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Originally Posted by botsapper
Counter-intuitive they say, that a wind-powered craft going straight downwind cannot go faster (no angled tacking/gybing) than the wind itself.
They built a three-wheeled land craft with a twin-bladed propeller/turbine, the Blackbird to test and prove their wind challenge. The bluff-body Blackbird first gets started by the wind from behind then the forward rolling wheels turn a geared sprocket that turns the propeller to push air rearward and provides added forward thrust! In their video you see the windsock showing the wind direction but the Blackbird's telltale streamer is waving backwards from an apparent headwind - it is moving faster the the wind.
Aerodynamicist Rick Cavallaro tweaked the Blackbird and achieved a top speed 2.86 times faster the the wind speed in 2010.
In 2012, they achieved a top speed 2.01 times faster the wind speed - headed UPWIND with a turbine rotating the opposite direction to transmit power to the wheels!
Blew my mind but it all made sense with the 'cylindrical Earth demonstration'
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1) from the first sentence, once the craft achieves a velocity equal to the wind velocity, there can be no net force acting on the vehicle. Thrust = zero.
2) during the runup, the wheels, acting through a transmission, could be feeding power to an accelerating flywheel, storing kinetic energy, in excess of that necessary to overcome any aerodynamic or rolling-resistance road load.
3) once up to 'speed', and the blades re-configured for thrust, the flywheel could be tapped, to power the craft to some given velocity, momentarily, however, at some point, this energy would be exhausted, thrust would be lost, with deceleration.
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What is not said would be of much interest. Like, for how much time can they demonstrate this feat?
Upon opening the kimono, we may just discover some parlor trick, which has nothing to do with aerodynamics.