Vibration Reduction Using Individual Blade Pitch Controls for Lift-Offset Coaxial Rotor Vehicles at Low and High Speeds

Vibration reduction using individual blade pitch controls (IBC) for lift-offset coaxial rotor vehicles is investigated in this study. A baseline XH59A coaxial rotor model has been developed first, and the rotor hub vibration characteristics are examined at a wide range of vehicle forward flight speed. An open-loop IBC control study is then conducted with a phase sweep to check its effectiveness on both 3/rev and 6/rev rotor hub vibratory loads. Closed-loop IBC controls are implemented using multiple IBC control input signals at 2, 3, 4/rev and 5, 6, 7/rev to reduce 3/rev and 6/rev hub vibrations. The results indicate that significant 3/rev hub vibrations reduction can be achieved. The overall vibration reduction is more than 90% at 120 Kts speed or higher, at speeds lower than 120 Kts is about 65-85%; however, the excessive peak IBC control inputs required to reduce the 3/rev hub loads indicate that an improved IBC control algorithm with constraints on IBC control effort is needed to make it practical. The 6/rev hub loads are very sensitive to 5, 6, 7/rev IBC control inputs. The peak required IBC control input signals are only about 0.6 to 1.3 degrees to achieve more than 90% reduction of 6/rev rotor hub vibrations.