Lift Computation Through Crossflow Measurement Behind a Rectangular Semiwing in Ground Effect

The lift force of a rectangular semiwing in stationary ground effect was computed based on the spanwise circulation distribution inferred from wake crossflow measurement and was compared with the direct force balance data at Re =$2.48$ × 10⁵. The lift calculated was found to be in good agreement with the force-balance measurement for ground distances h larger than 10% of the airfoil chord c. In close ground proximity (h/c ≤ 10%), a large discrepancy however existed due to the appearance of a multiple vortex system consisting of a tip vortex, a corotating ground vortex, and a counter-rotating secondary vortex. The counter-rotating secondary vortex offset the vorticity of the tip vortex. By considering its contribution additive, the discrepancy observed between the lift forces computed and measured with the force balance in close ground proximity became marginal. The impact of ground effect on lift-induced drag was also discussed.