Helicopter thickness noise reduction possibilities through active on-blade acoustic control

Abstract

A new theoretical preliminary design method, based upon linear acoustic theory, is presented to review the possibility of controlling helicopter thickness noise by on-blade control. Near elimination of thickness noise in the plane of the rotor, in the direction of forward flight, by using on-blade acoustic sources (monopoles) and in-plane forces (dipoles), has been theoretically shown to be possible at a single target observer location in the acoustic far field, with some limitations. The resulting control time histories are shown to be low frequency in nature and use the basic physics of the noise generating process to cancel the noise. When the rate of change of mass flow or the rate of change of in-plane force is the method of acoustic control, noise cancellation is effective above and below the rotor plane, but it is not as effective for small changes in the observer azimuth angle. Low harmonic controllers are effective in canceling noise over a larger off-target observer space but need larger control values, whereas higher frequency controls are effective over a small off-target region but are associated with lower values of control. Out-of-plane control forces are shown not to be an effective means of cancelling thickness noise. Copyright © 2009 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.