"A two-dimensional wind-tunnel investigation of the flow and force characteristics of four NACA 6A-series airfoils with thickness ratios of 4, 6, and 9 percent has been conducted in the Langley airfoil test apparatus at at transonic Mach numbers between 0.8 and 1.25. The Reynolds number range for these tests varied from 2.6 x 10(6) to 2.8 x 10(6). As was expected, the airfoils exhibited a smooth transition in force coefficients from a Mach number of 1.0 to the values obtained at the higher speeds. Lift-curve slope and maximum lift-drag ratio correlated very well on a basis of the transonic similarity laws at Mach numbers above 1.0, but below that value the correlation was not good" (p. 1).