Wing trailing vortex paths in formation flight

Abstract

Includes bibliographical references.Formation flight has been shown to reduce the induced drag for a formation of aircraft. The mechanism by which this is achieved is caused by the wake velocity field of the aircraft. This field is dominated by wing-tip trailing vortices. The paths of these vortices become too complex for rigid wake models downstream of the second aircraft in the formation. To tackle this problem, a combined vortex lattice and vortex filament numerical model was developed. For each simulation the vortex lattice model determined the lift distribution which was applied to the vortex filament model. The vortex filament model used the Burnaham-Hallock vortex profile with a core size of 5% of the wing span to eliminate numerical instabilities. Individual components of the model were verified successfully against literature and the overall approach was validated against wind tunnel data. The wind tunnel data was extracted from apparatus designed and build as part of this study. The apparatus consisted of two NACA 0012 rectangular planform wings mounted in various formation positions and a tuft grid placed downstream of the wings to visualise the vortex paths. Test were performed with both wings at 8◦ angle of attack. Span-wise wing-tip overlap distances were set at 38%, 10%, 0% and -10% of the span, where 0% implies wing-tip alignment and a positive value indicates a wing-tip overlap. Vertical separations were set at -3%, 0% and 3% of the span for each span-wise wing-tip overlap condition apart from 38% which was only tested at 0 vertical separation. The formation outboard vortex paths were predicted well within the 3% span accuracy of the tuft grid. The predictions of the paths of the formation inboard vortices, however were less accurate. The errors were attributed to a combination of bias errors in the experimental apparatus as well as the pseudo-viscous effects of the Burnham-Hallock vortex profile