Using Control Redundancy for Power and Vibration Reduction on a Coaxial Rotor Helicopter at High Speeds

This study uses the Rotorcraft Comprehensive Analysis System (RCAS) to examine the effect of control redundancy on power and vibratory hub loads of a lift-offset coaxial rotor helicopter operating at 230 kt cruise speed. An aircraft nose-up pitch attitude of 3° resulted in very low main rotor power (less than 10% of the total power), with the majority of the power consumption attributed to an efficient axial propeller. At this 3° pitch attitude, the rotor speed and differential lateral pitch, which are redundant controls, were parametrically varied, and low power (LP) and low vibration (LV) states identified. The LP state (80% Nr and 3° differential lateral) required 3.5% lower power than the LV state (90% Nr and 0° differential lateral), but the latter had substantially lower 3/rev vibratory hub loads. The lower power in the LP state is primarily due to reduced main rotor power on account of smaller drag on the advancing blade tip at lower rotor speeds. The rotor drag is comparable for the two states, but the LV state has larger drag contributions from the advancing side, whereas the LP state has larger contributions from the reverse flow region (accounting for 14% of the total rotor drag) due to higher pitch on the retreating side and larger reverse flow velocities. Even so, the rotor drag accounts for under 30% of the total propulsor thrust requirement, with the fuselage (and hub) drag being the dominant component. Rotor L/De values for the LP and LV states were 12.3 and 11.3, respectively.