Onboard quadrocopter failsafe: flight after actuator failure

As an additional potential recovery mode: ETH Zurich's Institute for Dynamic Systems and Control [0] has done amazing work on quadcopter recovery after rotor control loss [1] (I believe they are down to controlled flight with only one or two rotors left, but I cannot find the link right now). I've been waiting for this to filter into the real world but have yet to see it. [0] link [1] ▶ this video

This isn't quite true. Cheap drones may not have this capability, but controlled, safe landings of quadrotors with a single failed motor have been demonstrated, see this ETH Zurich video from 2014: ▶ this video Obviously there are still other single points of failure such as the battery or controller, but at this point there's no excuse for crash landing a commercial drone just because a single motor (or rotor) has failed.

Actually that's a benefit of the quad-copter design. It can sacrifice an axis of control in that situation (a "minor" one, like rotation about the gravity vector). In this case it would likely make the passengers sick, but it would still be able to fly in a controllable fashion. See this video: ▶ this video

You can't pull autorotation with fixed pitch blades, at least not in any meaningful manner. Light quads can fall pretty slowly with pinwheeling motors but that's not actually autorotation. The way a helicopter is able to autorotate is to pull negative pitch, which spins the rotors using the fall energy of the copter, then shortly before crashing pulling back to positive pitch and using the still spinning blades to slow down to a controlled landing. For motor failure ETH Zurich has some research out there about recovering to normal(ish) flight after a prop loss. The quad goes into a pretty rapid spin but it's still controllable, link below to an outside the lab test. ▶ this video