| If you can go downwind 3x faster than the wind, can you turn and go perpendicular to the wind using this mechanism? Can you turn again and go directly upwind? At a multiplier of 3x windspeed, it seems like the fact that there's wind at all is irrelevant other than to get off the starting line. You're generating your own wind power by pushing the propeller through the air. Is this correct? Because if this works other than directly downwind, you have a perpetual motion machine - scale up, drive in a circle, and hook a dynamo to a wheel. Which makes me think I've neglected something somewhere... Edit - from the Wired article: "Skeptics think that the wind is turning the prop, and the car is turning the wheels, and that's what makes the car go," Cavallaro said. "That's not the case. The wheels are turning the prop. What happens is the prop thrust pushes the vehicle." OK, now I'm really confused. I thought the prop rotation was screwing through the air slower than the relative airspeed and driving the wheels - now you're telling me that the wheels are spinning the prop to produce forward thrust? If he can go from sitting still: wind speed -10, ground speed 0, to W0/G10, and accelerate up to W20/G30, what happens if the wind drops to 0 mid-run? Can he keep going at W20/G20, isn't that easier than W20/G30? |
The propellers are being pushed into a spin by the difference between the airspeed and the groundspeed. (Note that the propellers are locked to the ground by the wheels.) As they spin, the blades generate lift. That lift then pushes the craft forward.
Take away the difference between airspeed and groundspeed, and nothing is keeping the blades spinning. Therefore the system will run to a stop due to friction.
The connection between spin and lift is mediated by aerodynamics, not a mechanical linkage. And aerodynamics allows the vehicle to be pushed faster than the spin. Which is why, if you get friction low enough, it can go faster than the wind. And not slightly faster, but a lot.