Originally Posted by teejayevans
Think of the propeller blade as a wing, but symmetrical, the forces (lift) would occur if the wind was either direction. The wind is not pushing the blades, its using the aerodynamic lift
of the prop blades.
An airfoil has a critical angle of attack that when exceeded will stall the airfoil defeating lift. Wind coming from 180 degrees or directly under (in this case aft of) the airfoil is an attempt to produce lift from a stalled angle of attack.
The answer is that they have a variable pitch
airfoil that starts the machine by spilling the air out to gain momentum and then as the apparent wind changes to 0 degrees the pitch
of the airfoil is changed so the air is still producing lift. This is evident as the driver stalled the airfoil about 0:45 into the run then re-adjusted the pitch at about 1:10 to regain kinetic energy in the drive. You will notice a dramatic increase in speed after the 1:10 minute spot on the video.
At first I thought this was BS but as I watched the pitch of the airfoils vs the speed of the machine it made more sense. Looks like the same guy (Richard Jenkins) broke the wind powered land speed record with another design... a sail!