Experimental and analytical investigation of contra-rotating multi-rotor UAV propeller noise

Abstract

Contra-rotating propellers could increase the efficiency and lifting capacity of multi-rotor unmanned aerial vehicles (UAVs); however, contra-rotating propellers can produce significant noise levels. This work experimentally investigated the noise from static contra-rotating UAV propellers in an anechoic chamber. The effects of propeller diameter were studied by testing different configurations of 12″ and 15″ propellers. The effects of propeller spacing, rotational speed and blade number were also investigated; around 1400 propeller configurations were tested in total. It was observed that interaction tones were a dominant contributor to the overall noise level, which also contained rotor-alone tones and broadband noise. A theoretical model for the far-field interaction tone noise produced by a contra-rotating propeller is presented and used to explain the tonal noise levels observed in the experimental results. It was found that the tones with a zero azimuthal mode order dominate the noise spectrum close to the propeller axis. It was shown that the amplitude and number of these tones with zero azimuthal mode order could be reduced by using contra-rotating propellers with mismatched blade numbers. It was also experimentally shown that increased spacing between the propellers generally reduced the amplitude of the interaction tones. The rate of this reduction in amplitude with spacing between the propellers was dependent on frequency and spacing. High rates of decay were generally observed at small spacings and lower rates at larger spacings. However, the decay was neither monotonic nor at a uniform rate for all frequencies, which suggests that there could be a change in the noise generation mechanism as the spacing increases. Finally, the validity of the indoor static results was demonstrated by comparing the results to an in-flight test of a quadcopter with contra-rotating propellers.