TILTROTOR VIBRATION REDUCTION THROUGH HIGHER HARMONIC CONTROL

The results of a joint NASA/Army/Bell Helicopter Textron wind-tunnel i nvestigation which was conducted to assess the potential of higher har monic control (HHC) for reducing vibrations in tiltrotor aircraft oper ating in the airplane mode of night and to evaluate the effectiveness of a Bell-developed HHC algorithm called MAVSS (Multipoint Adaptive Vi bration Suppression System) are presented. The test was conducted in t he Langley Transonic Dynamics Tunnel using an unpowered 1/5-scale semi span aeroelastic model of the V-22 which was modified to incorporate a n HHC system employing both the rotor swashplate and the wing flaperon . The effectiveness of the replate and the flaperon acting either sing ly or in combination in reducing 1P and 3P wing vibrations over a wide range of tunnel airspeeds and rotor rotational speeds was demonstrate d. The MAVSS algorithm was found to be robust to variations in tunnel airspeed and rotor speed, requiring only occasional on-line recalculat ions of the system transfer matrix. HHC had only a small (usually bene ficial) effect on blade loads but increased pitch link loads by 25%. N o degradation in aeroelastic stability was noted for any of the condit ions tested.