An Airfoil Shape for Efficient Flight at Supercritical Mach Numbers

An airfoil shape is proposed for delaying, at the usual cruise lift conditions, the subsonic drag rise well beyond the critical Mach number. The shape has a slot between the upper and lower surfaces near the trailing edge to delay shock-induced separation on both surfaces and incorporates negative camber ahead of the slot with substantial positive camber rearward of the slot to reduce shock losses. Wind-tunnel results obtained at Mach numbers from 0.65 to 0.80 for two-dimensional models of a 13.5-percent-thick airfoil of the proposed shape and an NACA 64A-series airfoil, used as a base of reference, indicate that for the design-section normal-force coefficient of 0.65 the proposed shape has a drag rise at a Mach number of 0.79 compared with a drag-rise Mach number of 0.67 for the NACA 64A-series airfoil. The drag at a Mach number just less than that of drag rise for the proposed airfoil is due almost entirely to skin friction and is approximately 10 percent greater than that for the 64Aseries airfoil. The sectio11pitching-moment coefficient for the proposed shape is substantially more negative than that for conventional airfoils. The proposed airfoil shape also significantly increases the stall section normal-force coefficient at high subsonic speeds.