THE EFFECTS OF HIGHER HARMONIC CONTROL ON BLADE-VORTEX INTERACTION NOISE AND VIBRATION

A rotor acoustics test was conducted to examine the use of higher harmonic control (HHC) of blade pitch to reduce blade-vortex interaction (BVI) noise. A dynamically scaled, four-bladed, articulated rotor model was tested in a heavy gas (R-12) medium in Langley's Transonic Dynamics Tunnel. Acoustic and vibration measurements were made for a large range of matched flight conditions where prescribed (open loop) HHC pitch schedules were superimposed on the normal (baseline) collective and cyclic trim pitch. A new sound power measurement technique was developed to take advantage of the reverberant field in the hard walled tunnel. Special calibrations permit straightforward acoustic scaling of the results to those one would obtain in air. Quantitative sound power results are presented for a 4/rev (4P) collective pitch HHC. By comparing the results using 4P HHC to corresponding baseline (no HHC) conditions, significant mid-frequency noise reductions of 5-6 dB are found for low-speed descent conditions where BVI is most intense. For other flight conditions, noise is found to increase with the use of HHC. Low frequency loading noise, as well as fixed and rotating frame vibration levels, showed increased levels with HHC.