Results of Flyover Acoustic Measurement of a Pusher-Puller Propeller Aircraft

Kurzfassung

Extensive flyover acoustic measurements have been performed with a Cessna F337F “Super Skymaster” at the DLR airport in Cochstedt, Germany. This aircraft is equipped with a pusher-propeller at the rear part and a puller-propeller at the front. It further features an H-tail. The test matrix comprised straight constant flyovers at an altitude of 90m over the microphone stations alternately from two opposite directions such that potential atmospheric effects can be averaged out later. The aircraft speed, the rotational speeds of the propellers, and the settings of flaps and landing gear have been varied during the test campaign. Overall 15 single microphones installed inversely on ground plates and on 1.20m poles acquired the flyover sound of the aircraft. The microphones were distributed over several measurement stations located underneath the flight path and laterally spaced to the flight path. The sampling rate was 48kHz. In addition to the onboard navigation systems, a ground-based optical device was used for improved localization of the aircraft. Besides the acoustic data, flight data also has been acquired. The resulting database can serve for validation purposes regarding numerical simulations of propeller aircraft noise or aircraft noise modelling, and also reveals key features of the Skymaster propeller noise and influencing factors. In this paper, data from ground-based microphones positioned underneath the flight track will be assessed. Results show that the spectral noise content is char acterized by distinct, high-amplitude tones at the harmonics of the blade passing frequency (BPF) which are also present up to the 30th BPF. On the one hand, an increase in the rotational speed of the front propeller and in flight velocity has the largest impact on the maximum noise level. Increasing the rotational speed of both propellers from 1800RPM to 2600RPM leads to an increase of about 6dB in maximum sound level perceived on the ground. Concerning the rotational speed, the maximum sound levels on the ground are observed when both propellers operate at the maximum rotational speed tested. On the other hand, the deployment of the landing gear and the deflection of the flaps raise the broadband noise level but does not significantly alter the maximum OASPL values as received on the ground.