Results are presented of an investigation made to determine the two dimensional lift and drag characteristics of nine naca 6-series airfoil sections at Reynolds numbers of 15.0x10⁶, 20.0x10⁶, and 25.0x10⁶. Also presented are data from NACA Rep. No. 824 for the same airfoils at Reynolds number of 3.0x10⁶, 6.0x10⁶. The airfoils selected represent sections haying variations in the airfoil thickness, split flap were determined in one instance, as was the effect of surface roughness. Qualitative explanations in terms of flow behavior are advanced for the observed types of scale effect.