One of the big worries regarding the widespread use of drones is that if their motors fail, they could plummet out of the sky and hit us on the head. Equipping them with parachutes is one option, although that adds weight and complexity. Meteorological forecasting company Meteomatics is taking another approach, in the form of a quadcopter that can spin its way down like a top.
The Swiss firm recently received a US patent for the "Meteodrone" concept, in which at least two of the drone's four symmetrically-arranged propeller arms are flat and twisted – not unlike propeller blades, actually. In the event of a motor failure, once the drone starts falling, those arms will act as passive lift elements. This will cause the aircraft to spin horizontally, around its yaw axis.
That spinning motion will keep the drone stabilized, so it comes straight down instead of tumbling erratically. Additionally, the motion will generate some dynamic lift, so the aircraft won't simply drop like a stone – that said, you probably still wouldn't want to be standing beneath it.
When and if the Meteodrone reaches production, plans call for it to be used for measuring weather parameters within the lowest region of the atmosphere.
Source: Meteomatics
Autorotation isn't JUST about the object "self spinning" the Blades need to be able to start the Spin from stopped, maintain the rotation (Drive)-consuming significnant Kinetic energy) while generating sufficient LIFT for the "fall" to not be catastrophically disastrous
Provision of Lift AND Drive is one reason why helicopter rotors are not [significantly] twisted like a propeller, but of course, helicopter rotors do not self-start well - even an autogyro needs "some" help in starting the rotation.).
A small Quadcopter Probably doesn't have enough Mass, nor the maximum potential energy to deliver significantly on any of these objectives, WHILE not creating a large aerodynamic penalty in normal operations. Possibly a lightweight parachute will be better (lower aerodynamic penalty)