Active control of aircraft vortex wakes by means of preset ailerons

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 50 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. On the basis of earlier experimental studies on four-vortex-wakes a rectangular wing with preset ailerons is designed. The resulting vortex wake consists of two counter-rotating vortex pairs. These vortex systems show stronger co-operative instabilities than co-rotating vortex pairs emerging from conventional transport aircraft. Cooperative instabilities are instabilities due to the presence of multiple vortices. The vortex system is investigated experimentally with Particle Image Velocimetry and the stability of the identified vortex system is analysed utilising the linearised Biot-Savart law as proposed by Crow. It is assumed that the vortex system attunes to the most unstable eigenform. The stability analysis now provides the most unstable movement pattern of the vortices together with the corresponding wavenumber. The co-operative instabilities lead to a disintegration of the vortex system. This disintegration is accelerated by perturbing the vortex system with the wavenumber resulting from the stability analysis. Each half-wing is equipped with two adjacent ailerons that oscillate in counter-phase. This configuration ensures that constant lift is maintained. It is shown that within 50 spans behind the wing a reduction of the induced rolling moment by approximately 50% can be achieved by an appropriate aileron preset alone. This is due to an increase of the effective vortex size resulting from the counter-rotating vortex pairs produced by the ailerons. Oscillating the ailerons induces a sinusoidal perturbation of the vortices and accelerates the onset of rapid decay. The reduction of induced rolling moment is obtained significantly sooner than without oscillation.