Variations in Thrust Sharing for Torque-Balanced Lift-Offset Coaxial Rotors

This paper presents numerical calculations, on coaxial rotor systems, that show the variations of rotor thrust share with lift offset, system thrust, and advance ratio. The calculations are based on a coupled analysis between the U.S. Army's Rotorcraft Comprehensive Analysis System (RCAS) and a viscous vortex particle method. The level of agreement between calculations and experimental data is good in hover and mixed in edgewise flight. In hover and zero lift offset–a condition in which the upper rotor typically produces more thrust than the lower rotor–changing system thrust does not significantly alter the rotor thrust share. For advance ratios up to approximately 0.25, independently increasing lift offset increases the lower rotor thrust; this is an aeroelastic phenomenon that is eliminated with rigid blades. Independently increasing advance ratio leads to three distinct regions with different thrust sharing behaviors; these behaviors are governed by changes to the longitudinal skew of the upper rotor wake and its proximity to the lower rotor.