Analytical study of an isolated coaxial rotor system with lift offset

Abstract

An analytical study, which is based on calculations from a rotorcraft comprehensive analysis of a notional coaxial rotor system in isolation, is presented to improve understanding of the aeromechanics of lift-offset coaxial rotors. The calculations include a trim analysis that enforces a fixed value for coaxial system lift in order to isolate the effect of lift offset. The quantities examined here are the blade pitch controls, coaxial system performance, blade airloads, blade structural loads, and coaxial system hub loads. Operating conditions examined form airspeed sweeps for four different values of lift offset. The maximum airspeed that the rotor may achieve trim is increased by increasing lift offset. For a given airspeed, increasing lift offset generally increases the coaxial system lift-to-effective-drag ratio as a result of a decrease in shaft power (despite an increase in drag power). Increasing lift offset decreases half peak-to-peak values of pitch link force, torsion moment, and in many cases, chord bending moment. While increasing lift offset increases half peak-to-peak flap bending moment for sufficiently low airspeeds, it may decrease this quantity for higher airspeeds. Lastly, increasing lift offset generally decreases the fixed-frame, vibratory, system hub longitudinal force and pitch moment.