Wind Tunnel Test on a slowed Mach-Scaled Hingeless Rotor with Lift Compounding

It is a well known fact that the forward speed of a single main rotor helicopter is limited because of the compressibility effects on the advancing side and reverse flow and dynamic stall on the retreating side. Compound helicopters are a viable option which could increase the forward speed while minimizing hover penalty. Much analysis has been carried out on compound helicopters but the test data are lacking. The present research aims to systematically bridge this gap, the first step of which was to study lift compounding. Slowing down the rotor and lift offset are two key ways to increase the forward speed. Slowing down the rotor requires the rotor to fly at high advance ratios and involves more complex aerodynamic phenomena. A series of wind tunnel tests were conducted to understand rotor behavior with hingeless hub at high advance ratios with a stub wing installed on the retreating side to balance the rolling moment. This allowed the rotor to operate efficiently at high advance ratios by producing more lift on the advancing side and less on the retreating side. The rotor was tested up to an advance ratio of 0.7 at different shaft tilt angles and wing incidence angles. The experimental results including rotor performance, controls, blade structural loads, and hub vibratory loads were measured and compared to predictions with in-house comprehensive analysis, UMARC. Comparison between different wing incidences at constant total thrust provided many insights into the compounding. Significant advantages of lift compounding were identified as the helicopter could fly faster with greater efficiency, requiring lower rotor collective. The blade flap bending moment was identified as a key constraint on the maximum speed.