Cost-Effectiveness Benefits of a Variable Diameter Rotor System as Applied to Advanced VTOL Aircraft

A variable diameter rotor system could provide many benefits to high speed rotary wing VTOL aircraft. A parametric mission analysi's of VTOL configurations varying from pure helicopter to compound lift systems to stowed Telescoping Rotor Aircraft (TRAC) has been conducted. to identify these potential benefits, particularly in terms of fleet life-cycle cost. The 400-knot stowed TRAC rotor aircraft shows the greatest potential for high productivity and low fleet costs, followed by the 300-knot full rpm TRAC compound helicopter. These benefits are in addition to special advantages that can derive from high cruise speed, such as increased mission versatility or reduced vulnerability. Despite the high cruise speeds, the fuel consumptions of the TRAC rotor aircraft are comparable in magnitude to the lower speed aircraft configurations because of substantially superior overall aircraft lift-drag ratios. Relative aircraft unit costs tend. to follow relative weights, with the pure helicopter being the least expensive because it is the lightest. Fleet costs, however, follow a different trend because fewer high-speed. aircraft than low-speed aircraft are required for a given fleet transport capability. Fleet life-cycle costs show the bene- fit to a greater extent than acquisition costs. For the transport scenarios studied, the TRAC rotor aircraft all have lower life-cycle costs than the conventional rotor aircraft. The 400- knot stowed TRAC rotor aircraft has the lowest fleet life-cycle costs, 20 percent below the best of the conventional aircraft. Wind tunnel testing of an aeroelastically scaled model has successfully demonstrated diameter changes and rotor starts and stops at 150 knots true air speed, as well as operation in the compound helicopter mode up to 400 knots true airspeed. These tests also provided quantitative information on substantial benefits to reduced vibration, gust response, and blade stresses that are available from variable diameter. More recent pletion of the preliminary design of a full-scale rotor system for test in the NASA Ames 40-by-80- foot wind tunnel or on the NASA/Army Rotor Systems Research Aircraft (RSRA). Laboratory tests of a full-scale jackscrew/nut system have also successfully demonstrated operation under full simulated flight load conditions. These accomplishments provide a solid technological base for the TRAC rotor system, giving a degree of realism to the parametric mission studies not available for many advanced rotor system concepts.