= NACA airfoil
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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 (aerodynamics)> 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>.<The Characteristics of 78 Related Airfoil Sections from Tests in the Variable-Density Wind Tunnel by Eastman N. Jacobs and others>[Summary, profile derivation and Apparatus and Methods]{ref}
The report describes duralumin models of 5-inch <airfoil chord> and 30-inch <wingspan>, tested at approximately $\mathrm{Re}=3\times10^6$ with tank pressure around 20 atmospheres. The moment reference was one quarter of the <airfoil chord> behind its forward end.<The Characteristics of 78 Related Airfoil Sections from Tests in the Variable-Density Wind Tunnel by Eastman N. Jacobs and others>[Apparatus and Methods; model dimensions, Reynolds number and reference point]{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 (aerodynamics)> 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.<Applied Computational Aerodynamics by W. H. Mason>[Appendix A.1, distinct four-digit, five-digit and six-series constructions]{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.<The Characteristics of 78 Related Airfoil Sections from Tests in the Variable-Density Wind Tunnel by Eastman N. Jacobs and others>[Discussion of experimental errors and air-stream turbulence]{ref}
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