= National Advisory Committee for Aeronautics
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= NACA
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The \b[National Advisory Committee for Aeronautics] was the United States' aeronautical research organization established in 1915. Its laboratories and personnel formed the core of the <National Aeronautics and Space Administration> in 1958. Its technical legacy includes <wind tunnels>, systematic <NACA airfoil> families and published measurements that let designers compare <aircraft wings>.<The National Advisory Committee for Aeronautics by NASA>[Agency history,1915–1958]{ref}
A particularly useful research strategy was to change geometry systematically while keeping test conditions comparable. <Eastman N. Jacobs>, <Kenneth E. Ward> and <Robert M. Pinkerton> varied <airfoil thickness-to-chord ratio> and <airfoil camber> in their related-section experiments, rather than testing only unrelated existing designs. The measurements connected an explicitly specified shape with <lift coefficient>, <drag> and <pitching moment>.<The Characteristics of 78 Related Airfoil Sections from Tests in the Variable-Density Wind Tunnel by Eastman N. Jacobs and others>[Summary and Introduction, systematic shape variables]{ref}
An original similarity calculation illustrates why the apparatus mattered. A small model with <airfoil chord> $0.10\,\mathrm m$ in air of <density> $1.2\,\mathrm{kg/m^3}$, <dynamic viscosity> $1.8\times10^{-5}\,\mathrm{Pa\,s}$ and <speed> $30\,\mathrm{m/s}$ has <Reynolds number>
$$
Re=\frac{\rho Uc}{\mu}=2.0\times10^5.
$$
A five-times-larger chord at the same conditions has $Re=10^6$. A <variable-density wind tunnel> can instead raise the model's air <density> fivefold to match that <Reynolds number>, approximately holding <dynamic viscosity> fixed. Matching it alone does not also establish matching <Mach number>, surface condition or <wind-tunnel turbulence intensity>.
The enduring contribution is a reusable experimental comparison: specify shape, operating conditions and how <forces> were measured, then publish results that can be checked against another experiment or <computational fluid dynamics> calculation.
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