INTRODUCTION
Previous investigations (references 1 and 2) have been conducted using boundary-layer control by suction on relatively thick NACA 6-series airfoil sections in an effort to bring about increases in the maximum lift coefficient. Substantial increments in maximum lift appeared obtainable by the use of boundary-layer solution, although the ultimate value of the maximum lift coefficient appeared to be limited by separation from the airfoil leading edge. Increasing the camber from zero to an amount that gave a design lift coefficient of 0.4 increased the maximum lift coefficient but did not change the nature of the stall. It seemed reasonable that if further increases in the maximum lift were to be obtained with boundary-layer control on these 6-series airfoil sections, some means of preventing leading-edge separation must be incorporated. The leading-edge slat has become recognized as one of the most effective deices for delaying leading-edge separation.
Tests have been conducted, therefore, of the NACA 64₁A212 airfoil section with a leading-edge slat, a double slotted flap, and a single boundary-layer suction slot at 0.40 chord to determine the increase in maximum lift coefficient attainable with this combination of high-lift devices. The optimum slat and flap configurations were determined, and the characteristics of the airfoil were measured for the high-lift devices operating individually and in conjunction with one another over a Reynolds number range from 1.5 x 10⁶ to 6.0 x 10⁶ in the Langley two-dimensional low-turbulence tunnel and the Langley two-dimensional low-turbulence pressure tunnel. The suction slot was placed at 0.40 chord inasmuch as this location was believed to be near the optimum location in conjunction with the slat, because the slat could be relied upon to delay separation near the leading edge. A suction-slot location closer to the leading edge might have a more favourable effect on the maximum lift of the airfoil without the slat; therefore, a few tests were made at a Reynolds number of 1.0 X 10⁶ in order to find the effect of suction-slot location on the characteristics of the plain airfoil.