Evaluation of the helicoidal surface theory for the noise prediction of small UAS propellers

Despite the great number of available models for the prediction of noise produced by large-scale propellers, little is known about their capability of predicting the noise of small-sized rotary-wings, such as found in drones. As these systems operate at much lower Reynolds numbers, different mechanisms of sound generation are expected to take place. This work investigates the capabilities of a gold standard theory, originally developed to assess the tonal noise of large propellers, at predicting the sound of small rotary wings. Thus, a model based on the helicoidal surface theory is considered. The first step of the analysis is carried out by conducting an experimental campaign to characterize the sound produced by drone propellers of different sizes and flight conditions. For all propellers considered, the comparisons between experiments and predictions in terms \(\Delta \) $\mathrm{\Delta }$ OASPL at different angles and rotational speeds showed deviations below \(\pm 2\) $\pm 2$ dB for radiation angles between 62.5 \(^\circ \) ${}^{\circ }$ and 135 \(^\circ \) ${}^{\circ }$ . A significant disagreement was observed for angles smaller than 62.5 \(^\circ \) ${}^{\circ }$ . A second analysis, considering the contribution of the individual sources encompassed by the theory, has suggested that the deviation at small radiation angles are due to an overestimation of the source term associated with the blade volume displacement, for which the contribution becomes negligible in the case of small propellers and higher harmonics. The results show that the helicoidal surface theory provides an accurate and inexpensive tool for assessing the far-field noise produced by small propellers, provided that the angles of interest are between 62.5 \(^\circ \) ${}^{\circ }$ and 135 \(^\circ \) ${}^{\circ }$ .