Aerodynamic Design Optimization of a Variable Diameter Tilt Rotor

Development of efficient proprotors is one of the keys to designing viable high speed tiltrotors. The Variable Diameter Tilt Rotor (VDTR) concept combines the advantages of a small diameter rotor in cruise and a large diameter rotor in hover and thereby reduces the rotor design compromise between hover and cruise performance. For a VDTR, obtaining a reasonable optimized configuration depends not only on optimizing design variables such as chord, twist, and sweep, but also the rotor cruise diameter/hover diameter retraction ratio (RR), hover tip speed, and hover design CT/σ. As a result, Sikorsky Aircraft has explored a new method to quickly identify optimized blade geometries for a specified tiltrotor mission. One unique aspect of the approach is the ability to address highly nonlinear distributions of blade chord, twist, and sweep. These parameters can be optimized at each location on a 16 station segmented blade. By using 48 separate aerodynamic design variables, chord, twist, and sweep patterns are found that improve combined hover and cruise efficiencies. This approach was used to calculate optimum rotor design trends from which a VDTR rotor, based on the early NASA Short Haul Civil Tiltrotor SH(CT) guidelines, and capable of cruising at 350 kts was designed.