Validation of acoustic directivity patterns from static and dynamic flight measurements

Two methods have been proposed to measure the acoustic emissions for small unmanned aircraft systems (UAS): 1) dynamic flight through an outdoor microphone array, and 2) static test cell measurements. Both methods produce source directivity patterns for predicting levels where recordings had been made during dynamic flights. In autonomous flight the engine rotations-per-minute (RPM) are constantly adjusting to maintain the commanded aircraft airspeed and altitude. This violates the assumptions of stationality in acoustic emissions over a single pass that are inherent in the dynamic flight method, which results in greater uncertainty in the predictions of levels on the ground. The static method can capture the acoustic emissions as a function of the engine RPM, which can then be used to more accurately predict the levels on the ground for a single pass given actual engine telemetry data. These results suggest that the prediction of levels on the ground for a future flight will require characterization of the variability of engine RPM with the corresponding acoustic emission patterns. The static method can quickly capture the required data, while the dynamic method may require an increased number of flight passes than are typically required for characterizing a manned aircraft.