This presentation discusses some new work being done at the Department of Aerospace Engineering, University of Maryland in the field of Rotorcraft Acoustics Analysis and its Active Control. This research is being conducted under the guidance of Senior Research Professor Fredric H. Schmitz. 

A new physics-based analytical approach to understanding and reducing near in-plane far-field impulsive rotor noise in hover and forward flight is presented. Using analytical approximations to the solution of the governing Ffowcs Williams and Hawkings equations, preliminary design estimates have been developed for low to mid frequency near in-plane harmonic noise in forward flight. These approximations, which work well for high aspect ratio blades up to moderate advancing tip Mach numbers (below delocalization, MAT < 0.85 for 9% thick airfoils), relate the time history and peak of in-plane noise to rotor design and operational parameters explicitly. Trends for linear thickness noise are studied in forward flight as a function of various governing design and operational parameters in both time and frequency domains. This analytical approach is then used to develop unsteady idealized active acoustic on-blade point controllers to cancel or reduce the in-plane far-field thickness noise. Point dipole (force) controllers are investigated in terms of their effectiveness is reducing the peak thickness noise levels in a region around the in-plane target location. Higher frequency controllers (4/rev and 5/rev) are found to require small peak-to-peak inputs but are only effective in a small region near the target observer; while low frequency controllers (1/rev to 3/rev) are found to cancel the noise over a larger region around the target observer but require larger peak-to-peak control inputs. 

The linear thickness noise generation process and control mechanisms in forward flight are found to be very similar to hover in principle, though noise levels and control requirements are lower in forward flight compared to hover at the same advancing tip Mach number.