It seems odd that I haven’t thought this one up before. Odder still that I can’t find it anywhere else.
Imagine a glider which embodies an electric motor with a propeller attached.
In today’s invention, the motor runs the propeller hard enough, in a single burst, to discharge the battery and get the aircraft in the sky (say 100m).
At that point, the engine and propeller unit is jettisoned, whilst the prop continues to spin.
The engine would also have plenty of padding, so that when it autogyros back onto the landing strip, it can be recharged and used in another engineless glider.