THE EVOLUTION OF SAILPLANE WING DESIGN

Abstract

As the sport of soaring initially focused on exploiting ridge winds to maintain altitude, and the level of structural technology was unable to allow large spans, the low sink rates required were achieved by wings having large areas and fairly low aspect ratios. By the late 1920's, the discovery of thermals led to the use of climb/glide sequences for cross-country soaring. Thus, the trade-off between low induced drag for climb and low profile drag for cruise became a critical issue in the design of sailplane wings. Theoretical guidance for these designs was provided primarily by the lifting-line theory of Ludwig Prandtl and the minimum induced drag, elliptical loading result of Max Munk. During this time, the need for greater spans and higher aspect ratios led to structural advancements in the primarily wooden airframes and the development of some very interesting wing geometries, such as the distinctive gull wings that were then popular. The evolution of wing design through this period continued slowly until the introduction of new materials and laminar flow wing sections led to very rapid advancements beginning in the late 1950's. The use of glass-reinforced plastic structures, and later carbon-reinforced plastic, allowed designers to incorporate much larger aspect ratios than had been possible earlier. By the rnid 1970's, the computational capabilities had improved io the extent that lifting-surface theories, such as vortex-lattice and panel methods, were utilized in the design process. In additiory non-linear methods were developed that could not only account for non-rigid wakes, but also optimize the wing geometry to achieve the greatest cross-country performance. These developments led to the adaptation of planforms having straight trailing edges and on to non-planar wing geometries and the, now commonplace, use of winglets. While it is not at all clear what directions wing design in the future will take, it will no doubt be influenced by technological developments such as the use of boundary-layer suction for laminar-flow control and conformable/adaptable wing geometries that "morph" to the optimum configuration for any given flight situation.