[RoadRaceJosh]

The rope and cart trick is irrelevant. The energy to move the cart is still transferred by the rope to the cart from an outside source.

With Blackbird rolling at wind speed where does energy enter the system to accelerate the car past wind speed? The propeller can't provide thrust or drag when the relative wind velocity is zero and the kinetic energy of the vehicle must be lost to turn the prop which decelerates the vehicle. You can fool yourself all you like with gearing, but there's no energy input to the system and the only energy that can be expended is kinetic and claims to the contrary are false. You could put a flywheel in the car and gain peak speed by storing kinetic energy, but the average speed is still lower than the wind speed.