Perhaps another way to think about it - tailwind. Aircraft can derive energy from the wind even when they're going faster than the wind-speed. The propeller ar

Perhaps another way to think about it - tailwind. Aircraft can derive energy from the wind even when they're going faster than the wind-speed. The propeller arrangement is a glider that is being pushed by the wind. The glider is tethered to a cart whose wheels in-turn drives the propeller to become a "powered" glider. Without a tether, the relative speed of the glider with respect to the wind would be zero. With the tether powering the propeller, the relative speed of the glider is greater than zero, which makes the cart go faster than the wind. Thinking about the "glider" and the tether as two separate systems makes more intuitive sense, imo.