In a helicopter if your engine stops working, you start falling. Falling produces wind on your rotor blades and causes them to spin faster. Blades spinning fast

In a helicopter if your engine stops working, you start falling. Falling produces wind on your rotor blades and causes them to spin faster. Blades spinning faster makes you to stop falling. This effect is called autorotation and you control it by varying the pitch of your rotor blades. Autorotation means that you don't automatically die if your engine fails. With plasma actuators (also quadcopters), battery faliure means death.