Optimum Spanloads in Formation Flight

Formation flight can result in large induced drag reductions. Optimum spanloads for a group of aircraft flying in an arrow formation were found using a discrete vortex method with a Trefftz plane analysis under constraints in lift, pitching moment and rolling moment coefficients. It has been shown that large reductions in induced drag can be obtained when the spanwise and vertical distances between aircraft are small. In certain cases this results in negative induced drag (thrust) on some airplanes in the configuration. The optimum load distributions necessary to achieve these benefits may, however, correspond to a geometry that will produce impractical lift distributions if the aircraft are flying alone. Optimum separation among airplanes in this type of formation is determined by such diverse factors as the ability to generate the required optimum load distributions or the need for collision avoidance.