NACA Four-Digit Airfoils
This series has one basic thickness form, with the maximum thickness at 30 percent chord. Camber lines were chosen so that the maximum camber occurred from 20 to 70 percent of chord. For any NACA xxxx section:
First integer: maximum value of the mean camber line, percent chord Second integer: distance from the leading edge to point of maximum camber, tenths of a chord
Third and Fourth integers: thickness of airfoil, percent chord
Thus, the NACA 4415 airfoil section shown in Fig. 5.3(b) has a maximum camber of 4 percent chord located at 40 percent chord with a thickness ratio of 15 percent.
NACA Five-Digit Airfoils
Five-digit airfoils were designed in the mid-1930s to improve the pitching-moment characteristics of four-digit airfoils. Accordingly, they have the maximum camber farther forward than the four-digit series. Both the four- and five-digit series have the same basic thickness form (i. e., maximum thickness at 30 percent chord) as the four-digit sections.
For any NACA xxxxx section:
First integer: the section-lift coefficient in tenths is 1.5 times the first integer; this first integer is indicative of the amount of camber
Second and Third integers together: twice the value of the maximum camber, percent chord
Fourth and Fifthe integers together: thickness ratio, percent chord
Thus, the NACA 23012 section has a design lift coefficient of 0.3, a camber ratio of 15 percent, and a thickness ratio of 12 percent. The design-lift coefficient is defined as the theoretical value of the section-lift coefficient at an angle of attack such that the oncoming flow is tangent to the mean camber line at the leading edge.