Ride Like the Wind (only faster) - 3 views
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yc c on 07 Jul 10Rick Cavallaro and John Borton have built a cart that moves 2.86 times the speed of the wind, moving straight downwind. That may seem impossible, but after a year of tinkering and some financial assistance from Google and Joby Energy, they did it. Don't believe me? Check out the video. Keep a weather eye out for the green flag at 0:35. Notice how it's blowing the exact opposite direction of the orange wind socks on the cart? That's because the cart is going faster than the wind. -
yc c on 07 Jul 10The cart is quite aerodynamic, so it makes a crappy sail, but it's still a sail. The wind gives it a push, and the car starts to roll, however slowly. As it moves along the ground, the wheels turn. At 0 kph the cart has an air speed (relative to the 10 kph wind) of -10 kph. As it gets rolling, it will catch up to the wind in velocity: at 5 kph ground speed, it has a -5 kph relative air speed, and at 10 kph ground speed, it has a relative air speed of 0 kph. At 10 kph ground speed, the cart is just like the balloon, and would not beat the balloon in a race. But unlike the balloon, the cart has a 17-foot propeller linked by a complicated drive train to the wheels. And it's the wheels that provide the work to turn the propeller. Remember that: The wheels turn the prop. Not the wind. Not magic pixie dust. The wheels turn the propeller. That's important. At 0 kph air speed, the propeller, sections of which have already been pulling on the car, really begins to bite on the air. It pulls the car forward exactly as a propeller pulls an airplane forward. The ground speed of the car increases, turning the wheels faster, which turn the propeller even faster, adding yet more acceleration. And now the whole project seems ridiculous,because everyone knows a perpetual motion machine is impossible. But the wind never stops adding power to the system. Come back to the difference between the relative air speed and the ground speed. In the example, the cart reaches a ground speed of 10 kph, and relative air speed of 0 kph. The propeller kicks in and the cart accelerates: Ground speed rises to 20 kph, with relative air speed of 10 kph; then 30 kph ground speed with relative air speed of 20 kph, then it finally reaches a top speed of 28.6 kph, with a relative air speed of 18.6 kph (meaning, going 18.6 kph faster than the wind). There's some loss to friction and to the drive train, but generally the wheels are always doing 10 kph-worth more work then the propeller, because the
