RPM Driven Extension-Torsion Coupled Self-Twisting Rotor Blades

This paper describes the effects of a composite coupled blade spar on the performance of a slowed RPM helicopter rotor in high speed edgewise flight. This study expands upon previous work that showed that antisymmetric composite coupling in the spar of a UH-60A-like rotor can provide a significant increase in the lift to drag ratio of a rotor when the RPM is reduced. The current analysis was performed using a full 3-D FEA based aeroelastic computational structural dynamics (CSD) solver, X3D, now with the inclusion of a freewake aerodynamics model. This was validated using existing UH-60A test data for high advance ratios. The current analysis shows that a blade with a nominal composite coupling can provide a maximum increase in the lift to drag ratio of approximately 1.3 at 85% of the nominal RPM (NR) of 27 rad/s and that hygrothermally stable layups do not degrade the rotor performance. However, close study of the strain in the rotor showed that a rotor with an extension-torsion coupled composite spar would unlikely be feasible at 27 rad/s as the axial strains felt by the rotor around the azimuth often exceed the allowable tensile strain of 6000 με. The details of what can be done to reduce the axial strain at all rotational speeds is part of an ongoing investigation.