On the use of perforated sheets to reduce propeller noise in ground effect

It is well known that a propeller hovering in close proximity to a solid ground plane can increase thrust levels for a given power. However, previous experimental studies have shown that this can lead to undesirable increases in noise levels, which can become an issue for Urban Air Mobility (UAM) when electric vertical take-off and landing (eVTOL) aircraft are taking off or landing from a vertiport landing pad. In this paper, we test a design for a vertiport landing pad which consists of a plane of perforated sheets placed some distance above a solid ground plane, thereby effectively increasing the distance between the propeller and the solid ground plane when taking off and landing. Experiments were undertaken in an anechoic chamber of a single propeller operating above a plane of perforated sheets which are supported below by a solid ground plane. Far-field noise measurements were undertaken to investigate the effect of the perforated sheets on the noise produced by the propeller. Preliminary results of the far-field acoustic measurements are presented and show that the perforated sheets have a negligible effect on the far-field noise.