Tip-vortex structures and lift-drag characteristics for delta winglet configurations

Based on a traditional wingtip endplate configuration, four wingtip variant configurations with delta winglet shapes were designed to explore their tip-vortex suppression effects. Each delta winglet configuration consists of a rectangular wing and a pair of tip-mounted delta winglets. The wingtip vortex flow characteristics of the wingtip endplate configuration and delta winglet configurations I–IV were experimentally investigated under different attack angles (α = 0–20°) and downstream cross-sections (x/c = 1.04–2.2) at Re = 154 032. The results indicate that the variation of α or x/c greatly affects the tip-vortex cores’ number, shape, area, location, vorticity level and circulation for the five wingtip configurations due to continuous interactions between their positive and negative vorticity regions. The wingtip endplate configuration’s total or parasitic drag always rises at a higher rate than those of the delta winglet configurations I–IV at α = 0–20°. Furthermore, for any of the five wingtip configurations, the induced-drag variation trend with α depends on the positive and negative vorticity regions’ circulation variations with α. At α = 0–20°, the maximum proportions of induced drag relative to total drag for the wingtip endplate configuration and delta winglet configurations I–IV are 39.41, 41.04, 37.70, 38.02 and 39.42%, respectively. By further comparing the lift-drag performances of the five wingtip configurations, it can be observed that the delta winglet configuration II exhibits better lift-drag efficiency due to its superior drag performance.