Full Scale: Flight Test Characterization of Aircraft Fuselage Surface Pressure Fluctuations

In flight testing there are a lot of noise sources present and each of them possibly contributes to cabin noise. One major source is the jet engine with its narrow band fan noise and its broadband jet noise. The other major source contributing to cabin noise are pressure fluctuations from turbulent boundary layer. On the surface of the fuselage pressure fluctuations from both sources are present simultaneously and they cannot be separated using a single microphone. In order to distinguish between the major pressure fluctuations present, the different propagation speeds between acoustic and hydrodynamic pressure fluctuations have to be evaluated using an array of transducers mounted on the surface. In the investigation presented, representative measurements from a flight test will be shown where both, the acoustic and the hydrodynamic pressure fluctuations are present. The different propagation mechanisms will be identified by means of speed and by means of characteristic appearance. An attempt to separate the pressure power provided by each of the two separate propagation mechanisms will be made. With the use of such tools, the identification and qualification of different possible sources responsible for surface excitation is possible. This information can be used for prediction of cabin noise using simplified structural models. The use of the tools is not limited to aircraft use, but has been successfully implemented on various types of transportation, such as trains and cars.