Flexible Wings for Vortical Gust Mitigation

Vortices appear throughout the natural world, and have a wide range of applications from propulsion to energy generation. One challenge posed by vortices is their tendency to impinge on aircraft and other vehicles, causing undesirable and potentially damaging disturbances. We report on experiments testing the ability of flexible foils to mitigate the effects of vortices as they impinge on a wing. Wings were created using silicone and tested in a water flume. Several vortices with differing encounter positions, strengths, sizes, and rotation directions were generated upstream of the test article using a separate upstream rigid foil. A total of ten different encounters, created by prescribing pitch and heave profiles on the upstream foil, were examined. The forces, moments, and deformation experienced by the downstream flexible foil were measured; a rigid foil with the same profile and dimensions was used as a control. The flexible foils were found to decrease the effects of stall and reduce the rolling moment during steady tests. In some vortex encounters, the magnitude of the lift response on the flexible foils was also reduced. In all cases, the lift disturbance induced by upwash from the vortex just before the encounter was smaller in magnitude for the flexible foils. This effect was also present in strong vortices close to the wing, and when the sign of the encounter location and circulation were opposite. In those same encounters, the flexible foils were shown to mitigate small-scale, semi-periodic fluctuations during their lift response. This highlights the potential utility of flexible materials in improving the performance of aerial vehicles during both steady flight and unsteady gust encounters.