Aerodynamic Loading of High Advance Ratio Rotors with Blunt Trailing-Edged Blades

The reverse flow region developing on high-speed edgewise rotors is an inherent aerodynamic limitation, confining the flight speed and the efficiency of modern rotorcraft. To enable high advance ratio flight, the impact of the rotor blade geometry on the aerodynamic blade loads in the reverse flow region was investigated in the present study. A conventional, sharp trailing-edged NACA0012 was compared to a novel, blunt edged elliptical airfoil. A single-bladed rotor rig equipped with a six-axis force-torque sensor was used to perform water tow tank experiments. Advance ratios between 0.40 and 1.00 at collective pitch angles from 13° to 25° with 0° cyclic pitch were studied. Highly unsteady blade forces were measured over the rotor azimuth due to the superposition of the rotational and translational blade motions in the three-dimensional rotor environment. At an advance ratio of 1.00, negative lift and drag forces were detected on more than 75% of the azimuth angle range on the retreating side of the rotor disk due to flow reversal and separation effects. The lift decreased to approximately zero on the retreating blade at an azimuth angle slightly above 270° at an advance ratio of 0.40. The elliptical airfoil reduced the lift magnitude, lowering the blade performance on the advancing side of the rotor disk. However, the elliptical airfoil decreased the magnitude of the drag, the rotor torque, and the pitching moment in the reverse flow region which increases the rotor performance. The current results highlight the significance of the blade cross-sectional geometry, specifically the geometric trailing edge curvature, and its impact on the rotor performance.