Propulsion Systems for Rotary Wing Aircraft with Auxiliary Propulsors

Pressure for improvement in both civil productivity and military capability has led to ever increasing demand for higher flight speeds for rotary wing aircraft while retaining their unique low speed operating attributes. In order to achieve higher speeds efficiently, it is necessary to supplement or replace the forward propulsion capability of the lifting rotor with auxiliary propulsion. Two promising concepts, that are in development, are the Sikorsky Advancing Blade Concept (ABC™) and the Bell Tilt Rotor. For different reasons, both require reduced rotor RPM at high cruise speeds. The reduction of the ABC rotor is needed to avoid compressibility losses on the advancing blade tips. The tilt rotor RPM must be reduced, in the propeller mode, to achieve efficient blade angles. In the ABC concept, conversion of engine power from lifting to propelling may be accomplished either internally or externally with respect to the engines. Special purpose engines, called "convertible engines", must be developed to accomplish the power transfer internally. External transfer, i.e. in the ABC drive system, may be accomplished smoothly and efficiently using conventional type helicopter turboshaft engines. The Tilt Rotor concept inherently accomplishes conversion externally by changing the function of its aircraft rotors from lifting to propelling, also with no internal change to its conventional type turbo shaft engine. It is concluded that internal conversion, which requires costly development of engines of limited applicability, is not necessary to achieve convertibility.