Fundamental Experimental Studies Supporting Active-Jet Acoustic Control of In-Plane Rotor Harmonic Noise

A new approach to reducing low-frequency, in-plane harmonic noise of helicopter rotors is explored experimentally, and results are compared with an extended theoretical model. The active-jet acoustic control methodology employs on-blade, tip-located, unsteady air blowing to produce acoustic waves that reduce the observed noise at targeted positions in the acoustic far field of the rotor system. An extended theoretical model of the subsonic air jet, which is idealized as a source of mass and momentum, is presented. Experimental measurements in the University of Maryland acoustic chamber for an approximately one-seventh-scale rotor, operated at a hover tip Mach number of 0.661, indicate that active-jet acoustic control is a viable option for reducing low-frequency in-plane harmonic noise. Good correlation between theoretical predictions and measured data for three valve control cases are observed. Model-scale limitations of the tip-jet blowing experiment lmited the peak noise level reductions to 30%. However, theory suggests that, if the limitations of the model-scale controller were mitigated, much larger noise reductions are possible.