It’s one of those annoying brain teasers that turns half the people in the room against the other half, a simple engineering riddle that even professional aeronautical engineers get wrong.
It’s well known that racing boats can travel faster than the wind when traveling to windward. The fastest boats, and especially ice boats and land yachts, can even beat the wind downwind by sailing off the wind at an angle. This is because a boat is not driven by the true wind, but by the apparent wind. When you move at some angle relative to the wind, the apparent wind changes its speed and direction. Lift and drag (including friction) all factor into the equation.
But sailing directly downwind poses a different challenge. The apparent wind’s angle never changes, and the apparent windspeed approaches zero the faster you go. And so, from a superficial examination, this proposition should be impossible. And, using a conventional sail, it is indeed impossible.
However, when you change things up by replacing the sail with a windmill that drives wheels, it is possible to go directly downwind faster than the wind. Since the windmill’s blades are spinning, there is always apparent wind that can be used to generate lift. Moreover, the tips of the windmill blades are moving much faster than the wind, so the true wind speed is actually not all that big a factor.
Foul, you may cry. Once the vehicle is traveling directly downwind at exactly the same speed as the wind, there is zero apparent wind. Thus, there’s no impetus for it to accelerate any faster. If the apparent wind on the spinning windmill blades caused the vehicle to accelerate, it would have to be an impossible perpetual motion machine. Right? Hmmm… Well, don’t worry. Internet forums where this problem has been discussed run thousands of posts long. Even professional aeronautical engineers can’t agree.
People have been testing this for a long time. The latest is a group working with the San Jose State University aeronautical department, Dead Downwind Faster Than The Wind (DDWFTTW, lots of stuff on their web site), with corporate sponsorship from Google and wind energy company Joby Energy. A video of one of their prototype’s vehicle’s test runs is posted here. They cite a test cart built in the 1960’s, as a result of such a disagreement between two engineers at Douglas Aircraft. Reportedly, it managed to travel downwind at 1.2x true wind speed. The DDWFTTW has traveled at more than 2x true wind, and is aiming for 3x.
The rules are simple. No stored energy can be used. The vehicle must travel directly downwind, not at some offwind angle. The only power allowed to drive the wheels is the windmill.
The hardest thing about this is to get your mind away from the concept of apparent wind on the vehicle, and think instead about apparent wind on the windmill blades. It’s way too easy to get stuck on this point, but you have to remember that apparent wind on the vehicle is not relevant to the blades’ lift, even though it’s coming from what intuitively seems like the “wrong” direction. It is relevant to the drag, and as such, it is still a variable in the equation. Friction, weight, and wind resistance must be minimized, because once you exceed the true windspeed, these all work against you.
But watch the video. Yes, it’s possible that SJSU, Google, and Joby Energy are all being hoaxed here. What do you think?