Yeah, my first inclination is that since the wind is turning the propeller and the propeller is turning the wheels via a gear system, the speed of the vehicle will be determined by the propeller speed, the gear ratio, and the tire diameter (ignoring friction and air resistance for a moment). The gear ratio is the critical component. It's not like the propeller is a simple sail. I could be completely missing something though.
Most critically: the wind is not turning the propeller.
If you watch the videos when the thing is first getting going, it is slow enough to see that the propeller is actually spinning in the opposite direction it would be if it was acting as a windmill.
Gear ratio *is* the critical component, specifically prop pitch vs. forward motion, but not exactly for the reason you describe. Xyla Foxlin built a working model and provides a bit more info here:
In short the working model she built has approximately a 2:1 ratio between forward wheel motion and propeller pitch. IOW if the propeller pitch is 5" per revolution, the wheels will require 10" of forward travel to spin it one time.
No, the wind is not turning the propeller, the wheels are.
The reason the wheels are able to turn the propeller without slowing the vehicle down is that the speed of the wheels over the ground is greater than the speed of the propeller through the air thanks to the tailwind.
The prop is spun by the wheel, not the wind. The tailwind provide push.
Think of the wheels as a power bank: It convert and temporarily store linear force to rotational force.
As long as it accumulated enough power to counter the rolling resistance, headwind drag, gravity, gearing loss, etc; the excess rotational force can be used to spin the prop. Which in turn generate additional push to be converted and stored.
Within this "loop", the vehicle got two linear input, tailwind and propeller.
This will allow continuous acceleration once the vehicle goes faster than tailwind up to the point where the additional force from the prop is completely negated.
