Sometimes you just have to say “sorry.” It appears that it’s my turn.
Yesterday, I saw a story on Wired touting the first full-scale demonstration of a wind powered machine that can move faster directly downwind than the wind itself is moving. The piece instantly got my perpetual motion scam meter ringing. So I did what any well-meaning physics fan would do — I flamed the shysters in the comments section of the article.
I now know I was wrong. I spent most of last night a drawing up page after page of force diagrams and pouring over videos like this . . .
. . . in an attempt to figure out how they managed what was clearly a deception. Instead, I have seen the light. I’m convinced that Rick Cavallaro and his buddies at Thin Air Designs and San Jose State University deserve a hearty pat on their collective backs for sticking with their convictions.
My belief that outrunning the wind is impossible stemmed from hours on sailboats as a kid. You can build up a lot speed while tacking across the wind, as these folks demonstrate.
But running with the wind always seemed so dull and slow. I tried endlessly to think of some way to speed up my downwind motion. I talked to countless sailors and physics profs, and ultimately came to terms with the inevitable fact that there’s no way to sail faster than, or even at, wind speed if it’s blowing straight at your back. That fact still holds true for sail-powered vehicles. Cavallaro, as it happens, didn’t build a sail boat, or sail car, or sail anything. I’m not really sure what to call it other than a faster-than-the-wind vehicle.
The problem with the new vehicle is at least in part the challenge of explaining the physics of the thing. In short, it exploits the difference in the motion between the air and the ground. There’s absolutely nothing wrong with that. There’s no perpetual motion or free energy involved. Once you take energy out of the system, you’re free to use it anyway you like, including outrunning the wind. If you can use a windmill to collect energy when your sitting still, then you should be able to continue collecting energy when you’re moving. You just have to be clever about how you do it.
The key to the new vehicle is that part of it has to be connected to the ground and part has to be immersed in the air, and that there has to be relative motion between the two. In other words, you can’t have a a propeller scavenge energy from the wind in order to turn a propeller that moves you along. Nor can you have a wheel that steals energy from your forward motion over the ground in order to turn drive wheels that move you forward. Those would be examples of perpetual motion machines. I could, however, build a machine that has a paddle wheel immersed in a running river and wheels on the adjoining shore that could outrun the river that powers it (maybe Rick Cavallaro could make one of those next). It would be a tricky thing to design, but it should work just fine.
I freely admit that my 180 degree conversion from yesterday to today has me a little nervous. If I could be so convinced that outrunning the wind was impossible at one moment, and then convinced that it’s entirely possible only 24 hours later, then what’s to guarantee that I’m right now? Epiphanies are like that I guess. But just to make sure, I’m going to build a model myself. Once it’s done, I’ll show it to my friends and colleagues here at the American Physical Society to see what they have to say about it. I’ll report back as things progress.
-Buzz