Aeroacoustics research in Europe: The CEAS-ASC report on 2005 highlights (edited by Spyros G. Voutsinas): 7.2. Acoustic and aerodynamic test results from HeliNOVI main rotor/tail rotor/fuselage test in DNW

In the framework of the European HeliNOVI project, an acoustic DNW wind tunnel study was conducted into helicopter tail rotor noise. The goal of the tests was to investigate (1) the importance of tail rotor noise for different flight conditions (2) main rotor/tail rotor interaction noise, and (3) tail rotor noise reduction concepts. Besides the conventional measurement techniques, such as an inflow microphone traverse, blade pressure transducers and PIV, an out-of-flow phased microphone array was applied to locate and quantify the different helicopter noise sources. The test results indicate that tail rotor noise is most important for climb and high-speed level flight. Furthermore, it is found that main rotor/tail rotor interactions only have a small effect on the overall noise levels. After a reduction of rotor tip speed, the most efficient tail rotor noise reduction concept involves changing the tail rotor sense of rotation from 'advancing side down' to 'advancing side up