Riffing on this a little bit based on my experiences with multirotors but not having any kind of "autorotation" capability with them: Most motors on multirotor

Riffing on this a little bit based on my experiences with multirotors but not having any kind of "autorotation" capability with them: Most motors on multirotors are BLDC motors that require active control to make them spin. Voltage gets applied to the motor phases sequentially to make it spin. However, when they're completely open-circuit, they actually spin quite freely! I've seen the ones I had (26" prop) windmill quite well in a mild breeze. The trick, though is that you're probably not going to get much descent-arrest out of open-circuit BLDCs. Another possibility is short-circuited BLDCs; in that condition, the rotos are quite tough to turn. My speculation is that the props wouldn't spin much at all in a short-circuit condition. Buuuuut... if ESCs were able to modulate the open/short-circuit condition (even without power to actively spin the motors), I suspect you might be able to get some kind of arrested descent situation. I've been thinking of building an ESC from scratch this winter... this might be a useful feature to experiement with!