DESIGN PHILOSOPHIES FOR SAILPLANE AIRFOILS WITH FLAPS

Abstract

Two different design philosophies for sailplane airfoils with flaps are compared. The first philosophy requires the greatest possible extent of laminar flow on the lower surface for the flap-up case. Because it is difficult to maintain the laminar boundary layer beyond the flap hinge, the flap chord must be small, in the range of 12 to 15 percent of the airfoil chord. For the flap-down case, the upper-surface currvature should be relatively continuous which yields good high-lift performance with 50 to 60 percent laminar flow. This feature results in a concave corner on the upper surface at the flap hinge for the flap-up case. The same is true on the lower surface for the flap down case. These corners cause local separations of the turbulent boundary layer with reattachment occurring aft of the hinge. Experiments have shown no increase in drag due to these turbulent separation bubbles, however. The second philosophy employs a wider flap of 20 to 22 percent chord. It is possible to design these airfoils such that, for the flap-down case, the laminar flow on the upper surface extends to the hinge, followed by a steep pressure recovery. A turbulator is then necessary forward of the hinge to insure attached turbulent flow up to the trailing edge. On the lower surface, the same situation occurs for the flap-up case. The concave corners at the hinge are milder on both surfaces and for most conditions, the turbulent separation can be prevented in the corners. Examples for both philosophies are presented. Theoretical and experimental section characteristics are compared.