A recent article in Make:Online and soon to appear in Make Magazine discusses the successful attempt by Rick Cavallaro to create a wind-powered vehicle that can go faster than the wind. This "land yacht", called the Greenbird, has been verified by the North American Land Sailing Association to have run at 2.8 times the speed of the wind. They have done this with various sensors, the data for which can be found on their website.
Rick was continuing the work of Jack Goodman who released a video of his toy DDFTTW cart in 2006 and created a huge internet buzz. Unfortunately for Jack, it also created huge controversy. People issued cries of "Perpetual Motion", "fraud", "its all done with string". Physicists claimed if this video were true it would break the basic laws of physics so it is clearly a hoax. One contributor to Make Magazine tried to reproduce the results and couldn't, and therefore concluded that Jack Goodman was a fraud.
You would think that Rick's full-scale reproduction of Jack's work verified by an independent body would be enough to silence the critics, but people continue to lose their minds over the impossibility of using the wind to travel faster than the wind. The cries of perpetual motion and breaking fundamental laws of physics continue. I find the hysteria just a little bit baffling.
Understanding how DDFTTW is possible is actually fairly straightforward. Once you understand what is going on, it is clear that what is causing all the naysaying is simply a failure of imagination.
Ordinarily, harnessing power from the wind is straightforward and it is something that has been done for millenia. You can use a sail or a turbine, but either way you have the wind push against it and get energy as a result. Connect the sail or turbine to a boat or sled or cart and you get the vehicle being pushed by the wind.
The trouble is that the energy harnessed comes from the relative speed of the air compared to the vehicle. As the vehicle speeds up the push from the air drops, until the vehicle reaches the speed of the wind (a little less due to friction, actually) and no longer gets energy from the air.
The first thing that is different in Rick's land yacht is that it does not get energy from the difference in speed between the air and the vehicle. Instead it gets energy from the difference in speed between the cart and the ground. As long as the wind continues to blow, the cart can continue to roll. As soon as the wind dies down it comes to a halt. So you can see there is no free energy of the perpetual motion kind here, just well understood harnessing of energy based on the difference in speed between two mediums, the cart and the ground. No matter what speed the car is going (other than when it is halted), there is still a difference in speed that yields energy.
But now the question comes as to how you harness that energy in order to provide thrust. The answer is that you connect the wheels of the cart to a propeller. The wheels of the cart are in contact with the ground, the propeller is in contact with the air. The wheels provide the energy, and the propeller provides the thrust by moving air.
The real brilliance of Jack Goodman and Rick Cavallaro is that they not only connected the air to the ground, they also inverted the source of the energy and the thrust. In a typical air-powered design, you get your maximum thrust when the cart is stopped. In this design, you get the least amount of thrust when the cart is stopped. But as the wheels turn faster they power the propeller to provide more and more thrust. As the propeller provides more thrust, the wheels turn faster and generate more energy. Rather than having less energy as the cart approaches the speed of the wind, it has more.
Does that mean that the cart will go faster and faster until it approaches the speed of light? Of course not, that is just silly. It is only harnessing the energy in the speed difference between the air and the ground, and that is limited.
You might be thinking that it isn't possible to design the cart to harness more of the energy from the air/ground speed differential than would be required to accelerate the cart to the speed of the wind, but that is easily disprovable. Imagine a more traditional design where you use a sail. Can you imagine the sail pushing a cart that is significantly heavier than the Greenbird? If you can imagine that then you can see that more energy is available with the right design.
So the problem is not having a vehicle that goes directly downwind faster than the wind. You only need a design to extract enough energy from the relative speeds of the air and the ground to accelerate your cart that fast. The one thing I can't figure out is that there are claims that the vehicle can start itself. Although the cart provides some surface area for the wind to push against, it would be surprising if it were sufficient to overcome the friction of the wheels, especially since the wheels are turning a big propeller. You might think (as I originally did) that the propeller initially acts as a sail just to get things going, but the comments on the article from Make:Online specifically state that there is a ratchet in place to prevent the propeller from ever driving the wheels. The video clearly shows that Jack Goodman's design needed a push to get going, so I have to admit I'm flummoxed on that score.
As far as a wind-powered cart running faster than the wind, it is a fine bit of out-of-the-box thinking combined with excellent engineering. Kudos to all concerned, but it is hardly going to revolutionize physics.