Directly downwind faster than the wind - part 3

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

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

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

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


Lack of imagination among wind-cart enthusiasts has prevented them from realizing that a simple modern invention can solve the problem of net forward air flow trying to stop the cart. That invention is--the air duct!

A swivelling duct would be able to take advantage of wind coming from any direction. A vane at the rear of the duct would automatically turn it into the wind. Even on a windless day, the lucky owner of this windmobile would only have to give it a push before leaping aboard, to create some relative air flow that would power up the fan and accelerate the cart. Who could have imagined that the answer to the problem of non-renewable resources could be so simple?

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


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


Three questions for cart enthusiasts:

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

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

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

Answers to (1) and (2) are clearly "yes." Answer to (3) can be determined empirically by blowing air at a small fan, first from the front, then from the back, and watching which way it turns. Answer to (3) will also be "yes."

Therefore, the reversed air flow will retard forward motion, the speed of the cart is self-limiting, and the claim is false.

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

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