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Also very little opportunity for regenerative braking.

Unless you use updrafts. Check out dynamic soaring. I have often thought about using updrafts above lava lakes and custom ESCs that can charge batteries to do quadcopter DS.

While quickly descending use the motors as a generator via autorotation?

I'm only joking, but there might in fact be practical application of the concept.

I don't actually know if it's possible to autorotate a quadcopter. I imagine you would have very little control during the transition. Quadcopters don't fly very high anyway, so there's probably little to gain from it.

ESC electronics usually doesn't support this, but I've wanted to try for many years.

autorotation requires control of blade pitch.

It certainly does in helicopters, which have large, relatively heavy blades you really don’t want to stop turning.

In a quadcopter the blades a small and light, so you could conceivably stop them and let them turn them in reverse as the airflow changes direction (from downwards to up through the blades). I think there would be real problem with control during the transition, and you’d need a lot of height to make it worthwhile.

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