The High Speed Challenge for Rotary Wing Aircraft

This paper reviews the problems associated with developing a vertical takeoff and landing (VTOL) aircraft that has desirable helicopter-like attributes in hover and low speed operation but is capable of efficient high subsonic cruise speed. A number of different configurations that have been proposed are reviewed and an assessment is made of the relative probabilities of future success. Factors considered to be important discriminators include speed potential, disk loading, empty weight fraction, the need for supplementary propulsion systems or convertible engines, and technical risk. The tilt-rotor configuration has considerable merit but will not achieve the highest speeds desired. It is concluded that incorporation of variable geometry, in the form of a variable diameter rotor system, has the best chance of providing the “ideal” VTOL. The variable diameter tilt-rotor adds on the order of 100 knots to the speed potential of the tilt-rotor and provides numerous other benefits as well. For highest speeds, the variable-diameter single stowed rotor configuration has the desired combination of attributes.