The development of industrial drones is expected to expand in the future. In particular, applications such as delivery services are expected to drive demand for low-altitude flights. As drones become increasingly integrated into human environments, various challenges may arise, among which noise reduction represents a critical concern. To address this issue, a propeller design incorporating an enlarged blade area was investigated, wherein additional curved plates were attached to the trailing edges of the propellers to mitigate noise. The planar shape of the curved plates was designed using several parameters in a non-dimensional coordinate system. We introduced a relationship between the target noise reduction level and the second moment of area, subsequently used to scale the designed structure. The results of simultaneous force and noise measurements of a single propeller yielded significant noise reduction for all propellers with enlarged blade area. Strong correlations were observed between noise levels and the attachment length in the spanwise direction, as well as the inclination angle at the wing root side in the designed geometry. The in-flight test was performed using a six-rotor drone, achieving an overall noise reduction of 8.1 dBA compared to standard propellers, with significant reductions in the tonal noise specific to drones and in the high-frequency broadband noise, which causes discomfort to humans. These findings are expected to significantly inform future design guidelines for noise reduction in drones.

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