Acoustic Scattering of Rotorcraft Noise

The scattering of helicopter rotor noise by the fuselage has not been a focal point of previous noise prediction studies. A new approach is presented which uses PSU-WOPWOP to calculate the rotor noise coupled with NASA’s Fast Scattering Code (FSC) to calculate the scattered noise field. The FSC uses the acoustic pressure and pressure gradient, supplied by PSU-WOPWOP at a specific frequency, to predict the manner in which helicopter bodies scatter noise. Representative BO-105 and heavy-lift quad tiltrotor cases are studied. The frequencies in each case was chosen such that the wavelength of the incident noise was comparable to the characteristic size of the scattering body. Significant tail rotor scattering was observed on an observer plane below the configuration, but the scattering of main rotor noise was negligible. Significant scattering was found on a plane beneath the configuration when the quad tiltrotor body was in the plane of the rotors. When the rotors were elevated so that the in-plane noise did not impinge upon the fuselage, little scattering was observed. All cases demonstrate the potential importance of acoustic scattering by the airframe components.