Acoustic Effects Of Rotor-Wake Interaction During Low-Power Descent

Helicopter blade-vortex interaction noise has been measured in partial-power descent using a microphone array mounted external to the vehicle. The array was designed to permit spatial location of dominant slap noise sources while mapping intensity levels with forward speed and descent rate. Vortex interactions were predicted using a free-wake aerodynami analysis, and show good trend correl a ti on with noise intensity variations wi th descent rate. However, the predicted intersection azimuth positions yield only fair agreement with position data. Vortex interaction aer odynami cs are modeled simply by two-dimensional, quasi-static theory and indicate that shock formation and stall may be responsible for intense slap noise.