The development of a coaxial acoustics test system for rotor noise

This paper describes the development of a new dual rotor test stand for the Open Jet Flow-Through Anechoic Chamber at the Penn State University. The Coaxial Acoustic Test System (CATS) can accommodate the testing of isolated, tandem, and coaxial rotor configurations operating in hover, climb, cruise, and descent flight conditions. The stand is equipped with a traversable microphone array that enables acoustic measurements to be acquired at a variety of azimuthal directivity angles. This paper highlights the design process as well as some of the encountered issues and their solutions. Sample results for two representative coaxial co-rotating rotor configurations are also presented. The first configuration consists of two vertically separated “stacked” rotors mounted on a single shaft. The rotors are mounted on separate shafts in the second configuration, so that the azimuthal position of the blades varies throughout the measurement. Close agreement was achieved in the average thrust and torque measurements for the two rotor configurations. The separated configuration had higher tonal noise levels than the stacked configuration, although broadband noise levels were similar.