Acoustic wind tunnel tests on helicopter tail rotor noise (HeliNOVI)

In the framework of the European HeliNOVI project, an acoustic DNW wind tunnel study was conducted with respect to 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 particle image velocimetry), 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. Aside from a reduction of rotor tip speed, the most efficient tail rotor noise reduction concept involves changing the tail rotor direction of rotation from "advancing side down" to "advancing side up".