Considering the kind of sensorless brushless motor and speed controller found in rc multirotors -
If the motor is connected to a windmilling prop will it fry the controller?
These things usually have an FET for each phase, and do some sensing trickery on the same windings.
The windmilling might be quite fast in the situation I'm thinking of; before I hang one out the car window does anyone know if these things are usually protected from such damage?