What I've learned about wind carts

As a follow-up to Rick Cavallaro's article on Make: Online about designing and making a wind-powered cart that can outrun a tailwind, I wrote a piece about wind cart enthusiasts and skeptics.

Is it possible to for a wind-powered vehicle to travel directly downwind faster than the wind?

The intuitive answer to this question is "of course not." Imagine tossing a balloon into a steady breeze. It will go along at the speed of the wind (or slightly less, due to drag) but it's inconceivable that it could go faster than the wind. How could it? If it were to go faster than the wind, it would be outrunning its source of power and move into a headwind, which would slow it down.

Think of a sailboat moving downwind. Once it gains enough speed to be moving at the speed of the wind, the sail will go slack, because the wind speed relative to the boat is zero. With no wind in the sails, how in the world could the sailboat go any faster? To claim that it could go faster than the wind is the same as claiming it could move forward with no wind at all!

People immediately began attacking Goodman's video, saying it was a fake. The video doesn't have a clear shot of the road ahead, so many commenters accused Goodman of towing the cart behind a car or bike with a piece of fishing line. Some said the cart was moving downhill; others said Goodman was deluding himself – the windsock changed direction because of propwash, not because it was moving faster than the wind.

Make Online: What I've Learned About Wind Carts