Faster than the wind

June 15, 2010
A wind-powered vehicle demonstrates that it is indeed possible to use wind power to travel faster than the existing winds.

You might be surprised to find that the five-meter-high propeller mounted on the “Faster than the Wind” car doesn't power any kind of electric generator. The car, fielded with a sponsorship from wind turbine company Joby Energy (see http://eetweb.com/wind/flyingwing061410/index.html) and Google, is the brainchild of Rick Cavallaro, chief scientist at Sportsvision Inc. When Cavallaro is not designing new techniques for sports television, he is an aerodynamicist, paraglider and kitesurfer.

Cavallaro convinced himself that it was possible to build a car powered by the wind that could also travel faster than the wind powering it. Cavallaro and a friend collaborated with the aerospace department at San Jose State University to build their ultra-light vehicle that would prove the theory.

The resulting car is made largely urethane foam. The wind pushes the vehicle forward. The wheels then turn the propeller which spins in such a way to push the wind back behind the car, thus pushing the car forward. The forward movement turns the wheels, and thus turning the propeller faster.

Reports are that the car hit a top speed of more than 2.85 times faster than the 13.5 mph of the prevailing winds. The next step is to have this verified by the North American Land Sailing Association.

Cavallaro was confident the car would work thanks to a study he produced that shows the amount of speed and power available for upwind versus downwind vehicles, as well as the equations balancing the power absorbed by the wheels and delivered to the propeller. He included losses such as the rolling resistance of the wheels, vehicle aerodynamic drag, gear train friction losses (he says the toughest part of the whole effort was the gearbox design), and propeller parasite and induced losses. He used an iterative solver to find the gear ratio where power required for the propeller and power available from the wheels are equal given the wind speed, car speed, and vehicle parameters.

More details, including a link to Cavallaro's study, are available at the Faster than the Wind site: http://www.fasterthanthewind.org/