Assessment of rotorcraft performance following an engine failure is necessary to ensure its safe operation. For single-engined rotorcraft an engine failure will result in
a forced, or engine-off, landing (EOL). For multi-engined helicopters, however, it is still necessary to assess the ability of the aircraft to continue flight particularly in critical areas of the flight envelope. In any event the pilot must react quickly to avoid loss of control and prevent the rotor speed from decaying below acceptable minimums. As the power fails directional control will go out of trim and the residual anti-torque moment will yaw the helicopter, generating additional sideslip that may possibly induce a roll if no corrective action is taken. (An undemanded yaw may be the first indication to the pilot that a power failure has occurred.) The rotor speed decay rate can be estimated by applying simple rotational mechanics to the problem. Now:
Q = I®
оо = — = rotor speed decay rate =—-j
where Q= rotor torque and I= moment of inertia of rotor system.
Thus reducing the rotor torque will reduce the rate of rotor speed decay. This can be achieved by reducing the blade pitch in order to decrease the rotor drag. In steady autorotation a given value of collective pitch will cause the helicopter to settle on a unique descent speed and rotor speed combination. The pilot can therefore control the rotor RPM using collective pitch – the lower the pitch the higher the rotor speed – but in practice the usable range of RRPM is very restrictive. If the rotor speed is too low the blade will stall and lose lift; too high and there will be excessive loading on the rotor hub and blade roots. The safe range is typically within 80% and 120% of the nominal power-on speed for transient excursions and between 90% and 110% for stabilized conditions.