On some aspects of the aerodynamic performance of ground-effect wings

With the advent of the hovercraft and hydrofoil, high-speed, overwater travel has become a practical proposition. Nevertheless, there would appear to be an upper limit to the speed of these craft imposed by, on the one hand, the intake momentum drag of the hovercraft and, on the other, the drag of the submerged foils of the hydrofoil- One method proposed for reducing these drags employs aerodynamic lifting surfaces to off-load the cushion or hydrodynamic lifting systems. These surfaces, which are referred to as 'ground-effect wings', fall into two main categories, namely 'open' and 'closed'. The latter type is defined as that which mays in theory, be designed for zero induced drag; the former type is that which may not. By employing the linearized lifting-surface theory the minimum induced drag of an open configuration, consisting of a planar wing with end plates, is determined. The results of this theory are in agreement with experiment in predicting that the effect of end plates is to reduce the induced drag. However, the indications of the experiments are that the reduction in induced drag is somewhat greater than the theoretical prediction. There is evidence that this is due to the tendency of the end plates to suppress harmful non-linear effects such as edge separations at the tips of the wing and the sidewash at the wing. A theoretical and experimental study of the lift and induced drag of a closed configuration, comprised of a substantially planar wing with end plates and not designed ab initio for zero induced drag, is described. The need to represent certain non-linear effects in the theory in order to obtain accurate values of lift is demonstrated. Further, it is shorn that with the type of closed configuration examined, namely that with chordwise camber, thickness and incidence which vary slowly round the configuration, the induced drag is small. In some cases, however, it is evident that the end plates are expected to provide a large thrust to offset a large drag contributed by the wing component. Thus, in these circumstances, it is important to ensure that the end plates are designed to sustain the requisite thrust. The image method was used in wind-tunnel experiments to simulate the effect of the presence of a water surface on the air flow about a wing. This technique has been criticized by various authors for a number of reasons. Nevertheless, experiments performed on representative configurations have indicated that the method is suitable for assessing the accuracy of many features of the theories.