Experimental Investigation of the Aeroacoustics of Small Rotors in Hover and Forward Flight

Abstract

The mechanisms of sound generation of broadband noise of small rotors are investigated for different flight conditions in an experimental study. A novel microphone array with 512 MEMS microphones is used to investigate the sound emission. Beamforming algorithms are applied to localize the sound generation areas of the broadband noise on the rotor blades. Additionally, the optical flow measurement techniques particle image velocimetry and background-oriented schlieren are used to investigate the aerodynamic mechanisms on which the noise is based. The experiments are supplemented by simulations using a panel method. The experiments show that trailing edge noise at 80% of the rotor radius is the dominant sound source mechanism for broadband noise in the frequency range 1 − 8 kHz in hover. In forward flight, the noise emission depends strongly on the flight condition. When the rotor is tilted against the incoming flow (decelerating maneuver), a C-shaped sound source area occurs in the front of the rotor disk, which can be attributed to interactions of blade tip vortices in the rotor wake with the subsequent rotor blades. In addition, two areas of sound generation on the advancing and retreating rotor side are detected when the rotor is tilted backwards by up to 10◦. These are caused by the interaction with the super vortices that form on both sides of the rotor during forward flight.