The lift force produced by an unsteady translating plate with a rotating tip

Dynamic effects of wing planform changes are investigated with the goal of gust alleviation. Force measurements are done on a low-aspect-ratio, high-angle-of attack translating wing having a rectangular planform that further incorporates a moving tip with in-plane rotation and sweep. The wing is towed in a water tank with various velocity ramps starting from a constant motion, which are categorized as step-up (increasing velocity) and step-down (decreasing velocity); ramps over distances of 1, 3, and 6 chords traveled are used for both types of gusts. The tip panel is rotated inward for step-up cases with an aim to lower the gust lift peaks, and similarly rotated outward for step-down gusts to reduce the negative force change. The forces from the actuation cases are compared with those of two reference geometries corresponding to pre-and post-gust wing shapes, namely rectangular and static-sweep where the tip panel is fully retracted or extended, respectively. Further, the sensitivity of the actuation effects to aspect ratio (A) is examined. The lift coefficient, C_L, uses the instantaneous main-wing translation velocity and the varying wing area to compare across all cases. The step-up gust C_L resembles that of the starting flow for the 1-chord ramp, as reported by others. The step-down gusts exhibit a C_L minimum then an increase to a circulatory-force peak. For both gust types, apart from the 6-chord step-up case, when the C_L curves are aligned at 50% of the gust-ramp travel, the post-gust circulatory peaks coincide. For the step-up gusts and tip-panel actuation out, the A = 2 case yields a more substantial C_L reduction (gust mitigation) but it is not sustained compared to the A = 4 actuation effect. Wingtip actuation at the 50% gust location produces a more sustained, lower C_L but does not affect the gust peak, while actuation before the peak for the longer ramp cases reduces the peak value but the mitigation effect is not prolonged. For the step-down gusts, actuation out is effective at increasing the C_L. For A = 4 the effect is greatest for the 1-chord ramp and yields a C_L plateau between the gust minimum and recovery peak that is higher than for A = 2. However, for the longer deceleration ramps the A = 2 actuation-out case produces relatively larger actuation peaks and earlier gust recovery.