352 



TABLE 346A.— FORCES ON AIRFOILS AT ANGLES TO THE WIND 



(FIGS. 14, 15) 137 



By suitably proportioning the thickness distribution over the chord of a plate, an airfoil 

 may be derived around which the flow will adhere even when the angle of attack is large. 

 Because the flow remains attached to the airfoil, high lift coefficients may be obtained with 

 low drag coefficients. 



The flow around a particular airfoil at a given angle of attack depends on the Reynolds 

 number, R, the Mach number, M, and the degree of surface roughness. The main effect 

 of increasing the Reynolds number is to change the maximum-lift coefficient and the 

 minimum-drag coefficient. When the surface of the airfoil is made rough, simulating the 

 surface of an actual airplane wing, the flow breaks away from the upper surface of the 

 airfoil at a smaller angle of attack and therefore results in a considerably smaller value of 

 maximum-lift coefficient. A rough surface increases the percentage of the chord over 

 which the flow is turbulent and tends to make the drag coefficient much higher (see 

 figure 11). As the Mach number is increased the variation of the local velocity from the 

 stream velocity is increased. 



On figure 14 are shown the force coefficients for two symmetrical NACA airfoils of 

 infinite aspect ratio plotted against angle of attack, a, for a Reynolds number of 6 X 10°. 

 Methods exist (see Method for calculating wing characteristics by lifting-line theory using 

 nonlinear section lift data, by James C. Sivells and Robert H. Neely, NACA TN No. 1269, 

 April 1947) for converting infinite aspect ratio data to finite wing characteristics. The 

 force coefficients of a 21-percent thick airfoil in the smooth condition and a 12-percent thick 

 airfoil in both the rough and smooth conditions are given. 



Figure 15 shows the variation in the force coefficients with Mach number for a sym- 

 metrical 9-percent thick airfoil at an angle of attack of 2° and at Reynolds numbers from 

 .35 X 10° to 75 X 10°. 



187 Abbott, Ira H., von Doenhoff, Albert E., and Stivers, Louis S., Jr., Summary of airfoil data. 

 NACA Rep. No. 824, 1945. Stack, John, and von Doenhoff, Albert E'., Tests of 16 related airfoils at 

 high speeds, NACA Rep. No. 492, 1934. 



(continued) 



SMITHSONIAN PHYSICAL TABLES 



