Patent No. 3687899
"is the magic word for a safety flight"
"Autorotation is a self-sustained operating state of the helicopter where the energy to drive the rotor comes from potential energy (altitude) converted to kinetic energy (main rotor rotation) from the relative descent velocity. In practice, autorotation is a maneuver that can be used to recover the helicopter to the ground in the event of an engine failure, transmission problems, or loss of tail rotor. It requires that the pilot ket the helicopter descend at a sufficiently high but controlled rate, where the energy to drive the rotor can be obtained by giving up potential energy (altitude) for energy taken from the relative upward flow through the rotor, thereby averting a ballistic fall."
Autorotative indices derived for several helicopters at standard sea level conditions (data source : various published helicopters specifications) The autorotative performance of a helicopter depends on several interrelated factors.
OTHER ROTORCRAFT BASED ON 3 OR MORE DISTRIBUTED PROPELLERS
CANNOT PERFORM AUTOROTATION
THE FLIGHT IS NOT SAFE