OT wind-powered vehicle travelling directly downwind faster than wind speed

Um. no. The wheels drive the prop in the downwind case. Only when going into wind does the prop drive the wheels

When the wheels are going in a forward direction, the prop will always push forwards, or push air backwards. At below wind speed this actually increases the effective drag

John posted a good video which explains in a more practical way, better than I can.

I liked Xyla's version, with real hands-on explanations.

Very long while since my model aircraft days:-)

I was quibbling about the initial conditions with the vehicle stationary. I suppose you could reverse the propeller pitch to get started.

Indeed, and that is the key point... if you view the area covered by the prop as a sail, then once the vehicle is travelling at wind speed you might initially think that there would be no net force "pushing" on it.

However that ignores the rotation of the prop which is developing a "wind" in the opposite direction to that of the prevailing wind. So there is still a nett forward thrust on the vehicle based on the vector sum of the natural wind speed and the prop generated wind speed.

So rather than the prevailing wind pushing against a static sail, it is in effect pushing on a sail that is in effect in constant motion toward the rear of the vehicle.

As long as the there is relatively little friction, there will be enough drag on the vehicle as a whole to get it moving even without the contribution of the prop.

The wheels drive the prop, not the other way around.

Basically it's a fan, not a turbine. The rotational direction is in the opposite direction to that that which would be the case if you disconnected the linkage between the wheels and the prop.

Only going to work if there is a nice wind through the building or it has a treadmill for a floor! (you need a source of energy)

There *is* a pitch control, but no mention if it goes negative, or if it is used at startup.

Thomas Prufer