The wheel friction and gearing allows the vehicle to tie the effective velocity of the sail (actually a propeller) to the velocity of the ground. Energy can be

The wheel friction and gearing allows the vehicle to tie the effective velocity of the sail (actually a propeller) to the velocity of the ground. Energy can be extracted from the interface between the ground at one velocity and the air moving at another velocity. A sail tied to the ground and extending into this wind will exert a force according to the drag equation: Force = 1/2 x drag coefficient x density x area x (differential velocity)^2 Typically, when going downwind, sailors ignore the ground (or the water) and just extract some energy from the difference in speed between the vehicle and the wind. Obviously, as the vehicle accelerates, this velocity difference asymptotically goes to zero where there is no more force and no more acceleration. For a more static example, imagine an enormously long vehicle with a linear bearing running the entire axis from the front to the back. Assume for the sake of argument the vehicle is ultra-light and has very low rolling resistance, some kind of cross between a top-thrill dragster and a train, hypothetically a kilometer long. On the axial bearing, mount a wind turbine with a cable and an anchor that can be fixed to the ground. Start with the wind turbine at the front of the vehicle, then drop anchor to fix the wind turbine in place. There is a velocity differential between the ground-referenced turbine and the air that will impart force on the turbine blades, use this to generate electricity. Use an electric motor to drive the bottom part of the vehicle forwards. When the turbine reaches the back of the vehicle, lift anchor - remember we're in spherical cow territory, so assume it's foldable and super-light so this part takes little energy and can be done arbitrarily fast - and push it back to the front of the vehicle, where you can drop anchor again. In this system, the energy you can extract only depends on the difference between the ground speed and wind speed, not the vehicle speed. Blackbird is just like this repetitively anchored wind turbine concept in that the point of reference for the wind energy extractor is the ground instead of the vehicle. But instead of dropping an anchor, you're tying to the ground speed through tires, and instead of shuttling the turbine from the back to the front, you're continuously advancing it with the gear mechanism, and instead of driving with an electric motor you're directly using the drag force.