Experimental and Computational Investigation of Stacked Rotor Acoustics in Hover

Abstract

Acoustic characteristics of two- and four-bladed rotors with a 1.108 m radius in hover were measured experimentally at rotor speeds up to 1200 RPM and tip Mach number up to 0.41. The Rotorcraft Comprehensive Analysis System (RCAS) using a viscous vortex particle method (VVPM) coupled with PSU-WOPWOP were used to simulate the acoustic characteristics of the same rotors. Simulations were also conducted for a stacked rotor over a range of azimuthal spacings with zero axial spacing. PSU-WOPWOP predictions included thickness, loading, and broadband noise. Both experiments and simulations showed that broadband noise was the dominant contributor to A-weighted sound pressure level at a distance of 8.25 radii from the rotor center. Simulations showed a sharp increase in the broadband and overall noise at higher thrust, but the experiments did not show such an increase. Higher harmonic components of the tonal noise were nearly as large as the blade passage frequency noise in the experiments, but were underpredicted in the simulations by up to 30 dB. Simulations incorporating wind showed closer agreement in the higher harmonic tonal components. The unweighted overall sound pressure level in the stacked rotor simulations showed 5 dB to 10 dB variations with azimuthal spacing, but the A-weighted tonal and overall SPL was insensitive to azimuthal spacing.