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Abstract
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 (advance ratio*tensile strain​). The details of what can be done to reduce the axial strain at all rotational speeds is part of an ongoing investigation.

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