[RedDevil]

Oh crikey. We already had a thread on this.

Ah well. I'll repeat my explanation in concise form:
The propeller on the craft is slowly turning backwards.
Its blades form a surface that still moves forward with the craft, but less fast than the car itself.
The wind still exerts pressure on that surface, even if the rest of the car is moving faster than the wind.

The propeller is driven by the wheels, but as the gearing reduces the speed the forward pressure on the blades is larger than the backward force on the wheels through those gears needed to spin the propeller.
As all other sources of friction are pretty minimal, that's enough to make the craft accelerate.

The important point is that the propeller spins slower than the wind. If it spins faster than the wind then the car would move backwards, straight into the wind.

Originally Posted by AeroMcAeroFace

...
in the beginning, the car isn't moving, but has a tail wind, the rotor spins, which in turn drives the wheels. the car accelerates partly due to the drag of the vehicle, and partly due to the propeller driving the wheels.

That is exactly what is NOT happening. The rotor spins against the wind!
It does not drive the wheels at any moment. instead it exerts a force on the wheels fighting its motion (but losing), or rather the ground forces the wheels to turn which through the gearing drives the propeller to turn slowly against the wind.
It is just the wind blowing against the blades of the propeller which moves it forward, like a sail would.

If the blades were fixed it would act just like a sail.

Now we add a mechanism that very slowly turns the blades backward.
It would still be mainly a sail.

It will remain working like a sail until you set the ratio such that the propeller moves the air backwards as fast as the craft moves forward, in which case the forces cancel each other out and the craft wouldn't move at all.