Wall Pressure Fluctuation on the Aircraft Fuselage: Part 1 Flight Test and Analysis of Anomalies

One of the main source driving the cabin sound level of an aircraft is the turbulent boundary layer (TBL), specially during cruise condition. The pressure fluctuations underneath the TBL along the external fuselage of aircraft have been studied extensively. Several engineering models have been proposed, mainly semi-empirical ones for zero pressure gradient but yet reliable models are still needed. Flight test measurements are mostly performed at the window location where complete flushness is not guaranteed. Other protrusions (sensors, steps, …) are also prone to affect the TBL but its influence in the downstream region is not known to the knowledge of the authors. The authors present the effect of the aircraft sensors on the TBL spectrum measured using surface microphones. The recording of the data is continuous during the whole flight test, then stable conditions in terms of Mach number, altitude, angle of attack and engine power settings are selected. Thanks to the continuous recording, anomalies in the overall sound pressure level and spectrum versus time were discovered. Sudden increases (switch on/off) of 3dB were observed, which was due to the change in angle of attack (AOA) after systematic analysis of the flight test data at several microphones. Thus a series of steady state CFD analysis was performed at different altitude and different AOA without any sensor. The surface streamlines starting from the surface microphones, also pass through the locations of aircraft sensor at specific AOA, providing the explanation of the switch on/off behavior.