty warehouse, decent fan, go for it.

But if we’re going to use the treadmill test, it needs to be equivalent in as many ways as possible. The inertial reference frame, which has been talked about by many on the forums, should transpose all elements of the actual conditions (where possible). Including any energy flows. If you think my analysis is flawed, that’s fine, I’ve been wrong many times before. But please address my analysis directly.
HarveyBoing

I encourage everyone to glance at this site on airplanes: http://www.livescience.com/technology/060828_how_planes_fly.html

The point is that the drag on the wing of a plane (or propellor) typically is much less than the amount of lift it can provide.

Not relevant. You are mixing up force with the actual net energy inputs and output.

For an airplane, induced drag is significantly less than lift only for relatively high airspeeds. In fact, at the lowest airspeeds, drag can be extremely high, assuming the wing can generate lift at all (at a low enough airspeed, the angle-of-attack required to generate the lift will exceed the wing’s stalling AoA).

So, sure…if you get the speed high enough, you can create some large amount of lift while generating far less induced drag. In fact, at best glide airspeed, the best airfoils can generate upwards of 50 times the lift as there is total drag (induced and parasitic combined).

But that’s a consequence of the fact that the drag is happening over a relatively large distance, while the lift happens over no distance at all. There’s no magical energy multiplier due to the use of aerodynamic lift to create the force. Conservation of energy still must rule.
ThinAirDesigns

Dust:
[blockquote](the ground is not pushing against the air)[/blockquote]

How do you figure that the ground is not pushing against the air? After all, the air can’t push against the ground without the ground pushing against the air — equal and opposite reactions and all that.

JB
airshowfan

I think I’ve “converted” someone. (Before, After).

And if any fellow DWFTTW “believer” is in the Seattle area (sorry, I have not checked you guys’ geographical location, so I have no idea where you are), let’s grab some coffee, or come take a tour of the world’s largest building.
airshowfan

Dang it, responding to this on ScienceBlogs (as on the JREF) would require more time than I have. And the fact that he calls DWFTTW proponents “bozos” makes me want to stay away. Which is too bad since in general I really like ScienceBlogs.

Notice that the ScienceBlogger does not doubt that the treadmill video shows what it says it shows. He just thinks that this cart cannot accelerate to wind-speed on its own, and recognizes that from there the treadmill experiment shows that the cart can accelerate. In other words, similar to this very post on BoingBoing (scroll up a couple miles), part of the confusion comes from thinking that the cart ought to be self-starting via a turbine mechanism. Once you say “We do not claim that this setup is self-starting. We have to push it to wind-speed, THEN we let go, and it does accelerate from there”, part of the confusion should go away. You can also explain that it CAN take energy from the wind even while going at wind-speed, since it has a propeller, not a turbine, so it can push on the air as shown by the treadmill experiment, which is equivalent to an outdoors wind test not only from an inertial-frame point of view but also from an energy point of view (the treadmill cart takes KE from its platform (the treadmill) and brings air closer to its platform’s velocity, and an outdoors wind cart takes KE from its platform (the Earth) and brings air closer to its platform’s velocity) like I showed in the Prius thought experiment a few comments ago.

You can also say “Power into the wheels = wheel traction force x ground speed, and Power out the prop = prop thrust x air speed, so Prop Thrust can be greater than Wheel Force even though Prop Power Out is smaller than Wheel Power In as long as the air-speed of the cart is smaller than its ground-speed”. I find that to be fairly elegant, and shows that it’s not perpetual motion since it would keep it from working on still air.

Ok, FIRST, I’m going to finish and turn in a report that I’m working on. THEN, I’ll go back to that ScienceBlogs page and see if it’s worth spending some time there… Or writing up a webpage from scratch or something where I gather my favorite thought experiments and Youtube videos…
GregLondon

sub@228: Well I read part of Greg’s pdf. His argument seems to be I don’t understand how this could work so it must be fake

Jesus, no, that isn’t what it says at all. It really works, its just that the guys who are using “intuition” to understand it really boils down to the simple fact that they can’t explain it. they have to wave their hands and show people videos and talk about it until, hopefully, the other person finally intuits it.

As an engineer, I’ve spent years working on projects where a screw up on my part means someone (or a lot of someones) could literally die a horrible ball of flame kind of death. And if there is one thing I’ve learned from that, it’s that intuition is crap and can get people killed.

I’m sorry if applying that level of engineering rigidity pissed off a lot of people, but it seems fairly clear that a number of people who have “intuited” how this thing works, really have no farking clue how it works.

I never said it doesn’t work. I started out very clearly saying I don’t understand and asking how the machine overcame this or that problem. No explanations ever really answered my questions or got me to understand this on a fundamental science (as opposed to intuition) level.

So, I started trying to reverse engineer how it works and it turns out that a lot of the smoke and mirrors used to explain it are complete bullocks. It works, but having the intuition that it works isn’t the same has having the understanding of how it works.

Once you understand it, it is about as interesting a device as a block and tackle. Which can be interesting at times, but I don’t know if I’d spend years on it, calling Mythbusters to “prove” it, and so on. I mean, once you understand it, asking Mythbusters to prove it is like asking them to prove that a lever lets you lift more weight if you pull on the long end and put the weight on the short end.
spork

“people talk about the air behind the cart. Has anyone considered the air in front?”

This is a KEY point. People talk about the cart “outrunning the wind”. But that’s a little like outswimming the water. The cart is immersed in a fluid (the air) that is all moving downwind at a constant velocity. This is a very advantageous place for the prop to be working.

As a related item, we think of a prop as creating a “jet” of air and therefore thrust. But in the ideal world the prop can produce thrust with arbitrarily small expenditure of energy. A bigger prop moving more air through a smaller delta-V (change in velocity) uses less energy to produce the same force. Evectively the prop can grab onto the wind just as you can grab onto a wall. Obviously in the real world the prop isn’t going to be infinitely large, and there will be energy losses to friction and drag. But if you run the basic energy numbers, it’s surprising how much loss you can tolerate and still make this thing work.

I’ll be happy to post that brief analysis for anyone that cares to see it.
FutureNerd

Mark asks, “How can you cause the string to wind around the yo-yo’s axle by pulling the string? It seems like the yo-yo will drag along, unwinding the string as it is dragged.”

Gotta set it up the other way.

Set up the yo-yo so the string is wound up halfway, and coming out the bottom of the yo-yo. So if it unwound, it would roll away. Now put it on the floor that way and start dragging it with the string. Hopefully it won’t veer off course.

Everybody who’s got a yo-yo TRY IT and REPORT BACK.
spork

Well, I’m glad to see Greg London is still in fine form – attempting to explain what he claims I don’t understand about the vehicle I designed from first principles. Supposedly he was unable to find any useful information from me on how it works or how to build it – despite the fact that I’ve posted the parts list and build notes here and many other places on the web. I’ve also explained it hundreds of times in hundreds of ways.

Thanks so much Greg for clearing all this up for us.
Takuan

(uhh, can we, like, grease them up a bit before we burn them?)
Takuan

the formulae appear complete and correct save the absence of the A and E coefficients (African and European)
ThinAirDesigns

GregLondon:

I don’t need experiements to prove the theory. check those two new drawings I added, and you’ll see intuition isn’t needed at all.

Yeah, I checked those two new drawings. Turns out you’re right – it isn’t intuition that’s needed … just common sense and high school physics.

Since I don’t rely on intuition, I don’t need experiments and trial and error to figure out whether the concept will work or not.

Yeah, we can sure see how that worked out can’t we.

I’m sorry if applying that level of engineering rigidity pissed off a lot of people,

Yeah, tell us about the “engineering rigidity” again would you.

but it seems fairly clear that a number of people who have “intuited” how this thing works, really have no farking clue how it works.

I think what’s fairly clear now in the wake of your total failure is that the whole “farking clue” was just a projection.

Was this a mail order degree?

Well Greg, was it?

Ok Antonius, I’ve endured quite enough crap from this wannabe engineer so in light of his monumental design disaster the we were left to expose I think he had it coming.

If he drops his “they’re clueless” routine I’m through with him — I can be a good boy, but others have to play nice also, no matter how long they’ve been around.

JB

PS: And Greg, I see you took your “I don’t need to test it ’cause I am rigorous and understand how it works” total failure of a DDWFTTW design off your website. We saved a nice copy of that .pdf just for future reference.
Mender

The source of the energy needed to propel the cart to a specific speed is the momentum of the air mass in relation to the ground that the cart is rolling on. One thing to remember, the propeller is always trying to slow down the speed of the air mass around it in order to harness that energy. A wind turbine slows the air to generate electricity and the amount of power from that momentum change is well documented. The cart also harnesses that energy but in a slightly different way.

Here’s a link to the article published about a DDWFTTW prop cart that was tested outdoors and on a treadmill. In the article, data is provided that can be analyzed.

http://www.ayrs.org/DWFTTW_from_Catalyst_N23_Jan_2006.pdf

The method used for measuring the thrust and drag is listed in the article.

The amount of forward thrust was taken directly from the scale which was attached behind the cart to restrain the cart from moving forward off the treadmill. That showed the force directly at various speeds. The readings were taken at 1 mph intervals between 4 and 10 mph.

The next phase of the test was conducted with two changes: the belt was repositioned so that the propeller turned the opposite direction from the wheels, and the scale and tether repositioned to restrain the cart from moving backwards off the treadmill. The reading from the first test was subtracted from the readings from the second test to remove the imbalance, then that result was divided by two to show the total drag of the cart during operation.

By adding the thrust measured at each point to the drag measured and calculated at each point, the difference between the drag that the cart produced was compared against the thrust for each interval.

The reading at 4 mph was zero and the drag part of the test showed 185 grams. Since the drag part showed the sum of the drag in two directions, the drag in one direction with all systems functioning and accounted for was 92 grams. A thrust of 92 grams balances the drag at that point for a lift to drag ratio of 1:1.

At 10 mph, the measured imbalance was 150 grams and the calculated drag was 402 grams, giving a thrust of 552 grams or a L/D ratio of 1.37:1.

I checked a wind energy calculator to see if the energy needed to provide the thrust measured could be captured from a wind moving at 4 mph given the size of the prop on the cart (40″ in diameter). After doing the conversions and using Betz’ law as a guideline for the max efficiency, the amount of power available to a wind turbine of the same diameter as the prop is 1.63 watts. That translates to 93.37 grams of thrust at 1.78 m/s, quite close to the test figure of 92.5 grams. For 10 mph, the numbers work out to 25 watts or 585 grams thrust at 4.44 m/s, again within experimental error of 552 grams as indicated by the treadmill test.

The small cart in spork and JB’s video tends to back those numbers up, with the 169 gram cart “climbing” a 4.4 degree incline at 10 mph, indicating an imbalance of 13 grams force in the forward direction. That would give it an acceleration rate of 2.47 ft/sec2 on level ground in a 10 mph wind when the cart is at 10 mph. The large cart would have an acceleration rate of 2.12 ft/sec2 based on a weight of about 2300 grams (five pounds). The break even speed on the little cart is 2.7 mph vs the large cart’s 4 mph, so a little better performance could be expected from the small cart.

So it appears that the energy available from the different wind speeds correlate quite well to the treadmill test. There is no mystery force needed to accomplish this, no violation of any physics, in fact pretty normal physics, just applied in a unique way.

Again, the propeller is geared to run the same direction at all stages of operation. The force needed to move the cart forward from a stop is less than the force needed to move the propeller backward (and subsequently the cart backward), so naturally the cart moves forward from a stop with the propeller trying to slow the air around it. Reversing the direction of the belt causes the cart to move upwind, because now the force needed to push the cart backward is less than the force needed to move the propeller (and the cart) forward, so the cart moves upwind instead.

Charles Platt clearly doesn’t understand how this is working (no insult; many people don’t). For those who are worried about the propeller, I’m planning on building a cart that doesn’t use a propeller. It won’t work quite as well but I’m hoping it will work well enough. As spork said, there is nothing magic about using a propeller. It simply works the best for the task required.
senbot

Many commenters here confuse energy with force, hence the perpetual motion accusations. You can multiply force without breaking laws of physics, for example using a lever or a pulley to lift heavy objects. I suppose it is the aerodynamics element of DWFTTW that makes the experiment too complex to be intuitive.

Here’s an experiment without aerodynamics: put a half-unspooled yo-yo upright on a flat surface (for example a table) in such a way that the string extends from underneath the spool towards your hand. When you pull the string parallel to the table and cause it to wind (sorry!) onto the spool, the yo-yo will catch up with your hand. It will be moving faster than the string that powers it – at a rate proportional to the ratio between the radius of the yo-yo and that of the spool.
HUDMAN

Greg, what is there to figure out? If it is efficient & produces thrust, it will probably work. If it is not efficient &/or doesn’t produce thrust, it will not work. And by “work,” I mean travel directly downwind faster than the wind.

That is the simplest way to explain it. Trust me, I have tried for about 9 months to find a more simple explanation.

If you want to explain it without using a prop, more power to you, but that might just confuse people even more.

If you want to design one without a prop, I will encourage it, but remember, it must produce thrust. That’s the 1st rule. Without producing thrust, it will not advance on the treadmill, which is the 2nd rule. It must advance on a treadmill.

Now, with that said, you might try other forms of thrust producing engines, like radial blade (squirrel cage) blowers. The problem with the radial blade though will be at startup. The tailwind will most probably turn the blade, and make the cart go in reverse. A simple flap, hinged from the top, covering the blowing hole at the rear of the blower may solve this, but now the thrust has to open that flap & still be able to provide sufficient thrust.

To put it more simply, some part of your cart must remain slower than the wind, while the cart itself is outrunning the wind. This should be the 3rd rule. And, that’s what the thrust does, it remains slower than the cart, while still “attached” to the cart, so that it can still interact with the tailwind that the cart is currently outrunning.

Another way to look at that would be to mount a windmill on a long threaded rod to the cart in a way that when the windmill turns, it will power the wheels forward. (The opposite of the DDWFTTW cart) Once the cart reaches windspeed, remotely activate a servo that locks a nut mounted to the long threaded rod, which will allow the windmill and rod to screw itself backwards, away from the cart. If the windmill were going exactly the wind speed, then the cart is now being accelerated forward by the threaded rod, and is now outrunning the wind, until the length of the rod is used up.

Of course in that example, exceeding the windspeed is temporary, but by using a prop, it is consistant, as long as the prop produces thrust.

Copyright Â© 2008 by HUDMAN â„¢ of HUDCO Â® Industries. <– Sorry just couldn’t resist…
ThinAirDesigns

Dust:
>So you’re going to address the validity of
>a model (the treadmill video)of the normal
>operation of the cart by proposing another
>model, which is even more confusing and has
>even less clear boundaries? That doesn’t
>seem like a very good idea.

I’m not proposing another model at all. It’s exactly the same model only physically bigger in size.

This exercise isn’t about the cart at the moment — it’s purely about the equivilancy of frames of reference.

I’ve been pretty good at answering your questions and it seems we are both capable of reasoned exchanges … will you humor me for this round and answer the question even though you apparently think it may be a unwise tangent.

Thanks

JB
airshowfan

At #135 and 136: The video of the treadmill test should show that this actually works, not just on paper or in people’s imaginations but in real life. Unless the video is faked, which I’m guessing it isn’t.

People; please don’t say that the treadmill test is somehow not equivalent to a wind-tunnel test; that was refuted a long time ago (like 300 years ago by Newton ;] ). Here’s another thought experiment to help out with that (easier than the “Whole town on a giant treadmill” one): Imagine a balloon in the air over the DWFTTW cart. Imagine that someone lowers a very large box from the balloon via a rope. The box is missing its bottom. Say the box is lowered until it touches the ground and “traps” the DWFTTW cart in it, the way a kid might trap a bug. (Imagine that the balloon then turns on an engine and prop, so that it stays at wind-speed despite dragging a huge box along the ground). Imagine that there is a person attached to the inside of the box somehow, like on a chair mounted on the side. That person is looking at a room of still air, with the ground moving, and with a DWFTTW cart in it. But to someone standing on the ground (hopefully not in the path of the box), they will see (say the box is transparent) the DWFTTW cart moving along the ground through some moving air, which is the outdoors test so many people want. Since the box is not moving relative to the air, it does not affect the cart in any way. (Well, if the cart can go DWFTTW, it will end up hitting the front of the box). In other words, if a room minus a floor could somehow be flown at wind-speed over the DWFTTW cart, then anyone in that room would observe the treadmill experiment, while anyone on the ground would observe an outdoors wind test. They really really really are equivalent. (Well, if you disregard the curvature of the Earth and its accelerations through the cosmos’ gravitational fields, of course).

I’ll side with the skeptics on one small and inconsequential point: To all the people who claim that airfoils can work as force magnifiers, i.e. can have lift-to-drag ratios in the range of 50 to 100; You’re saying the right thing, but for the wrong reason. Machines that multiply force (like a lever, and a wing) cannot multiply work (only transfer it, often with some loss). Saying that “the prop can magnify the force from the wheels, and that’s why the thrust is greater than the force of the ground spinning the wheel” would allow for a cart that works in zero-wind, i.e. a perpetual motion machine. So this cart works, but it does not work only because the prop can magnify force, since if that were so, this cart would be a perpetual motion machine that works in zero wind. Be mindful of that. The more relevant reason why the prop can provide extra work is best illustrated by comment #67, which has to do with the fact that the ground is going by much faster than the downwind velocity of the cart.

Some people (e.g. #141) seem to still think that, when moving at wind speed, there is no force between the air and the vehicle. They somehow miss the fact that the vehicle has a powered propeller. An airplane can start out at rest relative to the air (say, sitting on a runway in a day when there is no wind) and use its propeller to accelerate to higher speeds. When the cart reaches wind speed, that’s what it is doing.

And since not everyone is reading everything on these threads (quite understandably), let me point out some things I think are noteworthy, that will probably help people understand what’s going on (or at least “believe” that DWFTTW is possible):

– Comment #108, which I think does the best job of any comment so far in any of the discussions, as far as giving not only a mathematical model for where the energy comes from and where it goes and how it gets there, but also the numbers (both theoretical and experimental) to back it up. Comment #67′s robot-on-a-people-mover thought experiment is also quite good.

– Several people still doubt that a sail-boat (or land-sailing ice-boat or whatever) can beat a balloon to some down-wind spot. I think the links provided in this thread and the previous one show that this is indeed possible, pretty conclusively. Not only is the boat’s overall ground-speed way greater than the wind speed, just the downwind component of the boat’s velocity is greater than the wind speed. So it’s going DWFTTW. It will beat a balloon in a race to a point downwind of the start point. There are links to actual real-world examples of VMGs greater than one. These boats go DWFTTW, it’s that simple.

– There are lots of mechanical devices (shown in at least three Youtube videos linked from this thread) where two platforms (two boards, or a string and a surface, pieces of paper, etc) move relative to each other, and a clever system of gears and belts (or even just a single spool or a yo-yo) placed in between/around them will move in one direction FASTER than the surface that is moving in that direction. So “down-board faster than the board” or “down-string faster than the string” is clearly, clearly possible. The question of whether DWFTTW is possible then becomes not one of whether it’s fundamentally possible, perpetual motion, etc, but just one of whether friction and propeller inefficiencies can be overcome by a clever design. (And yes, energy must be put into moving the board or string, and some of that energy goes into moving the vehicle, just as the DWFTTW carts remove energy from the wind (or from the treadmill, depending on your point of view) by bringing the air velocity closer to the ground velocity).

– I was thinking about this in the shower this morning, and it occurred to me that, in a way, a turbine-powered vehicle that slowly crawls UP-wind (and these exist, you can buy a kit, perfectly non-controversial, etc) is equivalent to DWFTTW, just like the yo-yo and the other mechanical devices. From the point of view of a balloon, or if you do a treadmill test, such a vehicle goes “down-ground faster than the ground”. It is “ground-powered” in the same sense that the DWFTTW cart is “wind-powered”, clinging to the ground, getting energy from the passing air (which is passing at high speed) and using some of that energy to slowly move itself along the ground in the other direction. This is the same as DWFTTW but the air and the ground are reversed. In either case, it’s the energy available at the air-ground interface that makes it possible, the fact that the air and the ground move at different speeds. And yes, I see that comment #128 beat me to this.

– Comment #120 (the “silly DDWFTTW cart based on the yo-yo principle”) reminds me of my very first idea when I first heard of DWFTTW. I too imagined a wheel rolling, and realized that all the points are moving forwards (except the very bottom, which is in contact with the ground), and the points below the axle move forward less quickly than the overall wheel speed (the speed at the axle). So I imagined a big ferris wheel rolling along the ground, and instead of seats it had sails, and the sails were oriented perpendicular to the wind (vertical) when they were below the axle (the lower half of the revolution around the axle) and then oriented along the wind (horizontal) when on the upper half of their path. If the sails were efficient enough, the lower part of the wheel would move at wind-speed, and the top would move even faster (since the wheel levers around the point of contact with the ground), making for an average speed (and an axle speed) that is faster than the wind. Does anyone disagree that this is possible, or think it’s a perpetual motion machine?

Now I haven’t read the latest 20 or so comments since they were posted while I was reading this, so let me get to them. Just from skimming them, I am particularly curious about #142…
spork

“For those who are worried about the propeller, I’m planning on building a cart that doesn’t use a propeller. It won’t work quite as well but I’m hoping it will work well enough”

This will be a challenge, but it will be REALLY cool. I want to see the Mythbusters do a prop and non-prop race.
GregLondon

Greg London: “the whole thing is about as interesting as a block and tackle”

anon@231: Yes! Yes! You get it! … As a practical matter, I wonder whether the tumbleweed will actually go faster than wind,

I added two diagrams this morning. Page 5 shows a time lapse sort of thing and it clearly can go faster than the wind. Page 7 shows it in a rope/pulley diagram a’la high school science course, and that also clearly shows it can go faster than the wind.

The only difference between those drawings and the tumbleweed is the efficiency of the anemometer, as in how much slippage it will allow and how much drag it will have on the retreating side.

But theoretically, if the diameter of the anemometer is one half the diameter of the ground wheel, you should be able to go 2x your windspeed.

Whatever inefficiencies the anemometer has, you would eventually be able to overcome once you get enough windspeed. Since a box fan could push it up hill, I don’t think it should be too much a problem.

As for more experiments, yeah, I was going to try it today, but the air has been dead calm all weekend. Stupid weather. Really though, I don’t need experiements to prove the theory. check those two new drawings I added, and you’ll see intuition isn’t needed at all. Since I don’t rely on intuition, I don’t need experiments and trial and error to figure out whether the concept will work or not.

It might be that my math is wrong somewhere, but anyone could check that by simply reading the pdf.
ThinAirDesigns

Since I have built and operated several of these devices, I feel reasonably qualified to address Mr. Platts questions: Here goes.

>Three questions for cart enthusiasts:

>1. When the cart begins running slower
>than a tail wind, does the air move
>through the propeller from the back
>toward the front?

If started from rest, there will be an initial short period when the wind moves from back to front. However, once the ‘bluff body’ drag of the device causes it to begin it’s path downwind, this motion begins to spin the prop through the wheel gearing. Once the prop gets up to speed (but with the cart still well below the speed of the tailwind), the air is now moving through the propeller from front to back.

For the purposes of Mr. Platts question, the answer is “No”.

This answer conflicts with Mr Platt’s own answer of “clearly yes”.

Perhaps it is this lack of understanding relating to the cart’s dynamics that led Mr. Platt to provide reversed drawings and ultimately failure in his well documented experiment.

>2. If the cart can somehow accelerate faster
>than the tail wind (as its proponents claim),
>does this means that air will now move through
>the propeller from the front toward the back?

The air movement will continue to be front to back, just as it was when it’s speed was below that of the tailwind.

The answer here is “Yes”

>3. If the flow of air through the propeller
>reverses in this way, will it tend to
>reverse the rotation of the propeller?

Other than the brief moments when the cart’s prop is just beginning to turn, the flow is always front to back. There is no reversal of flow and thus no tendancy to reverse the rotation of the prop.

The answer to this question is: Not relevant as the flow doesn’t reverse.

Mr. Platt again:
>Answers to (1) and (2) are clearly “yes.”
>Answer to (3) can be determined empirically
>by blowing air at a small fan, first from
>the front, then from the back, and watching
>which way it turns. Answer to (3) will also >be “yes.”

>Therefore, the reversed air flow will
>retard forward motion, the speed of the
>cart is self-limiting, and the claim is
>false.

According to his MAKE article, MR. Platt was unable to get his version of the cart to move more than “about an inch”. Well, we’ve got miles on ours and we can state with confidence that his construction methods, testing methodology, results and conclusions are far from the reality that we have experienced.

I look forward to the day in the not so distant future when the mounting evidence will force Mr. Platt to remove the word “con” from his vocabulary as it relates to this craft.

JB
yesno

Spork,

Please provide the evidence that it is possible for a sailboat to tack downwind and arrive at point B from point A faster than something traveling windspeed in a straight line would.

My understanding had been that the greater distance caused by tacking cancelled out the increased speed, such that you don’t actually get to a point faster than the wind would.

Say the wind is blowing east, and you want to get to a point due east. The shortest distance between you and your destination is a straight line that is exactly downwind. You are claiming that you can tack back and forth and get to that point faster than the wind does?

I don’t know either way. It seems impossible, but since it’s possible to tack upwind (and arrive at your point B infinitely faster than the wind or something traveling with it would), why not?
GregLondon

Actually, it’s more like

(snip)

dude, seriously. I can admit I was wrong. I can apologize. I can tell you that you were right about this thing working. But don’t take that as me accepting your story about your totally selfless behaviour on these threads.

You were right. That doesn’t mean you were perfect.
GregLondon

good grief.
yesno

Spork,

Please provide the evidence that it is possible for a sailboat to tack downwind and arrive at point B from point A faster than something traveling windspeed in a straight line would.

My understanding had been that the greater distance caused by tacking cancelled out the increased speed, such that you don’t actually get to a point faster than the wind would.

Say the wind is blowing east, and you want to get to a point due east. The shortest distance between you and your destination is a straight line that is exactly downwind. You are claiming that you can tack back and forth and get to that point faster than the wind does?

I don’t know either way. It seems impossible, but since it’s possible to tack upwind (and arrive at your point B infinitely faster than the wind or something traveling with it would), why not?
airshowfan

DustBuster, and possibly others, seem to have trouble seeing the equivalency between a treadmill test and an outdoor wind test. I think I see the problem: They can see energy flowing “from the treadmill” and see this as being significantly different from energy flowing “not from the ground, but from the wind”. Our talk of the air-ground interface is not making sense to them. And this is understandable: From the point of view of a balloon, the air has no kinetic energy, even if the balloon is being blown by a wind strong enough to turn wind turbines below it. (The balloonist might claim that the ground going by has some energy that could be captured, but the wind turbine repairman down below would say that’s crazy). In the end saying “wind powered” or “ground powered” or “treadmill powered” all mean “Powered through the difference in velocity between the air and the ground”, which is either constantly fed by a treadmill motor, or is slowly subtracted from a large reservior of energy (the inertia of the wind and of the planet).

Let me try to explain this another way. Another attempt to reach out to the people who think that the treadmill does not represent what would happen outdoors in the wind.

In an outdoor wind test, the ground can add energy to the cart just like the treadmill adds energy to the cart in a windmill test. Let me demonstrate this via a three-step thought experiment. (Well, only the third step is really a thought experiment, since the first two parts happen around the world every day in real life).

1) Imagine you have a wind turbine, like the kind that generates electricity out in the hills. You could say “The wind slows down and some of that kinetic energy becomes electricity”, but that’s not the only way of looking at it. There is an equal and opposite force: In order for something anchored to the ground to pull on the wind, the wind pulls on that something too. So not only is the wind noticeable slowed down relative to the Earth, the Earth is imperceptibly pulled along with the wind. A wind turbine is like a giant air brake that makes the whole earth more closely match the local wind speed (by just a teeny tiny bit). With me so far?

2) Imagine you have a Prius driving down the highway at some steady speed. At some point the driver touches the brakes, not too heavily. This will turn on generators (not friction-based brakes) that will convert some of the car’s kinetic energy into electricity. From the point of view of the Earth: The Prius slows down, it’s KE becomes electricity (stored in a capacitor or as chemical energy in the battery or whatever). BUT NOW imagine you start out in an inertial reference frame that is flying in formation with the Prius. You see the Prius as stationary and the Earth as moving. When the Prius hits the brakes (turns on the generators) and thus becomes somewhat connected with the earth, it shoots off in one direction (towards its back). The Earth, meanwhile, slows down just a teeny tiny bit. That is, the ground speed of your inertial reference frame drops. When the Prius connected itself with the Earth, some of the Earth’s kinetic energy went to accelerate the Prius backwards, and some went into the Prius’s battery. So from an inertial reference frame going at the Prius’s original speed, the Earth lost KE, and the Prius got some KE and some electricity. (The Earth only loses a tiny fraction of its KE, though, so measuring the change in its speed would be pretty much impossible. But you can see it happens).

3) Here’s where it gets a little crazy, and more directly applicable to DWFTTW. Say you have wind blowing steadily and perfectly parallel to a road. Say you have a Prius driving at wind speed (so the driver feels no airflow) and a balloon overhead (the balloonist feels no airflow). Now say the balloonist drops a rope that hooks onto something in the car. And say the Prius now lightly hits the brakes (the generators). The rope becomes taut, and the balloon now pulls the Prius, and this set-up reaches some terminal velocity and generates energy. NOW HERE’S THE IMPORTANT PART: What would a second balloon (or an astronaut overhead in an almost-geostationary orbit, which originally was in formation with the balloon) observe? They would observe the Prius shooting off in one direction, dragging the first balloon away with it… and the Earth moving by at a slightly slower rate. The Prius and balloon act like a big air-brake, and as the Prius (which pulls on the earth) drags the balloon around, it slowly slows down the Earth. If the Prius did this for millions of years, the Earth would eventually come to rest relative to the air. The air is also being accelerated, which is why I brought in the astronaut.

If the balloon then had an electric engine attached to a prop, and could run it off the Prius’s battery, this would be just like the DWFTTW cart, but with an electrical (not mechanical) connection between the wheels and the prop.

If you doubt that this Prius-plus-powered-balloon contraption could go DWFTTW, please ignore that for a second to take in my main point, about where the energy comes from.

From a truly inertial reference frame, the Earth supplies some KE. When the DWFTTW is dragged along the earth being pulled by its propeller (or even if it only had a sail and weren’t a DWFTTW cart at all), it pulls on the earth, and the earth ends up moving just a tad more slowly than it did initially.

To really appreciate this, you have to be in a TRULY inertial reference frame. The balloon is not one (since all the air, through viscosity and separation effects, gets slowed down), and the ground is not one (since it is pulled to the side by the vehicle). You’d have to be an astronaut in deep space observing through a huge telescope, or an astronaut in an almost-geostationary orbit that is going at the same angular velocity around the earth as the ballonist was at the beginning of the experiment (and even that is not truly inertial).

So the Earth supplies some energy. So it’s the same as the treadmill. The difference is that the Earth supplies kinetic energy out of a huge reservoir (so you don’t notice that any of the Earth’s KE has gone missing, but some of it has), while the treadmill keeps making it from electricity and only holds a little at any given time.

Right?
spork

“If the forces were equal, there’d be no movement, there is, so the forces aren’t.”

No force doesn not imply no movement. It implies no acceleration.

“that energy must come from somewhere. It can’t be the wind, since its outputing energy to the air, the wind can’t put energy in to the cart. Thus, the cart is not a wind powered vehicle.”

Again, you’re making the mistake that energy is a parameter that relates to the outcome of an experiment. I know it seems extremely odd that the direction of energy flow could possibly come down to my whim, but it does exactly that. It depends only on which reference frame I choose for my analysis. It does not depend on whether I perform the test on a treadmill or the road in front of my house (which is also moving along at one heck of a clip).

“if you are right, then you have indeed managed something truly useful, even if you don’t realize it.”

Again, you’re misunderstanding. He is right, but there’s nothing amazing or useful about it.
arkizzle

“..I don’t need experiments and trial and error to figure out whether the concept will work or not.”

Correct or not, I can’t tell you how wacky that sounds to me (or just arrogant, maybe).
GregLondon

Greg:
>I wrote up a spec that describes how this
>bloody thing actually works, and how to
>build a device that will go directly
>downwind faster than the wind without
>any moving parts.

thinair:
> Perhaps you could put it on a treadmill
> in a still air room and post the video.

I haven’t tried it. I don’t have a treadmill and my tumbleweed is four feet wide.

But looking at the design, I don’t think it would work on a treadmill.

Air has to flow over the anemometer at the bottom to lever it forward. Because of the levering and gearing and the way it models as a rope and pulley system, the air can move over the anemometer at 5 feet per second while the wheel moves over the ground at 10 feet per second, and it still works.

Putting it on a treadmill in dead air would be like pushing it forward in dead air. It’ll just roll to a stop.

But just because it can’t work on a treadmill doesn’t mean it can’t go faster than the air.
And if that’s counterintuitive, then try reading my document and stop relying on intuition.
GregLondon

Greg, what is there to figure out?

everyone says the wheel pushes the prop. In the rope/pulley diagram I came up with, the wind pushes the wheel. Or the parachute at the end of the rope pulls the wheel. Or whatever.

So I’m missing something.

I don’t see how the wheels can turn the prop and make the wheels go faster.

Another way to look at that would be to mount a windmill on a long threaded rod to the cart in a way that when the windmill turns, it will power the wheels forward.

I think your infinitely long threaded rod is similar to my infinitely long piece of rope. They both model the wind in some way or nother.

I’ll try to pencil through your threaded rod analogy and see if it helps.
shMerker

@107

Just tried the yo-yo experiment. I set a yo-yo on my desk in front of me with the string completely unwound. I pulled on the string and the yo-yo began to roll forward immediately, rolling up the string and moving much faster than I was pulling the string. I reset the yo-yo several times to verify this.

I also tried starting with the yo-yo partially rolled up once with the string running under and once with the string running over the axle. When the string was wound under the result was the same, the yo yo wound up more string, moving faster than I pulled. When I wound the string over the yo-yo unwound the string, running more slowly than I pulled. On a last try I wound a very small amount of string over the top. The yo yo moved slowly until the string was entirely unwound. At this point it began winding much faster and winding up the string.

Science is fun.
ThinAirDesigns

Yesno:
>You are claiming that you can tack back
>and forth and get to that point faster
>than the wind does?

Sailboats:
http://www.boatdesign.net/forums/sailboats/tacking-downwind-faster-than-wind-24761.html

Ice-boats / Land Yachts
http://sports.groups.yahoo.com/group/2nalsa/message/161

JB
Mark Frauenfelder

senbot@53: “When you pull the string parallel to the table and cause it to wind (sorry!) onto the spool, the yo-yo will catch up with your hand.”

How can you cause the string to wind around the yo-yo’s axle by pulling the string? It seems like the yo-yo will drag along, unwinding the string as it is dragged.
HUDMAN

“1. When the cart begins running slower than a tail wind, does the air move through the propeller from the back toward the front?”

Of course the answer is yes. We could just leave it at that, but I believe a “just yes” answer here might lead people down the wrong path, so I will expand upon it. The tailwind flows through the prop, and one might think this would cause the prop to turn, geared to the wheels, which make the wheels turn, pulling the cart forward.

But, this is not true because of gearing & the advance ratio. If the wind turned the prop, the cart would actually go in reverse, sailing directly into the wind. (Which should be another article entirely) But, because of the gearing and advance ratio, the path of least resistance is for the cart to start moving forward, downwind, which turns the wheels, powering the prop, generating thrust. Now, once thrust is generated, the air passes through the prop from the front to the back. The cart does not have to wait until it outruns the tailwind in order to do this.

The propeller, like any thrust producing propeller, now has a low pressure in front of it, sucking it forward and a high pressure behind it, pushing it forward. Once it matches the wind’s speed, and this is the critical part, one might think that there’s no way it can go any faster, because if it does, it would be outrunning its power source.

But, again this isn’t true. Why? Because the whole cart is immersed in a “medium” of air movement, (tailwind) and now there is no relative wind on the cart, in front of the cart, or behind the cart. To the cart, it’s in still air, and just drifting along at wind speed like a neutral buoyancy helium balloon. (Also, just like the cart on the treadmill) But the prop is still producing thrust in this medium, much the same way a plane still generates thrust in a tailwind. For the plane, it’s ground speed = plane speed (air speed) + tailwind speed. For the cart, it’s the cart speed + tailwind speed – drag.

The drag is represented by minimal losses due to friction, and major losses from the wheels because they have to power the prop. The trick is to make the cart as efficient as possible by reducing friction and optimizing the advance ratio through gearing and prop pitch. So even though these guys make it look easy, it’s actually a very hard thing to do. But not only is it possible, they (and a few others) have achieved this.

“2. If the cart can somehow accelerate faster than the tail wind (as its proponents claim), does this means that air will now move through the propeller from the front toward the back?

Yes, as mentioned earlier, this will happen a few short seconds after the cart starts moving, and long before it reaches the wind’s speed. Once the cart moves faster than the wind, the relative headwind is now being accelerated by the prop, and is still moving from the front to the back of the cart, but again, the prop is doing no magic trick here. It’s a prop, and it does what a prop does, it generates thrust by creating a low pressure in front of the prop and a high pressure behind the prop.

“3. If the flow of air through the propeller reverses in this way, will it tend to reverse the rotation of the propeller?”

Not at all. The propeller is constrained to the wheels, and as long as the tailwind is present, the wheels continue to roll, powering the prop, and the prop continues to spin, generating thrust.

“Answers to (1) and (2) are clearly “yes.” Answer to (3) can be determined empirically by blowing air at a small fan, first from the front, then from the back, and watching which way it turns. Answer to (3) will also be “yes.”"

(1) Isn’t so clearly a “yes.” It’s only a “yes” for a few short seconds, then thrust is generated, and it becomes a definite and resounding, “No.”

(2) You’re on the money.

(3) If the fan was constrained to tires mounted on the fan, and the whole fan were rolling on these tires and moving in one direction, then it wouldn’t matter if you were blowing air from the front or the back, the fan would turn in the same direction because of it’s kinematic constraint to the tires. Another definite and resounding, “No.”

“Therefore, the reversed air flow will retard forward motion, the speed of the cart is self-limiting, and the claim is false.”

I know it was a long read, but hopefully you have read it through. If you don’t understand any part of what I have written, please let me know so I can explain that section in more detail. Reread it a few times, and I’m sure your final analysis will be, “…and the claim is true.”
jymdonovan

Four Principles of Thermodynamics; It’s not just a good idea, it’s the Law.
airshowfan

And for as long as the DWFTTW cartis being dragged along by its propeller (I say “dragged” because the wheels provide non-negligible resistance), it’s pulling on the Earth, slowing the Earth down, and continually drawing KE from it… just like the treadmill cart draws power from the treadmill.

(And up near the top of my previous post I meant “he treadmill adds energy to the cart in a TREADmill test” not “he treadmill adds energy to the cart in a windmill test”.)
spork

“We saved a nice copy of that .pdf just for future reference.”

I particularly enjoyed the fact that he made sure to copyright it so he got due credit for it (which I will certainly make sure of) – and even came up with a snappy name for the unworkable toy so it’d be ready for marketing for Christmas sales.

Fortunately, the insults are also preserved and will remain fresh for future generations.

http://www.hanggliding.org//files/tumbleweed_151.pdf
Anonymous

Could this be faster as wind speed if it drive in opposite direction?
arkizzle

OK Greg, I totally get the purpose of your design now. I also presume it would work, but would like to video showing the interactions of wind/friction/slippage.

I can see it working in mechanical theory, but can also see it just getting battered by the wind and pushed-by-force rather than as a function of the difference in rolling diameter vs anemometer diameter.
FutureNerd

A nice clear demo of the yo-yo phenomenon on YouTube: Pulling on a Wire Spool.

The analogy is roughly that the wire is the wind, and the bottom edge of the inner roll of wire (smaller diameter than the disks that roll on the floor) is the propeller.

Also check out Along the Paper Faster than the Paper with a touch of sarcasm in the voiceover.

But really people, try it with a yo-yo, string and floor that are under your control. Videos can be faked.
jackofsometrades

DB,

I’m beginning to understand that the flow of energy — like motion itself — is relative to a frame of reference.

Energy is force times distance (or force through a distance). From the cart’s frame of reference, energy flows into the wheels from the ground and is transferred to the propeller. From this reference frame, the cart is stationary, the ground is moving backwards. Taking energy from the ground would tend to slow it’s backward velocity. The energy transferred to the propeller causes still air to be accelerated backwards. This reference frame is more intuitive indoors on the treadmill (it corresponds with our own), but it is equally valid outdoors in the wind.

From the reference frame of the ground (or the moving treadmill), energy is flowing from the air into the propeller (it has a positive force and a positive velocity), and is transferred through the drive train, into the wheels and into the ground (which is presumably being accelerated forward by the force applied by the wheels of the cart). This reference frame is more intuitive outdoors in the wind, but is equally valid indoors on the treadmill.

I apologize for being so narrow in my perspective. I was so focused on convincing people of the equivalence of the indoor and outdoor scenarios, I instinctively put myself in the reference frame of the vehicle in both situations. Both views are correct, and equally valid.
airshowfan

Sorry, comment #189 was meant as a follow-up to comment #187. That may not be clear with comment #188 in the middle.

And yes, if the net force is zero, this means “no CHANGE in velocity”, not “no motion”. Duh. ;]
Anonymous

Theoretically, it is possible. Practicality is another matter, but the following jives with the little catamaran experience I have.

How to beat the wind using low-friction wheels or a fast sailboat.
- start downwind, raise a sail, and keep it nearly perpendicular to the wind
- as you pick up speed, turn away from the wind (angle theta) until you are driving nearly perpendicular to the wind
- wait until groundspeed ~ windspeed/sin(theta) gets high
- lower/reef/furl the sail and turn downwind until groundspeed ~ windspeed
- tack the other way and repeat

The trick here is not to rely on a fixed profile but to (passively) flap like a bird.

People regularly clocked GPS speeds higher than windspeed on Clinton Lake, KS; but I don’t think their mean downwind groundspeed exceeded the wind.

- DH
airshowfan

I meant “while I was writing this”…
Anonymous

Hey guy above me. Yes you. I hope you’re being facetious because you just described how a propeller based DDWFFTW vehicle would work. Two rigid sails rotating around a pivot equals a propeller. No wheel drive is needed the prop will work just like two airfoils.

BTW for doubters/skeps it doesn’t take much for an average beach catamaran to sail faster in a perpendicular direction than the wind that propels it.
spork

“everyone says the wheel pushes the prop. In the rope/pulley diagram I came up with, the wind pushes the wheel. Or the parachute at the end of the rope pulls the wheel. Or whatever.”

Greg, there are some real subtleties to this thing. In the prop-cart example, the prop pushes the cart – which of course pushes the wheels, but this allows the wheels to apply torque to turn the prop. In other words, if the wheels lose traction they’ll tend to skid in a braking sense, not in a pulling sense.

“I don’t see how the wheels can turn the prop and make the wheels go faster.”

The prop only makes the wheels go faster by pushing the cart faster. In the end, it’s the road that makes the wheels go faster.

Essentially the parachute/yo-yo example is the same thing. If you imagine the yo-yo experiencing aero drag, the yo-yo will want to skid the same way our cart wheels do. The rope pulls the wheel, but the road applies the torque to it (I realize that’s an over-simplification in that the rope and the road provide a moment couple – but hopefully it still helps).
ian9toes

This would have to come down to the efficiency of the propeller versus the drag of the wheels and bearings and pulley etcetera. So my comment is only on how to increase that efficiency. I’m talking about the cart with the big swivelling funnel and the exhaust leading out the back. What about having the exhaust go straight down, so when there is relative tail wind no wind can go up the exhaust and turn the prop the wrong way. In actual fact no wind would ever go up the exhaust as the swiveling top section is greater than the small exhaust opening, but by pointing the exhaust downward rather than backward we don’t hinder that exhaust coming out in the case of a relative tailwind. By having the exhaust pointing down this will also create a ventouri, which will help ‘suck’ the exhaust out.
Anonymous

I didn’t read every post so maybe this has been mentioned, but isn’t there a type of propeller that has non-tilted blades that are airfoils, and rotates in the same direction even if the airflow is reversed? The propeller is symmetrical when viewed edgewise.
rasz

sorry, but now Charles Platt just comes out as an ass. If he is so sure that it doesnt work why dont he buy that $40 “impossible to work” model and see it for himself? If I remember correctly Spork even proposed to send him a kit for free.

At least now we know something – if you come out with a breakthrough that brakes current rules of physics or common sense people will reject it no matter what. Only way to prove people wrong is to design a cheap way of reproducing said breakthrough and spread the plans/kits.
spork

Anyone up for a real challenge?

http://scienceblogs.com/goodmath/2008/12/windpowered_perpetual_motion.php
ThinAirDesigns

JYMDonovan:
>Four Principles of Thermodynamics; It’s
>not just a good idea, it’s the Law.

And we are nothing if not law abiding citizens – either that or we have diplomatic immunity, I’m not sure which.

JB
semiotix

I agree that such a device is possible, strictly so that I can then post

DWFTTW FTW!
Lithoman

Dust:
The energy source for the prop is the belt. The energy source for the belt is the axle. The axle is turning because of the wheels turning. The wheels are turning because of friction and the relative speed difference between the cart and the surface under the wheels. It doesn’t matter if the cart is moving over the surface or the surface is moving under the cart, the energy in the axle is due to the wheels turning.
Wind speed is a measurement of the relative speed difference between the fluid and the surface being measured from. Air moving 10mph over the ground would have the same force as stationary air over a surface moving 10mph.
GregLondon

I uploaded another rev.

It fixed a couple of typos.

And I added a “bonus section” which is just add on info for folks who may be interested in it.

The bonus section contains a flip-chart animation of the yoyo pulled by a rope with a parachute on teh end.

It also contains an idea I’ve been pondering for making a sail-barge version. Mostly I just wanted to see if I could figure out a way to open and close the sails in a rigid manner. I think that part works. Scaling it to a workable vehicle might be a challenge.

It also contains a description of the “electro cart” mentioned above. I liked the idea enough that I put a page in at the end that describes it. It makes me smile just thinking about it.

Enjoy

http://www.greglondon.com/tumbleweed/
macegr

I might have a way to do it. Maybe.

The problem is that every is assuming the propeller will be mounted to the body of the vehicle. Once the vehicle is going the same speed as the wind, no more energy can be extracted from it.

However, some parts of a rolling vehicle are always going slower than the rest of it. Namely, the part of each wheel that is lower than the axis of rotation.

Imagine a vehicle that is mostly big wheels. The top half of each wheel is shielded from the tail and head winds. Inside the wheel is a multitude of propellers such that when spoke with a propeller is touching the ground, the propeller can be turned by a tail wind. The relative velocity of the propeller to the wind progress from wind speed to zero speed to wind speed as the spoke rotates out of the shielded area. Instead of a shielded fender, the propellers could be feathered during the top half of the rotation in order to minimize resistance.

It sounds far-fetched, and friction losses would probably kill it, but it DOES place propellers in a place where the wind speed is faster than the propeller speed while the entire vehicle is in motion at wind speed.
farkinga

Here is a direct link to Spork’s build plans:

DWFTTW Build Plans

…and this is the article I posted to RTFA that contains the PDF above:

downwind-faster-than-the-wind-dwfttw-plans

I want to see a third-party test this idea out… I can’t wait to see this in Make magazine and on Mythbusters! Enjoy!
HUDMAN

To quote the David Blaine parody that is all over YouTube, “What the ‘F’?” “What the ‘F’?” “What the ‘F’?” What the ‘F’ is Greg London smoking?

Are you kidding me? So, you admit that your measly design won’t work on a treadmill, but will work in the “Real World?” Then… you won’t provide proof because no proof is needed. “Just look at the pdf file,” is what you say. Are you kidding me? Or, are you kidding yourself?

WAKE THE “F” UP!!! Dude, if it won’t work on a treadmill, then it just simply won’t work! (I can end here by the way, as anything else I say will not matter) But, let me try to explain it for you.

The cart (any cart) that tries to go directly downwind faster than the wind will need (make that require) some component of itself to travel slower than the wind powering it, in order for it to outrun that wind. In your design, what component will travel slower than the wind, in order to use that wind to help drive it beyond the speed of the wind it’s in? Since you’re obviously baffled by this question, let me help… NOTHING! Nothing is traveling slower than the wind. So… Since nothing is traveling slower than the wind its in, it cannot travel faster than the wind.

Now, let’s take another look at JB & spork’s cart. What’s going slower than the wind it’s in, in order for it to outrun that wind? Thrust. That’s it, thrust. Period, and end of sentence, thrust. The thrust is propelled backwards, and is traveling slower than the wind, so the wind can act on this thrust. The wind can no longer act on the “frame” of the cart, but it can (and does) act on the thrust from the cart. Because of gearing & advance ratio, the wind “pushes” on the thrust, (more accurately though, the thrust is pushing on the still air behind it, kind of like the treadmill test) which is coming from the cart, and the cart is able to travel faster than the wind.

Since no part of the “Greg” cart is traveling slower than the wind, and no part of the “Greg” cart is producing any thrust, and no part of the “Greg” cart can still “use” any part of the tailwind, then obvioulsy the “Greg” cart has exceeded its power source, and the “Greg” cart cannot travel faster than the wind.

You can pull all the ropes in the World through all the pullys you can muster, but that cannot change the fact that your cart is worthless & useless, and cannot achieve what you claim it can.
Nelson.C

Now that I’ve seen this video, and read a couple of well-written explanations from others, I’m 80% convinced that this thing could work as advertised; I only need to see an experiment where the windspeed and vehicle speed are well-documented and I’ll have lost all but my last quantum of doubt.

Spork, you and ThinAirDesigns are just rubbish at explaining things. You act as though you’re trying to explain to the Internet, a single intelligence, and are frustrated that we keep forgetting what you told us before. Whereas, in fact, you’re berating people who have never come across this before and need some help getting up to speed (insert self-start DDFTTW joke here). And, yeah, some of us are skeptical, and others are downright disbelieving, but this is normal and something you should have learned to cope with by now, you’ve been doing this long enough.

I, and others, keep making suggestions for how you could make things easier for yourself if you were really interested in proving your case, but you keep insisting that you’ve made your case â€” elsewhere. When asked where specific bits of information are, the reply is either vague handwaving at the Internet, avoidance, or rarely, links to fragmented and inadequate bits and pieces scattered hither and yon.

For goodness’ sake, write a FAQ, already. Set up a website, it’s not that hard or expensive. Consolidate the information that’s out there, roaming free across the internet prairie, so when someone asks a question you’ve heard before, you can just point to the FAQ. Or, you know, just ignore everything except the opportunity to pick fights with random internet strangers.
dustbuster7000

JB, you say its the same model, but that’s difficult to assess, since its defined very informally. Whereas the treadmill test is straight forward and easy to assess, for all the reasons I mentioned above. I don’t know if the town-treadmill thought experiment is equivalent, I’m not even sure what you mean in your description of it. Why even bother with it. This discussion isn’t, or at least shouldn’t be, about humouring anyone. You designed the treadmill test (or someone else did and you support it) as a response to criticism of the outdoor runs, to control variables. It does that nicely. You say that the treadmill test is an inertial frame of reference shift from the normal outdoor operation of the cart, equivalent in all controllable respects.

The town-treadmill may or may not be equivalent to either the treadmill test or the normal outdoor cart operation, its impossible to tell from the description you gave, and it add a whole bunch of new variables (scaling issues is a major one, since you’re riding it in your thought experiment, it can’t be the one in the treadmill test). Its not arguably an unwise tangent, its a complication of something that is already complicated. Go back to the treadmill test. Pick apart my analysis.
GregLondon

arkizzle: I totally get the purpose of your design now.

Cool. was there any particular part that gave you the “a-ha” moment? Just curious.

but can also see it just getting battered by the wind and pushed-by-force rather than as a function of the difference in rolling diameter vs anemometer diameter.

Yeah, it’s a question of how much drag there is, but the point is that the concept of how to go faster than the wind is now understood in a simple mechanical system. Ropes and pulleys show how everything works.

The only difference is how much does the air “rope” slip over the anemometer “pulley”.

I had the idea that the anemometer could be improved by making them hinge. Since you only want them to engage the wind at the bottom of the turn, you could have them on some kind of flap or hinge so that when they’re on the bottom of the wheel, they fall down and pick up the wind. and then when it rotates up the front fo teh wheel, it would drop back down against the center drum.

Seemed like extra comlication and I wanted ot try to have a design with no moving parts, but if I really need more efficiency, that would probably be the first thing I’d try.

But like I said, the point for me was to understand the concept from a purely mechanical process with no voodoo, and I think my diagrams explain the process conceptually.
Anonymous

The issue of propeller direction is solvable. Just have “selection” so the propeller can be geared to power wheels in the forward direction with the wind coming from forward. Once you get it going, there’s no reason IMHO it couldn’t keep doing that for awhile, but power enough to fight the wind resistance associated with now moving into the air the opposite way is going to make it slow down to wind speed eventually.

tyrannogenius
shMerker

So if I understand this correctly the vehicle is actually still pushed by the wind once it gets going because the wind is able to push on the air that the propeller is pushing behind it?
Anonymous

@225

Greg London: “the whole thing is about as interesting as a block and tackle”

Yes! Yes! You get it!

For those who didn’t click through, Greg designed the “tumbleweed”, a direct analog to the spool and thread with a curved blade impeller contacting the wind replacing the inner spool contacting the thread. As a practical matter, I wonder whether the tumbleweed will actually go faster than wind, and the data reported so far (“I put a box fan in front of it, and was able to blow it up an incline”) doesn’t say. I look forward to further tests. Regardless of whether the tumbleweed works in practice, it’s an excellent intermediate in principle, and could be helpful instructing any remaining skeptics.
ThinAirDesigns

NelsonC
>For goodness’ sake, write a FAQ,
>already. Set up a website,

For goodness sake, stop trying to tell people on internet forums what to do.

We’ve done a lot. You’ve done nothing See the delta?

JB
GregLondon

there are some real subtleties to this thing.

I’m starting to get that impression too.

the yo-yo will want to skid the same way our cart wheels do

That might be good to know.

thanks
ThinAirDesigns

Greg:
>I wrote up a spec that describes how this
>bloody thing actually works, and how to
>build a device that will go directly
>downwind faster than the wind without
>any moving parts.

Perhaps you could put it on a treadmill in a still air room and post the video.

JB
ThinAirDesigns

Dust, you may not realize it, but I’m asking you a very relevent question to the debate:

You claim the cart behaves differently on a treadmill vs in the street. I’m simply asking you to tell me how you know this.

Give me just one example of a test that can be performed without outside reference to determine whether you or I or any object is on a treadmill or not.

Since you’re convinced there is a difference it should be very easy for you to describe such a test.

Please do.

Thanks

JB
jackofsometrades

AirShowFan,

I really like your “Ferris wheel with sails” idea; I believe it should work, and is easy to visualize. I have been visualizing a similar (but more complicated) vehicle, something like a skate board with multiple sails moving backwards, driven by the wheels. Your idea is much easier to visualize, and hopefully will convince the skeptics that DWFTTW is clearly doable. Thanks,

Jack
shMerker

This whole conversation is making me think I should give out some copies of “The Way Things Work” for Christmas. Every conversation should be this much fun.
technogeek

I think it’s a perpetual motion machine, and I think that’s demonstratable by picking the right reference frame.

Set relative airspeed to zero, as it would be if the vehicle is moving as fast as the wind; in that state you’re not able to extracting any energy from your tailwind. You’re now trying to to extract enough energy from pushing the vehicle to a higher speed to maintain that speed. This would require perfect, lossless transfer of energy from somewhere — I don’t care where — and I think that’s pretty clearly a second-law violation.

Note that this is a different case from travelling at an angle to the wind. Iceboats, I’m told, *can* attain ground speeds exceeding windspeed — but not when travelling directly downwind.
bop

ictharus (#36) is of course right. the secret behind travelling faster than the wind lies in temporary energy storage by a spring, rubberband etc. it would definately work and give a vehicle an instant velocity greater than the wind. however, the avarage speed would be lower than the wind, when the time used to charge the energy storage + the time used to travel a certain distance is accounted for.
spork

“Please provide the evidence that it is possible for a sailboat to tack downwind and arrive at point B from point A faster than something traveling windspeed in a straight line would.”

Please see my post #95. It refers primarily to ice-boats but the principle is the same. High performance sailboats can do this also, but to a smaller degree. I’ll find evidence if need be.

“My understanding had been that the greater distance caused by tacking cancelled out the increased speed, such that you don’t actually get to a point faster than the wind would.”

This is a very common view, but it actually can be done, and is perfectly routine for ice boats and land yachts. In fact high performance sailboat do not run directly downwind for this very reason. They get to their destination faster by tacking downwind.

“Say the wind is blowing east, and you want to get to a point due east. The shortest distance between you and your destination is a straight line that is exactly downwind. You are claiming that you can tack back and forth and get to that point faster than the wind does? ”

That’s correct.

“I don’t know either way. It seems impossible, but since it’s possible to tack upwind (and arrive at your point B infinitely faster than the wind or something traveling with it would), why not?”

In fact, tacking upwind is actually the same thing. A sailboat is just a vessel that exploits the energy at the wind/water interface – it’s got one wing sticking up (the sail) and one sticking down (the keel). What a fish sees when a sailboat tacks upwind is a keel that’s tacking down-current faster than the current.
GregLondon

spork: And I’m dying to see someone make a non-prop design that can work.

I forgot to ask. Did you look at the tumbleweed version on page 14? It uses “saloon doors” to do the flapping. I think that would actually work, I just have to figure the foot switch to close the doors.
Caveman Joe

Hmm. In the bottom illustration which shows what happens when the cart is travelling faster than the wind (IE the wind coming in from the opposite direction will try to spin the wheels backwards), I reckon that particular problem could be sorted out with some sort of ratchet or clutch system.

But something tells me that it wouldn’t matter either way. :)
Inkstain

“At least now we know something – if you come out with a breakthrough that brakes current rules of physics or common sense people will reject it no matter what. Only way to prove people wrong is to design a cheap way of reproducing said breakthrough and spread the plans/kits.”

And hope that the other 99 guys before you who did similar things and were debunked were just unlucky.

The people who reject have a long history of being right. Unless you really can run a car on Brown’s Gas…
JakeTheSnake

Why hasn’t the question been asked:

Can you go DWFTTW on a treadmill and still take off?

Does anybody have a good way of going up wind with a propeller powered by the wind? Snake thinks not.
cmpalmer

Can Spork33 or whoever from the other discussion at least chime in and confirm that this drawing is completely backwards? If the drawing is correct and that is the kind of machine they’re claiming, it’s obviously BS. However, I don’t think that mechanism is drawn right (the prop should rotate the other way).
JakeTheSnake

There is a similar problem, that would have the same answer as this one:

Can you use power generated from the flow of water power your way up stream?

Remember: air is a liquid.
cmpalmer

@SHMERKER (#60)

That is the only explanation that makes any sense. I’m still not sure if it works or is real, but you’ve had the most succinct description of it so far.
spork

Wow Greg – you’ve kind of gone off the deep-end.

Your suggestion that we don’t understand how this works is particularly entertaining in light of your recent proposal of a cart that can’t possibly work even in theory. I have many times described and proven that our cart works through math, vector analysis, analogies, and now repeatable physical proof.

You’ve offered NONE of those.
Anonymous

this has been done to death on Runryder, the model helicopter site….members have working demo’s on youtube.

eni fule kno the propellor is just 2 sails moving at right-angles to the true wind,

boats travel considerably faster than the wind and also into wind.
all basic stuff, really and a member has made a DIY kit so you can prove it for yourself
jeffareid

>Please provide the evidence that it is possible for a sailboat to tack downwind and arrive at point B from point A faster than something traveling windspeed in a straight line would.

Note that the apparent crosswind on a sailcraft is independent of the sailcraft’s forward speed. The apparent crosswind is the component of true wind perpendicular to the heading of the sailcraft and therefore independent of the sailcraft’s speed. The apparent crosswind equals the true wind times the sin(angle between wind and sailcraft heading).

Say there’s a 10 mph wind and a sailcraft traveling at 30 degreees (downwind) or 150 degrees (upwind) offset from the wind. The apparent crosswind is 10 mph x sin(30 or 15) = 5 mph. As long as the sailcraft holds it’s heading (and wind direction doesn’t change), the sailcraft experiences a apparent crosswind of 5 mph regardless of it’s speed. High end “Skeeters” have a lift to drag ratios of around 7 to 1. Thus a 5 mph crosswind translates allows forward speed with an apparent headwind of 35 mph. The downwind speed would be 35 mph x cos(30) = 30 mph, 6 times faster than the wind.

Note that like a wing, the sail converts the apparent headwind into thrust, and at high effective glide (lift to drag) ratios, with a small angle between apparent wind and apparent headwind, most of this thrust results in a net upwind component of flow off the sail (else there would be a net aerodynamic drag on the iceboat).

Again note that the power source, an apparent crosswind is indpendent of sailcraft’s forward speed, and so the limit is simply the effective lift to drag ratio (including ground related as well as aerodynamic drag effects).
cmpalmer

Personally I’m beginning to think it has something to do with the enigma about being able to suck a strand of spaghetti into your mouth, but not being able to blow it out.
spork

“A nice clear demo of the yo-yo phenomenon on YouTube: Pulling on a Wire Spool.”

That was posted just the other day by one of our hang gliding buddies. It was intended to prove the case to the guy that posted this unintentionally humerous video:

http://www.youtube.com/watch?v=G2S_jHro9Bk&eurl

“Also check out Along the Paper Faster than the Paper with a touch of sarcasm in the voiceover.”

This was posted by one of our proponents on the JREF forum. With Terry as a neutral observer, it’s pretty hard to dispute his results.

Long ago, I diagramed a silly DDWFTTW cart based on the yo-yo principle:

http://www.putfile.com/pic/2794071/?action=zoom

The whole yo-yo thing is sort of a brainteaser all by itself. The reason it works is because a rolling wheel actually rotates about the contact point – not the axle. So the torque being applied by pulling on ANY point above the contact point is in the correct direction to move toward the force. If that force is below the axle, simple mechanical leverage will cause the wheel to move toward the force at a faster rate than the item pulling it.
Daemon

Ok, I officially declare this particular design to be incapable of achieving the stated design goal. I hadn’t thought of the relative speed before, but this design wouldn’t even be able to move at the SAME speed as the wind. (same speed = effectively no wind)
spork

>> Page 5 shows a time lapse sort of thing and it clearly can go faster than the wind.

Your tumbleweed design can’t and won’t go faster than the wind (I just hope no one dies in a horrible ball of flame as a result).

>> Since a box fan could push it up hill, I don’t think it should be too much a problem…As for more experiments, yeah, I was going to try it today… Really though, I don’t need experiements to prove the theory.
< <

Good - in that case you should be happy to take my bet before doing the experiment. You name the amount.

>>But looking at the design, I don’t think it would work on a treadmill.

If it won’t work on a treadmill it won’t work on the road in a tailwind. The two are one and the same. Again, please tell me what engineering designs I should avoid if I don’t want to die in a fiery death at your hands.

>>
But just because it can’t work on a treadmill doesn’t mean it can’t go faster than the air.
And if that’s counterintuitive, then try reading my document and stop relying on intuition.
< <

This is rock solid proof that you don't understand one of the most basic principles of physics - the principle of equivalence of inertial frames.

>>
the point for me was to understand the concept from a purely mechanical process with no voodoo
<<

Fail.
Nelson.C

Contra Charles Platt, I keep thinking about the change from below windspeed to FTTW, but I thought the situation was complicated enough that I decided to concentrate on the core issue of whether it was possible for the device to go FTTW at all.

My intuition says no, but I can’t find out enough about the cart to prove or disprove the notion to my satisfaction.
Mathleticism

If you turn the pedals of a bike backwards will it stop or pedal backwards. Unless its an old POS from generations ago, then no. If there is a catch in the axle then you could possibly use that wind from the opposite direction to your favor and somehow convert it to usable energy in the “faster than air game”. Im still with the “I dont know” crowd and Im not a physics major but I think its still possible. For this scenario, does the wind have to be constant in velocity and direction?
spazzm

I’m in the ‘don’t know’ camp – ultimately this will be solved through well-designed, peer-reviewed and reproducible experiments, not online forum “debate”.

It’s the scientific way.

My initial hunch, however, is that it’s impossible. But if it’s one thing my life has taught me is that, when it comes to physics, I should never trust my initial hunch.
jeffareid

“apparent crosswind is 10 mph x sin(30 or 15) = 5 mph.”

Make that sin(30 or 150).

“High end “Skeeters” have a lift to drag ratios of around 7 to 1. Thus a 5 mph crosswind translates allows forward speed with an apparent headwind of 35 mph. The downwind speed would be 35 mph x cos(30) = 30 mph, 6 times faster than the wind.:

Make that 3 times faster than the 10 mph total wind, as opposed to 6 times faster than the 5 mph crosswind.
ThinAirDesigns

Mark F, here is another version of the Yo-yo demonstration that we’ve used numerous times:

http://www.youtube.com/watch?v=dCBpTv0DsPI

JB
Anonymous

The way to tell the difference is easy – moving objects experience a Coriolis force, and ones stationary relative to the Earth don’t. It’s also not relevent. The treadmill in still air is the same as the street with wind for practical purposes.

Whether using the treadmill as a test is a different matter, though. In one the plane starts motionless relative to the air, and in the other it starts motionless relative to the ground. This doesn’t make a difference as far as the energy available from the ground and air interfaces.

However, it does make a difference as far as ‘flywheeling’ goes – that is, motion using the kinetic energy of its moving parts, instead of energy it’s picking up. Here the history is important, so there can be a small difference between the two situations.
GregLondon

Some of my favorite Greg London quotes

I’m sorry spork.

I’m not sure exactly how many times I’ve apologized at this point, but I’ve decided I’ve done it enough. If you want to hold onto stuff after this, you’re doing it because you choose to.

I’m sorry.

And I’m done apologizing to someone who has no interest in accepting my apology.
spork

One of my favorite passages from the famed “Tumbleweed” document:

“Why in the world do all the designs out there right now use propellers?
I don’t know the answer to that one.
I believe it might be because the people who built the propeller driven machines didn’t understand what was going on exactly and so just tinkered with something until they got it to work.”

I truly want to thank you for this document Greg. It now has the prize position in my “Museum of DDWFTTW FAIL”

I eagerly await your test results – even though you don’t need test results to validate your theory.

Greg London says:
“As an engineer, I’ve spent years working on projects where a screw up on my part means someone (or a lot of someones) could literally die a horrible ball of flame kind of death.
…
I’m sorry if applying that level of engineering rigidity pissed off a lot of people, but it seems fairly clear that a number of people who have “intuited” how this thing works, really have no farking clue how it works.
…
Really though, I don’t need experiements to prove the theory. ”
joelphillips

The propeller in the diagram appears to be the wrong way around.

The questions all assume that the wind turns the propeller, which turns the wheels, which pushes the cart forwards. In fact, the wind pushes on a large object, which accelerates it, which turns its wheels, which turns the propeller, which accelerates it more.

At sub-wind speeds, the action of the wind rotating the propeller *does* slow the cart down, but that’s okay, because the action of the wind pushing the cart along is stronger. At close-to-wind speeds, there is no longer any effective wind, so the only thing happening is the wheels driving the propeller.
LSK

Out of curiosity: What if some hypothetical device behind the propeller limits air flow in one direction?
spork

It’s interesting to note that each of the “DDWFTTW” concepts involve pushing air backward relative to the cart, but allowing it to still go forward relative to the ground. This is what leave the slower moving air in the wake of the cart (the air from which some of the energy was robbed).

This is what we refer to as the “advance ratio”. The distance the prop would theoretically move through the air in a single rotation without slip divided by the distance the wheels move along the ground in that same single rotation of the prop is our advance ratio.

To build a cart that goes directly downwind faster than the wind the advance ratio must be less than 1.0. An advance ratio of greater than 1.0 will produce a cart that goes directly upwind (and can also go faster than the wind).

As the advance ratio gets closer to 1.0 (from either side) the maximum theoretical speed increases – but too close to 1.0 and you’ve traded off too much of your force for speed – and you’ll no longer overcome the frictional losses of the real-world.

On another note…

Is it me, or is the tide turning? It seems like there are a lot fewer people calling B.S. on our little brainteaser.
misterfricative

@ Spork #203 Good luck with that because unfortunately I don’t think Mark C is listening. Maybe he needs to see it working in the wild on Mythbusters….

@ Airshowfan #175 — Yes, I’d be very glad if you could check out my post #142. I tried to make it as accurate, rigorous, clear and complete as I could, but if you see any problems or obscurities or errors, then please advise.
Mender

I’ll start on my build during the Christmas break. I suspect that a non-prop version capable of running on a treadmill will be a little tricky …
GregLondon

Thanks, Greg. I’m having trouble visualizing how pulling the string of a yo-yo on the ground will cause the yo-yo to move faster than the string. It seems to me that the yo-yo will just unwind as the string is pulled.

Mark, I tested a draft version of my document on a friend and he had the same confusion. What we did was put something together in a couple of minutes with two paper plates for wheels, an empty soda can for an axle, and some dental floss (neither of us sew, so we didn’t have thread).

He was weirded out when he actually saw it turn, but then it clicked for him. I was thinking of adding something real quick for anyone really stuck by that part to suggest they actually build the yoyo and then mention that paper plates, empty soda can, thread, and tape is all it takes. Most people probably have them around the house. Propellers? Not so much.

Once he played around with that, he seemed to understand the rest.
idontcomment

you guys think the greg cart won’t work? it looks like a version of airshowfan’s ferris wheel, which seems like it could work in theory at least.

greg: if it works in the wind it will work on the treadmill, these scenarios are identical from the perspective of the vehicle. though I can imagine it might be hard to find a 4′ wide treadmill! On the treadmill the sails will be being propelled backwards, the air will resist them, the vehicle will lever around in a forward direction… perhaps.

Also, lets not knock intuition over math too much – certainly the formal description of a problem is important. The formulae can take you from a fully specified problem to a conclusion, but its meaningless (or worse, wrong AND convincing) if one doesn’t have the (intuition?) to set up the problem right in the first place. Just on this issue we have seen many people manipulating symbols “proving” that this vehicle is impossible. One can convincingly prove just about anything in a problem like this mathematically, when making subtle errors in assumption.
xzzy

Out of curiosity: What if some hypothetical device behind the propeller limits air flow in one direction?

Let us call this hypothetical device a “sail”.
dustbuster7000

JB,

If the forces were equal, there’d be no movement, there is, so the forces aren’t. Please be consistent.

Airshow,

Comparison to planes stop when you note that planes carry fuel.

JB, I will agree that ground at rest with wind of 10 mph is equivalent to air at rest with a treadmill at 10mph. This is the source of your (and other) equivalence claim. However since there is no energy exchange (neither changes speed or position), thats as far as it goes. If we accept that the cart acts as a conduit of energy from one to the other (and the cart’s energy content changes, since it moves) then for the propeller to increase the energy of the air by pushing on it and thus moving the cart, that energy must come from somewhere. It can’t be the wind, since its outputing energy to the air, the wind can’t put energy in to the cart. Thus, the cart is not a wind powered vehicle. Its being powered by its wheels. Since the wheels don’t have any intrinsic energy, they in turn are getting it from somewhere else. The ground. The only time energy from the ground can enter the wheels is when the wheels are driven by the treadmill. This not the case outdoor, since the ground cannot add energy to the wheels. Thus the treadmill is different to the outdoor case.

And that’s pretty much all I have the time or energy (ha) I have to add to this discussion for now. I wish you luck, because if you are right, then you have indeed managed something truly useful, even if you don’t realize it. But as you’ve no doubt guessed, I wouldn’t bet on it.
idontcomment

Ah, thanks JB (261), makes sense, I wasn’t thinking about the top vanes, which airshowfan removed with the hinging!
yesno

Spork,

Thank you. I am convinced that downwind travel at a speed faster than the wind is possible, with tacking.

Now, the question is just about whether the cart does it without tacking– but I care much less now. All the cart would do is provide a new, clever way of doing what is already in principle possible. I don’t see how you can claim with a straight face that accomplishing the feat through tacking is possible, but no other way is.

I think more skeptics could be won over if you provided this explanation up front. A lot of people think not that the cart specifically is impossible, but that all dwfttw travel is impossible.
af2008

Joelphillips hit the nail on the head. Detractors don’t even have the right polarity.
Teresa Nielsen Hayden / Moderator

Reasonably civil. Good on you all.

Remember, just click on the eyeball if you need a moderator.
GregLondon

spork: And I’m dying to see someone make a non-prop design that can work.

spork: Your new cart is reminicent of a design I use strictly as illustration:

so, you’re telling me I should have bet you a hundred grand before I sent you the link?

that was my retirement plan, dang it.

I’m pretty sure that both the torque and thrust increase roughly with the square of the RPM on a prop. But generally the efficiency also increases – up to a point.

Hm, so I need to convert a tailwind into reverse torque which I can then put into the ground wheel. And so the idea is to pick a prop rpm that has good efficiency so most of the tailwind gets converted into reverse torque.

Is that about right?
Mark Frauenfelder

Joel Phillips@7: “In fact, the wind pushes on a large object, which accelerates it, which turns its wheels, which turns the propeller, which accelerates it more.”

Does that mean if you put two propellers on it, it will accelerate twice as fast?
idontcomment

Hey guys, here’s something to help you visualize how this works, its worked for explanations IRL maybe it will help online also!:

You have a robot on an airport style people-mover which moves at 10mph. The robot can use 1 watt to move forward at 2mph on the mover, making a total ground speed of 12mph. To keep this up, it puts out a generator wheel to contact the ground. This wheel sees a total speed of robot_speed + mover_speed. The extra force of this generator rolling along the ground is going to make it harder for the robot to crawl forward, which is going to slow its crawl. What makes this work out is that the generator is using the total_speed, but the robot only needs to produce robot_speed. So they will come to an equilibrium where the robot slows down until the energy its using to move forward is equal to the energy produced by the wheel. Even in a very inefficient setup, the wheel will be producing some power since its moving at at least 10mph, and the robot can use that to make some forward progress.

If the people-mover stops, then you’re back to the standard perpetual-motion situation where you’re trying to power a car from a generator based on the wheel speed of the car – doesn’t work. When the people-mover is going, however, you have a base wheel speed that’s powered externally, and that’s what makes it work.

Now on to the wind. The people-mover is the wind, the robot is the vehicle. The robot’s wheels are the propeller, worming its way slowly along the wind (i think worming slowly through the wind as a worm gear would is easier to picture than the aerodynamics). The propeller is powered off the vehicle’s wheels (as the robot was powered off its ground-touching generator).

With the wind situation it is very similar, only it is easier to imagine losses and inefficiencies (for the robot it is easy to stick to and crawl along the people-mover, far easier than to grab onto the wind!). However what’s nice here is that we’re not dealing with a perpetual motion situation, we have an energy source we can pull from that doesn’t directly impact our output. A simple sail-based vehicle won’t quite keep up with the wind, due to friction etc. In our case not only do we have friction, but an added force from sapping some power through the wheels.

How will the power we take from the wheels increase our speed past the speed we would have been without taking that power, you ask?! Go back to the people-mover example! We’re taking power from the sum of the wind just regularly pushing us along, plus the extra speed we get by clawing our way forward through the wind with the prop, but we only have to put that power back into the extra bit of speed from the prop. We have to make sure that as we do this we’re more than making up for our losses due to friction etc., but this is no problem – we can always “add more sail”, as it were, by say switching to a bigger, same weight prop, to get more wind power for the same wheel friction. What’s the theoretical max top speed? The wind energy available is based on the wind speed RELATIVE TO THE GROUND going through an area with a cross section of your vehicle – the top speed, then is, how fast you can make your vehicle go with this energy (that’s based on standard things such as friction and drag and efficiency).

Criticisms:
–when you’re at wind speed the relative wind speed is zero. you have no energy, its the same as being in a room with no wind!–
The difference is that the ground is moving past at wind speed! This is an arbitrarily large source of energy. Sure, the more energy you pull out of the ground the more you slow down. But the amount you slow down is not the same as the amount you can use that energy to speed yourself up! eg, it might take 1 watt to propel you an extra 1mph. With your current setup it might take 1mph of slowing down to generate that 1 watt. no gain! no problem, double the wind cross section, now you get 1 watt with only 0.5 mph slow down, and you’re in business! Remember you’re pushing off the wind, clawing your way past it – a small amount of energy for clawing, say 1mph, will increase you past the wind!

–the propeller needs to switch directions as you pass wind speed–
ok this is specific to that particular design, but here goes: as people have written here already, the direction of the propeller is such that as the vehicle rolls forwards, it’s pushing air backwards. The concern, then, is that when the vehicle is stopped, a tailwind will spin the prop and make the vehicle roll backwards. THIS DEPENDS ON THE GEAR RATIO. It’s hard to think about with air, but use this to visualize: consider a vehicle with wheels on the bottom connected by gears to a tread on the top (wind paddle wheel, if you will ;) s.t. if you move the tread towards the front of the vehicle the wheels propel the vehicle backwards. For a low gear ration (large track motion -> small wheel motion) sliding your hard forward along the tread will make the vehicle move backwards. For a higher gear ratio (great than 1:1) sliding your hand along the top will make the vehicle go forwards, at a speed greater than your hand (your hand will slip off the back as the tread pushes it backwards). Same for props.
soupisgoodfood

@#25

Helicopters have collective and cyclic pitch control, which makes them vastly different to a fixed pitch propeller. I’m not sure about the exact aerodynamics of autorotation, but I know that they are different enough to this situation. For starters, gravity plays a major roll in heli autorotation, which it doesn’t in this situation. Airspeed is another factor of a heli autorotation, which means it’s not directly down wind.
yesno

Spork,

Thank you. I am convinced that downwind travel at a speed faster than the wind is possible, with tacking.

Now, the question is just about whether the cart does it without tacking– but I care much less now. All the cart would do is provide a new, clever way of doing what is already in principle possible. I don’t see how you can claim with a straight face that accomplishing the feat through tacking is possible, but no other way is.

I think more skeptics could be won over if you provided this explanation up front. A lot of people think not that the cart specifically is impossible, but that all dwfttw travel is impossible.
spork

“Its not arguably an unwise tangent, its a complication of something that is already complicated. Go back to the treadmill test. Pick apart my analysis.”

O.K. I’ll let JB take his approach, and I’ll go your way (we’ll double team you).

Your analysis says that the treadmill test and the outdoor test are fundamentally different. I claim that to the cart there is absolutely no difference between being on a road with a 10 mph tailwind vs. being on a road that’s moving backward at 10 mph in zero wind. This is one of the most fundamental principles in physics. Galileo, Newton, and Einstein tell us that no experiment can be done to distinguish between the two situations, because the situations are in fact identical. They only seem different to us because we are so used to using the ground as ground-truth.

What is moving and what is not is actually nothing more than a case of human perception. All velocity is relative, and there is no absolute frame of reference to measure *any* “true” velocity in.

Galileo:
http://en.wikipedia.org/wiki/Galilean_invariance

“the fundamental laws of physics are the same in all inertial frames”

“using the example of a ship traveling at constant speed, without rocking, on a smooth sea; any observer doing experiments below the deck would not be able to tell whether the ship was moving or stationary. Today one can make the same observations while travelling in an aeroplane with constant velocity. The fact that the earth on which we stand orbits around the sun at approximately 30 km/s offers a somewhat more dramatic example.”

So your contention that the treadmill is different from the road directly violates this principle.
aj

So if it succeeds, will the next blog post be DWFTTW FTMFW?

And if it fails, DWFTTW WTF?
Takuan

what would be the very best basic physics primer?
GregLondon

I’ve only had two people with no knowledge fo this device read the document so far. One got the yoyo on a rope without building it, the other didn’t. I think it’s probably one of the trickier bits because if you’re not thinking in newtonian mechanics but in everyday intuition, then it takes a bit of unlearning your intuition and learning newton.

His first reaction was it would spin out like a car stuck in the snow and then roll backward. Which is useful intuition for dealing with cars stuck in snow, but doesn’t actually apply here because the yoyo doesn’t slip.

I wonder if I could improve the time-lapse somehow…. hm. Maybe I could put numbers along the edge to show rotation more clearly.

I’ll ponder it some more, see if I can come up with a beter way to explain it.
DrieStone

Curiously the pic trying to disprove the theory made me realize how it could work. It might be easier to think of this if the cart were moving without any wind. The wheels would turn the propeller at the same speed as the still air around it (effectively the propeller cuts through the air without moving the air forward or backward). Now if the wind is blowing behind it is pushing against these blades and moving the car forward. Even if the air is moving against cart (due to forward velocity), the propeller is unaffected and only the air in relation to the ground pushes against the propeller.
bbblaker

Though correct, I don’t think most of the explanations given directly address what is proposed. #51 is helpful, but it’s not a full explanation. In my attempt to understand this, here is my best shot:

First of all, instead of considering a cart with the wheels attached directly to the propeller, imagine the wheels are connected to a generator, connected to a battery, connected to a fan.

Release the cart into the wind with the fan turned off. The wind will cause the cart to accelerate, but as the cart speeds up, the wind speed relative to the cart will decrease. It takes force to turn the generator, which can only be taken from the kinetic energy of the cart, so the cart will reach a maximum velocity when the force needed to turn the generator is equal to the force exerted by the wind.

In other words, some of the force that could have gone towards accelerating the cart to equal the speed of the wind is being redirected to power the generator, and the cart will be at a steady state speed less than the speed of the wind.

Then we turn the fan on. The fan is able to draw the power stored by the generator, and accelerates the cart. The cart could easily surpass the wind speed if the fan is powerful enough. However, notice that in order to go faster than the wind, the cart had to go slower than the wind while the battery charged up. The average speed will equal the speed of the wind because the cart is just shifting kinetic energy from one time period to another.

If the fan draws exactly as much power as the generator can create in an instant, we can get rid of the battery. In this case, the cart will move as fast as the wind, because any kinetic energy taken from the cart to power the fan is exactly what will be returned by the fan to the cart (assuming away inefficiencies). Hopefully it is clear that a propeller attached mechanically to the wheels is equivalent to this.

Caveat: Not a physicist. Just a math/econ major trying to understand.
zafner

The only way this contradicts the laws of physics is if the energy available in a horizontal cylindrical column of air with the same radius as the prop moving at windspeed relative to the ground is less than the amount of energy needed to overcome all the forces holding the cart back. Nobody has proven this. Since one is based on mass and velocity and the other is based on design, it should be relatively trivial to make a cart that will do this: just make a really light cart with an efficient prop and very good bearings.

Which they have done. Congratulations, guys!
misterfricative

These (D)DWFTTW devices — both the carts and the iceboats — are all counterintuitive to me, and I was skeptical at first, especially of Jack Goodman’s free-running DDWFTTW cart. Also, I have to say that I found many of the posted explanations very misleading. So now, in the spirit of a born again believer who has made an honest effort to think this through, I’m going to try to shed some helpful light on what’s really going on with these puppies.

(And yes, I know this post is an Essay. Sorry.)

Some prefatory points. First: I won’t be invoking iceboats.

Second: the inertial reference frames for Spork’s treadmill minicart and JG’s free-running cart are equivalent, so I’ll only discuss one of them — the free running cart, because even though the frames are equivalent, this is the one I personally found hardest to understand.

Third, I’m going to do this without resorting to formulae. I’ve nothing against formulae — far from it! — but here I’m going to try for a rigorous verbal description instead.

Lastly, a quick definition. I’m defining ‘positive’ prop rotation to mean that the prop is acting as a propeller, ie that it’s rotating in a direction that will accelerate air backwards and thus drive the system forward. Or in other words, that it’s acting exactly like the kind of propeller that you see on the front of any prop-driven airplane. (Contrariwise, if the prop is being driven (blown) *by* the wind, ie acting as a turbine, then the rotation would be in the opposite direction ['negative'].)

OK, from a standing start, Jack Goodman’s cart, flat surface, 10 knot tailwind, here’s what happens.

The tailwind pushes (accelerates) the entire cart system forward. As the system moves forward, its wheels turn. Because of the gearing (and the gearing is absolutely critical!) the turning wheels drive the prop in a ‘positive’ direction so that it accelerates (blows) air backward — ie, the prop is functioning just like a prop on a conventional prop-driven airplane. Note that the tailwind that pushes the system forward is *also* acting to produce ‘negative’ prop rotation (ie the wind is acting on the prop as though the prop is a turbine). The gearing MUST be such that the force transmitted from the wheels is great enough to overcome the turbine effect and drive the prop in a ‘positive’ direction. This is already counterintuitive to me, but this is what you can see happening both in JG’s video and also in one of Spork’s videos http://www.youtube.com/watch?v=QTAd891IpRs . Contrary to expectations, the carts’ props don’t ‘windmill’; instead they start slowly rotating in a ‘positive’ direction.

Obviously, the downwind cart velocity and, consequently, the ‘positive’ prop rotation is slow at first, but as the tail wind continues to act on the system, cart velocity and ‘positive’ rotation both increase.

The cart system will continue to accelerate until it’s traveling at something close to tailwind velocity. With a 10 knot tailwind and everything working pretty efficiently, let’s say the cart reaches this point when it’s moving downwind at 9 knots. At this point, the tailwind is no longer able to directly apply much force to push the system forward because the system is pretty much outrunning it. However — here comes the clever part! — consider the cart itself. The cart is moving forward with a groundspeed of 9 knots, and this forward motion along the road is making the wheels turn. And the wheels are driving the prop, which is now spinning quite fast in a ‘positive’ direction. So from the point of view of this stable, moving frame of reference (ie the cart at 9 knots) the cart is sitting in still air — actually it’s experiencing a slight (1 knot) tailwind — and its spinning prop is grabbing the air in front of it and accelerating this air rearwards. That is, the cart’s spinning prop is acting to propel the cart forward through the air that surrounds it. Now although the air surrounding the cart appears to be stationary from the cart’s perspective, this same air is in fact moving downwind over the ground at 10 knots. Consequently, as the prop continues to accelerate the cart through this mass of air, the cart’s ground speed will soon exceed the downwind velocity of the wind.

At this point you have achieved DDWFTTW. (Yay!)

In this example, final equilibrium (ie final downwind groundspeed) will be 9 knots (ie the speed at which the tailwind is no longer able to push the cart directly) plus something close to whatever speed the prop would impart to the cart if the system were moving through still air. (Note that this will be lower than the system’s ‘free flight’ speed because there will be a retarding effect from the wheels. Note too that the system will not continue to accelerate indefinitely because, even under the most favorable design configurations, and with 100% efficiency and zero friction, there’s an upper limit to the rotational velocity that a prop can reach. In practice, for the most elegant configuration, the final equilibrium velocity of the cart should coincide with the prop rotating at its ‘sweet spot’ cruising rpm.)
GregLondon

Greg, you have to decide if you’re an open minded guy willing to learn something, or a guy that knows everything and you’re just here to insult others.

You are in no position to give advice.

I accept your apology. And if you start it up again, we’ll go through it again

if you bury the hatchet and keep a list of GPS coordinates and a map of all their locations, then you’re just hoarding grudges underground.

If y ou call it anything else, you may be fooling yourself, but you’re not fooling me. You either let it go or you don’t. Obviously, you haven’t.

I get my integrity bcause I apologized, not becaue you accept it. I’d rather you accept it, but if you want to use my apology as future leverage against me, well, you only get to do that if I let you. And I’m not letting you. You got your apology. Keep trying to hold it as a weapon, and I’m done with you.

Now I’ll admit, Mark’s was a world-class apology. I don’t intend to ever hold anyone to that standard.

OOooh, a world class apology, eh?

You are unbelievable.
Brian

So here’s a thought experiment that suggest (to me at least) that DDWFTTW isn’t impossible. Imagine you have a very long skateboard with a person on it pointed downwind. He folds up a wind turbine and brings it to the front of the skateboard and plants it on the ground. The wind turbine is connected to a motor on the skateboard which propels it downwind.

When the turbine reaches the back of the skateboard the guy folds up the turbine and brings it to the front, repeating the process.

The act of moving the turbine around doesn’t fundamentally consume any energy, and since the turbine is stuck in the ground, it doesn’t cause a backward force on the skateboard. I can’t think of any argument to prevent this machine from going downwind faster than the wind (perhaps others can).

The key difference between the thought experiment machine and the machines in the video is that the propeller is stationary in the thought experiment. That may be a key difference, but I’m unclear on what that would matter.

I’m not totally clear on if the machines shown in the videos can go DDWFTTW, but I think it’s not impossible to build a machine that does.

I think where diagram (2) gets it wrong is that the propeller is supposed to bush back against the wind, thereby decreasing the speed of the wind relative to the ground, which is an overall decrease in energy of the wind. The question is whether that decrease in wind speed can be put into useful work (ie. increasing the speed of the car). I have a hard time arguing either way.

The transition between going downwind slower than the wind the then transitioning to faster than the wind may rely on wind gusts in order for it to happen (if it happens at all).

Here’s another question that I think is related. If you have a very long treadmill (like those ones at airports, but without the handrail) can you build a machine in contact with the treadmill and the stationary ground next to it that goes faster that the treadmill without using an external power source? I’m pretty sure that this is possible.
spork

“Spork, you and ThinAirDesigns are just rubbish at explaining things. You act as though you’re trying to explain to the Internet, a single intelligence, and are frustrated that we keep forgetting what you told us before”

Thanks for the nice compliment after pointing out that MY video just convinced you. As to the notion that we are frustrated by people that keep forgetting – you’ve got that all wrong. We get frustrated that nearly everyone has to start by calling us fools. Those that arrive with an open mind and a modicum of curiousity get the answers they request.
davidglover

Success at El Mirage, DDWFTTW proven NALSA record to be ratified, time for a new article,
http://www.youtube.com/watch?v=BvJZKZsUqgI
http://www.fasterthanthewind.org
http://www.thinairdesigns.com
@thinairdesigns on twitter
dustbuster7000

JB,

You claim the cart behaves differently on a treadmill vs in the street. I’m simply asking you to tell me how you know this.

I don’t know it, I think that this is the case, based on my analysis of the situation posted above.

Give me just one example of a test that can be performed without outside reference to determine whether you or I or any object is on a treadmill or not.

If on the cart, in apparent still conditions, travelling forward, I would, in the absence of a headwind, think I was travelling at the same speed as a tailwind (or that there was no air, but lets ignore that one). After examining the cart for on-board power sources, and finding none, I would think that this tailwind was responsible for my current velocity or that some other energy source, no longer present, had provided sufficient impulse so that I was currently at tail wind speed.

If the cart maintained speed and there was no apparent change in wind conditions, I would conclude that the tailwind was indeed responsible for my current condition. If the cart slowed, I would think that the wind alone was not enough and that the cart was losing energy to the ground. I would observe a slight apparent tailwind rise as the cart slowed.

If the cart accelerated and I observed a headwind, I would assume that energy had been added to the cart from outside and thus I might be on a giant treadmill. Or some other source of energy had been applied to accelerate me. If I accelerated, but observed no headwind, I would think that the tailwind had increased and might be responsible for my increase in speed.

That’s one possible test. It’s not complete (I haven’t covered all the possible known sources of energy that might speed me up, nor covered the cart in a vacuum question, nor unknown sources of energy), hence the use of ‘might’ in certain phrases.

This meets you request. If might not satisfy you completely, but it meets you request. Please meet mine and provide a critique of my analysis of the treadmill test in post #144.
spork

>> you guys think the greg cart won’t work?

I’m confident enough to offer him my tired old $100K bet.

It’s based on a sound principle improperly executed by GL.
GregLondon

Oooh, it would take some reorganization, moving chunks of text around, but if I could turn the timelapse into a flip-chart animation that you can watch by flipping the pages of the document, that might be pretty cool, actually.

You could see it by paging through it in the PDF too.

I’ll give that a try.
GregLondon

I understand the rope and pulley drawing

Hm, I didn’t actually label the “yoyo” in that drawing. It’s the double axled pulley with the star hanging off the left. The only difference is instead of pulling on it to the side, it’s being pulled *up*. It still has the same leverage effect, and it still moves in the direction it is being pulled. *up* in the pulley diagram. Or rotate that whole drawing 90 degrees clockwise, and it’s being pulled *right*, same as the yoyo.

If you get the pulley diagram, turn your head and look at it siddways and it’s the same as the yoyo drawing.
Anonymous

Hello! Interesting convo on boingboing today so I just had to weigh in.

My qualifications on the topic are a B.S. in Ocean Engineering and being an instructor in advanced sailing theory.

If you look at how energy works and is transfered it IS POSSIBLE! However, not likely with the rig proposed for reasons mentioned before.

Wind energy is .5*Density*area*velocity^3
where as kinetic energy is .5*mass*velocity^2

See the difference in cubed to squared thats why this is possible. It goes with the fact that energy is being transfered from fluid to kinetic. Another example is that we can convert acoustic to electrical energy all the time. However, the relative speed of sounds and electrons are not related(and about constant).

To further derive the proof the ratio of energy is

Ew/Eo=eff*(.5*Density(wind)*area(wind)*velocity(wind)^3)/(.5*mass(object)*velocity(object)^2)

taking out all the constants and you get.

Ew/Eo=eff*((Velocity of wind)^3)/(velocity of object)^2

as your energy ratio. Ew*eff=Eo, So,

Velocity of wind=(Velocity of object^2/3)

When your not looking at any of the properties of the wind or object.

Ta da! velocity of wind is less than velocity of object(go ahead plug in numbers). This requires that the efficiency is high, the mass is high and the wind speed is high enough to cover energy lost with a large area of wind being converted. That why sail boats can achieve this at an angle, they are super efficient. Downwind a sailboat stalls and is much less efficient. So its not doable as far as I know, but is it possible!
spork

From Nelson’s first post:

“My intuition says no, but I can’t find out enough about the cart to prove or disprove the notion to my satisfaction”

If ONLY there were a way to click on my name from the posted video and ask me directly – man that would be great.

Oh – that’s right, there is. And many have done so.

You can claim the evidence isn’t available, but I can (and would have happily) pointed you to pages and pages of clear descriptions on the topic.
GregLondon

spork: Two problems here. I’ve never offered a $100K bet I wasn’t absolutely sure I’d win

spork, it was just a joke.

me: “Hopefully by the time most people finish reading the explanation, they don’t need to build it to prove it to themselves.”

spork: But that’s exactly what you claimed about your first design. In fact you explained that unlike us, you understood the theory so well that you needn’t test it.

Think of it as a learning curve. Someone discovered this propeller cart thing. (I don’t even know who. Was Goodman the first person?) They published pictures from the top of that mountain. Other people saw those pictures and figured out how to get up that mountain on their own.

What is really left to do other than find an easy way up for anyone else who would like to see it?

If I hit a deadend, I mark it on the map for other people to avoid. If I fall down a loose slope, I can warn people about it. And if I find a way up, that is a fairly gentle slope, and manage to map it out for others to follow if they want, I did what I wanted to do.

People don’t have to use my map, but it’s available if they want it. And if we take this map I made and drop it into some ongoing argument about whether this device is possible or not, and the argument stops, I’ll assume everyone followed it to the top, and everyone understands, and I did what I wanted to do.

Those who follow the unicycle club rule might see me screw up and laugh. Whatever. If I eventually make it to the top, and write up a map that lets everyone get to the top, then I did what I wanted to do.

Some people get to teh top and look at that as the final measure of their accomplishment. “Look what I did.” That’s fine. On this thing, my measurement isn’t whether I made it to the top, but how many people I can help get there too.

If when I finish this paper, we take it to some place where there is an ongoing argument about the propeller cart, and drop it in there, and the argument ends, I’ll assume people used the map to get to the top and now understand it. And then I’ll have accomplished what I wanted.
spork

“so, you’re telling me I should have bet you a hundred grand before I sent you the link?
that was my retirement plan, dang it.”

Two problems here. I’ve never offered a $100K bet I wasn’t absolutely sure I’d win (although I’ve offered plenty of those). And $100K ain’t gonna be much of a retirement unless you’re already 94 years old.

“Jack Goodman gives all the necessary details about his cart”

And I’ve posted the plans for ours in several forums. Probably including this one.

“Hopefully by the time most people finish reading the explanation, they don’t need to build it to prove it to themselves.”

But that’s exactly what you claimed about your first design. In fact you explained that unlike us, you understood the theory so well that you needn’t test it.

“I’ll start on my build during the Christmas break. I suspect that a non-prop version capable of running on a treadmill will be a little tricky …”

I think it will be a real challenge – but not impossible. Knowing your background I would expect you to have as much chance as anyone.
SamSam

To the proponents of FTTW, who say that the wheels drive the propeller:

When the cart is moving at close to the speed of the wind, what is the speed of the air molecules directly behind the propeller?

The air molecules are all moving, relative to the ground, at some speed, X. They are hitting the cart, and that is what is pushing the cart forward. The propeller is pushing the air molecules backwards relative to the cart, and so relative to the ground the molecules are traveling slower than X.

So you’ve gained some acceleration by Newton’s 3rd law, but you’ve lost the acceleration that those air molecules directly behind the cart would be otherwise contributing to if they were pushing the cart at speed.
ratwerks

I have followed a number of these threads to their conclusion (my interest is less in the problem itself than in the ways people succeed in getting other people to change their mind about mistaken beliefs) and I am both saddened and gladdened. I am made glad by the ultimate victory of reason over unreason and by those with the courage to announce that they have changed their minds. I am made sad by the poor grace shown by many of those ultimately persuaded who will not then stand up and say clearly “Gee, I was wrong here, sorry for being such an arrogant twit and thanks for enlightening me.” Mostly the only sign that reason has won is that the traffic tapers off.

Perhaps the very best thing would be for the person starting the thread with “look at this impossible thing these fools believe” would add a note at the top of the thread right after that along the lines of “Edit: the thing is not impossible after all and the believers are not fools, see below for explanations.”
ThinAirDesigns

GregLondon:
>Anyway, I think it would work if you could
>do it with minimum drag. ANd it would be
>pretty cool to have something that only
>has two moving parts.

It will work great as long as no actual testing is required.

JB
HarveyBoing

The questions all assume that the wind turns the propeller, which turns the wheels, which pushes the cart forwards. In fact, the wind pushes on a large object, which accelerates it, which turns its wheels, which turns the propeller, which accelerates it more.

Agreed. I had seen others complain about this “reversing prop” issue, but I didn’t realize that Platt’s own objections included that (I was unable to find his article comments on Make’s web site…just the link to his article mentioning those comments).

I find it obvious that this device must be expected to work without the wind ever driving the propeller directly. And I’ve seen nothing from the proponents of the design that would suggest otherwise. I agree that if they had made such a claim, that would be an easy thing to refute. But all they’ve ever said is that the wind pushes the whole vehicle, and that the propeller is always driven by the wheels, not the wind.

I remain skeptical that the proposed vehicle would actually accomplish what is claimed. I believe that the videos of tests done on a treadmill are either faked, or those tests are somehow overlooking some important factor in the working of the vehicle. I’m willing to believe the latter, but haven’t figured out exactly what that might be.

But it’s counter-productive to try to argue against the vehicle by making up stipulations about the design that were never proposed by the proponents. It’s vitally important that if someone’s going to argue against the possibility of such a vehicle, that they do so solely within the framework described by the proponents. And I don’t think that talking about a design where the wind drives the propeller at any speed does that.
dustbuster7000

Spork,

My analysis shows you a non-equivalence in the two cases. Energy flow is not equivalent in both cases, thus the cases are not equivalent. Everything else seems to be equivalent, as best as I can tell and for reasonable requirements (I don’t ask that the treadmill surface be the same material as the road surface for example).
Anonymous

This is crazy. The conveyor belt demo in part 2 works because the vehicle is not experiencing any wind due to its forward motion. This lets the propeller add energy and push the vehicle forward. It has nothing to do with perpetual motion as the treadmill is providing the energy to make it move forward.

This is also the reason why it won’t work when out on the street (and why the part 1 videos are so obviously fake). It only works if you take the air movement caused by the motion of the vehicle out of the equation but allow the air movement caused by the propeller to add energy.
GregLondon

That’s the interesting thing about the car

That’s what you find interesting about the car. There is no such thing as “science in a vacuum”. It always involves people with whatever subjective personal goals that are driving them. And what is “interesting” about the car is subjective.

The car works or it doesn’t. The car goes faster than the wind or it doesn’t. But the car doesn’t care, and the propeller doesn’t care, and the ground doesn’t care, adn the air doesn’t care. They just do or don’t do whatever they do. That’s science in a vacuum.

As soon as you get people involved in science, there’s who understands something, who doesn’t, does anyone understand something, does everyone, do I understand something, do they understand something, and so on.

And which of those you make important is up to you.

The thing I find interesting is coming up with a way to explain this to someone who doesn’t know anything about it, and having them get it. The more the better.

If you find it interesting understanding how the car works, that’s fine too.

If I may offer a word of advice… prepare yourself for continuous merciless personal attacks from people that don’t have a clue what you’ve done or what you’re talking about.

If we’re handing out advice to one another, I’d say it’s all about the narrative. It’s not about whether or not you understand how it works. It’s about whether or not you can understand where the other person is stuck. If you never get stuck or you try to avoid getting stuck or if you try to banish memories of getting stuck, how do you help someone who is stuck?

And how do you think they take it when you ask them to bet you 100k? Are you helping them get unstuck? Or are you taking advantage of the fact that they are stuck? Just because someone doubts the car doesn’t mean its a personal attack on your integrity. I think it was the part 2 thread where you said something about getting upset about people doubting your veracity. Doubting isn’t personal.

I swear, if you’d discovered this thing a couple hundred years ago, you’d be shouting “Pistols at dawn!” to doubters because you seem to take anyone’s doubts as a personal attack on your character. Even if random-Joe-public thinks you’re a lying cheating sob, that doesn’t actually define who you are.

If people don’t understand, I’ll try to fix the document. If I get to the point that a bunch of people read it and most undersstand it, and no change I make seems to improve it, I’ll probably leave it at that. That’s the best I could do. I couldn’t explain it to the others. I failed to teach them. But I’d be willing to admit that my explanation failed to explain it to them without taking it as some personal attack on me.
ThinAirDesigns

Mark F.

Spork was referring to the advance ratio of the cart a bit above.

Here is a drawing I’ve posted in several discussions to demonstrate those principles.

http://www.mediafire.com/imageview.php?quickkey=m7yytmonird&thumb=5

The drawing demonstrates how by changing the “gearing” on the yo-yo, you can change the speed and ultimately the direction of travel of the yo-yo, all while pulling the same direction and speed.

We can do the same thing with our cart — make the wheels smaller and it goes faster and faster until it stops working. If we make them smaller still, the device begins to back up *into* the wind when released rather than go downwind.

JB
Count One

Imagine a cart with a lightbulb on top instead of a propeller. Suppose the lightbulb generates 5 watts of light. The energy to create this light comes from the wheels and therefore the kinetic energy of the cart must be reduced by at least 5 watts (and probably a lot more).

Whatever propulsion unit you have that is powered by the wheels must drain more energy from the kinetic energy of the cart than it can ever emit otherwise you are violating the laws of thermodynamics.

I believe that the propeller does not act as a source of propulsion, but rather acts as a fly-wheel. The fly-wheel absorbs energy as the cart is being accelerated and emits energy when the cart is being decelerated.

This causes the cart to move forward on the treadmill when the speed of the treadmill is slowly reducing, and also would make the cart slow down less quickly than expected when pushed with a fork.

I think the treadmill “experiment” would still work in a vacuum.
Takuan

are these planeontreadmill/downwindfaster arguments a new meme? They show every sign of proper meme-hoodishness. Or are they just revisiting angel-pinheads? They smell new to me but I’m too lazy to do the thinking. They aren’t arrow-tortoises and they aren’t perpetual motion that I can see. TANSTAAFL thermodynamics has always served me well… why can’t we get a simple lucid summary here so I don’t have to read all this?
GregLondon

spork: I eagerly await your test results – even though you don’t need test results to validate your theory.

Seriously? Ropes and pulleys have to be demonstrated in the lab before we’ll believe they’ll pull like ropes and spin like pulleys?

You’ve got a Masters in Aero? You expect me to believe that? When you sit down and draw a wing flying into the wind, you have to stop, and build a wing to make sure that wings still produce lift? Every time?

Was this a mail order degree?

How about kinetic energy? When I calculate energy of a moving body, do I then have to stop and confirm that conservation of energy is still valid?

Really and for true?
SubductionZone

I worry about engineers that do not understand the equivalence of inertial frames. I suppose that they can still solve the problems given to them, but it shows a shocking lack in basic physics education. As I said before I was a great skeptic of this idea but as soon as I saw spork and JB’s first video I realized my mistake and offered them my congratulations and an apology. I think I asked before, what is your mathematical objection to the treadmill?
GregLondon

Oh, I should probably label the “ground” in the pulley diagram. If you look at the pulley diagram, there is the ground block at the bottom of the page. There is only one rope mounted directly to the ground. That rope goes up and wraps around the outer wheel of the pulley.

If you turn the diagram 90 degrees clockwise, you can label that rope the new “ground”. What used to be the 5 lb mass of the star becomes whatever inertia is in teh pulley/yoyo itself.

Hm, If I make a whole other diagram that simply turns the pulley diagram 90 degrees and then labels the new “ground”, the yoyo, and everything else, that might help. If people get one diagram (yoyo or pulley), then showing how they relate might help them get the other.

I’ll put that in the next rev.
Anonymous

DDWFTTW is impossible. The only way for it to be possible wouldï»¿ be to use a sail that travels perpendicular to the wind direction and can therefore experience constant acceleration. Which is impossible. Unless…….the sail is made rigid and accelerates around a pivot! How about two or three rigid sails that rotate around a pivot and power the wheels of the craft! By Jove! I have it, I shall call it the Propelorator! (I doubt someone else has thought of this, so I call dibs!)
arkizzle

Once you actually make it to the top , of course :p