On a quadcopter, there isn’t this point of inefficiency, but having to continuously slow down and speed up the props to remain in the air does waste a lot of energy (however regenerative braking would reduce this wastage, but not many models do this at the moment). What’s interesting is that the effect of this was very evident when we were tuning our quadcopter – our quadcopters were designed to lift about 1kg of payload, but they could be made to lift a brick (about 2.3-2.6kg) ONLY if the stabilization tunings were relaxed so that the craft was less stable in the air, but wasted less energy trying to stay absolutely level. Either way, aerodynamically, larger and slower spinning props are more efficient than small fast-spinning props.

This comes down to a lot of aerodynamics, but in effect it’s to do with the fact that the formula for kinetic energy is 1/2mv^2: it takes four times more energy to move a mass of air at twice the speed; compared to only twice the amount of energy to move twice the mass of air. In both cases, the same amount of momentum is conferred (m*v). So it turns out that it takes less energy to exchange momentum with a large amount of air at slow speed than it is to exchange momentum with a small amount of air at high speed. Therefore a large, slower-moving prop is more efficient than a small large-moving prop, especially at hover.