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

This study examines a compound helicopter with an articulated main rotor, focusing on the high advance ratio, low disk loading environment attained at a flight speed of 225 kt. The objective of the study is to examine how aircraft power and hub vibrations can be minimized through the use of redundant controls available on a compound configuration. The compound model is based on a UH-60A derivative with 20,110 lb of gross weight with simulations performed using the Rotorcraft Comprehensive Analysis System. Untwisted blades are compared against –8° linearly twisted blades. It was observed that for the rotor with –8° twist, the minimum power and minimum vibration states corresponded to very different redundant control settings and main rotor behavior. The low-power state occurs at lower RPM, lower stabilator pitch, and higher auxiliary thrust, with lower forward flapping of the main rotor. At a state with reduced vibrations, main rotor forward tilt is higher, resulting in weaker blade–vortex interactions over the aft section of the rotor disk. For an untwisted rotor, it was shown that the controls for minimum power and minimum vibrations are more similar, and the rotor operates best with lower rotor speed and reduced flapping relative to the rotor with twist. Untwisting the rotor and providing proper control settings can reduce the power requirement by 7–16% and the hub vibrations by 50–80%, relative to a –8° twisted blade with proper control settings. Control of lift offset through the use of ailerons did not provide any further reductions for either twist case.