Tonal noise produced by a UAV propeller due to unsteady rotational motion

A number of recent studies have shown that unsteady rotational motion of small unmanned aerial vehicle propellers can produce tonal noise. In this paper, time- and frequency-domain methods for calculating this noise are presented and validated against one-another. The unsteady loading on the propeller blades, required for the predictions, is calculated using both blade element momentum theory and computational fluid dynamics simulations. The noise prediction methods are validated against measurements. The propeller unsteady motion during these experiments was measured using a rotary encoder and this measured rotational motion was used as input to the noise prediction methods. The results presented in this paper focus on a case where the electric motor drives the propeller in unsteady rotational motion where the unsteady motion is almost sinusoidal with a frequency equal to 14 times the shaft rotation frequency. Predictions show that this unsteady rotational motion produces high amplitude tones at the frequency of the dominant fluctuation speed and adjacent harmonics of the blade passing frequency — confirming the findings of a previous study. These predictions are shown to be in generally good agreement with measurements. In addition, the polar and azimuthal directivity of this tonal noise is investigated.