Hovering helicopter rotor blade/vortex interaction noise characteristics in ground effect environment

To grasp rotor aeroacoustic radiation characteristics during helicopter flight close to the ground, firstly, based on the compressible Reynolds average Navier-Stokes equations and the Ffowcs Williams-Hawkings equations, a set of aerodynamic and aeroacoustic analysis method suitable for rotor/fuselage interaction in ground effect state was developed. The reliability of the established method was verified by comparing the ground effect test of Lynx tail rotor and the NASA rotor/fuselage interaction test with calculation results. Then, the influence of fuselage and hub on the rotor flowfield was investigated. It was found that the fuselage could change the rotor aerodynamic load distribution and increase the aeroacoustic radiation level of the intermediate order (8th–12th order). The aeroacoustic contribution of the blade middle section (r=0.4R–0.7R, R is the radius of rotor) could be weaken by the hub. Finally, the aerodynamic and aeroacoustic characteristics of the rotor at different heights (h) above the ground were investigated and a “critical ground height” with Blade/Vortex Interaction (BVI) noise was found. The results show that the rotor thrust and fuselage lift decrease with the increase of ground height. When h>1.8R, the influence of ground effect can be ignored. When h=0.6R, the inflow of the rotor will be changed by the blocking effect of the ground and the fuselage. Especially, load fluctuations are induced severely in the blade tip area and obvious BVI noise is received at the characteristic observation points. According to the time/frequency characteristics, this paper suggests a “critical ground height” of approximately 0.7R, which provides a reference for the noise suppression when the helicopter flies close to the ground.