Radiation and Directionality Characteristics of Helicopter Blade-Vortex Interaction Noise

摘要

The geometric factors that govern the radiation of helicopter blade-vortex interaction (BVI) noise are explored using simplified aerodynamics and wake modeling. Formulation of the BVI problem is restricted to a constant strength, epicycloid tip-vortex system that is initially confined to a plane parallel to the tip-path-plane at a fixed distance below the rotor. The resulting equations, when solved, illustrate the connection between the trace Mach number profile of each interaction and the phased summation of acoustic sources for each individual BVI. Areas of peak acoustic radiation are identified on a radiation sphere surrounding the helicopter as a function of advance ratio and hover tip Mach number for a two-bladed rotor. BVI noise radiation patterns show that the peak noise of each distinct BVI is very directional and is primarily a function of the advance ratio of the helicopter. The directivity of the peak level of each interaction changes in a continuous manner as advance ratio is increased―first radiating predominantly in the direction of the flight path near the plane of the rotor for oblique BVI, and then increasing in level and radiating more downward to the advancing side of the rotor for near parallel BVI. When the miss-distances of the tip-vortices are allowed to vary, the closeness of the younger shed vortices associated with oblique BVI tend to make the intensity levels of the oblique and near parallel interactions similar for many operating conditions. For flight along constant angles of descent at varying advance ratios, the direction of peak helicopter BVI becomes discontinuous with advance ratio―suddenly switching when different BVI become dominant.