Higher Harmonic Control: Wind Tunnel Demonstration of Fully Effective Vibratory Hub Force Suppression

Closed-loop Higher Harmonic Control (HHC) has been demonstrated successfully on a dynamically scaled model of the three-bladed CH-47D Chinook rotor in the Boeing Vertol V/STOL Wind Tunnel. Vertical and in-plane 3/rev hub forces were suppressed simultaneously, completely, and continuously as the model was “flown” throughout a wide test envelope which included trimmed flight up to 188 knots. This is the most effective multicomponent suppression which has been achieved with HHC. The suppression was accomplished with a fixed-gain feedback control law which is much simpler and faster than the adaptive control laws which have been used recently by other investigators. The controller applied small amounts of oscillatory swashplate motion to produce multi-harmonic blade pitch variations up to ± 3.0 degrees at very high speed. This control caused only small changes in rotor system loads and performance. A closed-loop control response bandwidth of 1 Hz was demonstrated, which is sufficient to suppress vibration in maneuvers and gusts. These results were obtained in part by very precise swashplate control at the scaled 3/rev frequency, 69 Hz. The test was also successful in demonstrating swashplate actuators with hydraulic seals which showed little wear after 15 million high-frequency cycles, about 25 percent of the desired service life for a full-scale production design.