Vibratory load predictions of a high-advance-ratio coaxial rotor system validated by wind tunnel tests

Abstract

To investigate the aeromechanics of coaxial counter-rotating lift-offset rotor systems, a comprehensive analysis model of a laboratory-scale torque-balanced rotor designed for high-advance-ratio forward flight was developed. Measured blade and control system geometries and structural properties were input to the model. Lower-order aerodynamics modeling with a free-vortex wake method was used. While previous analytical studies on this coaxial rotor test rig have focused on performance and control requirements, in this current work, vibratory hub and pitch link loads, the influence of rotor–rotor phasing and the effects on blade deflections and tip clearance were investigated. The analysis was validated by wind-tunnel tests at advance ratios of 0.21–0.52 and for a lift offset varying from zero to 25%. Coaxial rotor performance, pitch link loads, unsteady thrust and rolling moments correlated well with the measurements. Pitching and rolling moment 2/rev and 4/rev harmonics correlated well for all lift offsets and advance ratios, whereas the vibratory torque was significantly overpredicted. The correct trends for varying lift offset and advance ratio were predicted in drag, side force, and thrust harmonics. Corresponding magnitudes were also predicted well, although an underprediction of the side force 4/rev harmonics was observed. Good correlation was found for the predicted blade tip clearance between the rotors over the entire range of lift offset and rotor–rotor phase angles, showing that advance ratio had little effect and judicial use of rotor phasing can increase the critical tip clearance.