Helicopter rotor behaviour after engine failure in forward flight

Abstract

Calculations have been made to find the changes in rotor speed following engine failure in forward flight. Several particular examples were investigated, based on the parameters of the S-81 helicopter. The effect of pilot's control movement was included. A rapid loss of rotational speed occurs with practically no change in forward speed of the helicopter, thus the tip-speed ratio increases rapidly. This may lead to stalling of the retreating blade and/or interference of the blades with the droop stops, either of which are dangerous conditions. The time available to the pilot to reduce the collective pitch after engine failure is very short throughout the speed range and engine failure constitutes a danger to safety on this type of helicopter. Some form of automatic pitch reduction or power failure warning system is necessary.