Eastman N. Jacobs (1902–1987)

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.^[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.^[ref]

An original scale comparison makes the distinction concrete. At mean speed $U=30m/s$, an illustrative fluctuating velocity with root mean square $u_{\mathrm{rms}}=0.6m/s$ has intensity $u_{\mathrm{rms}}/U=2\%$. A second test with the same mean speed, density, dynamic viscosity and model dimensions but $u_{\mathrm{rms}}=0.03m/s$ 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.