Ducted fans installed on vertical takeoff and landing vehicles experience significant ground effect during takeoff and landing. The aerodynamic characteristics of a ducted fan hovering and transitioning in the ground effect are studied using numerical simulations in this paper. The flowfields are obtained by solving Reynolds Averaged Navier–Stokes equations with the Multiple Reference Frame approach. When a ducted fan hovers in the ground effect, the blade thrust increases due to the combined effect of the increase in the effective angle of attack of the blade and the increase in ambient pressure; the duct thrust decreases due to the combined effect of the decrease in the effective angle of attack of the duct and the increase in ambient pressure. Stall occurs at a certain advance ratio and angle of attack when transitioning in the ground effect. The ground effect delays the occurrence of stall at some advance ratios. The ground effect is hardly detectable at angles of attack less than 30° even if the height drops to 0.5 times the duct exit diameter. At this height and high angles of attack, the different positions and influence regions of the ground vortex at different advance ratios contribute to the different variation trends in the ducted fan performance.

The aerodynamic performances and control characteristics of vertical takeoff and landing aircraft will be significantly affected by the ground effect during takeoff and landing. The longitudinal aerodynamic characteristics and main flow features of a fixed-wing vertical takeoff and landing aircraft hovering in ground effect are investigated in this paper, using Multiple Reference Frame based numerical simulations. The aircraft is propelled by three ducted fans. Results show that the overall morphology of the flow is characterized by a fountain, ground vortex, reingestion, and recirculation. When the thrust distribution between the front and aft ducted fans changes, the flow features change as well. As the aircraft approaches the ground, the ducted fan thrust decreases, the fuselage lift increases, and the total lift first decreases and then increases. The maximum lift increases by 9.6% and the minimum lift decreases by 3.0% compared with that out-of-ground effect. The magnitude of the drag is about 1% of the lift, which has little influence on the aircraft’s performance. The pitching moment gradually changes from the equilibrium state to a significant nose-up moment. The total power consumed increases at a specific rotational speed.