Aeromechanics Analysis of a High-Advance-Ratio Lift-Offset Coaxial Rotor System

The aeromechanics of a model-scale, lift-offset rotor system designed for high-advance-ratio forward flight were investigated by means of a numerical comprehensive analysis as well as hover and wind-tunnel testing. The 2-m-diam rotor system was tested in single-rotor as well as in coaxial counter-rotating rotor configuration. The focus was on the development of the computational model and its validation for advance ratios of up to 0.5, at lift offsets up to 20%. Blade structural characteristics were modeled according to the measured elastic properties, and reduced-order aerodynamics modeling using a free-vortex wake method was used to ensure computational efficiency. Numerical predictions of coaxial rotor performance, cyclic controls, steady hub forces, and blade clearance at multiple advance ratios and varying lift offsets in general correlated well with the measurements and particularly well for medium to high advance ratios (mu = 0.3-0.5). Discrepancies at high lift offsets may be related to an underprediction of the cyclic controls. Vibratory hub loads correlated well with the measurements in both trends and magnitudes for the single rotors. For the coaxial configuration, good agreement was found in the vibratory thrust and drag, whereas the side forces captured the trends but underpredicted the magnitudes seen in the experiments, which included significant higher-frequency content.