Lift and Tip Vortices Generated by Tapered Backward-Swept and Forward-Swept Wings under Stationary Ground Proximity

An experimental investigation of the aerodynamics and near-field tip-vortex flow field behind tapered backward- and forward-swept wings with a stationary ground effect was conducted at Reynolds number $(\mathrm{Re})=1.81\times {10}^5$. The results showed a large lift increase of 26.7% and 12.3% for the backward-swept wing (BSW) and forward-swept wing (FSW), respectively, with reduced ground clearance, along with a significant drag reduction of 45% and 30% for the BSW and FSW. For the BSW, a multiple-vortex system appeared in close ground proximity, consisting of a tip vortex, a corotating ground vortex, and a counterrotating secondary vortex. The ground vortex strengthens the tip vortex, whereas the secondary vortex negates its vorticity. For the FSW, the multiple-vortex system was not readily identifiable due to its unique geometry, which always keeps the inboard region of the wing at a close ground effect while leaving the tip region less affected by the ground effect. The root stall of the FSW also produced a continuously strengthening tip vortex with the increasing angle of attack. In contrast, the tip stall of the BSW led to a monotonically increasing vortex strength only up to the static-stall angle. Regardless of the wing model, the weak tip vortex also translates into a small lift-induced drag compared with the total drag. Finally, the lift force computed through the integration of the spanwise circulation distribution, inferred from the cross-flow measurements, at selected ground distances was also found to be in good agreement with the direct wind-tunnel force-balance data, with 101% and 98% consistency for BSW and FSW, respectively. The aerodynamics and tip-vortex measurements of both wing models outside the ground effect were also acquired to serve as a comparison.