Slowed hingeless rotor wind tunnel tests and validation at high advance ratios

Abstract

In forward flight, slowing down a rotor alleviates compressibility effects on the advancing side blade tip, extending the cruise speed limit, and inducing high-advance-ratio flight regime. Previous wind tunnel tests have shown that an articulated rotor trimmed to zero hub moment generates limited thrust at high advance ratios, because the advancing side needs to be trimmed against the retreating side with significant reverse flow, where the rotor is ineffective in generating thrust. A rigid hingeless rotor with lift offset may help overcoming this problem. A series of wind tunnel tests were conducted to investigate the behavior of slowed hingeless rotors at high advance ratios. Performance, control angles, hub vibratory loads, and blade structural loads were measured and compared with comprehensive analysis predictions. The results demonstrate that a hingeless rotor with lift offset is more efficient in generating thrust and exhibits higher lift-to-drag ratio at high advance ratios. The blade structural load level is significantly higher compared with an articulated rotor, especially its 2∕rev flap bending moment, which can pose a critical structural constraint on the rotor.