NACA Six-Digit Airfoils
Six-digit airfoils were designed in the early 1940s with the objective of encouraging a longer run of laminar-boundary-layer flow along the surface to reduce the skin – friction drag. As described in detail herein, early transition from a smooth laminar flow to a turbulent flow leads to increased drag. In this airfoil configuration, the mean camber line and the thickness distribution are designed to generate a specific pressure distribution at a certain value of lift coefficient. These sections were the first to reflect the importance of delaying boundary-layer transition on the airfoil surface. The value of this design approach was demonstrated in the highly successful North American P-51 fighter of World War II fame; the high speed and impressive range were directly attributable to such drag-reducing design features.
For any NACA xxxxxx section:
First integer: identifies series (i. e., 6)
Second integer: chordwise location of minimum pressure in tenths of a chord for a basic symmetrical thickness distribution at zero lift
Third integer: range of Q (tenths) above and below the design-lift coefficient for which favorable pressure gradients exist on both airfoil surfaces Fourth integer: design-lift coefficient in tenths (reflects the amount of camber) Fifth and Sixth integers together: thickness ratio of section, percent chord
Thus, the NACA 64,3-212 airfoil is a six-series section with minimum pressure at 40 percent chord, a favorable pressure gradient for a range in lift coefficient of 0.3 above and below design, a design-lift coefficient of 0.2, and a thickness ratio of 12 percent.
There are variations on many of these airfoil shapes, and the nomenclature becomes more complicated. For example, the letter A appearing in the six-series designation means that the contour was altered near the trailing edge. Abbott and Van Doenhoff (1959) is a useful collection of results for NACA airfoils and also provides details of the airfoil-numbering system. The Riegels (1961) book is a comprehensive catalog of airfoils developed at NACA and elsewhere, as well as a review of airfoil theory.
It often is useful to have a computer program that produces the coordinates of the NACA series. Program NACAFOIL is provided with the software package accompanying this text for that purpose. This program generates airfoil coordinates for the NACA four – and five-digit-series airfoils and stores them in a user-defined text file. This file is readable by other programs in the software package that can use airfoil data.