Downwind faster than the wind - Part 4

placidbeau

Thanks (and apologies) to Dusty Springfield and “Windmills of Your Mind”

Like a cotton reel thatâ€™s spinning
Against all logic spun.
We read to see whoâ€™s winning
An argument un â€“ won.
By George, we think heâ€™s got it,
But Louise must disagree:
A polite conjecture,
Against Terryâ€™s decree.
Counter or anti, which way?
All must have their say.
And give personally
their own yeah or nay.
Like the circles that you find
In the windmills of your minds.

Beelzebuddy

There is no claim of perpetual motion.

Then you’re welcome to explain how the friction of a cart slowing down can speed it up. So far we’ve covered ‘it just works, oh em gee’ and ‘it slows down the Earth,’ of which the former is useless and the latter is silly.

From my perspective, translational kinetic energy (cart motion) is transferred into rotational energy (shaft) and back to even more translational kinetic energy via the propeller. Despite those of you waving your arms and going “TA-DA!” like lovely assistants, where’s that energy coming from?

Chrs

@Beelzebuddy, others willing to accept tacking DWFTTW, but not DDWFTTW:

Propeller blades are effectively tacking as they spin. Thank you for pointing this out in #98, Twoshort.

The DWFTTW problem is in exactly the same class as tacking upwind, which is a more well-understood phenomenon. In either case, you are extracting energy to move against the motion of the air relative to the boat.

Of course, this raises another question:

UWFTTW?

Seriously though, please don’t post on that one.

LightningRose

Unfortunately, this video has nothing to do with DWFTTW.

Once a vehicle going downwind reaches the wind velocity, it “sees” *no* apparent wind. If a vehicle goes down wind faster than the wind velocity, it “sees” an apparent headwind.

The bottom line is *you cannot capture wind energy if your velocity is the same or greater than the wind*.

Anonymous

I’m going to make this comment again, although it’s rehashing previous discussions: the direction of energy flow depends on reference frame. That’s what makes this problem tricky. In the reference frame of the ground, the only energy source is the wind; in the reference frame of the wind, the wind itself can no longer power anything. So does that mean the energy source disappears? Obviously not, it has to still be there, by Galilean invariance. The same energy is now available from the ground. Nothing fancy, no mysteries, no first or second law violations (it’s an open system, duh), no practical uses, just a system which is explicitly designed to lead you into a specific flawed argument (“but *at* wind speed, there’s no force!”). It’s a physics brainteaser, that’s all.

TwoShort

Beelzebuddy:
Let’s say the wind is coming from due north. You steer south-east, your sail is oriented across the wind, east-west. Ignoring friction, your speed downwind South) levels off at the speed of the wind ( 10 knots in our example). If you were going straight downwind, you would not be moving at all compared to the air; your sail is hanging limp. But you’re actually going 10 knots east “across” the wind too. As far as your sail can tell, there’s now a 10 knot wind coming from the east. So you “pull in the sheet”: the “sheet” is the rope that moves the sail, so when you pull on it it turns the sail to cut across that east wind, and use it to push the boat harder southward. So the boat wants to accelerate southward, but the runners constrain it to going southeast, so the wind from the east intensifies, and you pull in the sheet a little farther and accelerate a little more…
And then you get into the dynamics of runners, which need to be pushed ever harder into the ice to stop from slipping sideways, which makes their forward resistance rise dramatically, so you don’t actually go infinitely fast; though 50 mph in a 50 lb. boat feels close to it.

If you’re sitting in the boat, it seems like the faster you go the stronger the wind gets, and the more it’s direction shifts around toward the front of the boat. Which, in the reference frame of the boat, is actually true. What angle you set the sail at and what course you steer to get up or down wind fastest is actually a very complex, continuously changing equation, which is why some sailors are better than others.

Michael C

Spork’s videos showing how to make the cart are brand new. He goes into a lot of detail.

alowishus

At long last! the answer to the ONE QUESTION that has been BURNING ETERNAL in the back of my mind!! I can now rest.

spork

Beelzebuddy, why the attitude? You’ve got a pile of people here patiently trying to explain this thing to you.

You want to know how to make a wind powered vehicle that goes directly downwind faster than the wind? I explained one way that’s pretty darn hard to argue against. I offered links to an article showing that ice boats can maintain downwind VMG of 3X to 4X windspeed. This is supported by vector analysis, and by GPS data. It’s also proven by my own simple vector analysis here:

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

I can also point you to ice-boating forums where this question has been put to them directly.

Moving on to directly downwind… just put two of these ice boats side by side and keep them on alternate tacks. Put a telescoping pole between them and sit on the middle of it. Done.

So instead of ignoring the answer to your request – how about responding that you either understand now, or telling us exactly where you see the problem.

Once you get this, it’s easy enough to explain exactly how it relates to the prop cart. But no point going into that when I’ve already explained this more basic approach twice without any traction.

airshowfan

Badger500,

Yeah it can get a little tricky. See if this helps:

Work done (energy) equals the force times the distance MOVED IN THE DIRECTION OF THE FORCE, or the distance moved times the component of the force in that distance.

In other words, if you have a force and it caused some movement, then the work done is “force times distance” along the SAME direction. If the force and the distance are not in the same direction, then one of the two has to be decomposed into the component that is parallel to the other (i.e. the whole thing has to be multiplied times the cosine of the angle between them, which is 1 if the angle is zero).

So if prop thrust is “forwards” (down-wind), then we care about the speed of the air in the horizontal direction (up-wind/down-wind). Either the air before the prop accelerates it, or the air after the prop accelerates it, either one.

What I call “wheel resistance” is the traction between the wheel and the ground, so yes, the static friction between the wheels and the ground. The ground pulls back (up-wind) on the wheels, and the cart pulls forward (down-wind) on the ground via the wheels. This is because the wheels power the prop (and some gears and axles and so on) so they see some resistance; that’s what powers the machinery on the cart.

Yes, if the axles and gears had more kinetic friction, and if the prop saw more drag as it spun, then you’d need more power into the wheels to make the machinery spin around, and the wheels would then experience a greater force pulling the cart back. (I.e. when you hit the brakes, things slow down).

Anonymous

anon@211: You people have built two completely different models of the problem in your mind.

I think there are probably way more than just two groups.

Unless, of course, this whole thing is still not being explained clearly.

Yeah, that’d be my take on the issue. I’m currently shuffling a bunch of stuff around on my document to try and explain it with as little jumping around as possible. I’ll put out another rev soon.

rat@212: Point being, what explanation fails to work for you is not therefor bad, and what works for you is not therefor good for others.

The point being that I was kidding. Once you understand the prop-cart, the shortest explanation is “ta-da!”, cause you hear the explanation and you understand the prop cart after you hear it.

course, you understood it before you heard the explanation too, but that’s what’s so funny about it.

spork

“Repeat the cycle indefinitely. Average speed 1.5 x wind speed. Is it plausible?”

You’re math is off just a bit. You’ll get 1.25X wind speed. But I don’t think there’s any theoretical reason it won’t work.

TwoShort

“where’s that energy coming from?”

The energy comes from the motion of the air relative to the ground. You can take energy out of that differential and use it to move relative to the ground, even upwind. You can also take energy out of that and use it to move relative to the air, even downwind.

ratwerks

Lightningrose @ 136

Unfortunately, you are mistaken. Moreover, your counterargument has been made and addressed many times.

The bottom line is *one should understand what they are talking about before attempting to instruct others*.

slamorte

@ #90

the energy is coming from the speed differential of the two conveyer belts: the wind and the earth, moving at different speeds.

you cannot doubt our little ruler cart above, right? it makes sense that you can set gears that would harness the differential of the upper belt (ruler) and lower belt (table top). and if there was no relative motion, the cart would not move. it only moves when one of the belt moves. yet there is no functional difference between moving the upper belt (ruler, actual conveyer belt, or wind) and moving the lower belt (table, treadmill, or earth). ANY differential works.

the key is this: a propeller is a special gear for connecting to the wind. here’s how i think it works:

the propeller has props. these props are set at an angle. let’s say you spin that propeller at 5 mph at the center of the props. because the props are set an an angle (say 45Â°), this means the *blades of the prop are effectively moving backwards 2.5mph from the cart!*

from the point of view of a vector of wind approaching from the rear of the cart, they are encountering an object that is moving slower than the cart itself. so, the wind pushes the slow object, which in turns pushes the cart, which spin the wheels, which then turn the propeller even faster.

in this way, a propeller really is a kind of gear for engaging the wind.

Chrs

@168: You must have missed mine, at least.

Beelzebuddy

Ah, informative stuff. So it sounds like you get your greater-than-wind-speed speeds by presenting a different surface between upwind and downwind vectors. I can buy that, but that isn’t at all similar to how these carts are supposed to work.

Still, I’ll concede that it is possible, at least when using a tacking strategy.

GregLondon

anon@211: You people have built two completely different models of the problem in your mind.

It’s probably way more than just two models.

Unless, of course, this whole thing is still not being explained clearly. But that could NEVER happen.

Yeah, I’m in that camp. I’m in the process of shuffling my document around quite a bit, trying to put things in an order that requires the least amount of intuitive jumping. Oh, and I put in a table of contents at teh beginning. I hope to have a new rev soon.

rat@212: Point being, what explanation fails to work for you is not therefor bad, and what works for you is not therefor good for others.

Point being, I was kidding.

The shortest explanation for someone who already understands the prop-cart is “ta-da!”. because they hear the explanation and they understand the cart after hearing it.

The fact that they already understood it before hearing the explanation just makes me chuckle.

I could go around the web and find two dozen different generic sites and say “This explains it perfectly”. ANd if you understand how the cart works, sure, it’ll explain it.

And in working on my document that tries to explain the prop-cart, I ran into that website fo simple machines, realized everything in the cart is explained on that generic page, and I had to chuckle.

spork

“the propeller has props. these props are set at an angle. let’s say you spin that propeller at 5 mph at the center of the props. because the props are set an an angle (say 45Â°), this means the *blades of the prop are effectively moving backwards 2.5mph from the cart!*”

The prop has “blades”. These blades are really just airfoils that create lift (in this case thrust) exactly as a wing does. The blades have twist to them so that any point on the blade is trying to move forward a specified distance over one full rotation. This is the “pitch” of the prop.

To say the blades present a “backward moving surface” to the air has a definite intuitive attraction, but tends to lead to an incorrect explanation of lift (namely that air molecules bounce off the lower surface of the wing or back surface of the prop blades). Nevertheless, this backward moving surface theory does correctly describe the theoretical max speed of the cart in terms that are almost identical to the ruler cart of the above video.

spork

SeattlePete @ 160 says:

“Except there is no gound/wind interface that can be exploited. There is only the interplay between wind/device/ground. Throwing the device in there creates drag against the ground and the best you can do is simply get very close to the wind speed.”

Of course there is such an interface to exploit. Or perhaps our cart doesn’t understand physics, because it DOES go faster than the wind – directly downwind.

As a good friend said to me yesterday – “sure it works in practice – but can you prove it in theory?”

#167: “Spork, have you thought about a long plexiglass wind tunnel?”

I’d LOVE to do this in a long plexiglass wind tunnel. If someone has one I’ll send them the cart.

#168: “I read this entire thread to see if there was a single person who, after claiming it was impossible (precisely my initial intuition) then followed up to say “Sorry, I was wrong.”

It’s VERY rare. Typically people either dissappear, or they explain that they actually knew it all along, or when they finally agree, they explain that the whole problem is how poorly we explained it, but now that they understand, they’ll explain it properly (followed by a horribly faulty explanation). That’s the typical script.

On the other hand we have had two world-class apologies from people that started out insulting us and later came around. The most amazing part – they were both Mark C. (two different guys).

“Anyhow, I just wanted to say wow – I kinda get it now! Thanks for showing how my initial theories were wrong, and filling me in. I really appreciate the patience and detail people, especially Spork and Airshow, have put into explaining this in a whole bunch of different ways.”

Thanks for having an open mind and a great attitude. As you’ve observed, both are relatively rare.

#171 “I called the plane on a treadmill instantly. And this one I see no issue about either… I call it impossible.”

Then you’re batting 500. That would be pretty darn good in the majors.

#174: “MichaelC’s videos are meant to answer but one of many mistaken objections and does so with elegance, wit and charm. A device of transparent simplicity is shown to outrun its power source.”

Very well put. I consider Michael’s videos as excellent companions to our own. Ours are not terribly illustrative (and aren’t intended to be). Michaels are.

#175: “Ok I’ve sketched a wind based version of the ruler example above, don’t see why it wouldn’t work.”

I buy it. It would be challenging to build a working model, but I don’t think that’s your point.

#176: “My sailor friends are laughing hysterically at the idea that the wind “pushes” a sailboat”

“pushes” is a very broad verb. If you want to get extraordinarily technical, lift is strictly about air pressure pushing on a wing (which is what a sail is).

TwoShort

“that isn’t at all similar to how these carts are supposed to work.”

It definitely took me a while to see it, but it’s exactly the same. The iceboat uses it’s motion over the ground to moves its airfoil (the sail) side ways through the air by moving the whole boat. The cart uses it’s motion over the ground to move its airfoils (the prop blades) sideways through the air by spinning the prop.

Chrs

@#181, final point: Boiling the concept of aerodynamic lift down to pressure differentials makes the most sense to me, in terms of being able to write a simpler balance. For people who are only barely familiar with aerodynamics, that way is probably easier to explain.

Beelzebuddy

Beelzebuddy, why the attitude?

I apologize if you’ve taken offense, I was merely responding in kind to ratwerks. I tend to counter snark with snark, as I find it an enjoyable way to argue on the internet and those who dish it out can generally take it.

As far as my disbelief goes, you’ve yet to put forth a convincing argument as to why it works. Specifically, how this is not defying the second law of thermodynamics. I haven’t looked over the boingboing guidelines lately, but I think we still believe in that here. Your cart loses momentum through the wheels, then transfers it to the propeller through which it gains even more? I don’t buy it, Jim.

you cannot doubt our little ruler cart above, right? it makes sense that you can set gears that would harness the differential of the upper belt (ruler) and lower belt (table top).Actually, I can. Please note my comment above, describing the equivalent force differential and how it’d cancel out any gains you get from distance.

spork

Not dopey questions at all. These get right to the heart of it.

The first thing to understand is that “energy” or “work” (sort of interchangable) is a force x a distance. But not any force times any distance. Work done = the force applied times the distance that force moves through – in the same direction of the force. Now you have to bear with me because energy and work are weird (seriously).

I can push on a wall all day and do no work if I don’t manage to move that wall. Remember, it’s the force I apply on the wall times the distance the wall moves in the direction I’m pusing it.

I can also lift a suitcase and carry it for a mile, and the only “work” I’ve done is lifting the suitcase a foot off the ground. That’s because I lift it upward and it moves upward. The suitcase may weigh 300 lbs, but walking for a mile with it moves it horizontally, while the 300 lbs acts vertically. Remember the work is the force times the distance that force acts *in the direction of the force*.

So, getting back to the cart…

The work on done on the wheels is equal to the force the road applies horizontally to the wheels times the distance the wheels roll. The vertical force does no work because the cart doesn’t move vertically. Now this horizontal force on the wheels will be partly due to rolling friction and other real-world losses, but the most “important” component of that force is that which is required to turn our prop shaft.

Now, for the purposes of analysing the cart, we’ll treat the prop as a prop-disk for now. That disk produces an aft thrust, and it moves through a distance relative to the medium in which it’s operating (the air). Because there’s a tailwind, it moves through a smaller distance in its medium than the wheels move on their medium (the road). So… the wheels can “take in” more energy than the prop needs to “output”.

This is good, because in the real world there are frictional losses, aero losses, etc. Also, the prop isn’t really a disk – it’s a prop. So the prop does swing through the air doing work on the air that’s not in the direction of the cart. Some of this work is related to “induced drag” which is effectively the minimum the prop needs to do its job, but there’s also “parasite” drag on the prop. This is just another real-world loss that gets lumped in with the frictional losses.

So… if we simplify a bit – the prop pushes aft on the air and it pushes that air a certain distance (over a given period of time) and the wheels are pushed on by the road, and they are pushed a certain (horizontal) distance in that same period of time. But since the road is going by faster than the wind, there’s extra energy available to take care of all the frictional losses.

spork

“Still, I’ll concede that it is possible, at least when using a tacking strategy.”

Excellent. Then we’re just a few short steps away from the prop cart.

If you consider an ice boat on a downwind tack, you’ll realize it’s actually tracing out a giant circle – because it’ll eventually go around the earth (let’s assume the earth is frozen over with ice).

Now let’s pretend the earth is a cylinder rather than a sphere. If the wind is blowing along the cylinder, our ice boat will just make one long spiralling downwind tack around and around that cylinder. And it will be moving along our cylindrical earth faster than the wind itself is (as evidenced by the downwind performance of ice boats).

Now let’s imagine we have two ice boats – on opposite sides of our cylindrical earth. They both spiral their way downwind – along the cylinder – faster than the wind.

Now it’s just a matter of changing scale. These two ice-boats might as well be prop blades making their way down a big cylinder that isn’t actually the earth. As long as they maintain a 45 degree downwind course at all times, they can outpace the wind along the cylinder.

Finally, we replace the cylinder they ride on with a simple structure that enforces the same kinematic constraint. This structure is a simple cart that gears the prop to the wheels. The gearing is such that the prop tips must move 1 foot around the circle when they push the cart 1 foot forward. So now we have two sails that are constrained to follow spiralling 45 degree downwind paths, just like our ice boats that circled the cylindrical earth.

Looking at it this way, you can see that the prop doesn’t exactly power the wheels any more than the wheels power the prop. The prop does push the cart forward, and the wheels do provide the torque to turn the prop – in exactly the same way that the sail pushes the ice-boat downwind, while the skates make it follow the 45 degree downwind course.

Side-note for technical accuracy: energy and work are bizarre quantities that have more to do with mathematical bookkeeping than they do intrinsic measurable quantities that an object has. As a result we can conclude that the air does all the work, the ground does all the work, or anything in between. This just depends on the frame you choose to do the analysis in – but really doesn’t have much to do with how or why the cart works.

BadStoryDan

Okay, given that there are now 178 comments in this thread I doubt anyone will read this and I’m sure this has already been said in a dozen other ways, but here goes anyways:

Imagine a tube in which sits a propeller in a housing that completely blocks all air flowing around its sides, and which has wheels which press against the sides of the tube – the one in my head looks like a Tunnel Boring Machine crossed with a turbine. Now push it from behind: The wheels drive the propeller and it sucks air from in front of it and moves that air to behind it.

Now replace the “pushing it” with the wind coming from behind it in the tube.

If you remove the seal from around the outside of the propeller, it doesn’t change the fact that the propeller still sucks air from in front it.

I totally grok this now. Or I could be wrong. No, I think I’m right. The wind effectively pushes the craft as a whole up to wind-speed and the propeller sucks on the air in front of it..

Where I had problems initially was realizing that the wind doesn’t turn the propeller in the direction I expected, so the argument that the spinning propeller will eventually meet the velocity of the wind is moot.The wind moving against the prop tries to push it like a spinwheel, but meets resistance and instead pushes the craft forwards, which turns the wheels and thus the propeller in the _opposite_ direction, which increases the resistance of the craft as a whole, and pushes the wheels faster. Effectively, at top speed, the work done by the wind passing through the area of the cross-section of the prop pushes the vehicle forward relative to the ground, at a ratio dependent on the gearing between the wheels and the prop.

Sorry to all those who find this post redundant.

LightningRose

Ratwerks @138

Show me the cart in a well calibrated wind tunnel.

Sailboats and iceboats can sail across the wind faster than the wind, but neither can sail faster than the wind if going directly downwind.

spork

Looks like Airshowfan beat me to the punch. He’s got it completely right (of course), so you can get two takes on it.

Beelzebuddy

The iceboat uses it’s motion over the ground to moves its airfoil (the sail) side ways through the air by moving the whole boat. The cart uses it’s motion over the ground to move its airfoils (the prop blades) sideways through the air by spinning the prop.

That’s not the same at all. Iceboats would work best if the surface were totally frictionless and only used as a guide to keep the boat pointed in the same direction. Propeller carts wouldn’t work at all in a frictionless environment – they depend on friction to drive the propeller to overcome the loss due to friction. Aside from both involving touching the ground, they seem pretty different to me.

Beelzebuddy

Spork: I’m sorry, you started losing me at “imagine the Earth is a cylinder,” lost me at “the prop doesn’t exactly power the wheels any more than the wheels power the prop,” and I just barely reached energy being “mathematical bookkeeping” before I was done. If I still give a damn about any of this in the morning, I’ll try to puzzle out the physics then. ‘Night.

spork

#178: “So, spork, JB, others: is that about right?”

It’s only part of the story, but it is exactly right.

“Issue 1: Energy Bookkeeping. It seems like that the prop’s thrust will rob the same amount of energy from the wheels, slowing them down such that there will be no net gain in cart speed. I thought this way, too, but now I suspect it is a confusion of terms (speed, force, thrust, work, energy, power…it becomes physics salad!). Can you help me plug in the *right* terms here and show that it is not just shuffling energy around, but that the gearing causes this to work?”

First of all, you got one important element exactly right – energy is all about bookkeeping. Energy is NOT like mass. Energy depends entirely on the frame you’re measuring from. To make a long story short, the force on the prop and the force on the wheels must be equal for the cart to maintain steady state (at any speed). The cart is moving much faster over the ground than it is through the air (because it is in a tailwind). Energy is force times distance (power is force times speed). So there is significantly more power available at the wheels than is needed at the prop disk (which is moving more slowly through its medium). Remember, energy is not like force or momentum. To put it one way – energy is weird.

“Issue 2: Strange Loop. If the turning wheels power the prop, and the prop then creates thrust to speed the car faster, that will cause the wheels to turn faster and power the prop more, which will cause the cart to go faster, etc… There’s a loop, and so it suggests the cart would accelerate constantly…”

This is in fact a key part of what makes it a mind bender. Let me try to unbend it a bit.

Again, I have to point out that “energy is weird” and that it is entirely dependent on the frame in which it’s measured. That being said, it’s better to think of it a little differently. The prop pushes the cart forward, and the wheels and transmission force the prop to turn an equal amount to the distance THE PROP pushes itself forward. It’s not a matter of prop “powers” the wheels and wheels “power” the prop. The wheels and transmission in fact provide exactly the same kinematic constraint on the prop that the blades provide for the sail of the ice boat. In other words, they simply define the path the prop tips MUST follow if the prop moves at all. This is easier to explain if I get to wave my hands and use lots of colored markers. Sorry text doesn’t do it justice – but probe further if necessary. There’s no wind for hang gliding or kitesurfing right now, so I’ve got nothing but time.

“I also respect the time and patience you’ve all put into this.”

You probably can’t imagine how much I appreciate such comments (amid all the personal attacks and demands). Thanks.

dculberson

I do know this: if you disregard the fact that it’s wind, and just say you have a kinetic energy input of X newtons, at Y km/h, there’s no reason why you can’t move a given mass at Y * 2 km/h.

Say you have a 1kg mass and a 1N force. After one second, that mass will be moving at 1 meter per second.

Now, perform the same experiment, but with a 1/2 kg mass, but still with a 1N force. After one second, that mass will be moving at 2 meters per second. Right?

..

Separate point:

Moving forward in a still wind is exactly the same as moving faster than a moving wind. Exactly.

..

Separate point:

I have no firm opinion on this matter, just those two previous _very_ basic physics point to make. Kisses.

..

Pure conjecture: If you have a device that can capture 1N of energy from the wind, but the total device weighs 1/2kg, why wouldn’t it go 2 meters per second after one second? Give a formula explanation and not “it just can’t work” or “it’ll stutter.” as I said, I have no firm opinion, I just want to know.

..

P.S. Science: It works, Bitches!

dculberson

Beelzebuddy, only addressing your “‘it slows down the Earth,’ [..] which [..] is silly.”

Actually, stepping away from this problem, if you move a machine in the same direction as the earth’s rotation, you are removing some kinetic energy from the earth. It’s minuscule, tiny, unmeasurably small, but it’s a fact.

I’m not saying it has a bearing on this problem. Just saying it’s true.

That’s the “every action has an equal and opposite reaction” rule.

If you move in the opposite direction, you’d be adding a little eentsy teensy tiny bit of energy.

spork

“The other day it was amusing how adamant were the people who hadn’t grokked yet. Today it’s just a frustrating example…”

I originally posted this problem as a brainteaser 3 years ago. I hope you can imagine how frustrated I’ve become for the same reason. This is compounded by the fact that I’m attacked personally as being a fool and a scam artist. I say this only so people can understand my hair trigger – not that I have any valid excuse.

Nevertheless, I have great respect for people like Michael C (and several others) that can keep an even keel in the face of incredibly frustrating exchanges.

“However, on the net, where every video looks faked, there is still a place for this sort of reasoning.”

No doubt you’re right. The ironic thing is that this is exactly why we made our videos. We thought surely we can make some simple videos that are sufficiently clear to show this thing is real and is just a simple novelty. It’s amazing how wrong we were.

“Nah, I’m sorry. It still sounds like you all are trying your best to talk around the fact that you don’t actually know how it works.”

I assure you there are several (perhaps many) people that understand this surprisingly well. I’ve watched others explain it and answer questions. Some are close, some are frequently wrong; but some are right on the money every time.

[quote]I’ll reserve judgment for now in hopes that maybe by the sixth thread or so, you’ll have a more cohesive (and less analogy-based) explanation[/quote]

Personally I find the “analogy-based” explanations to be far more convincing than the mathematical sort. I’ve seen far too many mathematical proofs that tell us this is impossible. If you’re going to let math convince you of the truth you’d better be darn sure you don’t forget to carry the 2. That being said, here’s a reasonably rigorous mathematical treatment: http://home.san.rr.com/tadhurst/DWFTTW.htm I don’t find this approach terribly compelling, but it is reasonably accurate.

spork

#179 “I’m going to make this comment again…
Nothing fancy, no mysteries, no first or second law violations (it’s an open system, duh), no practical uses, just a system which is explicitly designed to lead you into a specific flawed argument (“but *at* wind speed, there’s no force!”). It’s a physics brainteaser, that’s all.”

Your statements about energy bookkeeping are EXACTLY right. I’m sorry to have to tell you, you’ll be taking plenty of heat for that. Your other observations are perfectly accurate as well. I noticed you didn’t sign your post – are you me? : )

spork

“Spork: I’m sorry, you started losing me at “imagine the Earth is a cylinder,”… If I still give a damn about any of this in the morning, I’ll try to puzzle out the physics then.”

There are many ways to tackle this thing. It’s very real, and very useless. It’s a fun toy, but it’s not perpetual motion.

Hope you’re back in the morning.

Frank_in_Virginia

I like the monkey and mouse peer review.

Michael C

I just got up to find my little video posted here with 100 comments after it! I can’t reply to everybody individually, but I’d just like to say to catastrophegirl (#40), or anyone else who’s wondering: Terry is indeed a librarian.

I will also not hide the fact that my machines are inspired, in part, by the great Leonard of Quirm, who invented such marvels as the Going Under The Water Safely Device and would certainly be capable of creating a Cart That Goes Down The Wind Faster Than The Wind.

ldcroberts

The whole argument is pretty funny. It’s impossible. By definition traveling downwind means you have wind behind you pushing. If you travel in the same direction as wind, and go faster, you are then traveling UP-WIND. So you can’t claim you are traveling downwind anymore so the whole question is irrelevant.
To the folks that talk about traveling faster than wind on an angle, you’re speed can be thought of as two directions, with the wind and orthogonal to it. Your orthogonal speed is faster but your travel in wind direction is limited by the speed of the wind. The interesting thing is that your orthogonal speed is against the wind.
Any design that can be pushed by wind to travel faster is going to have to expand in size and contract (e.g. like a bird wing flapping)

spork

“I just got up to find my little video posted here with 100 comments after it!”

Pretty cool – huh?

We’re just glad you and the others in your videos are using your powers for good rather than evil. One shudders to think…

How long do you suppose until we have everyone on the entire internet converted?

David G

Lightningrose @ 137

The posted video is an excellent demonstration of how a device can travel faster than the input velocity. Unfortunately, like many others, you find it difficult to see how this equates to the wind powered cart.

I have posted a demo of another cart that also travels faster than its input but with a setup more similar to the wind powered version:

http://www.grogware.com/ddwfftw/cart_picture.html
http://www.grogware.com/ddwfftw/cart_video.html

Like the wind powered carts, as the wheels turn so does an aft facing shaft. In the wind powered version this shaft is attached to a propellor. In my version the shaft is attached to a threaded bolt and a nut is used as the input. As you can tell, pushing on the nut causes the cart to move forward faster than the nut is being pushed. I don’t think most people would find it surprising that the cart moves faster than the nut.

The wind powered version works the same way but instead of a thread and a nut there is a propellor and air. The propellor is not 100% efficient but there is more than enough efficiency to move faster than the wind. An example of inefficiency for the cart I made would be if the nut slipped one thread for every two that it had gained; it would still move faster than the nut just not as efficiently.

spork

“What do you want, a vector analysis of a DWFTTW ice-boat…”

I have provided that several times now (here: http://www.putfile.com/pic/8419299 ). For me personally, that is the most compelling evidence, but I realize different people respond to different approaches.

“…and a vector analysis of a DWFTTW cart’s propeller blades”

It turns out these are exactly the same. It’s simply a matter of scale. The ice boat goes around the world, while the prop blades go around the axis of the prop-shaft. But the aero analysis about each is identical.

“Since the one thing that is missing on my Blogspot page is more info on how ice boats do it… Are there any online resources you’d recommend about ice boats”

I think you might want to add these:

http://www.btinternet.com/~sail/dwfttw02.htm

http://sports.groups.yahoo.com/group/2nalsa/message/161

“Show me the cart in a well calibrated wind tunnel.”

A wind tunnel is a device for studying vehicles that go into the wind – without those vehicles getting away from you. To test a vehicle such as our DDWFTTW vehicle what’s needed is a treadmill, or better yet a large turntable. You’ve probably seen our treadmill videos. I may make a turntable so we can see the steady-state faster than wind performance. Unfortunately, our cart will simply shoot off the front of the treadmill before reaching steady state (or at least before doing so convincingly). In a wind tunnel the thing would simply go screaming downwind out of the test section in a matter of seconds.

“Sailboats and iceboats can sail across the wind faster than the wind, but neither can sail faster than the wind if going directly downwind.”

Some sailboats and most (if not all) ice boats can still outpace the wind directly downwind all day long. They just have to tack to do this. Plenty of evidence of that has been provided in this very thread.

dragonfrog

Lightningrose

As has been explained before: Iceboats sail downwind faster than the wind – that is, the direct downwind portion of their speed, on its own, is faster than the windspeed.

By tacking, the portion of their speed that is across the wind cancels out over a pair of tacks, and their velocity is directly DWFTTW.

The use of a rotor – a rotating axle with two or more windfoils on it – achieves exactly the same end. The windfoils move at an angle to the wind, such that their speed across the downwind direction cancels out over one rotation of the axle, and their overall speed is sufficient to make their velocity end up directly DWFTTW.

sforslev

This reminds me of a spinning wheel, where the drive wheel is much larger than the spindle. Depending on the fiber you’re spinning, you want the spindle or the flyer (depending on the type of wheel) to spin 10 times or more for each spin of the the drive wheel. 10 times may be good for wool but you might want 100/1 when spinning cotton on a hand turned charka.

This little cart moves faster than the ruler because of the ration of the big wheel to the little wheels. It looks roughly 4 to 1 to me.

spork

“The whole argument is pretty funny. It’s impossible. By definition traveling downwind means you have wind behind you pushing. If you travel in the same direction as wind, and go faster, you are then traveling UP-WIND.”

You’re confusing true wind and relative wind. It’s true that our cart feels a headwind when it goes downwind faster than the wind, but it’s still traveling downwind in the sense of the true wind. If you were standing beside the road when our cart went by you’d see it is going in the same direction as the wind. That’s all that’s meant by “downwind faster than the wind”

“To the folks that talk about traveling faster than wind on an angle, you’re speed can be thought of as two directions, with the wind and orthogonal to it. Your orthogonal speed is faster but your travel in wind direction is limited by the speed of the wind.”

That’s a very common misconception, but it’s just that – a misconception. Ice boats can maintain downwind velocity components of 3X to 4X wind speed.

spork

“Since the one thing that is missing on my Blogspot page is more info on how ice boats do it… Are there any online resources you’d recommend about ice boats”

I made a post responding to this but it dissappeared. You might want to add these:

http://www.btinternet.com/~sail/dwfttw02.htm

http://sports.groups.yahoo.com/group/2nalsa/message/161

ossix

i haven’t read each and every comment in the other posts, maybe this is totally obvious and has been stated before, but here is my 2 cents on the main topic (not directly related to this ruler-thing post which is worth discussing on its own):

at first let’s state that a machine that is only powered by the wind cannot move if no wind is present.

also every linear differential equation has the trivial solution 0, at which nothing moves and nothing ever changes.

two systems which only differ in a constant velocity-vector are galilei-invariant. that means the same forces and rules apply to both.

at first our machine starts with zero speed, to reach a speed faster than the wind at one point it has to be the same speed of the wind. at that point the only difference between the “same as wind” speed system and the “no wind” system is the constant velocity speed of the machine, which means both systems are galilei-invariant (ignoring kinetic energy stored in the wheels, which cant accelerate the cart further).

that means that there is no way to reach a faster-than-the-wind speed without storing any energy.

arguably the wind is an “infinite” resource of energy and it would be feasible without any problem to construct a machine that converts the wind-energy from the back and stores it into a battery. then the vehicle reaches windspeed, folds in the device that generated energy from backwind and uses the energy to accelerate even further with the energy stored in the battery.
the stored energy of course cant be infinite, but it will be enough to generate significant “frontwind”, which is a relative airmotion that only needs to be converted to energy again to accelerate even further, which increases relative airflow, which increases speed… you get the deal.

to make it simpler: if you put that machine that converted frontwind into acceleration into a room with perfectly still air and gave it a little push, it would accelerate forever which is a thermodynamic impossibility.

mikefinch

What was demonstrated was a transmission.

Think of your being on a boat. Is it possible to extract more energy from the wind than the wind carries? no. So its impossible to use the wind to travel faster than the wind unless you create stores of potential energy for use later.

If your “cart” has a sail and you ride in the wind it will go as fast as the wind. If you put a transmission onto the set up and change the gearing of wind to thrust or something you would still only go as fast as the wind. Once you reach the speed of the wind it ceases to exist. Your power is gone.

If you attempt to drive a prop from the movement of the ground beneath you you will not speed up. You will slow down.

Lets say your being blown by the wind at 5k and you start your wheel power now. The rolling energy gathered by any apparatus would be LESS than the energy gathered by the wind – your prime source of power.

What is being proposed violates newtons laws of mechanics.

You cant generate 5energy out of a system that only provides 3energy. No matter what you are being powered by the wind – if you are traveling at wind speed VIA wind speed any “drive or prop” utilizing the potential energy presented by the difference in speed between you and your surroundings WILL NOT WORK ITS ENERGY FROM NOTHING – it cant drive anything with any more energy than the wind can.

Energy that can be derived from your wheels is in direct relation to the speed of your vehicle. If your vehicle goes faster your tires go faster.

Energy = 1/2MV If your velocity depends on the wind then there is no way you are going to derive MORE energy from some internal reaction.

ratwerks

DWFTTW is more than feasible, it’s been done and demonstrated and all that. See http://www.ayrs.org/DWFTTW_from_Catalyst_N23_Jan_2006.pdf and anything by spork on youtube.

I love this guy’s utterly charming videos. I hope they help break people out of the “impossible” mindset so they can work on “how” instead.

spork

“Is it possible to extract more energy from the wind than the wind carries?”

No, and nothing we propose would suggest otherwise.

“So its impossible to use the wind to travel faster than the wind unless you create stores of potential energy for use later.”

Wrong.

“Once you reach the speed of the wind it ceases to exist. Your power is gone.”

Unless you’ve got a clever way to exploit the energy at the ground/wind interface where there’s still plenty of delta-V.

“What is being proposed violates newtons laws of mechanics.”

Wrong again. At least read a few of the posts on this thread if not the thousands upon thousands of posts on thread everywhere on the topic.

Boy it gets tiresome to answer people that come along 300 posts into a thread that just want to substitute their intuition for physics, and then call it proof.

TomSwirly

There’s an interesting technique called Sevin’s Principle of Virtual Work that you use for analyzing perpetual motion machines, for example, that very much simplifies the issue.

Imagine all the parts are friction-free and “ideal”.

Then perturb the system slightly in each degree of freedom and see what happens to it. If work is performed then that means it “would run”.

Making all the parts “Platonic” doesn’t actually change the outcome, it merely makes it “more efficient” (perhaps making something “practically impossible”). Once you’ve established how the underlying structure works, you can then put the weight on again and do the analysis.

Airshowfan’s model is perfectly good but can be simplified. Throw away all the parts and think of a fan on wheels – the fan spins the “outside” of a generator one way, the wheels spin the “inside” of the generator the other way.

“Does it run”: is work performed?

If you think of it just as “the fan vs the wheels” you can see all sorts of cases where “work is performed” (because the wheels are going very fast compared to the fan) even though “the fan is going backwards” (the speed of the system is greater than the speed of the wind).

The ruler video is a perfect analogy – and charming to boot.

Reverend Loki

“Terry, of course, knows exactly what’s going to happen, but he’s not going to tell.”

Terry is a prick, and I’m pretty sure I had a class or two with him in college…

The Black Adder

Get a laser pointer

Hold it at arm’s length and shine it at hill or distant building.

Move your arm slowly. Oh look the red dot moves hundreds of metres in a second.

Have you created some miracle way of making things move faster than the source of their motion?

no.

You have discovered the lever. It hsa been known for millenia, as has this “demonstration”

Kieran O’Neill

I hope dearly that this guy makes science/tech educational programs for kids for a living. He has a gift.

sartre

A vehicle moving downHILL could travel faster than the wind.

Perhaps this is part of the confusion.
Very few road surfaces are perfectly flat.

DownHILL + DownWIND = faster than wind. (even in theory!)

UpHILL + DownWIND = slower than wind.

Have any of these carts been tested on uphill surfaces?

Kieran O’Neill

And a beautiful accent – tiny traces of Germanic through otherwise flawless BBC English.

spork

“Have any of these carts been tested on uphill surfaces?”

Yup – ours will climb uphill all day long

http://www.youtube.com/watch?v=xHsXcHoJu-A

sforslev

In my previous post, I meant ratio, not ration.

Now that I’ve thought about it a little more, I think the problem with a wind powered vehicle like this is that once the vehicle starts going faster than the wind, maybe due to a big drive wheel differential, the motive force drops off and it will start to slow down until it’s the same speed as the wind when it’ll pick up again.

This would not affect a ruler driven vehicle so I definitely think that the energy department should look into a ruler drive ASAP.

JT Montreal

I think DWFTTW is possible, and allowed by physics, and the wheel thing provides a nice analogy.

Think levers. Observe instantaneously a point just above the ruler; at a given instant, the big wheel will rotate about that point, rather than the point where a regular wheel and a surface touch.

How far offset this point is from the touching point is given by the sizes of the large and small wheels. The speed of the cart is thus a fixed multiple of the speed of the ruler.

HYPOTHESIS: a DWFTTW will at equilibrium attain a speed that is a fixed multiple of the wind speed, minus inefficiencies of the mechanics; the multiple will be determined by the angle of the prop and gearing of the drive.

No funny free energy is needed.

airshowfan

MikeFinch #146: “Once you reach the speed of the wind it ceases to exist. Your power is gone.”
– - -
LightningRose #137: “The bottom line is *you cannot capture wind energy if your velocity is the same or greater than the wind*.”

No. At that point, you are capturing GROUND energy, if you prefer to think of it that way.

Sartre #149: “UpHILL + DownWIND = slower than wind.”

Actually, no. The little cart can (barely) go DWFTTW and uphill, as shown in one of the treadmill videos. The math is here.

MikeFinch #146: “If you attempt to drive a prop from the movement of the ground beneath you you will not speed up. You will slow down. “

That is not necessarily true, especially if the air is going by more quickly than the ground. The force on the wheels can be smaller than the prop’s thrust even though the power into the wheels is greater than the prop power. The math is here.

LDC Roberts #141: “The whole argument is pretty funny. It’s impossible”

No, it’s not.

LDC Roberts #141: “Your orthogonal speed is faster but your travel in wind direction is limited by the speed of the wind. “

That is incorrect: 1, 2.

ratwerks

greglandon @ 210: The very simplest example is a venturi, air inside the tube beats air outside the tube downwind. I have seen people point out mockingly that explanations of levers are beside the point. Or gears. Or transmissions. Or sailboats. Or inertial frames of reference. Or analogies. Or treadmills. Point being, what explanation fails to work for you is not therefor bad, and what works for you is not therefor good for others.

anon @211: I don’t think that is a useful characterization of the disagreement. Personally, I think of the problem as a failure to see the cart in 4 dimensions. My most helpful mental image is of a double helix traced by the prop tips as the cart moves downrange. If you want an example of something very small moving directly downwind faster than the wind see my comment to greglandon, above.

arkizzle

Can we make a rule?

If you want to make fundamental suggestions about the feasibility, or not, of DWFTTW, or the carts previously shown; you have to read every comment of the last 3 threads..

Please! Its all been said before (obviously, not literally, but certainly everything in Ossix’s #3).. go read!

spork

“Look, Terry and friends are on http://www.bestofyoutube.com/”

COOL! Looks like they have the #1 spot too.

prom77

I just want this guy to record some bedtime stories. He has a lovely, sonorous voice.

airshowfan

Sorry, in #151 I meant to say “especially if the ground is going by more quickly than the air“.

arkizzle

I said it on Coolan’s YouTube page, but I’ll repeat it here for completeness:

Louise is a hottie.

TomSwirly

Lightningrose is actually right until the very last step:

Once a vehicle going downwind reaches the wind velocity, it “sees” *no* apparent wind. If a vehicle goes down wind faster than the wind velocity, it “sees” an apparent headwind.

That’s absolutely true!

Now, stop for a moment. Consider that the ruler is exactly the wind:

Once a vehicle going down ruler reaches the ruler velocity, it “sees” *no* apparent ruler motion. If a vehicle goes downrule faster than the ruler velocity, it “sees” an apparent ruler going the other way.

This is also a true statement. In fact, the video exhibits exactly this phenomenon.

Look at the ruler. Look at the cart. The ruler moves forward – the cart moves forward faster.

Sure, the one point where the cart touches the rule, the rotating surface is moving backward.

So what?

You could do just the same in wind with a fan set horizontally with one side shielded from the wind! It would be dreadfully inefficient but everything’s platonic and you’d get a system where the wind was always “faster than” the fan.

But you only actually care about what happens to the center of gravity of the cart. The fact that one single point of the wheel is going backwards is irrelevant, you have to agree that the cart is overall moving forward.

At each step you need to squint your eyes and get the big picture.

Here’s the biggest picture of all with your eyes almost closed – the first law of thermodynamics.

You can extract work from the difference in velocity between the ground and the wind – and you can do anything you want with it. In particular, you can impart arbitrary amounts of power to things that are arbitrarily light and might them go arbitrarily fast – faster than the wind.

It is ridiculous to suppose that if you’ve constructed a system to capture a lot of wind energy and direct it to a fairly small payload which includes the energy capturing system itself that you could continue to be able to extract energy from the system as a whole and yet be unable to accelerate the payload past a certain speed!

Let me come up with a thought experiment that will accelerate you personally as fast as you like this way.

If you want some SFnal fun (and I demand an autographed copy of any book or story that uses this idea!)

Let’s suppose that we’re the Culture and have near-Platonic engineering capabilities – and you’re bored and want to go very fast on even a tiny breeze.

You build a very big sail with a very long stiff boom. It’s immovably fixed at one pivot point and can rotate only a tiny amount, a degree or less.

There’s a monorail track that goes from a terminal point close to the pivot point to you where, where you are sitting in your luxuriously-appointed cabin sipping Mai-Tais and preparing for high-G a thousand kilometers away.

Your cabin is attached frictionlessly to both the boom and the track so it can slide along either of them independently. You’re at the apex of a big isoceles triangle, where one side is the monorail, one side the boom, and the base the tiny distance between the pivot point and the monorail terminal point.

Everything’s set up. You wait for the slightest breath of wind in the direction from the sail to the track, and then allow the sail to rotate.

What happens? Your huge sail very much wants to rotate towards the track. But because your triangle is so very tall and thin, the lines are almost parallel, so small changes in the angle result in extremely large changes in the position of the intersection between the points – which is your monorail.

So in order for the sail to perform its work, it has to accelerate your tiny car extremely fast toward the pivot point. You are small, the sail is huge, with a big enough sail you can propel yourself as fast as you like in any wind.

If you actually wanted to make an amusement park ride out of it, you’d have the monorail track and the boom be symmetrically slightly diagonal to each other and give the boom only a small degree of freedom. If you set up the geometry right, you’d sit at one end in complete stability for a while and then if the wind shifted just a bit, wham! you’d suddenly be propelled incredibly fast to the other side and then if it shifted back, wham, you’d flash back. It would be interesting as a form of Russian Roulette…

If the wind nearly always came head on this system it would be inherently unstable and you might go back and forth a huge number of times in a small period.

The essence of the model is that the monorail car being anywhere in the middle is extremely unstable. Tiny displacements of the system which result in huge quantities of work also result in huge displacements of the car. Unless the wind blew exactly head on, any position between the two end points is impossible.

So here’s a plot then. Suppose this was set up as an amusement park ride with a Russian Roulette feel. But suppose in the middle of the ride the wind became so stable that the car was stuck out in the middle — still experiencing dislocations because even tiny instabilities could result in huge displacements but hovering right between one way and the next – a metaphor for heaven and hell.

Well, let me give it a whack first before anyone else does. :-D

GregLondon

more of Terry’s friends

Hehe. The last shot where everyone is watching the video on the laptop made me laugh. Nice video.

phadrox

Well, I can see this has been idle for a while, but if you go here -

http://www.greenbird.co.uk/about-the-greenbird

You’ll see pictures and explanations of a car that can go “at between 3 & 5 times the real wind speed, depending on the surface traction”. Booya. Not just a little more than the wind speed, up to 5 times the wind speed.

ill lich

Ow. brain hurt. need beer.

yesno

You should also read why Mark Chu-Carroll changed his mind and now acknowledges that DWFTTW is real:

http://scienceblogs.com/goodmath/2008/12/the_real_bozo_attempts_to_aton.php

The guy knows his stuff.

ThinAirDesigns

MikeFinch:
>What is being proposed violates
>newtons laws of mechanics.

Mike, it’s not “being proposed”, it being *done* … and no it doesn’t.

JB

Plethora_Jenkins

Possibly it’s the handful of nootropics I scoffed around midday, but where this topic resulted in comments that I got some kind of entertainment last time, this time I can only find frustration and angst, in response to all those people who continue to use incorrect physics, or an abject lack of basic physics knowledge to attempt and fail to back their claim that DDWFTTW is impossible.

The other day it was amusing how adamant were the people who hadn’t grokked yet. Today it’s just a frustrating example of the kind of thinking that powers religious bigotry: people who can’t countenance anything other than what they spout because they just _know_ they’re right, and so needn’t consider any other point of view.

Try this: Take the spork vehicle. Replace the propellor with something that looks like a propellor, but with flat blade that have no pitch.

The cart goes downwind almost as fast as the wind.
Due to friction losses, it goes a little bit faster still if you remove the gear from the prop shaft.

Imagine the gear is back, and the spinny thing that’d be a propellor if its blades had pitch. The cart is moveing with the wind, almost as fast as the wind. The spinny thing is spinning. Now give the spinny thing’s blades a few degrees of pitch.
What happens?

Now give the blades the opposite pitch.
What happens?

DougO

This just shows that gears of different sizes can move a machine faster or slower than the transit of the drive shaft circumference, but it does not say anything about the energy it takes to move the cart or the effect of crossing that critical threshold from being pushed by the wind vs pulled by the wind.

ratwerks

Guy Fleegman: Did you guys ever WATCH the show?

Gwen DeMarco: Let’s get out of here before one of those things kills Guy.

Anonymous

He’s moving the ruler opposite the direction of the car relative to the car. The big wheel acts as a gear, multiplying the speed of the ruler as it reaches the ground. Because the wheel is rubber, the ruler sticks and the movement of the car forces him to move his hands.

But look at the top of the wheel and the markings on the ruler. The ruler is moving, relative to the wheel, in the opposite direction of the cars travel. Everything is to be expected. At best, he’s shown that movement into the wind is possible. He certainly hasn’t shown anything about dwftw.

Alpinwolf

@ #’s 163 & 164, ARKIZZLE and RATWERKS:

(and Michael C. also)

Silly me, shoulda checked his YouTube profile. Now that it’s been said, I do hear those differences of the German, but after living with some German exchange students, (who told me with a chuckle that I speak German with a Russian accent, and really I don’t speak either), I am shamed that somehow I never picked up on that before your tip. But am I right that the English he is speaking is of a Commonwealth flavor?

I’d say Gaiman’s English is one of the lightest accent variants to my ear. I tried listening to archival samples from near or in West Sussex, but very few of those sounded like him either. I believe he does have parents that immigrated from the continent. Erm.. Polish Jews, if I recall?

I’ve come to the conclusion that there’s something about the calm reasonableness of the cadence of their speech that makes them sound most similar. That, and a plush monkey named “Terry” to hold vigil at the proceedings.

…

Oh and, to stay at least a smidgen on topic, I say it all works. Plus I blathered all over “Part 2″ about the ‘how’ already.

k thx tschÃ¼ss!

Clayton

All this pontification and ambiguous reasoning about something which is testable and observable and there’s still only ONE reference to Zeno’s folly? Boing Boing community, you disappoint.

yesno

Also, Dave Munger has collected objections to the cart, and refutations of those objections, at his blog, here:

http://wordmunger.com/?p=1002

spork

>What is being proposed violates
>newtons laws of mechanics.

>> Mike, it’s not “being proposed”,
>> it’s being *done* …

That’s it!!! I’m calling the physics police.

Takuan

you missed it

Landowner

That’s great and all, but the word is counter clockwise.

Anonymous

I used to do this as a kid. And with experimentation of all the variables involved, you’ll soon learn you can make the cart go either direction regardless of which way you move the ruler. Let’s just throw out two F words for ya: finesse and friction.

airshowfan

Dang it. Just when I thought I was going to be able to get some sleep again…

@#3: “…it has to be the same speed of the wind. at that point the only difference between the “same as wind” speed system and the “no wind” system is the constant velocity speed of the machine…”

No, there is another difference: The “same speed as the wind” system (unlike the “no wind” system) has the ground rushing past and spinning the wheels, and this can be harnessed to spin a propeller.

@#11: “…crossing that critical threshold from being pushed by the wind vs pulled by the wind…”

No, the cart is always pushing on the air with its propeller. It goes from pushing on air that is coming at it from behind (like an airplane about to take off into a tailwind) to pushing on air that is coming at it from the front (like an airplane about to take off into a headwind).

And for those who wish they had the time to read all the comments in the previous thread: I did my best to capture the whole debate (and all the associated thought experiments and Youtube videos) here:

http://dwfttw.blogspot.com

Now I’ll go add this video. It’s even better than the spool one. Must be all the extra peer-review ;)

FutureNerd

Two people complained of the ruler demo: “That’s just a demonstration of gear ratios.”

Right. And that’s the main idea of the cart. There are gears between the wheels and the prop. And the prop itself is a kind of imperfect gear against the wind (both props and certain gears are called screws).

“The wind is a constant force.”

The wind isn’t a force per se. The force between the wind and an object depends on how fast the thing is moving, what its area is, whether it’s a turning propeller, and so forth.

“Moving forward in a still wind is exactly the same as moving faster than a moving wind. Exactly.”

Yes, as far as the difference between you and the wind. But there is also the difference between you and the ground to consider.

About relativity: the difference between the velocity of the wind and the velocity of the ground is independent of the velocity of the observer (or close enough at non-relativistic speeds).

Mark Frauenfelder: “Instead of simply speculating.” I like the experiments, but applying the laws of physics is at least informed speculation! This cart isn’t based on any new physics or challenge to physics. So this isn’t so much an experiment a demo.

FLG

Awesome video :D

This is going to be one of those things, like when mythbusters did the airplane on a treadmill, that when it gets busted/proven, EVERYONE is going to say that they knew it all along.

Clayton

Takuan, do you have Boing Boing linked to your brain? Seems like you’re here 24/7.

Anyway, “missed it”? Missed what, exactly?

hidflect

I’m sure that ships do move faster than the wind all the time because they don’t just use the simple thrust of the wind to propel themselves. When the wind angle is appropriate, They use the sail as a vertical wing-shaped surface using the wind blowing over the front of the billowed sail to generate a vacuum in front of the sail thusly pulling the ship along. While the speed of the wind does alter the vacuum force, it’s not a direct 1:1 ratio and with a large sail they can generate a very large vacuum force. #68 sez it also.

Daneel

@13 – no, it isn’t.

The narrator certainly sounds like a Brit, so why would you assume he’s speaking American rather than English?

Meh. Personally I prefer widdershins.

arkizzle

Landowner.. *sigh*

Is this thread not partisan enough? You may as well start mentioning Legos and Daylight Saving..

Antinous

Shouldn’t that be ‘Legoes’?

yasth

#8

I would argue rather strongly *NOT* to read the other threads, read the science blog, but not the threads.

The problem is that too many people on the threads have the right answer but the wrong work. Most people seem to explain it with broken models, and clumsy wording. It is very hard to be convinced by something with giant holes, or that is just wrong.

FoetusNail

Greg, 50 pages while arguing with Jonathan; when do you sleep, or eat for that matter?

arkizzle

Daneel, personally I think it either spins Godwise or Devilwise.

Anonymous

Please, let a poor, anonymous coward explain what’s really bothering him about this whole DWFTTW discussion:

You people have built two completely different models of the problem in your mind.

The, “No, it doesn’t,” Camp are treating this strictly as a one-dimensional problem. There is no such thing as tacking, because the cart’s velocity vector can only be aligned with that of the wind. They say, “No,” and they’re right, for all the reasons stated.

The other camp — the “Yes, it does”ers — are treating this as a two-dimensional problem. This is the Sailor’s camp, and they relate what every sailor knows. Yes, you can sail at Broad Reach, and the component of your velocity that is aligned with the wind is greater than the wind velocity. They say, “Yes,” and they’re right, for all the reasons stated.

Unless, of course, this whole thing is still not being explained clearly. But that could NEVER happen.

Here’s an interesting challenge: To the sailors, please prove that DWFTTW is possible for the one-dimensional case. To the nay-sayers, please disprove the possibility of the two-dimensional DWFTTW sailing problem.

arkizzle

Yasth,
it’s just a pain when someone has the issues completely backward and thinks they are chiming in with the latest cutting edge assessment of the situation.. but it is just wrong and has been addressed before.

I don’t think you should necesarrily read those threads for the correct answers – just to be aware of what has already been said 10 times.

airshowfan

@ Badger500 # 178,

Doesn’t #132 answer all those questions to your satisfaction?

spork

I assumed the reason for the fake accent was the same reason we see the faces of your cohorts, but never yours. I just assumed you’re running from the law, and wound up in Germany because they have no extradition agreement. But I have bad news for you my friend. If you and your fury band of ne’er to wells go breaking the laws of physics, and flaunt it in our faces by posting “physics law snuff videos” on YouTube, you will ultimately be brought to justice – just as O.J. will one day find the real killer, and W. will find Bin Laden.

I’m impressed that Terry refuses to say anything. I wonder what accent he would attempt? : )

GregLondon

Someone else said it’s about as interesting as a transmission. I said its about as interesting as a block and tackle.

Normally, a block and tackle lets you lift 100 pounds with only 50 pounds of force by requireing that you pull in 2 feet of rope for every 1 foot the weight goes up.

This thing flips it around. You reel in 1 foot of rope and the weight goes 2 feet. The tradeoff is that you’ve got to put 100 pounds of force on the rope just to move 50 pounds.

You can easily visualize this working with the wind using a yoyo and a string and a parachute. There are some drawings here:

http://www.greglondon.com/tumbleweed

From there, converting to a prop is a matter of realizing that a prop is just a different gear ratio that you have to deal with.

Transmission, block and tackle. whatever you call it.

tobergill

The wheel ratios have nothing to do with this – the linear velocity at the wheels circumference is the same. If there’s no slippage in the system, ruler moves 5 cm, big wheel point on the circumference moves 5 cm, little wheel point on the circumference moves 5 cm – but in the opposite direction. The aggregate movement is therefore twice 5 cm which is 10 cm WITH REGARD TO THE FIXED TABLE TOP. If you use the machine as the reference point, both ruler and table top move 5 cms but in opposite directions.

Alpinwolf

Frakking brilliant, Michael C, no matter where you’re from. ;o)

Do you teach university and/or secondary school physics? If not, I think the world needs you to.

Your examples remind me of an old PBS program on which my science teeth were cut back when I was short, called “Eureka!”

(BTW, was listening to Gaiman read his Graveyard Book shortly before ambling over to BB and finding your vids. I still say the similarities are uncanny, and it’s wonderful for presentation. :)

TwoShort

I have gone downwind faster than the wind using only wind power.

At iceboat races I attended in high school, we would often have someone at the windward mark release a bit of streamer as the lead boat passed him. This would blow along at essentially wind speed toward the finish, directly down wind. If the winner was cocky, they would attempt to park their boat and acquire a beverage before catching the streamer. Simply beating the wind wasn’t even a challenge. Iceboats (and a few very fast water boats) do it sailing about 45 degrees off of straight down wind, then turning to the opposite angle to come back to straight down wind.

The cart linked by Ratwerks appears to use the same principle to move straight down wind.

Don’t think of it as using the wind to move downwind. Think of it as using the ground whipping by underneath you to move through a stationary air mass. At least, that’s what helped me figure it out.
You’ve got two media (ground/air) moving relative to each other. You can exploit that differential for energy which you add to your overwhelming traction vs one of the media to move through that media. The cart uses energy from the ground going by to move through the air.

spork

First of all, Michael C makes some darn fine videos. Nice and simple, and plenty clear. And people can argue about what’s happening, but they don’t accuse him of cheating. Very good. He does us “downwinders” proud.

Mike’s video is 100% legit, and it does in fact demonstrate that it’s possible to “outpace” your power source. People may counter that it works differently with wind than it does with a ruler, but the principle is really quite similar. And the energy of the wind over the road is essentially unlimited as well. It’s all a matter of how you harness that energy.

“to make it simpler: if you put that machine that converted frontwind into acceleration into a room with perfectly still air and gave it a little push, it would accelerate forever which is a thermodynamic impossibility.”

This is a very common misconception. It would be true if the vehicle simply harnessed the energy of the wind relative to the vehicle itself, but that’s not how it operates. The prop-cart acts as a sort of lever between the ground and the wind. It then trades off some of the available force for speed. And it can and does go directly downwind faster than the wind – steady state.

GregLondon

yoyo being pulled by a string: page 3

yoyo pulled by string moving at 5fps, yoyo moving at 10 fps: page 5

yoyo pulled by string attached to parachute. Chute moving at 5 fps, yoyo moving at 10 fps. Page 6

(once you get this diagram, everything else is just implementation)

rope and pulley diagram showing that the forces balance out and we’re not getting something for nothing: page 7

translating rope and pulley diagram back to yoyo/parachute diagram: page 9

A cart that implements faster than the wind motion using parachutes and rope, exactly like the yoyo did: page 25

yoyo/rope/parachute moving at exactly the speed of wind, from the frame of reference of the yoyo: page 28

converting parachute/rope cart to a propeller cart: page 32

http://www.greglondon.com/tumbleweed

GregLondon

Mark, did your question here ever get answered?

You were going to dig out a yoyo and play around with it.

Once you see the yoyo move faster than the string is being pulled, it’s fairly easy from there to see how this cart is working.

I made a “yoyo” with two paper plates for wheels and a soda can for the axle, and used tape to put it all together. I taped a piece of string to teh can, then slowly pulled on the string and watched the yoyo move faster than the string was being pulled.

I used that to demonstrate the idea to a friend of mine. Once he got that, I told him to visualize a parachute at the end of the string instead of my hand, and he got that the whole idea of the cart was possible.

Get some tape and paper plates and a soda can and some string if you want the most basic demo of all.

Takuan

being far too indigent to read and think, my default postition regarding perpetual motion machines has always been: “Drive by in your new Bentley and I’ll believe you”

This isn’t (maybe)perpetual motion, but I am prepared to believe when someone drives by in their working machine.

Oren Beck

TANSTAAFL cannot be denied. Even if the meal is wind.
But yeah- you can still get a hell of a deal in some cases. And those cases include Savonius rotary airfoils on the sail’s “spill edges”. Geared to Hovercraft skirt fans for example. With bleed air for thrust. The delta between “faster” and equal speeds likely will be a sawtooth or uneven sine function at best. For there’s an inescapable truth of relative motions being a cold equation.

alowishus

There is only one way to solve this dilemma . . . Mud wrestling!

Milo Minderbender

Congratulations! You just invented the transmission.

Anonymous

There’s a common confusion that when you’re rolling along at wind speed, the energy available is limited to the kinetic energy of the cart. This, however, is not true at all. Suppose you’re rolling along as fast as the thing that’s pushing you. The pushing force drops to zero, true. But now drag your feet. You don’t slow down at all, but the force pushing you goes way up. Doesn’t seem like that would be the case? What if the thing pushing you were a locomotive? It’s pretty clear that it would be true then, right? Without slowing down in any perceptible way, you can get a huge amount of energy (a steam engine’s worth), and you can use that to push yourself away from the train (by dragging a wheel that’s connected to something that can do useful work).

Same deal with the wind. You slow down a smidge, and your sail is suddenly being pushed on really strongly. If your sail happens to be a spiral-shaped thing like a propeller, you can take connect your wheel to the propshaft, and use it to screw yourself forward through what would otherwise seem like still air.

[A greatly abbreviated version of "The Parable of The Presence", which got clobbered by the ice-storm-induced power outage that hit my house a week ago and is still ongoing.]

J-J

darue

I would think that analyzing the energy inputs and outputs would suffice to show it’s not possible. The momentum transfer would diminish the closer to wind-speed the vehicle got. If it somehow was accelerated to move faster than the wind, the wind wouldn’t be able to push it anymore, and it would feel the wind-like force pushing against it’s direction of travel slowing it down (drag) become greater than the force of the wind from behind. If it were to keep going faster where would the energy be coming from? It would have to come from somewhere other than the wind.

Probably such a vehicle would exhibit a stuttering motion speeding up then slowing down, then speeding up again. perhaps it could establish a stable balance, but the wind wouldn’t be stable either…

Now if you extend the concept of vehicle to include a track and a long line of pairs of ground mounted windmills that turn heavy rubber wheels that have stored up momentum and speed the vehicle as it’s shoved between each pair in turn… maybe yes?

ThinAirDesigns

@27:
Contratulations! You just confused a demonstration with an invention.

JB

Michael C

For those who are speculating about my accent and where I come from, I feel obliged to point out: my YouTube profile correctly states that I live in Germany. This does not say anything about my actual nationality…

In any case, thanks for all the friendly comments. I never imagined that this little video would generate so much interest. There’ll be more: I have the idea for the next one, but I need to find the time to make it. I’ll post a link here when the next episode is online.

FCAlive

downwind faster than the wind != infinite acceleration

twofedoras

Magnet under the table.

That is all.

Michael C

If Terry said anything, it would be “Ook”.

spork

“This isn’t (maybe)perpetual motion, but I am prepared to believe when someone drives by in their working machine.”

Where do you live (and do you get much wind)?

“The delta between “faster” and equal speeds likely will be a sawtooth or uneven sine function at best.”

Just like Michael C’s ruler driven cart, our prop cart can go signficantly faster than the wind steady-state, for as long as the wind blows. It’s plenty easy to try it out yourself. Here are the build plans:

http://www.rtfa.net.nyud.net/wp-content/uploads/2008/12/dwfttw_build_plans_147.pdf

I hope to make a build-video and post on YouTube this weekend.

Michael C

Tobergill (#115), you’re not looking at the gearing correctly. The essential point is that the bigger wheel is touching the inner parts of the cotton reels. The linear velocity at this point is less than that of the outer edges. The size of the bigger wheel is unimportant: its only function is to translate the movement to the ruler, in the right direction. The radius of the cotton reels at their inner parts is approximately half the radius of the outer parts: this means that the top point of the top wheel moves forward at approximately half the speed of the cart. This condition is essential for the cart to move faster than the ruler: the ruler must be acting on part of the cart that moves slower than the cart, but in the same direction.

When the ruler moves 5 cm forwards (relative to the table top), the machine moves approximately 10 cm forwards (relative to the table top). If you use the machine as the reference point, the ruler moves back 5 cm and the table top moves back 10 cm, in the same direction.

arkizzle

Darue

Read YesNo’s links @ 10 and 12, and AirShowFan’s link @ 15

tp1024

I found the solution. It is possible.

Bernhard Schmidt, inventor of the Schmidt telescope, build a ship that is capable of sailing directly against the wind without having to change direction or needing an extra power source. (I was sure i had read about this, but didn’t suspect it was a book about Schmidt telescopes – of all things.)

A quote of him, from

http://www.friedensblitz.de/sterne/schmidt/Schmidt3.html

â€žMit meinen Segelversuchen bin ich nun schon dazu gekommen, daÃŸ ich sogar direkt gegen den Wind mit Windkraft allein vorwÃ¤rts fahren kann, zwar nicht gerade schnell, aber immerhin komme ich schneller zum Ziel, als ein entsprechendes Segelboot durch Aufkreuzen gegen den Wind.”

Rough translation:
In my sailing experiments I even managed to go forward right against the wind. Not exactly fast, but faster than crossing against the wind. (As far as i could find out the english equivalent is “sailing upwind with tacking” – but I’m certainly not a sailor)

So, it is possible to go forward AGAINST the wind – as he proved beyond all doubt.

What does that have to do with the topic on hand? Well, because of friction the wind will always be slower on the ground than some way above it. (Otherwise there could be no Magnus effect, or sailing ships driven by Flettner rotors.) Now, when a cart moves AS FAST AS the wind, that means, that it will experience a HEADWIND near the ground. And as we know, a headwind is perfectly sufficient to propel a ship (and for that matter also a cart) forward. But if the cart is already moving as fast as the wind and can still create a force driving it forward, then it MUST GO DOWNWIND FASTER THAN THE WIND.

The problem, as you see, has already been solved more than 80 years ago.

ThinAirDesigns

Fcalive:
>downwind faster than the
>wind != infinite acceleration

Yeah, well … except for that little drag issue that crops up at something just bit faster than the wind.

JB

Anonymous

Place a ping-pong ball on a flat surface. Blow on it to get it moving. Now stop. The ball continues to roll even though the wind speed has gone to zero. Its works because energy gets stored. (1/2)mv^2.

badger500

Spork and Airshowfan,

Sorry to not get back on this thread, but for some weird reason I couldn’t log in to comment. In any case, the last to explanations you both provided were very helpful. I think most of the work of my “getting” this comes with practicing the ideas, and I think I have enough material now to practice with. But I’m basically there. (it is surprising how much “slippage” is possible when ideas are not well practiced. I’ve encountered this in neuroscience, too).

I should also note, in probably leaving the thread now, something that too few people have said to the guys who made these carts (including Jack in the outdoor video, spork, JB, and anyone else on the team, I hope I’ve got this close to right). And that is: it’s clever! Simply put, it’s clever, you guys are clever and are obviously aerodynamics gurus, and you’ve taught a lot of people two great lessons. One is just a nice basic physics lesson. Two is: don’t be so sure of your “certainties”. Thank you! To be honest, I still can’t say with 100.0% certainty this is right, but that is my shortcoming, not yours (just in the same way that I can’t say with 100.0% certainty that many of the claims of science are correct), and I am basically convinced now that it works as claimed.

Michael C,

Another delightful video! I love when you speak about Esther’s perspective, it is so helpful for people to get the relative frame issue. And that they were all watching it on your laptop is just comedic genius. I hope you infect people with your warm and measured rationality merely by watching these videos!

Takuan

you tease! Post a design link that works!

spork

“I would think that analyzing the energy inputs and outputs would suffice to show it’s not possible.”

Interestingly, analyzing the energy inputs and outputs is sufficient to show that it IS possible. We can see in the video that it works with Michael’s ruler cart, and you can see in our video that it also works for our prop-cart:

http://www.youtube.com/watch?v=xHsXcHoJu-A&fmt=18

If there were no wind, the wheels would never “collect” enough energy rolling on the ground to spin the prop fast enough to continue pushing the cart forward. But this cart is operating in a tailwind. Therefore, the energy available at the wheels is significantly greater than needed by the prop to keep the vehicle moving downwind faster than the wind steady-state. I’ll be happy to post those simple equations if you like.

“The momentum transfer would diminish the closer to wind-speed the vehicle got.”

Again, this would be true if the cart simply harnessed the energy of the wind relative to the cart itself. But it doesn’t – it actually exploits the energy available at the ground/wind interface. This continues to be possible even when the cart is going downwind at the speed of the wind.

“…and it would feel the wind-like force pushing against it’s direction of travel…”

“Probably such a vehicle would exhibit a stuttering motion speeding up then slowing down, then speeding up again. perhaps it could establish a stable balance, but the wind wouldn’t be stable either…”

Not true. It proceeds perfectly steady-state faster than the wind.

It’s very much like the ice-boats that can achieve a steady state downwind tack such that they have a downwind velocity component several times the wind speed.

sabik

@tobergill #115: you’re missing that the little wheels are cotton reels: while a point on their spindle moves 5cm, a point on their outside circumference will move around twice as far, 10cm.

Frank W

This thread reminds me of the Dead Parrot Sketch.

No it doesn’t.

Anywayâ€¦ Terry knows what will happen, but he isn’t going to say.

arkizzle

Tak, link works for me..

desprez

Irrelevant.

The ruler is not providing the energy. The energy is coming from the person moving the ruler.

The cart speed to the ruler speed is a gearing thing, not a look-i-just-extracted-more-speed-out-of-the-rulers-speed thing.

Personally, I’m mostly in the (sustained) DWFTTW doesn’t work camp. I’m not going to declare it impossible, because I’m still wrapping my head around the problem, but I will be quite surprised if it turns out to be true.

But I will say that so far every “experiment” I’ve seen so far is deeply flawed.

For instance, a treadmill in still air is not equivalent to moving at the same speed of the wind.

Why can’t somebody just make a nice controlled experiment already?

darue

SPORK, you’ve convinced me.

what is the top speed? I mean what it the best possible ratio?

ThinAirDesigns

Great, the guy who failed miserably on his own had to be outed as an incompetent is here to present our long and oft used examples, (yo-yos, parachutes, etc) to explain to everyone how it’s done.

No testing required if you understand things as well as you do, right Greg?

JB

spork

“you tease! Post a design link that works!”

http://www.rtfa.net.nyud.net/wp-content/uploads/2008/12/dwfttw_build_plans_147.pdf

And this is how it works:

http://www.youtube.com/watch?v=xHsXcHoJu-A&fmt=18

Michael C

Twofedoras (#116): you can do all sorts of fun things with magnets under a table, but I can assure you that it’s much easier to make this video as I did, using the mechanical gearing of the cart and the motive force of the ruler pulling on the wheel, than it would be if I tried to fake all that with magnets.

spork

I just completed and posted my videos last night. I am in the process of making 9 carts for a few people that requested them. I used the first cart as the basis for the videos. It would be nice if it was one single video, but YouTube has a 10 minute limit, and I can’t talk that fast.

Here they are:

Build video 1 of 3:
http://www.youtube.com/watch?v=T-ArigMKhi4&fmt=18

Build video 2 of 3:
http://www.youtube.com/watch?v=p0rhgop5wEM&fmt=18

Build video 3 of 3:
http://www.youtube.com/watch?v=gSHNqrF93MU&fmt=18

catastrophegirl

hrm, i am wondering if the fact that his orangutan toy is named terry is an homage…. could terry possibly be a librarian?

spork

“Great, the guy who failed miserably on his own had to be outed as an incompetent…”

For the record, there is no crime in being incompetent. Everyone starts somewhere. What frankly bugged me was taking the insults and being called incompetent by such a person.

Michael C

A DWFTTW regatta: that would be fun. Maybe enough people will make carts so that we can organise a race?

If you want to make your own cart, here’s where to start: Spork has made a series of videos that show how to do just that. The first one is at http://www.youtube.com/watch?v=T-ArigMKhi4

airshowfan

@#25, Takuan:

It’s not perpetual motion.

See http://dwfttw.blogspot.com/#5

(Although IMHO it’s not practical enough for anyone to earn a Bentley off the idea, either.)

@#28, Darue: “I would think that analyzing the energy inputs and outputs would suffice to show it’s not possible”.

It WOULD, except when you do analyze the power in and power out, you see that the power needed for a prop to push the cart at (or just faster than) wind speed is less than the power that is available to the wheels from the ground.

Again, http://dwfttw.blogspot.com/#5

spork

“For instance, a treadmill in still air is not equivalent to moving at the same speed of the wind.”

Galileo, Newton, and Einstein claim that a road moving 10 mph backward beneath still air is exactly the same as a 10 mph breeze going over a stationary road.

“Why can’t somebody just make a nice controlled experiment already?”

Well, Jack Goodman did his test on the open road. Everyone complained these were not controlled conditions. So we decided to do ours on a treadmill to address everyone’s concerns. And many complain this is “different”. But one of the most basic laws of physics tells us they are in fact one and the same.

“what is the top speed? I mean what it the best possible ratio?”

For the prop-cart the top theoretical speed relative to the wind is determined by the advance ratio (the theoretical distance the prop advances in one rotation divided by the distance the wheels rotate in that same single rotation of the prop). As the advance ratio approaches 1.0, the cart speed approaches infinity – but the margin for internal losses diminishes rapidly. I think a practical limit of 2 or 3X wind speed is probably achievable.

Anonymous

Spork,

I can think of at least one practical application: moving ships upstream. Just stick wheels on either side of the ship to rest on the bank on each side of the river, and stick the propeller under water with reverse gearing, and viola, a ship that steams upstream all by itself.

davidspargur

The way it works all has to do with the inner and outer radius of the spool.

SeattlePete

@ Spork in 146:

“Unless you’ve got a clever way to exploit the energy at the ground/wind interface where there’s still plenty of delta-V.”

Except there is no gound/wind interface that can be exploited. There is only the interplay between wind/device/ground. Throwing the device in there creates drag against the ground and the best you can do is simply get very close to the wind speed.

Also, as has been stated elsewhere in this post, once you are going faster than the wind speed, you’re traveling into a headwind. At this point there is no wind speed, in fact the wind speed is negative so traveling faster than it is nonsensical.

Imagine a hallway with a fan at one end. The fan generates a 10 knot wind which fills the hallway. Relative to the walls, floor and ceiling of this hallway, the wind is moving 10 knots. True wind speed = relative wind speed = the only wind speed. Now imagine a cart the exact width of this hallway going 11 knots. This is true DWFTTW. Once the cart gets to 10 knots, there is no more wind relative to the cart. The cart and the hallway are, in effect, stationary. There is no sustained energy to be had from the wind at this point and the carts progress stalls. Friction pushes it back a little, it catches a little bit of tail wind, and speeds back up close to 10 knots.

Don’t bother responding to my post because I’m certain what I’ve said has been said before. Also, it would appear that everything I say is true can be refuted by an ice boat.

Blame Canada.

airshowfan

@#37 Desprez; “The ruler is not providing the energy. The energy is coming from the person moving the ruler…”

Right.

“For instance, a treadmill in still air is not equivalent to moving at the same speed of the wind.”

Wrong.

See http://dwfttw.blogspot.com/#6

(How handy to have a page with all the answers ready to be linked to! I hope everyone appreciates that this is not spam. If I wanted to get hits on my website or AdSense clicks off of all this, I would have put that page on my website rather than creating a Blogger blog to put it on, or would at least have put ads on the blog. Right?)

Alpinwolf

Does anyone else, (American, British, or otherwise), think the guy in this vid sounds just like Neil Gaiman?

(I am obviously tipping my hand as an American by asking this, and no, I don’t think all Brits sound alike, just these two specifically.)

spork

“…and BoingBoing not once not twice but three times.”

Airshowfan – time to update your page : )

airshowfan

Indeed.

ratwerks

desprez @ #37

What nice controlled experiments do you propose beyond:

http://www.youtube.com/watch?v=xHsXcHoJu-A

http://www.youtube.com/watch?v=4pgDs50A-Yw

http://www.youtube.com/watch?v=1BRvYZd81AQ

http://www.youtube.com/watch?v=kWSan2CMgos

and, in general, anything found at:

http://www.youtube.com/user/spork33

I’d like to point out that I am unaware of the existence of any sort of wind tunnel sufficiently long for this sort of testing but if you know of a handy free one please let us know.

spork

“The, “No, it doesn’t,” Camp are treating this strictly as a one-dimensional problem. There is no such thing as tacking, because the cart’s velocity vector can only be aligned with that of the wind. They say, “No,” and they’re right, for all the reasons stated.”

I think you’re confusing 1D for 2D and 2D for 3D. That being said, this vehicle will do very nicely without tacking.

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

spork

“How handy to have a page with all the answers ready to be linked to!”

Indeed. I haven’t studied it yet – but you are among the few that seems to be completely accurate in all your responses on this topic.

“I hope everyone appreciates that this is not spam.”

Nope – it had to be done.

desprez

“Galileo, Newton, and Einstein claim that a road moving 10 mph backward beneath still air is exactly the same as a 10 mph breeze going over a stationary road.”

For frames of reference, yeah, but not for where the power is coming from.

The power is coming from the treadmill, where as power to turn the wheels geared to a prop on normal ground, the power is coming from momentum that was initially provided by the wind – which after you hit the airspeed is now actively working against you.

The ground providing power is not analogous to the wind providing power.

Chrs

I’m glad the DWFTTW people have gotten better at explaining the energetic element.

It took me a while to make sense of it, quite frankly, and I didn’t believe it initially, thanks to the zero relative wind speed argument.

Explaining it as “sort of a lever”, and “squeezed between the wind and the ground” are some of the ways people have mentioned to intuitively describe what’s going on, once you have a feel for it. However, because the motive power does finally have to come from the wind, these are somewhat unsatisfying from the balance of forces perspective. The energy is there; the kinetic energy of the wind passing through the area of the propeller, relative to the ground, is larger than that needed to accelerate the cart. The force balance is what kept nagging at me.

The way that finally makes sense to me is this:

The wind is not directly pushing against the cart. It is pushing against the wind being propelled backward from the cart.

First, the cart needs to give relatively little energy to the air surrounding it to propel it slightly faster in the same direction. This is relatively intuitive. Now, however, the wind behind the cart’s immediate area is pushing against that stream of slower air pushed by the propeller. This force is transmitted through the air behind the propeller, to the propeller, to the ground.

The wind is acting on something traveling slower than the wind, then, even if that thing is air.

GregLondon

Spork’s videos showing how to make the cart are brand new.

Good job, spork.

He goes into a lot of detail.

I watched all three. They’re pretty good. I cringed a couple time at potential safety violations. the man likes to put his fingers right up next to the power tools. I hope he was wearing safety goggles.

;)

Otherwise, the three vids plus a parts list and the world will soon be overrun with propeller carts.

darue

that’s just beautiful guys, thanks for building this! Allow me to issue everyone involved: mad props

Hey ya know, miniature carts might make nice toys

arkizzle

Alpin, I was gonna jump in with “Noooo!”, but on another listen there is definitely something similar there. MichaelC has a little something else in his accent though, a faint german/norse/euro something giving it that extra lilt. Whereas Gaiman is pretty clear English, with maybe some slight immigrant-parental influence.

Hmm.. maybe they are more similar than I’m letting on.

morehumanthanhuman

Reminds me a bit of a Brennan torpedo, a torpedo with a propeller driven by unwinding a large spool of cable with a shore based steam engine.
http://en.wikipedia.org/wiki/Brennan_torpedo

I just built a model of the cart in the video out of Lego, and the verdict is: it doesn’t work. IF YOU HAVE ANY LEGO BUILD IT NOW, TEST IT YOURSELF!The cart will move in the same direction as the rod very slowly, with wheels dragging. But I figured out how the trick could be done. Tilt the rod slightly while moving the rod in the direction of motion, causing the cart to be pushed to the other end faster than the speed of the rod.(Though I suspect the maker of this video used string to turn the wheels or something with reversing the video)

In fact the rod can have zero velocity along it’s length and be use to move the cart.

Maybe if I reduce the friction it will work, but then again, perpetual motion researchers have been saying that for a long time…

arkizzle

MichaelC, do you mind telling us whether you have a mixed accent, and if so, where the non-English bit is from?

Thank you!

For what it’s worth, mine is Irish with a flavour of Australian, and recently some bits of English coloquialism.. :)

Napkins

If we harnessed the energy of all the keyboard strokes, this argument provides, we would have enough energy to cook me burrito. It’s late and i am hungry

Chrs

MHTH: If anything, you need to increase the friction on the ground wheels. How close is your ratio to that shown in the video?

ratwerks

http://www.youtube.com/user/coolaun says he’s German. Just sayin’

Richard Kirk

DWFFTW can work. It may seem like madness, but that’s because it is hard to mentally untangle what is going on because everything is happening at once. Perhaps we can clear up the argument by imagining a very different mechanism where the different actions happen one at a time…

Imagine you have a creature that has a part like an umbrella, connected to a part like a spring, connected to a part like a suction cup…

The suction cup grips the ground, the umbrella opens and catches the wind. As the umbrella moves off downwind the spring is stretched between it and the stationary suction cup.

The suction cup lets go. The spring pulls the suction cup towards the umbrella. The suction cup is a lot smaller than the umbrella, and so has much less wind resistance, so the umbrella is almost stationary with respect to the wind, and the suction cup and the spring are accelerating towards it.

When the spring goes slack, the suction cup and the spring must have travelled faster than the wind to catch up with the umbrella. The whole creature has a net forward momentum. The creature then closes up the umbrella, reducing its wind resistance. The whole creature can now travel for a while through the air faster than the wind.

When it slows down, the suction cup grabs the ground again, the umbrella opens, and the whole process repeats.

Okay, to make this creature work, I have had to add some actions and they will take some energy. The suction cup will not work itself, though the umbrella can be largely opened by the wind. Nevertheless, the spring gathers energy in each cycle, and then converts it to kinetic energy. If the creature is very light, and we have put no restrictions on the mass of this creature, so it might be very light, then we could tap off or recycle a bit of this energy to power the other actions.

Okay – this creature does not actually exist as far as I know, but you can find stranger things in a drop of pond water. If you think this works, then the cart ought to be possible too.

PS:
The real way to test this is with an experiment, and not sit around like Greek philosophers arguing why an arrow flies. However, on the net, where every video looks faked, there is still a place for this sort of reasoning.

arkizzle

Good catch Rat! Thought so :)

spork

“I cringed a couple time at potential safety violations.”

I’m an old man with nothing left to live for – so I live on the edge.

“Otherwise, the three vids plus a parts list and the world will soon be overrun with propeller carts.”

I have posted a parts list, but I tried to call every part out specifically in the vids so they’d stand on their own.

I just posted the URL’s for all 3 vids, but they seem to be held up in purgatory.

ThinAirDesigns

DesPrez @47:
>For frames of reference, yeah, but
>not for where the power is coming from.

Someday perhaps you will come to understand how nonsensical that statement really is.

The very principle of inertial frames of reference says that there is no device nor test that can tell which object is moving and which is still. This obviously also precludes determining which one of the duo is powering the motion.

If the DDWFTTW cart behaves differently on the treadmill than on the street, you have just discovered a way to differentiate inertial frames of reference. Forgive me if I don’t believe you have discovered such a breakthrough.

JB

Chrs

@160:

Missing element: Wind/local air/device/ground. Local air provides the leverage.

Similar energetic proposal to yours: It is impossible to tack upwind.

Takuan

would the wind from the burrito propel you faster than a normal walk?

arkizzle

Napkins: WIN!

Michael C

Here’s a new video staring some more of Terry’s friends. It’s called “Under the ruler 2: the ground moves”:

http://www.youtube.com/watch?v=9Yt4zxYuPzI

Michael

airshowfan

@#47 Desprez: “The power is coming from the treadmill, where as power to turn the wheels geared to a prop on normal ground, the power is coming from momentum that was initially provided by the wind…”

No. The power comes from the KE of the Earth, just like the power that comes from the movement of the teadmill.

Will you PLEASE read this…

http://dwfttw.blogspot.com/#6

(especially the three thought experiments)

… and then come back if you STILL don’t see how the Earth (with air moving relative to it) and the treadmill (with air moving relative to it) are equivalent? Please?

GregLondon

Clayton Anyway, “missed it”? Missed what, exactly?

the thread has been idle a couple weeks or so. The only reason I posted the newtonian physics paper here was because I had to relearn newtonian physics to figure out the propeller cart in this thread.

Foetusnail when do you sleep, or eat for that matter?

It’s a bit of a problem, I’ll admit.

spork

“The way it works all has to do with the inner and outer radius of the spool.”

Well shoot! That would have saved us thousands of posts across scores of threads on tens of forums. I wish you’d spoken up sooner.

spork

“The wind is not directly pushing against the cart. It is pushing against the wind being propelled backward from the cart.”

This is a common way to look at it, but not really accurate. In fact the propeller is simply acting in a medium that is moving aft more slowly than the road beneath the tires. This provides the necessary advantage. But there is no convergence of air-flows behind the cart. This could be shown with tell-tales. The prop behaves exactly like the prop of a plane taxiing down the runway (when it goes faster than the wind), or sitting still on the tarmac (when the cart is going exactly the speed of the wind).

drm1974

The treadmill demonstration, I think, is conclusive. And this wonderful ruler vehicle helps explain why DDFTTW is possible. The most interesting thing about this is why DDFTTW is so unintuitive to our dysfunctional brains. I’ve tried to explain the reason here:

http://david-royal-martin.blogspot.com/2008/12/directly-downwind-faster-than-wind.html

lava

this is just like the jet on the treadmill runway question

TwoShort

A vehicle is embedded in medium A near it’s interface with medium B, which is moving relative to A, and the vehicle uses some of the energy from that motion to move relative to A.

When A is water and B is air, the vehicle is a sailboat and nobody is surprised it can move relative to the the water.

When A is air and B is dirt, their intuition fails, but it’s no different. The vehicle (pretend it’s a blimp if that helps), sits their in the air mass, reaches out a little wheel to touch the ground whipping by and gather up some energy. It uses that to move through the air. That motion added to the motion of the air mass it is floating along in equals DWFTTW.

spork

“this is just like the jet on the treadmill runway question”

You mean in that it’s annoying?

FutureNerd

A nice demo might be to build the cart with a big, and hard, screw-shaped propeller. One with a single helix (instead of two blades) that goes a full 360 degrees. Then, get a stick with a little wheel on the end and push on the helix from behind.

Spork, have you thought about a long plexiglass wind tunnel? Also, instead of streamers maybe a bubble blower? here’s one.

Beelzebuddy

Oh, my stars and garters. How does this topic deserve four friggin’ threads, when my elegantly worded theses on darksucker theory, square-Earth and the insect-men living among us get tossed?

This video is a great demonstration of gear ratios. Unfortunately, that’s ALL it is. See, what Bloggy McYoutube isn’t telling us is that the “twice as fast as the ruler” speed isn’t exactly free. It’s kind of like stepping down a few gears on your bike: each rotation of your pedals moves you farther, but it’s much more difficult to pedal the thing. Similarly, it takes twice as much force to turn the big wheel as it would just to push the cart. Bloggy pretends the extra force is free; it isn’t. Any constant force (say, the wind) will only turn the big wheel half as far, but it’ll move the cart twice as far as that, leading to a net gain of precisely bubkis. This is why stuffed monkeys are considered unable to perform adequate peer review.

As for the wind-cart itself, I will echo Takuan’s “put up or shut up.” You’ve all made it abundantly clear that reason alone won’t sway you, so perhaps the salty tang of negative results will do the job. Besides, self-experimentation is the most prominent scar on the face of SCIENCE! Go out and make ol’ Victor proud.

Anonymous

I read this entire thread to see if there was a single person who, after claiming it was impossible (precisely my initial intuition) then followed up to say “Sorry, I was wrong.”

It would appear not. Which is sad, given the extreme effort people are putting in to explain this (nifty!) thought experiment, all the way to the building real experimental examples.

People suck. And the ability to defend one’s first intuition as if it were immovable truth is saddening.

Anyhow, I just wanted to say wow – I kinda get it now! Thanks for showing how my initial theories were wrong, and filling me in. I really appreciate the patience and detail people, especially Spork and Airshow, have put into explaining this in a whole bunch of different ways.

I love learning why I’m wrong! Nothing quite so thrilling as a chance to learn something new!

Antinous

Anonymous @ on top of me,

This is the fourth thread on this subject and some commenters have already grovelled in mortification. But, yes, being wrong is, in the long run, more interesting than being right.

ratwerks

David Martin @ 122

While you are correct that the treadmill demonstrations are conclusive, I believe your focus on the wheels as flywheels is a little off target. This cart would still work exactly as demonstrated if you could some how build massless wheels and maintain their friction with the ground. The amount of energy stored by the cart is incidental to its working theory, just like the internal friction of its moving parts. All that function requires is that the propeller’s travel through the air be properly constrained by the cart’s movement over the ground.

It fascinates me how many roads people take to understanding in this exercise. But some of those roads rely on false assumptions and should perhaps be relayed through the territories of the true so as not to lead others astray.

GregLondon

If you want to make your own cart, here’s where to start: Spork has made a series of videos that show how to do just that.

Are these new? Or old videos reposted?

Beelzebuddy

Nah, I’m sorry. It still sounds like you all are trying your best to talk around the fact that you don’t actually know how it works. I’ll reserve judgment for now in hopes that maybe by the sixth thread or so, you’ll have a more cohesive (and less analogy-based) explanation.

airshowfan

This is not as ambiguous as the airplane on a treadmill question. At least I’m fairly sure it’s not. Unlike the airplane thing, in this case when people misunderstand the DWFTTW claims, the DWFTTW people go “No, no, you’re not imagining it right, it’s like this”…

And Spork, regarding the prop pushing on the air and where the energy comes from and whatnot… I really think that the people who say “But the treadmill is supplying power, and when you’re moving along with the wind, what is supplying power other than the momentum of the cart?” will not be satisfied until you tell them that, from the point of view of the wind, i.e. from a balloon, i.e. from the frame of reference that is equivalent to standing in a room with a treadmill… the EARTH is providing power. Relative to the air, the Earth is going by underneath you at slightly slower speeds as the cart travels (since the cart is slowly bringing the air and the Earth to the same speed; It acts like a big air-brake for the earth, just like a wind turbine does). So just as the treadmill supplies energy, the Earth supplies energy. Except that the treadmill keeps making more from electricity using a motor, while the Earth has a huge store of KE and you don’t really feel it when some is taken by a little cart.

A more detailed explanation is on that Blogspot page I made (now updated to say “four times” ;] ).

Ito Kagehisa

If NausicaÃ¤ and I were traveling at the speed of the wind, and I released a device from the rear of our vehicle with a backward velocity, so that said device were still traveling forward but not as fast as the wind, and this device were capable of capturing some fraction of the wind’s energy and transmitting it, perhaps electrically, through the rapidly unwinding tether connecting it back to our vehicle, and I used that power to increase the speed of my fans, then jettisoned said device (or caused it to fold into a tiny needle and pulled it back in) would we be able to travel faster than the wind with wind power? Why not?

Energy can be gathered, and wind energy and air resistance are strongly affected by surface areas and configuration. It seems that games could be played…

GregLondon

This might be the shortest explanation of the propeller cart yet:

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

“it is also possible to trade a large input force through a small distance for a small output force acting through a large distance. This is also useful if what you want to achieve is a higher velocity.”

And it even shows the basic machines in play, a wheel and axle (wheel with pulley and belt) and a screw (propeller), among others.

ratwerks

Beelzebuddy @ 61

How much more put up do you want than:

http://www.youtube.com/user/spork33

Maybe you could build one yourself and enjoy the sweet sweet taste of positive results?

OpSiN

Well, I called the plane on a treadmill instantly. And this one I see no issue about either.

At the point the vehicle is travelling faster than the wind, the only available wind will be coming from the other direction, becoming a headwind. At which point, unless we’re talking about a reversible propeller, it will start to slow the vehicle.

The video above only works gearing wise because the source of power is never going to reverse. And when it does, the cart travels the opposite direction.

I call it impossible.

FutureNerd

Another thought experiment.

Run the cart with a sail for a minute. Gear a generator to the wheel and charge a battery. The most efficient speed is about 1/2 of wind speed.

Now fold up the sail and use the battery to run an electric motor that turns the wheel. Let’s say that with the sail folded, the cart is very lightweight, streamlined and efficient, and averages 2 x wind speed for a minute before the stored battery charge runs out.

Repeat the cycle indefinitely. Average speed 1.5 x wind speed. Is it plausible?

airshowfan

@#61 Beelzebuddy; As I explained briefly in my comment 62 and more at length here, the energy in a DWFTTW cart is not free. Just as the ruler poses some resistance to the hand pulling it (so that hand must input energy) and just as the treadmill-tested DWFTTW carts pose a resistance to the treadmill’s rolling (so that the treadmill must input energy), a DWFTTW cart running outside would (from the point of view of the wind, e.g. as a balloonist sees it) take kinetic energy from the moving Earth, since there is resistance by the cart to the Earth’s movement towards the back of the cart, as if the cart were an air brake. So the Earth slows down. There’s your energy.

darue

ok, what if there’s no wind and it’s pulled… it would resist being pulled or?

slamorte

all,

assume for a moment that the wind cannot push a sail, if that sail is going faster than the wind. now imagine you could make a sailboat with a kind of battery-powered paddle-wheel looking device with dozens of little sails on it, and that you could spin that paddle-wheel backward. now the wind has something slow to push against, and your sailboat could go wind-speed + paddle-wheel-speed. you now have DWFTTW sailing and it violated no laws of physics.

now instead of a battery-powered paddle-wheel, make it a paddle-wheel that is powered by the wheels spinning on the ground. for every x-revolution of the ground wheels, the paddle-wheel makes 2x turns backwards.

with this system, you can STILL have wind-powered DWFTTW only if the follow condition is true: will you capture more energy from the wind hitting the paddle-wheel than you expend spinning the paddle-wheel with the ground wheels?

the final answer is “yes.” the cost of spinning the paddle-wheel is less than the wind hitting the paddles, and the cart accelerates, up to a max speed limited by inefficiencies and frictions in the system.

in fact, looking at is this way, the faster the cart goes, the easier it becomes to spin the paddle wheel, since there’s less and less of a tailwind to fight against as you speed up.

all that’s left to do is change out our very inefficient paddle-wheel for a well-engineered propeller.

@ spork

thanks for the correction regarding my definition of the spinning prop. what i was trying to avoid was the worse analogy of “the wind pushes against the buffer of slower wind from the prop.”

morehumanthanhuman

CHRS I am probably using a higher ratio, as in a wheel that is a bit larger. I tried using lego pulleys with and without tires, slides either way, slides less without tires.

TwoShort

“this is just like the jet on the treadmill runway question”

Nah, that describes a nonsensical question; people come up with different ways of pretending it makes sense, and hence different answers. Then they assume the people with different answers are idiots. The people who actually understand go away, because nansense questions are not interesting.

This one is a perfectly reasonable question. It has a real answer. Some people have come up with the wrong answer, which is reasonable because it’s tricky. Unfortunately, many of them assume people with the other answer are idiots and don’t even try to understand their explanations or hear their reports of positive experimental results.

For Example:

I have personally gone downwind faster than the wind in a wind powered vehicle.

FutureNerd

Doh! Late for the DWFTTW regatta!

Michael C

Look, Terry and friends are on http://www.bestofyoutube.com/

I’m proud of them!

spork

My links have cleared customs and now appear at #230

arkizzle

And in tonight’s forth performance of DWFTTW, GregLondon will be played by Beelzebuddy.

ldcroberts

I read the iceboat example and it did make me think. If you have a sail against the wind and you are going diagonally across the wind there appears to be two possibilities. In one, the sail is pushed by the wind, giving you power in on direction, which you are exploiting to get more horizontal speed. In the other, the sail could theoretically be pushed by the relative wind, which also gives you power. The relative wind in this second example is in the opposite direction to the wind observed by a stationary observer if you are traveling faster downwind than the wind. So provided you had existing speed you could use the winds relative speed differential to capture energy.

Cragsavage

Someone should give this guy a science program for kids.

And then let me know when it’s on so I can watch it religiously.

Beelzebuddy

Ratwerks, I’m a little confused why you’d claim it works, above, yet link me to someone with a video who claims it doesn’t. You understand that I think it’s hooey, right?

Airshowfan: the rotation of the Earth? Seriously? What if it’s moving east-west, where moving faster than the wind would actually speed the Earth up? What about a north-south orientation, eliminating rotation altogether?

Actually, I think I see something worth discussing, as a sort of mini-problem en route to the correct answer. As Twoshort says, iceboats and sailboats can tack faster than the wind (but not directly downwind). They can also go directly downwind (but not faster than the wind). They can build up speed by tacking and then turn downwind, going faster than the wind for a short period of time (but not on average). Is there anyone here who thinks a system can be rigged up by which a sailboat can *sustainably* move downwind faster than the wind? How?

GregLondon

OK, so, after having to re-learn a lot of the newtonian physics stuff that I’d forgotten since college, I decided to write up an introduction to newtonian phyics that is intended for people who learned it once and since forgot it, or need to learn it for the first time, and would like to get all the information in a single spot.

It covers everything from Newton’s three laws of motion, conservation of momentum, conservation of energy, inertia, force accelerating a mass, work, energy, power, and then takes all these concepts and shows how they can be applied to solving collision problems.

It’s a bit of a work in progress still, but I figured it was ready for some feedback. If anyone wants to give it a read, and help flush out any typos or confusing or incorrect bits, you can read it here:

http://www.greglondon.com/newton/

It’s about 50 pages to cover the basics. It’s licensed CC-attribution-sharealike.

Enjoy.

infraspace

One thing the doubters seem to be doing is assuming that the wind is like a solid object pushing from behind. If the cart were to outrun the wind in this case, it would lose it’s energy source. In reality the wind is all around the cart at all times. Even though the cart is travelling faster than the wind, it is still WITHIN the wind and “touching” it.

ratwerks

Michael C,

You must stop. Each of your videos is more marvelous than the one before, if you continue you will violate laws of nature.

ratwerks

Beelzebuddy @ 124

One reason you may be getting more snark than you prefer is that your arguments have all been made and answered several times. Not your fault, but true.

I do not expect “our” argument to ever cohere because “we” are not in any way organized. There is no test to pass, there is no quality control, there is no governing body. “We” are merely a random collection of relative strangers who believe that this can work. Many of us who believe it can work are confused about how it can work but nevertheless eager to explain. There are a handful of believers who do understand how it works and who have provided clear and complete explanations. The problem for you is how to sort the wheat from the chaff. The signal to noise ratio on all threads that I’ve followed has been pretty ugly, with plenty of noise coming from all sides.

If you start with the question of “how can this work” rather than “how can this be shown impossible” you’ll find your own way. The phenomenon is counterintuitive but does not require more than a basic understanding of how things work.

Beelzebuddy

(sorry for the double post)

I have personally gone downwind faster than the wind in a wind powered vehicle.

But not on average. Not sustainably. There’re many ways of storing wind power, from a flywheel to a generator and a battery, and releasing it later to go faster than the wind. But it takes more time to charge your boost up than the speed you get from it. You have to tack for longer than you can coast faster than the wind.

tosscandy

The key weakness in the pro-DWFTTW argument is the energy extraction from the ground. They claim the ground applies a force to the wheel which generates torque to turn the prop. The ground force on the wheel bottom would be opposite the direction of travel. The cart exerts a force on the wheel axle in the direction of travel. Ah ha! What exerts the force in the direction of travel of the cart? THE PROP THRUST or a LOSS in velocity of the cart! This cute system in NO WAY harnesses energy from the wind/ground velocity difference. It does lose energy in the prop and via mechanical friction, so in the “real world” this DWFTTW cart would be a dismal failure.

lukus

What if the vehicle is dropped off the edge of a cliff?

tosscandy

“PROPS” to BEELZEBUDDY for bringing rational thought & discourse to this thread.

airshowfan

Beelzebuddy, the previous topics discuss the boats thing thoroughly. That’s the one thing I have not added to my Blogspot page in details, but sail devices CAN move downwind faster than the wind. They do move at an angle, but the downwind component of their velocity (what sail-people apparently call VMG, “velocity made good”) is greater than wind speed. In other words, in a downwind race, they could beat a balloon. The previous posts have links to stats on this, and soon (maybe tomorrow) I’ll gather those links and add them to the end of my Blogspot page.

And no, not the ROTATION of the Earth. The SPEED of the Earth as it goes by under a vehicle that is moving with the wind. So, not the movement of the earth as seen by a very distant astronaut, but the movement of the Earth as seen by a balloonist. Think about it. Take a wind turbine. A wind turbine doesn’t just “slow down the wind and convert some of the air’s kinetic energy to electricity”, it brings the Earth closer to the local wind speed. Just as the wind turbine pulls on the wind, the wind pulls on the wind turbine, so the wind turbine is like a big air brake. Just as a wind turbine brings the whole Earth’s movement just a little closer to the local wind speed (i.e. “brakes” the earth by pulling on the wind), so does the DWFTTW cart.

Again, this is explained on the Blogspot page with more detail and more analogies (like a Prius driving downwind and then hitting the brakes/generators… Pretty please, do check it out).

tosscandy

AIRSHOWFAN is pimping that blogspot page hard. LOL. I wonder why.

ratwerks

Opsin @ 171,

So you must’ve claimed the POTR would never fly? Cuz, really, this is JUST like that! There’s, like, a TREADMILL in some of these demos?!

More seriously, MichaelC’s videos are meant to answer but one of many mistaken objections and does so with elegance, wit and charm. A device of transparent simplicity is shown to outrun its power source. The ruler here is entirely analogous to the wind in the dozens of spork/thinairdesigns videos. Both devices require energy and both exploit a speed difference between two media by coupling those media to provide that energy.

Not impossible, not even unlikely. Surprising for sure.

Anonymous

Anyone who’s ever gone landsailing or windsurfing knows that you CAN move faster than the downwind windspeed…just not in the same direction as the wind.

Bloodboiler

Nice brain teaser, but nothing to do with impossible wind powered carts.

airshowfan

Okay, I’m back.

Beelzebuddy, I’m not sure why you say that the people explaining how it works don’t understand why it works. I don’t know what kind of answer would be satisfactory. Here’s the most thorough one I can think of, and which I’ve written before:

From the point of view of the cart (imagine you are riding the cart), you have the ground going by under you at a good speed, and a slight breeze blowing in your face. The ground pulls back on the wheels, trying to slow the cart down, while the propeller pushes the whole thing forwards, trying to speed it up. As long as the forwards thrust of the propeller is greater than the backwards pull on the wheels, the cart will not slow down. And because the air and the ground are going by at different speeds, this is not perpetual motion. Since the ground is going by very fast, the wheel can produce a lot of power without a big slowing-down force. And because the air is going by very slowly, the propeller does not need a lot of power to generate a big thrust force. If you had no wind, then you’re right, it would be impossible.

Here’s a balance of the power (energy per second) and forces involved:

Six parameters are key to understanding it.

- The force exherted by the prop pulling the cart forwards (Prop Thrust);

- The force exherted by the wheels pulling the cart backwards (Wheel Resistance);

- The work done (energy per second) by the prop to pull the cart (Prop Power);

- The work received (energy per second) by the wheels as they are spun by the ground (Wheel Power);

- The speed of the ground going by, which is high (Ground Speed); and

- The speed of the air going by, which is low (AirSpeed).

(And yes, there are two different AirSpeeds; the one in the air overall, and the one in the air being affected by the prop, which is pushed back a little faster than the general slipstream. You can use either one. For the purposes of this analysis, that doesn’t really matter).

Power (energy per second) is force times speed. (For example, if an F-22 requires 320 kiloNewtons of thrust to go at MACH 2 (about 650 meters per second), then this requires just over 200 megaWatts of power (energy expended per second), since in this case “force times speed” is 320,000 times 650, giving a power of 208,000,000, give or take).

In order for this to be a DWFTTW craft, the Prop Thrust must be greater than the Wheel Resistance (at least when going at wind speed, maybe not when going significantly faster).

In order for this to NOT be a perpetual motion machine, the Prop Power must be lower than the Wheel Power, since some of the energy provided by the wheels is lost to friction in the gears and other inefficiencies.

To simplify;

Prop Power = AirSpeed x Prop Thrust

Wheel Power = Ground Speed x Wheel Resistance

So how can Prop Power be less than Wheel Power even though Prop Thrust is more than Wheel Resistance?

Simple! AirSpeed must be smaller than Ground Speed.

Think about it.

Say Ground Speed is a huge number (i.e. strong wind) and AirSpeed is a tiny number (i.e. the cart is going barely faster than the wind). This means that Wheel Power can be quite large, and Prop Power can be quite small, with Prop Thrust still being a little greater than Wheel Resistance.

This means that you don’t need to make energy out of nowhere in order to get a cart to propell itself faster than the wind.

It DOES mean that you need SOME wind. If AirSpeed equals Ground Speed (and if Wheel Power is greater than Prop Power, i.e. no perpetual motion), there’s no way to make Prop Thrust bigger than Wheel Force.

The faster the wind, the better. The more wind you have, the more different AirSpeed and Ground Speed will be. And the more different they are, the bigger Wheel Power can be relative to Prop Power while still allowing Prop Thrust to be bigger than Wheel Resistance. If power is force times speed, then the greater speed difference you have (between the air and the ground, i.e. the cart’s Ground Speed), the easier it will be to multiply a low Wheel Resistance by a big number and get lots of power which can then give lots of thrust given the low AirSpeed relative to the cart.

It may not be easy to juggle six variables at once. But try it for a little while. Remember, the Prop Thrust has to be greater than the Wheel Resistance (so that the cart can accelerate past wind speed) and the Wheel Power has to be greater than the Prop Power (otherwise it’s a perpetual motion machine). How can that be done? The answer is, with a low AirSpeed (which means the cart is moving barely faster than the wind) and a high Ground Speed (which means a strong wind, so the cart is going fast to keep up with it).

If you want some numbers to go along with it, check out Mender’s comment here.

And all the text above is, of course, from here. Which I bet you haven’t read yet.

So why, exactly, would you say that I don’t know how this works? What do you want, a vector analysis of a DWFTTW ice-boat and a vector analysis of a DWFTTW cart’s propeller blades, looking at how they interact with the air around them and with the vehicle they’re on, showing how the math is the same? I guess I could try to make one. Let’s see how much free time I end up having today. I have to admit that “evangelizing” DWFTTW is an addictively fun challenge.

spork

“And Spork… I really think that the people who say … will not be satisfied until…”

Well you’re right of course, but I can assure you even that explanation won’t satisfy a lot of them. JB is working that angle right now on another blog with little success.

Not wanting to understand is a pretty powerful force.

GregLondon

Another revision.

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

a table of contents, a lot of stuff got rearranged, hopefully the order is better and requires fewer intuitive leaps.

I gotta stop uploading at 2 am.

ugh.

spork

“They can build up speed by tacking and then turn downwind, going faster than the wind for a short period of time (but not on average).”

Ice boats can maintain steady state downwind courses such that their downwind velocity component is 3X to 4X the wind speed.

Here you go – vector analyses, GPS plots, and everything you need:

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

The ice boat racers really get a giggle out of this silly question.

“Is there anyone here who thinks a system can be rigged up by which a sailboat can *sustainably* move downwind faster than the wind? How?”

Click on that link.

Then if you want to go DIRECTLY downwind faster than the wind, just put two of those ice boats side-by-side on alternating tacks with a telescoping pole between them. Sit yourself in the middle of that pole and go directly downwind at 3X the wind speed…. or just continue poking fun at people that understand basic physics.

TwoShort

Beelzebuddy:
Yes on average. Yes sustainably. An ice boat sailing on a broad reach can go fast enough that just the component of it’s motion that is in the direction of the wind is greater than the wind speed. You can start from a dead stop and get downwind faster than the streamer floating along at the wind speed. Much faster. You’re not gaining speed and then coasting, you’re maintaining a constant heading.

You sail at about 45 degrees from straight downwind. Say the wind is 10 knots. Because runners provide almost no resistance to forward motion, the boat quickly starts moving 10 knots downwind. But the runners constrain it to that 45 degree heading, so it must also move 10 knots across the wind (that’s 14.142 knots forward, but let’s keep thinking of it as the two separate component vectors). Now think of yourself sitting there in the boat. Your motion downwind means you’re not moving relative to the air at all in that direction, but your motion across the air mass means you’ve got 10 knots of wind going past your sail. So you pull in the sheet and accelerate. Now you’re going forward faster than 14.142 knots, which means you’re going downwind faster than 10 knots.
Now, runners aren’t actually frictionless, so you can’t go infinitely fast. But a good sailor in a good boat can go forward at 4-5 times the wind speed, for windward-component-of-motion of about 3 times the wind speed.
It really does work and it’s not a minor effect in any sense; it’s quite dramatic. (and fun)

slamorte

ok, i think i got it. i have a slighter better telling of the balloon comment above.

imaging you are in a hot air balloon (i imagine a steampunk balloon with lots of brass, but ymmv). assume you are traveling 1 foot off the ground, at 10 mph relative to the earth, blown by the wind. let’s say you want to go faster. you happen to have invented a big propeller that is hooked to some gears which is hooked to a rubber wheel. you lower the wheel to the earth, which is running under you at 10 mph relative to you. your little wheel catches the earth and spins your propeller, which provides a few pounds more thrust! you are now moving slightly faster than the wind.

then the gears, like my analogy, break down. parts fly off in the balloon, shearing off my sanity, and i am left stabbing my little drawing of cory doctorow in a cape, and pulling my hair out. because i can’t figure out *what is pushing the cart!*

Mender

The ultimate unmovable object.

tosscandy, he’s doing that so people can get past the usual “can’t work because I say so” comments that come from “I haven’t put much thought into it but I think I’m smarter than everyone so I must be right” people.

GregLondon

No testing required if you understand things as well as you do, right Greg?

Well, if you and spork want to live in the past, here is a little nugget from some guy named my_wan the Randi forum from November of 2008:

“I defended the idea on physics forums but unfortunately spork (the OP poster) wouldn’t refrain from rants about being attacked and the thread got locked twice”

Spork got two threads locked ranting (to use the words from someone trying to defend him) about this just recently. And you guys want to bust my chops? that’s pretty funny.

airshowfan

@#73;

Because that page compiles the answers to the questions and misconceptions that keep coming up in this debate, that’s why.

Honest.

I promise I will not later redirect the Google-juice towards my own website or anything like that. (Not that I think that Blogspot page is getting any Google juice at all anyways).

What kinds of motives are you implying?

Ok, I’m going to sleep. I’m sure there will be more DWFTTW fun to be had later…

airshowfan

Spork;

Since the one thing that is missing on my Blogspot page is more info on how ice boats do it… Are there any online resources you’d recommend about ice boats (or any other vehicle that can pull this off) with a VMG greater than one? Theory of how they do it, accounts and data of them doing it, etc. I’d really appreciate it if you could help me gather some.

So far (thanks to you) I have

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

and

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

which are excellent. Do you know of any more good stuff like that?

And I think you also mentioned some forums we should go check out, with comments by the people who make and operate these vehicles. Where exactly are those forums? (or webpages or whatever).

Thanks.

TwoShort

Spork: Thanks for the link; wish I’d seen it before I typed up my post.

Man, I gotta get the DN running again. Well, after I move back to somewhere with ice and not snow.

ldcroberts

Ok I’ve sketched a wind based version of the ruler example above, don’t see why it wouldn’t work:
http://www.flickr.com/photos/32410508@N06/3105949643/

Ito Kagehisa

My sailor friends are laughing hysterically at the idea that the wind “pushes” a sailboat, and they say look up Bernoulli’s principle. Pedro the Cruel says the only time the wind pushes the sail, is when you are running dead downwind with a flat sail. Actually, that might not be what he said, I am attempting to translate from a lot of sailorly gibberish involving bags, beams, yards, and something about a bermuda rig in a death roll with 30 degrees of something or other else spinney. I’m not really following it all. OK, now they are singing:

with tar in our pigtails
and blood on our rapiers
we’ll fly the skull and crossbones
by God we’ll take no prisoners!
and its high ho away boys
we’ll sail for Trolley Bay boys
and hoist the Jolly Roger
at the break of day

I must go, they are breaking furniture. Something about a golden leg.

GregLondon

The way it works all has to do with the inner and outer radius of the spool.

Yeah, actually, it’s really as simple as that. I think the propeller and gears obfuscate just how simple this thing really is.

If you start with the question, “is it possible to have a vehicle go downwind faster than the wind?” and then show the yoyo being pulled by a parachute, I think a whole lot more people would immediately get that, yes, it can be done.

If you start with the propeller cart, it’s a lot of complexity that isn’t really needed to explain how a machine can go downwind faster than the wind. And a lot of people wonder if it isn’t inertia or something else going on.

To me, that’s the problem of demonstrations. In 1770, the Turk, a chess playing machine, was demonstrated to the public. It was demonstrated repeatedly for years after that. It wasn’t until 1857 that it was explained to be an elaborate hoax.

Demonstrations are not sufficient. People don’t believe this works because they don’t see how it could be possible, and they know there are plenty of ways to fake a demonstration. If people can understand how it might be possible, then all that’s left is figuring out how to implement it.

It really is as simple as the difference between the inner and outer radius of the spool.

Guysmiley

We’re using a ruler along the top of a wheel as an analog for wind?

Wow.

The point about moving FASTER THAN THE RULER (ooooh aaaaah) is due to something called gear ratios. Specifically in this case the ratio of the size of the big wheel to the small wheels. It takes just as much work (of the physics definition of work) to move the car with the ruler as it would to push it from the back with your finger. You actually lose some efficiency due to the friction of that center wheel.

This has nothing to do with wind at all. Fallacious argument is fallacious.

dragonfrog

Beelzebuddy – first I want to confirm that you do now concede DWFTTW is possible, as evidenced by the fact that iceboats and some ridiculously impractical sailing skiffs regularly achieve it.

The question is merely whether the described propeller cart is capable of embodying the same mechanics, when its outward appearance is different.

Sez Spork @ 100

The iceboat uses it’s motion over the ground to moves its airfoil (the sail) side ways through the air by moving the whole boat. The cart uses it’s motion over the ground to move its airfoils (the prop blades) sideways through the air by spinning the prop.

Sez Beelzebuddy @101

That’s not the same at all. Iceboats would work best if the surface were totally frictionless and only used as a guide to keep the boat pointed in the same direction. Propeller carts wouldn’t work at all in a frictionless environment – they depend on friction to drive the propeller to overcome the loss due to friction. Aside from both involving touching the ground, they seem pretty different to me.

You explain why it’s exactly the same, and miss the point.

Iceboats would work best if the surface they ran on were completely frictionless in the direction of the skates, and had effectively infinite friction perpendicular to the direction of the skates.

That is, the perfect iceboat would harness all the available power from the difference between ground and wind speeds, and lose none of it through friction that would prevent it turning that captured power into movement of the airfoil. The reason they go so blazingly fast is that skates get surprisingly close to that property.

The cart works exactly the same. The perfect cart would capture all the available power from the same speed difference (the tires would have perfect traction, never slipping relative to the ground), and lose none of that power on the way to turning it into movement of the airfoil (the machinery that turns rotation of the axle into rotation of the propeller would be perfectly efficient).

Beelzebuddy

So you pull in the sheet and accelerate.

If you don’t mind, what’s meant by this maneuver? I’m afraid the only nautical nonsense I’m familiar with comes from Spongebob.

I still don’t see it – if the wind is coming at N knots, and you’re 45 degrees to it moving at N*sqrt(2) knots (such that your downwind speed is N knots as well), the drag component that faces directly upwind should be N. So your speed should level off regardless of friction, no?

mikeen

Nice…
But actually the ruler moves to the left relative to the center of the blue wheel (the cart is faster than the ruler so the ruler moves to the left relative to the cart). Which naturally makes the blue wheel go counter-clockwise and turns the rollers clockwise => the cart goes to the right.
The Blue Wheel Experiment shows an example of how gears may work.

A winddriven cart can move faster than the relative windspeed – just like some sailingboats can. The aerodynamic forces around a sail, propellor- or wing-surface are so strong – its the same forces that keep aeroplanes in the air and race cars on the ground – that they can pull optimally designed vehicles with amazing speed.

To clarify (and (over)simplify):
There are two propulsion sources at work here: The wind and the propellor. The propellor functions roughly like a sail and is not driven round by the wind but by the wheels.
If the cart moves downwind the applied wind-force on the cart will aproach zero as the speed of the cart approaches that of the wind. But as the speed increases, the wheels move faster and ad propellor-thrust to the cart through the gear-system. This is a complex interacting system but what we get is essentially a gearing of the applied wind force, enabling the systems topspeed to be higher than the relative air speed. Just as it does (in a different way) for sailing boats that tack.
It is not perpetual motion in any way. The cart will stop accelerating at some certain point and when the wind dies off, the cart will slow down and stop.
Its just a complex mechanical system that optimizes the use of wind power in this specific context.
Essentially what happens may be better understood if one were to see the whole thing as a hybrid drivechain. If the propellor was changed to some sort of sail-like surface, and the wheels where connected to an efficient charger-battery-motor combination it becomes fairly obvious that such a combination would be able to keep moving for some limited time after the wind had stopped.

Takuan

A capital ship for an ocean trip
Was the Walloping Window Blind.
No gale that blew dismayed her crew
Or troubled the captains mind.

The man at the wheel was taught to feel
Contempt for the wildest blow.
And it often appeared when the weather had cleared
That he’d been in his bunk below.

The boatswain’s mate was very sedate,
Yet fond of amusement too;
And he played hopscotch with the starboard watch
While the captain tickled the crew.

And the gunner we had was apparently mad
For he stood on the cannons tail,
And fired salutes in the captains boots
In the teeth of a booming gale.
The captain sat in a commodore’s hat
And dined in a royal way
On toasted pigs and pickles and figs
And gummery bread each day.

But the rest of us ate from an odious plate
For the food that was given the crew
Was a number of tons of hot cross buns
chopped up with sugar and glue.

We all felt ill as mariners will
On a diet that’s cheap and rude,
And the poop deck shook when we dipped the cook
In a tub of his gluesome food.

Then nauticle pride we laid aside,
And we cast the vessel ashore
On the Gulliby Isles, where the Poohpooh smiles
And the Anagzanders roar.

Composed of sand was that favored land
And trimmed in cinnamon straws;
And pink and blue was the pleasing hue
Of the Tickletoeteasers claws.

We climbed to the edge of a sandy ledge
And soared with the whistling bee,
And we only stopped at four o’clock
For a pot of cinnamon tea.

From dawn to dark, on rubagub bark
We fed, till we all had grown
Uncommonly thin. Then a boat blew in
On a wind from the torriby zone.

She was stubby and square, but we didn’t much care,
And we cheerily put to sea.
We plotted a course for the Land of Blue Horse,
Due west ‘cross the Peppermint Sea.

FutureNerd

I wonder whether most people would agree that with a different gear ratio, this thing will go upwind? Upwind without tacking sounds like an almost practical application… especially for the wind-powered car I’m building– I hate having to cut back and forth across traffic like I did with the previous prototype.

ratwerks

Beelzebuddy @ 68

It would be better if you’d read more and write less. I understand you think DWFTTW is hooey. I understand that it is not and I understand how it works. The link I provided ( http://www.youtube.com/user/spork33 ) is to a collection of video’s made by two people who set out to demonstrate a working model and succeeded in doing so. They’ve got a bunch of videos there including videos of _other_ people’s ideas that don’t work.

Start with:

http://www.youtube.com/watch?v=xHsXcHoJu-A

to see their sort of summary video.

Really, these guys have built a bunch of working models and helped a bunch of other people build working models and spent countless hours explaining how it works. There is no claim of perpetual motion. Hell, there is no claim of practical usefulness of any kind.

spork

“I wonder whether most people would agree that with a different gear ratio, this thing will go upwind?”

I know there are several people across the different forums that seem very well versed in how and why this thing works as it does. I’m confident they would agree that lower gearing to the wheels will produce an upwind cart. Perhaps I’ll put some small wheels on my cart and demonstrate this.

ratwerks

Mark Fraunfelder @ 1 – 4 topics

Why do you keep posting this without any sort of summary at the top? I find it a bit disheartening to see exactly the same misconceptions dragged out anew each time. I guess, to look at it another way, I also see a whole fresh group of people on their way to that ‘ah-ha’ moment when they see how it can work.

badger500

@ airshowfan # 187,

“Doesn’t #132 answer all those questions to your satisfaction?”

You’re right! It should! But I am still a little shaky, bear with me… You said up there:

“Prop Power = AirSpeed x Prop Thrust

Wheel Power = Ground Speed x Wheel Resistance

So how can Prop Power be less than Wheel Power even though Prop Thrust is more than Wheel Resistance?

Simple! AirSpeed must be smaller than Ground Speed.”

Why is Prop Power = Airspeed x Prop Thrust? You said power = speed x force, but why is it the speed of the air around the prop and not, say, the speed of the blades moving, or the speed of the air that is accelerated by the prop?

Also, why is wheel Power = Ground Speed x Wheel Resistance? I can understand why the speed comes in, but why the resistance? By this, do you mean the friction of the wheels against the ground? So with a higher friction you have more power? Why?

I know these are incredibly dopey questions, but I might as well understand it and you guys have been supernaturally patient with us. I’m closing in on it, thanks.

slamorte

@83 guy, i don’t think it’s fallacious at all.

the system is counter-intuitive for many reasons, one of which is that we are used to thinking of wheels as something you give power to, and get propulsion from. here it’s reversed, i think; though i’m only about 80% sure dwfttw is possible at this point. i still don’t understanding it completely because i can’t make a good analogy, but i am close. that math proves out, but it violates our intuition.

try this: is it possible to make a cart that is powered by a conveyer belt, such that the cart runs faster than the dead air around it? yes it is: make the wheels of the cart power a propeller that spins, and the cart will accelerate relative to the air. the belt powers the prop.

now take the two-belt system in the video above. by changing the gears of the wheels that contact those surfaces, you can change the direction the cart/gear combo moves. you take energy from the differential, and make your cart move. nothing wrong with that so far.

the wind really is just another conveyer belt, albeit in another form. there is one surface (wind, upper belt) that is moving relative to another surface (earth, lower belt). the air is a gas instead of a solid, and has very different surface properties, but it is still a conveyer. a propeller in this case is just a kind of wheel that is used to make contact with the conveyer that is the wind. different tool for a different surface.

airshowfan

In short: There is a difference between static friction and kinetic friction. Duh. ;)

badger500

My attempt at an analogy-less explanation, in 3 steps (and then I have some questions for spork or JB)…

Assume cart on open road, 10mph wind.

1. Wind pushes on prop surfaces from behind. This attempts to cause 2 effects:
a) Force on prop shoves the cart forward.
b) Prop spins such as to retard the wheels.

2. But (a) dominates because of a gear ratio that makes it much easier for the wheels to turn the prop than for the prop to turn the wheels.

3. And so the cart is shoved forward, and as it does, the spinning wheels turn the prop so as to produce thrust, which works to move the cart forward even faster.

So, spork, JB, others: is that about right?

If so, where some people get confused is, I think, step 3. There are two issues there that are sticky:

Issue 1: Energy Bookkeeping. It seems like that the prop’s thrust will rob the same amount of energy from the wheels, slowing them down such that there will be no net gain in cart speed. I thought this way, too, but now I suspect it is a confusion of terms (speed, force, thrust, work, energy, power…it becomes physics salad!). Can you help me plug in the *right* terms here and show that it is not just shuffling energy around, but that the gearing causes this to work?

Issue 2: Strange Loop. If the turning wheels power the prop, and the prop then creates thrust to speed the car faster, that will cause the wheels to turn faster and power the prop more, which will cause the cart to go faster, etc… There’s a loop, and so it suggests the cart would accelerate constantly, so given a 10mph wind it would eventually get up to 300mph and more. I know that is not what is occurring, so what is the error here?

For what it’s worth, I started off thinking this couldn’t work, now I suspect it does but still can’t fully “grok” it, but would like to. I also respect the time and patience you’ve all put into this. I’ve been following the story on various blogs since Good Math/Bad Math’s recent brouhaha.

Downwind faster than the wind - Part 4

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