A Physics-Based Approach to Trim Optimization of an Articulated Slowed-Rotor Compound Helicopter in High-Speed Flight

The present study considers a compound helicopter with an articulated main rotor and uses a physics-based approach to arrive at, and understand, the optimal trim states at high-speeds in the 225-250 kts range. Simulations are based on a compound derivative of a modified UH-60A helicopter with a 20,110 lbs gross weight operating at sea level. The study shows that it is critical to use ailerons on the wing to counteract the propulsor torque and reduce rotor lateral cyclic pitch requirement and lateral cyclic flapping. Also, the stabilator needs to be used to fly the aircraft at a nose-level pitch attitude and bring the wing-lift fraction in the range of 75% and upward. With the ailerons and stabilator used to counteract propulsor torque and adjust aircraft pitch attitude, rotor RPM and propulsor thrust can be used to seek low power states. At both 225 and 250 kts the lowest power states correspond to high wing-lift fraction, low rotor RPM (advance ratio greater than 1), negative longitudinal cyclic flapping (blowback) producing an upwash through the rotor disk and putting the rotor in a near-autogyro, or even a windmill state. The high advance ratios increase the prominence of the reverse flow region, which can account for 30-40% of the total rotor drag.