AEROACOUSTIC FLOWFIELD AND ACOUSTICS OF A MODEL HELICOPTER TAIL ROTOR AT HIGH ADVANCE RATIO

Some results, relevant to rotorcraft noise generation process at high advance ratio, are presented in this paper from Schlieren flow visualization and acoustic tests of a model tail rotor. The test was conducted in the anechoic environment of the Acoustic Research Tunnel at United Technologies Research Center under National Rotorcraft Noise Research Program Contract NASI-17146 issued by NASA Langley Research Center. The measured in-plane noise trends are consistent with the growth of the tip supersonic region seen in the Schlieren visuals. Schlieren flow visuals reveal a propagating pressure wave in the second quadrant. Simultaneously measured acoustic data and the results of two-dimensional transonic Blade-Vortex Interaction analysis code ATRAN-2 indicate that this pressure wave is attributable to BVI activity in the first quadrant. This paper establishes that the transonic Blade-Vortex Interactions contribute to noise at high advance ratio level flight conditions.