Aerodynamic Characteristics of a Ducted Fan Hovering and Transition in Ground Effect

Abstract

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.

Longitudinal Aerodynamic Characteristics of Ducted Fan Propelled Fixed-Wing VTOL Aircraft Hovering in Ground Effect

“…It can be seen that as the height decreases, the duct thrust decreases, the blade thrust increases, and the total thrust decreases; the power consumed by the ducted fan increases and the power loading decreases. The variation trend of these performances is similar to that of an isolated ducted fan subjected to ground effect [30]. The normalized performances of an isolated ducted fan are almost constant in a large rotational speed range at a given height [34].…”

“…This increases the pressure difference between the fore and aft part of the duct, resulting in an obvious increase in drag. No recirculation is observed in the case of an isolated ducted fan hovering in the ground effect [30]. In the height range of 3D-4D, besides the ground vortex formed by the front and aft ducted fan jets, the ground vortex is also formed when the jets of the two front ducted fans collide and move upstream along the symmetry plane.…”

“…For a single ducted fan, the MRF method requires two separate zones connected by frame interfaces inside the computational domain: the rotating zone containing the block near blades and the stationary zone containing the rest block, as shown in Figure 3b. The boundary conditions are shown in Figure 3: all surfaces of the aircraft model were set as no-slip walls; three of the four side surfaces, except for the symmetry plane, were set as pressure outlets with a value of 0 Pa relative to the standard atmospheric pressure; the top surface was set as the velocity inlet with a value of 0.5 m/s to reduce the order of residuals magnitude [30]; the bottom surface was set as the non-slip wall. The air is assumed to be the ideal gas with properties of the standard atmosphere at sea level.…”

“…The duct thrust decreases due to the combined effect of the decrease in the effective angle of attack and the increase in ambient pressure. A more detailed analysis can be found in another paper [30]. Figure 26 presents the comparison of pressure contours and streamlines on typical sections in and out of ground effect of the front CG case.…”

“…The aft ducted fan is only used during the VTOL phase and can be fully enclosed into the fuselage during the flight phase with the door closed. Both the front and aft ducted fans have eight blades, and the distributions of the twist and dimensionless chord [30] along the radial direction of the blades are identical. The maximum thrust of a single front ducted fan is 8820 N, and the maximum thrust of the aft ducted fan is 5880 N. Such a configuration can achieve a better balance between aircraft size, structural weight, allowable center of gravity range, and available control power.…”