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This paper presents a detailed investigation into the tonal noise produced by an unmanned aerial vehicle (UAV) propeller operating with a circular strut mounted just downstream. Experimental measurements of the acoustic pressure in the near- and far-fields are presented and it is observed that the measured pressure signals contain a strong impulse caused by the propeller-strut interaction. The magnitude and shape of this impulse varies significantly with observer location. It was also observed that the magnitude of these impulses was reduced by increasing the distance between the propeller and the strut. In order to investigate the physics of the noise generation mechanism, a number of computational fluid dynamics (CFD) simulations were performed and a numerical method was developed to predict the radiated acoustic tones using the CFD data. This analysis showed that the unsteady loading sources on the strut and the propeller both made significant contributions to the total impulse and that the directivity of the sound radiated from the strut was different to that radiated from the propeller. Therefore, the addition of the signals from the propeller and the strut produced a pressure impulse which had a complex directivity. Analytical models were also developed to estimate the unsteady loading on the propeller and strut from which estimates of the radiated noise field could be made. Predictions made using these models are compared with CFD predictions and experimental measurements of the unsteady loading on the strut and also the total radiated noise field and show reasonable agreement.

This paper presents an analysis of the noise produced by contra-rotating propeller systems used on small unmanned aerial vehicles. Selected results from an extensive experimental campaign are presented. In these experiments a commercially available contra-rotating propeller with 2- and 3-blade propellers was statically tested in a large anechoic chamber. It was observed that the noise spectrum contained a large number of tones which occurred at the sum of integer multiples of the blade passing frequency of each propeller. These tones exhibit some interesting features including: the amplitude of the interaction tones decays rapidly with increasing propeller spacing; tones for which the azimuthal mode number is equal to zero generally have a dipole like radiation pattern radiating strongly along the propeller axis. This is in contrast to the tones at the blade-passing frequency which radiate strongly in the plane of the propeller. A simple theoretical model for predicting the tones produced by the contra-rotating propeller is presented and is used to explain some of the observed phenomena.

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