= Eastman N. Jacobs
{c}
{title2=1902–1987}
{wiki=Eastman_Jacobs}
\b[Eastman N. Jacobs] (1902–1987) was an aerodynamicist at the <National Advisory Committee for Aeronautics>' Langley laboratory. He joined in 1925 and led its <variable-density wind tunnel> section from 1928 to 1939. His work connected systematic <NACA airfoil> geometry with experimental measurements and later helped drive low-turbulence <airfoil> research.<Eastman N. Jake Jacobs by NASA>[Biography, entry to Langley and tunnel-section leadership]{ref}
The scientific problem was that a <wind tunnel> can influence the very phenomenon being studied. Higher air <density> raises the model's <Reynolds number>, making a small <airfoil> more representative of a larger one. But a large <wind-tunnel turbulence intensity> can alter <boundary layer> transition, preventing an otherwise appropriate <Reynolds number> match from reproducing flight behavior. NASA's account describes this difficulty as a motivation for Jacobs's low-turbulence tunnel program.<Eastman N. Jake Jacobs by NASA>[Variable-density tunnel turbulence and the low-turbulence program]{ref}
An original scale comparison makes the distinction concrete. At mean <speed> $U=30\,\mathrm{m/s}$, an illustrative fluctuating velocity with <root mean square> $u_{\rm rms}=0.6\,\mathrm{m/s}$ has intensity $u_{\rm rms}/U=2\%$. A second test with the same mean <speed>, <density>, <dynamic viscosity> and model dimensions but $u_{\rm rms}=0.03\,\mathrm{m/s}$ has intensity $0.1\%$. Both have the same <Reynolds number>; their fluctuation levels differ twentyfold. These stipulated values are not historical tunnel measurements.
Jacobs's program thus illustrates why <dimensional analysis> and careful apparatus design complement one another. A <laminar flow> claim needs evidence about the flow environment as well as a smooth-looking <airfoil>. His collaborators <Kenneth E. Ward> and <Robert M. Pinkerton> supplied further experiments on related sections and their measured performance.
\Image[https://upload.wikimedia.org/wikipedia/commons/2/25/Variable_Density_Wind_Tunnel_-_GPN-2000-001242.jpg]
{title=Eastman N. Jacobs seated at the far-left control panel during airfoil testing in the variable-density wind tunnel,15 March 1929. NACA photograph.}
{height=480}
{source=https://commons.wikimedia.org/wiki/File:Variable_Density_Wind_Tunnel_-_GPN-2000-001242.jpg}
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