Noise reduction mechanism of a bio-inspired rotor blade with forward–backward swept tip

Aiming at addressing the noise control challenge of small unmanned aerial vehicle (UAV) rotors, this study proposes a bio-inspired blade design with forward–backward swept tip (Bat blade). Its aerodynamic and aeroacoustic performance in hover is systematically investigated using anechoic chamber measurements and large eddy simulations. Results show that the Bat blade maintains aerodynamic efficiency comparable to the baseline blade while achieving significant noise reduction. The overall sound pressure level is reduced by up to 4 dB, with the broadband component attenuated by ∼7 dB across all directivity angles. A combined acoustic and flow-field analysis elucidates the noise reduction mechanism. Flow-field analysis reveals a delta-wing-like control mechanism: the forward–backward swept geometry induces leading-edge vortices that stabilize the flow over the central wave-crest region, inhibiting boundary-layer transition. This flow stabilization effectively suppresses the generation of turbulent structures in the outboard trailing-edge region. Near-field acoustic source analysis confirms a substantial suppression of pressure fluctuations in this region, which fundamentally weakens the mid-to-high-frequency trailing-edge scattering noise. This work provides new insights for the design of low-noise rotors for small UAVs.