Effects of number of blades on propeller noise

This paper presents the effect of the number of blades on the aeroacoustic characteristics and aerodynamic performance of propellers utilized in urban air mobility vehicles through an extensive experimental study. A detailed parametric study is conducted for two, three, and five-bladed propellers. Investigations are carried out under static thrust conditions and incoming axial flow velocities for various rotational speeds. The spectra and directivity of the far-field noise are studied in detail for broadband and tonal components. Results show that two-bladed propellers produce higher tonal noise levels than other blade numbers for the same tip speed, while five-bladed propellers produce the highest broadband noise levels. The highest level of noise radiation is noted in the upstream and downstream directions for broadband noise, whereas tonal noise is radiated mainly in the plane of rotation. Results also indicate that high-frequency broadband humps are observed for higher advance ratios, likely caused by the formation of laminar separation bubbles, and that amplitude increases with blade numbers. The results of this study provide valuable insight for further investigations of propeller noise and aerodynamic performance.