Studies on the influence of outboard flaps on the vortex wake of a rectangular wing

Aircraft trailing vortices constitute a hazard to following aircraft, and are therefore one of the main concerns for airport capacity constraints. At the Institute of Aerospace Engineering (ILR) experiments on wake vortices up to a distance of 60 spans behind the model of a rectangular wing are conducted in a towing tank. The motivation behind the presented experiments is the alleviation of the rolling moment induced on following aircraft by constructive means at wings and flaps of the preceeding aircraft. Following the approach of Ortega et al., the influence of the geometry of outboard flap extensions on the wake of a rectangular wing is investigated. Towing tank experiments using Particle Image Velocimetry are conducted, and the induced rolling moment is calculated from the experimentally determined downwash. In addition a stability analysis is performed for the experimentally investigated four-vortex configurations utilizing the linearized Biot-Savart law as proposed by Crow. It is assumed that the vortex system attunes to the most unstable eigenform. The theoretical results help giving an interpretation of the experimental data. It is shown that a significant reduction of the induced rolling moment by more than 50% can be achieved by the use of outboard flap extensions. This is due to an increase of the effective vortex size resulting from the counter-rotating vortex pairs produced by the flaps.