A Physics-Based Approach to Trim Optimization of Coaxial Helicopters in High Speed Flight

Optimal trim was investigated for a lift-offset coaxial helicopter in high speed flight. An RCAS model of the XH-59 Advancing Blade Concept demonstrator aircraft was developed for the study and flight test trim conditions at 250 kts were used for baseline trim. The effects of varying redundant controls including rotor speed, auxiliary thrust, and differential lateral pitch were examined by trimming the aircraft at a variety of different trim states. Low power and low vibration trim states were identified and the performance and vibratory loads of these trim points were compared to the baseline trim condition. Reduction in power requirements of 17% were observed for a reduction in rotor speed combined with reduced differential lateral pitch. Power savings came in part from rotor slowing and the corresponding re-distributing of rotor lift and avoiding drag divergence at the advancing blade tips. The optimized trim state also resulted in an aft tilt of the tip path plane and upwash through the rotor disk, further decreasing main rotor power. Decreasing the blade pitch on the retreating side through a reduction in differential lateral pitch resulted in reduction of reverse flow drag. Rotor slowing was limited to 70% of the nominal RPM because of resonance of the rotor first flap and chordwise modes at the 2/rev frequency. Further reductions in rotor speed produced a small reduction in power required but large increases in vibratory loads.