INTRODUCTION
The NACA 65-210 airfoil section was believed to offer a reasonable compromise between high maximum lifts and high critical speeds. The greatest amount of data available on the application of high-lift flaps to fairly thin airfoil sections has been obtained from ests of the NACA 23012 airfoil. Data on this airfoil with various types of flap have been presented in references 1, 2, and 3. These data were obtained with a simple slotted, flap, two slotted flaps with slot lips extended to 90 and 100 percent of the airfoil chord, and a double slotted flap. Reference 2 shows that the maximum lift and the pitching moment increment obtained with single slotted flaps increase as the length of the slot lip is increased. The double slotted flap of reference 3 produced maximum lifts about the same as the slotted flap with the longest slot lip (Fowler flap) and pitching moment increments about the same as the single slotted flap with the 90 percent-chord lip extension. The use of long slot-lip extensions presents structural problems that are accentuated by the very thin rear parts of the present NACA 6- series section. The flaps used in the present investigation were designed to provide as much thickness for structure in the slot lip as was considered compatible with reasonable flap thicknesses.
The present paper covers an investigation in the Langley two-dimensional low-turbulence tunnels of a series of slotted flaps similar to the flaps of references 1 to 3. Tests were made to develop optimum flap configurations for maximum lift of three slotted flaps with varying slot-lip extensions and of a double slotted flap. Scale effect, the effect of roughness on lift characteristics, and pitching-moment characteristics with the flaps deflected were also determined.