HIGHER HARMONIC AND TRIM CONTROL OF THE X-WING CIRCULATION CONTROL WIND TUNNEL MODEL ROTOR

A wind tunnel test of a 1/6 scale aeroelastic X-Wing model rotor was recently completed in the 18 foot section of the United Technologies Research Center Main Wind Tunnel. The model was tested at normal rotor speed up to 175 knots and at 150 knots for a range of steady rotor speeds typical of conversion. One of the key objectives of this test was to demonstrate that higher harmonic control (HHC) could effectively suppress fixed system vibration (4P) throughout a wide range of rotor speeds and forward velocities. A flexible man-in-the-loop HHC and trim control system was employed to simultaneously reduce stand 4P accelerations and rotor balance 4P loads while maintaining rotor trim via automatic control of the 24 blade root pressure valves. With some exceptions, it is shown that 70% to 95% reductions in fixed system vibrations are realized. The control system also proved to be an invaluable aid to achieve rotor trim and blade load reductions when required. Two additional objectives were to characterize blade vibratory loads behavior during HHC and to evaluate any potential for unstable HHC-trim coupling in future closed loop HHC systems. Projections show that to achieve highly effective vibration control via closed loop HHC may require attention to trim-HHC coupling. It is also shown that vibratory blade bending moments are consistently reduced even though blade moments are not directly controlled.