Wind Tunnel Test on a Slowed Mach-Scaled Hingeless Rotor at High Advance Ratios

Slowing down the rotor in forward flight is a viable means of extending the cruise speed of a rotorcraft by alleviating compressibility effects at the advancing side blade tip. It was shown by previous wind tunnel tests that an articulated rotor trimmed to zero hub moment generates limited thrust at high advance ratios, because the advancing side of the rotor needs to be trimmed against the retreating side in the reverse flow state, where the rotor is ineffective in generating thrust. Therefore, a hingeless rotor that allows the advancing side to generate more thrust can be rewarding in overall thrust potential. At the University of Maryland, a rotor test stand was modified for hingeless rotors and two wind tunnel tests were conducted to investigate the behavior of hingeless rotors at high advance ratios. The experimental results, including performance and control, hub vibratory loads and blade structural loads, are presented in this paper and compared with predictions of the in-house comprehensive analysis, UMARC. The performance results demonstrate that a hingeless rotor with lift offset is capable in generating more thrust at high advance ratios, and the blade structural load results reveal that the predominant structural constraint on the rotor is in 2/rev flap bending moment, combined with 1/rev flap bending moment from the lift offset.