PROPULSION SYSTEMS FOR ROTARY WING AIRCRAFT WITH AUXILIARY PROPULSORS.

To achieve flight speeds of 200 to 300 kt, rotorcraft require auxiliary propulsion, such as propellers. To power them, the transfer of engine power from lifting to propulsion may be accomplished within the engine, by pilot variation of internal geometry to shift power from rotor to propeller drive, or outside the engine by control of variable features of the aircraft drive systems described. Special-purpose 'convertible' engines of limited application must be developed for 'internal' transfer. Conventional general-purpose turbine engines may be used in rotorcraft designed for 'external' transfer. Two 'external' examples are the Sikorsky ABC and the Bell Helicopter Textron Tilt Rotor. Both require rotor RPM reduction of about 30 per cent at 250 knots. The earlier anticipated problem of loss of power turbine efficiency, hence power, is dissipated by the ram recovery of modern turboshaft engines which is sufficient to compensate or reverse the loss. It is concluded that the costly and time-consuming development and production of special-purpose 'internally' variable convertible engines are not necessary to accomplish the desired power conversion. Conventional, general-purpose modern turboshaft engines are capable of doing the job in rotorcraft designed for this purpose and, importantly, also in conventional helicopters.