Wind powered vehicle nearly 3 times faster than downwind

Beautiful. Absolutely beautiful. Ten minutes of feeling like an idiot for not getting it, then a moment of glory when it finally became clear.

For those who still feel like idiots, here's the post on Autopia that made me a believer:

"Let me get this straight. It is common knowledge that a sailboat can tack at an angle to the wind sufficiently fast to reach a directly downwind point faster than the wind itself. Yes? And yet people actually thought that merely doing this in a straight line would be “impossible”?

Assuming the former is true and commonly accepted, all you have to do is imagine an arbitrarily small (narrow) “zig zag” tacking pattern — which is roughly the equivalent of your “cylinder earth” thought experiment. If the zig-zag is small enough to be contained with in the width of your “boat”, that’s all you need, in principle.

The rest is just engineering (and using a propeller is much better than a zig-zagging mechanical device of some kind). I had never heard of this controversy, and halfway through the article I’m thinking “this is just obvious.”"

http://www.wired.com/autopia/2010/08/ddwfttw/2/

Tacking within the width of the boat is the key, right? Now, think about changing the angle of the sail not by swinging it horizontally back and forth, but by TURNING THE SAIL UPSIDE DOWN. In other words, constantly spin the sail at the correct rate, and it's the same as tacking back and forth; and we already know that you can beat the wind that way.

Here is a fun and straightforward demo of the principle behind this:

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

This is an interesting wikipedia article on the subject:

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

In particular, iceboats can sail at up to 5x the speed of the wind, according to this: http://iceboat.org/elements/faq/apparentwind.jpg

Hmm, this might be a good way to look at it (or it could be wildly inaccurate!).

1) Assume for the sake of argument that your friction through the air is the same as your friction of sliding on the ground (this isn't very accurate, and is variable, but shouldn't invalidate the basic thought experiment).

2) instead of the two mediums being air and land, consider them to be two parallel conveyer belts, going in opposite directions (i.e. moving relative to each other).

3) Your vehicle is straddling the two conveyer belts, a set of wheels on each belt.

4) Couple the set of wheels on one belt with the set of wheels on the other belt, along with a transmission to engage/disengage the wheels from each other.

5) Lock the wheels on one belt. This is effectively "going the same speed as the wind".

6) The wheels on the other belt will be spinning furiously. Engage the gears appropriately so that you make headway on the belt that you're relatively stationary on. You're now going 'faster than the wind'!

Ok, it was easier to understand in my head :)

Does 50mph in 20mph wind!

Video in action: http://www.youtube.com/watch?v=5CcgmpBGSCI

(looks like it's right out some post-apocalyptic anime ie. Ergo Proxy)

Related, a similar vehicle travelling directly into the wind:

http://news.ycombinator.com/item?id=1874846

The key point that's missing in this headline is that it attains that speed 'directly downwind'. Going crosswind is easy to attain speeds like this. The confounding 'magic' is that it can do this directly downwind.

As a mind experiment to explain why this seems impossible, think about what happens when the vehicle gets up to exactly the speed of the wind. At that point, there's no net wind pressure on the vehicle (if you were on the vehicle, there would be no apparent wind on your back or on your front), yet the vehicle still continues to accelerate!

You could probably build such a propeller to a boat as well, having the wind drive the underwater propulsion (only with worse efficiency than with rolling wheels, though).

You could build a similar device that travels upwind as well, albeit slower, right? The energy extracted from headwind by the propeller needs to be greater than the energy needed to keep the vehicle stationary against the headwind.

Using a Darrieus or Savonius turbine on the vehicle or boat would allow for more flexible headings without having to point the propeller to headwind direction.

Is it just me, or are other people also annoyed by the incorrect usage of "2.8 times faster than the wind"?

The actual speed record is 27.7 mph in a wind of 10 mph. That's 1.77 times faster than the wind, not 2.77 times faster. (It's impressive and cool as hell; I'm not quibbling with that...)

So the wind pushes the cart forward, which turns the wheels, which spin the propeller, which pushes the cart forward even more - a feedback loop. Eventually various frictions and inefficiencies equal the input energy from the tailwind and you get a stable speed.

To go faster than the tailwind you need a gearing ratio that spins the propeller quite a bit faster than the wheels are turning - leverage.

I couldn't find meaningful explanation of how they do it. The only explanations I could find sound like "Flux capacitor" explanations from "Back to the Future" comedy.

My intermediate conclusion is that it's some sort of hoax.

When faced with actual evidence/proof, some people will still not accept it. If the naysayers would spend $20 like the school kids did and make their own model they could prove/disprove it themselves.

This sound like an experiment that mythbuster could replicate.

The wheels are powering the fan blowing rearward I assume at a 1 to 1 ratio into the wind. The cart requires an initial "push" from the human. The wind pushes both the fan blades and the cart. So the net push from the wind > net drag with the wheels powering the fan. The faster the cart goes, the faster the blades spin, so the cart is being perpetually pushed by the wind on the spinning blades no matter how fast it goes. The spinning blades appear to be stationary from the perspective of the wind.

The blades spinning tax the speed of the vehicle, but the payback is that the spinning blades are stationary relative to the ground.

This cart would not move if the ground was moving at the same speed and direction as the cart. First law of thermodynamics has not been violated, the cart is simply using the energy from the wind on a virtual stationary object to power itself.