WINGLETS FOR SAILPLANES

Abstract

There is a good deal of reliable experimental evidence that winglets can increase sailplane performance with measured improvements of 2 to 5 points in glide ratio. Adding winglets to an exi:sting sailplane will increase skin friction drag which is most important at high speed. However, with properly designed winglets, the speed at which skin friction drag of the winglets cancels out the induced drag reduction is well above normal cruising speed, and even at higher speeds the drag increment is almost too small to measure. The concept behind the 15 meter span limitation is not violated by winglets because winglets only provide a finite performance increase. Winglets are inherently small in size-which limits both their cost and the cost of supporting structures. ln contrast, performance increase due to incieased span is limited only by weight, structural problems and cost. Winglets provide a substantial improvement in aerodynamic efficiency without an increase in wing span, both for the Open Class case where span is limited by structural considerations and for 15 meter and Standard Classes where there is an arbitary span limit. Winglets are particularly attractive for retrofit on the many aircraft which already have short span extensions, such as the ASW-20, Nimbus, Ventus, DG-600, LS-3, for example. The winglets allow the low speed advantage of the span extension to be retained while at the same time keeping the high speed advantage of the shorter span.