Tilt Proprotor Composite Aircraft, Design State of the Art

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Abstract
The design of tilt‐proprotor composite aircraft is reviewed, and the performance and mission potential of this type of aircraft are indicated. Design and model‐test data for the Model 266 Composite Aircraft are used to illustrate the current state of the art. This aircraft design is the product of a recently completed Exploratory Definition performed under contract to the U.S. Army Aviation Materiel Laboratories (AVLABS), Fort Eustis, Va. The Army specified that the aircraft be optimized for maximum payland‐ton‐miles per dollar, that it hover out of ground effect at 6000 feet on a 95°F day, that its disc loading not exceed 13 psf and that it have a minimum level‐flight forward speed at sea level of 340 knots. Power‐management and flight‐control systems have been designed to provide conventional helicopter control procedures in that mode, natural straightforward convension procedures, and turboprop airplane procedures in the high‐speed mode. Proprotor‐pylon‐wing dynamic‐stability requirements are met by a redundant design approach which combines a “focused rotor” with stiff pylon mounting. Dynamic model tests conducted to substantiate the design correlate closely with full‐scale predictions. Proprotor aerodynamic considerations for efficient operation in hover and cruise are discussed. It is shown that the proper selection of rotor solidity, blade twist and tip speeds can result in high figures of merit and propulsive efficiencies. Performance and cost comparisons for light transport missions show that tilt‐proprotor aircraft could significantly improve the reaction time and the overall cost‐effectiveness of air mobility. The application of current design technology to a high‐speed mission shows that good propulsive efficiency, and consequently high mission cost‐effectiveness, can be achieved in the 400‐knot speed range. An ultimate speed potential in the 450–550‐knot range is forecast for proprotor aircraft. The design technology necessary to produce cost‐effective high‐performance tilt‐proprotor aircraft now exists. A demonstrator aircraft program is the next logical step in this development.

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