Actually, I have less problem understanding how the cart could go upwind (against the wind) using a propeller or turbine. It's just that going down wind, I would expect a "zero" crossing point as the cart goes from being pushed by the wind to "pushing on the wind" using power from the wheels and with no apparent airflow across the propeller. Once going faster than the wind, I can believe and possibly model sustaining the propulsion, it's just getting past the passive, being pushed mode, to the actively pushing using energy from wheel motion and airflow that is giving me the most trouble.<quoted text>
Actually from an aerodynamic perspective sails can act either as propellers or turbines. When a sailboat is on a downwind reach it is acting as a propeller. The wind will be moving faster relative to the sail after interacting with it. When a sailboat is on an upwind tack it is acting as a turbine, the wind moves slower relative to the sail after it interacts with it. In both cases the wind moves slower relative to the ground after interacting with the sails.
If you consider the path of any point on the propeller you will find that it takes a corkscrew path through the air. If you "unrolled" that corkscrew into a straight line you would see that the path is always one at a constant angle to the wind, exactly the same as a boat on a downwind reach.
This is extremely counterintuitive. The man who has the official record will even readily admit that he may not have been the first one to travel directly downwind faster than the wind. A man did it in the sixties, but his indicators were simple yarn tell tales that could be argued to be in the path of the rotors acceleration of the wind.
Here is video of possibly the first ever DDWFTTW vehicle. Turn up your speakers:
I am beginning to suspect an "assist" of some sort to cross that "zero point". If I am right about that, it wouldn't be cheating ... much ... but it would mean going faster than the wind downwind would not be possible without artificial assist at some point.
Again, I haven't carefully modeled it, all thumbnail speculation at this point.