Acoustic Phasing and Amplification Effects of Single Rotor Helicopter Blade-Vortex Interactions

Simplified mathematical models of Blade-Vortex Interaction (BVI) noise are developed to help gain insight and understanding of the BVI noise radiation process. These models highlight the importance of BVI wave coalescence, Doppler effects and dipole directivity. It is shown that two major BVI events dominate the radiated noise for the 2-bladed OLS rotor in a normal descent to landing condition: a supersonic decelerating interaction and a broadside interaction. Both interactions are capable of radiating large amounts of acoustic energy away from the rotor. Longitudinal and lateral directivity profiles are presented in order to illustrate the highly directional nature of acoustic waves radiating from these BVI interactions. The supersonic decelerating interaction is shown to be a strong function of Mach number and radiates near in-plane BVI noise directly ahead of the helicopter’s flight path.