Mitigation of propeller noise in ground effect with perforated sheets

This study investigates the aeroacoustic performance of propellers operating in ground effect, with a focus on the influence of perforated sheet surface treatments. Three scenarios are tested: a solid ground plane, a perforated ground plane, and a perforated ground plane placed over a solid ground plane. Experiments were carried out in an anechoic chamber with a Mejzlik 12” × 6” propeller in a pusher configuration, where noise was measured using a polar array of microphones positioned in the far field, alongside near-field hydrodynamic measurements. The results show that using a solid ground plane increases far-field noise, particularly at lower propeller-ground distances, by up to 12 dB. In contrast, replacing the solid ground plane with a perforated one, reduces the additional noise generated by ground effect by as much as 6.5 dB and up to 100% of the ground effect noise. Spectral analysis reveals that the amplification of low-order blade passing frequencies under ground effect conditions is linked to increased steady loading on the blades, while higher harmonics and broadband noise are influenced by vortex interactions and propeller downwash near the ground. When a perforated sheet is combined with a solid ground plane, spacing and perforation size have minimal effect on tonal levels, suggesting that reflection and interference mechanisms dominate. These findings offer important guidance for designing quieter vertiports for Urban Air Mobility (UAM) vehicles, facilitating their safe and efficient integration into urban environments.