Fair enough, I'll give it a try.
Okay, the cart is standing still, and the wind causes
(a) an aerodynamic force forward on the entire cart including the propeller. This force together with the friction between the tire and the ground creates a torque that wants to turn the wheels forward.
(b) an aerodynamic torque on the propeller that wants to turn the wheels backward.
Now the question is: Which torque is bigger?
If the prop is small enough and the wheels are big enough the propeller will lose. Therefore the cart will start rolling forward. As soon as this happens, two things follow:
(a) The cart has a forward velocity, which lessens the relative tailwind and therefore the countering torque on the propeller
(b) The prop starts spinning and therefore creating thrust, adding to the forward aerodynamic forces on the cart, and increasing acceleration.
This situation is true for all speeds below wind velocity, so the cart will reach wind velocity (QED).
At speeds higher than wind velocity there must be an equilibrium between thrust on one hand, and friction and drag forces on the other. Since thrust in non-zero at wind speed (as evidenced by the treadmill experiment), the equilibrium speed will be higher than wind speed.
My €0.02
Well, thank you, but I was thinking of evidence of performance. The only evidence I have seen, is a cart being blown in the wind at some indeterminate velocity. The direction of the propeller is also vague and aliasing makes it impossible to be sure.
I'm sorry, I cannot answer that question. I've only read pages 1-4, and the last two.
Also, I don't understand the question.
My turn to say fair enough.
The treadmill and cart are set running at a belt speed of 10mph.
This is put ina van, and driven downwind to wind speed of 10mph.
The behavior of the cart is then said to agree with those conditions, validating the model, and by inference, the equivalence of the 'treadmill wind' to still air.
What then would indicate a failure, and how would you explain it?