 NACA airfoil

A NACA airfoil is an airfoil from a family developed by the National Advisory Committee for Aeronautics. The designation identifies a geometry family; it does not by itself give the lift, drag or aerodynamic stall behavior of a manufactured wing under every flow condition. The NACA four-digit airfoil family makes thickness and camber explicit geometric parameters.

The early experimental programme made those parameters useful engineering choices. In their 1933 report, Eastman N. Jacobs, Kenneth E. Ward and Robert M. Pinkerton varied thickness and airfoil mean camber line form systematically, then measured lift, drag and pitching moment coefficient in the NACA variable-density wind tunnel.^[ref]

The report describes duralumin models of 5-inch airfoil chord and 30-inch wingspan, tested at approximately $\mathrm{Re}=3\times 10^6$ with tank pressure around 20 atmospheres. The moment reference was one quarter of the airfoil chord behind its forward end.^[ref] Increasing gas mass density helps reach a larger Reynolds number with a small model: $\mathrm{Re}=\rho Uc/\mu$, where $\mu$ is dynamic viscosity. This is a reason to control the test medium, rather than enlarge the model alone.

A NACA 0012 section is nominally symmetric and 12 percent thick; NACA 2412 adds a specified camber while retaining the nominal thickness ratio. Those are geometric comparisons, not measured equalities of their flow fields. Later NACA families use different designation rules, so the four-digit decoding must not be applied to every NACA number.^[ref]

The report also discusses tunnel corrections and the influence of turbulence when applying model measurements to flight. Matching a code or nominal Reynolds number alone therefore does not reproduce every experimental condition.^[ref]