premature decay of wake vortices with differential- and oscillating flap setting

Abstract

This paper gives an overview of the research on wake vortex decay using Differential Flap Setting (DFS) and Oscillating Flap Setting (OFS). The concept of DFS deals with the small deflection of flaps and elevator to produce additional vortices which interacts with the vortices from wing, elevator, flaps and wing-body-junction. Numerical calculations with the A340-300 model at this topic are presented in the first part of the paper. On the other hand the OFS concept works with the periodic movement of DLC (direct lift control) flaps and/or ailerons to trigger inherent instabilities of the vortices. The basic mechanism of the introduction of the periodic disturbance into the flow field is a periodic alternation of the spanwise lift distribution, i.e. of the circulation and spacing ratios of the distinct vortex pairs in the extended wake vortex nearfield. This can be achieved both by pitch oscillation and oscillating DLC flaps. Pitch oscillation alone are sufficient for accelerating Crow instability as proven already in the 1970s with small aircraft. One can distinguish between long-wavelength and short-wavelength instabilities. The second part of this paper is truncated on short-wavelength instability examinations i.e. the Crow instability.}