[37] In the 1930s the wind tunnel evolutionary tree had split into two main branches:
1. The branch concerned with scale effects and the reach toward the higher Reynolds numbers characteristic of actual flight. The Langley tunnel species growing on this branch were the variable density tunnel, the full-scale tunnel, and the 19-foot pressure tunnel.
2. The branch dealing with high-speed effects, as represented by the 24-inch high-speed tunnel and the 8-foot high-speed tunnel.
A third branch sprouted unexpectedly in the late 1930s when Eastman Jacobs and his associates at...
....Langley were assessing the performance of wings developed in the VDT. For some unexplained reason, the wings usually performed better in actual flight than wind tunnel tests had predicted- a strange turnabout because one generally expected laboratory tests to be more optimistic that flight results. Careful research demonstrated that the performance gap was due to undetected turbulence in Langley's wind tunnels. The atmosphere outdoors was actually quieter and more homogeneous than that in the best wind tunnels.
In contrast to wind gusts and other large-scale turbulence in the atmosphere, the wind tunnel's fans and air-guiding structures induced fine-scale random fluctuations in local air velocity and flow angle. This microscopic "weather" disturbed the thin boundary layer of air next to the surface of the wind tunnel models. Lift, drag, and other measurements were compromised in ways that could not be corrected for.
Wind tunnel designers employ two techniques to tranquilize microscopic air turbulence. In the first, the airstream is simply squeezed into a duct with a much smaller cross-sectional area. In effect, the squeezing or contraction irons out some of the disorderly [39] airflow-an aerodynamic mangle, as it were. Modern low-turbulence tunnels usually have a contraction section in which the flow area is reduced by a factor of 15 or more. The second technique uses a settling or stilling chamber upstream of the contraction section. In this chamber, baffles and screens (some with wire as thin as human hairs) smooth out the flow by breaking up the eddies.
Although these two principles were recognized in the late 1930s by the NACA engineers contemplating their first low-turbulence tunnel, no one had ever built a large tunnel using high contraction ratios in combination with a settling chamber packed with honeycomb and fine-meshed screens. Would such a tunnel work at the high Reynolds numbers demanded? Before investing in a full-scale, pressurized tunnel of such novel design, it seemed wise to build a cheap model to work out any unexpected engineering problems that might arise.
At this period (the late 1930s), the desirability of low turbulence in wind tunnels was not widely appreciated. Funds for a "low-turbulence" tunnel would have been difficult to justify. Aircraft icing, however, was a "hot" topic. The model of the low-turbulence tunnel was therefore designated the " NACA Ice Tunnel." Fabricated from plywood with an inner lining of sheet metal, the ice tunnel was completed in
April 1938. The contraction ratio was 19.6 to 1, with a test section 7.5 feet high and 3 feet wide. Airspeed was 155 mph. True to its announced purpose, the tunnel walls were insulated with a thick wrapping of crude insulation, and refrigerating equipment of sorts was added. This consisted simply of an open tank of ethylene glycol cooled by blocks of dry ice, with the cold mixture pumped through coils that cooled air drawn from the tunnel. Ice actually did form on the leading edge of an airfoil during one of the early, rather perfunctory tests, and the ice tunnel fulfilled its announced purpose.
By October 1940, however, aircraft icing had been forgotten and an array of honeycomb and screening had been installed upstream of the test section. As the tunnel designers had hoped, the air in the test section was almost devoid of turbulence, and a new horizon for aerodynamic research was opened.
The plywood and tin model did its job well. Not only was it employed to perform useful research in its own right, but it also served as a design base for a more permanent facility-the so-called Low Turbulence Pressure Tunnel (LTPT). In the LTPT, a heavy steel shell replaced the flimsy plywood and tin because the tunnel was to be pressurized to 10 atmospheres. The test section was 7.5 x 3 feet. The contraction ratio was a bit smaller than the model...
...(17.6 to 1), but 11 screening elements were installed so that the turbulence levels approached those encountered in the natural atmosphere.
When the LTPT commenced operation in the spring of 1941, it began war work on a crash basis. With its unique low-turbulence-flow characteristics, it was an ideal tool with which to explore the capabilities of a revolutionary type of wing-the newly conceived laminar-flow airfoil. The practical consequences of the new wing were far reaching and of utmost importance in the war effort. It was "fortunate" that Langley engineers, via their ice tunnel, had just the right instrument on hand at the right time.
What was behind this prescience? As noted earlier, Osborne Reynolds had demonstrated in 1883 two types of fluid flow in his classic pipe-flow experiments. The first, turbulent flow, is characterized by high skin friction, which duly translates into high aircraft drag. The second, laminar flow, occurs when the layers of air slide smoothly over one another without breaking up into swirls and eddies. Skin friction in laminar flow is very low-typically one-fifth of that in turbulent flow. If the airflow over a fighter or bomber wing could be made mostly laminar, its range could be increased markedly because less fuel would be expended in fighting drag. The low- turbulence pressure tunnel was made to order to explore laminar flow because its airflow was so quiet and smooth that the layers of air sliding over the test wings were not disturbed by tunnel- induced turbulence.
Eastman Jacobs and his associates at Langley knew that the laminar flow of air over a wing was inherently unstable and that it broke up into turbulence just beyond the leading edge of the wing. However, Reynolds, Prandtl, and other aerodynamic theorists had predicted that if the layer of air closest to the wing surface (the boundary layer) was moving into a region of decreasing pressure, the laminar nature of the flow could be stabilized. Pursuing this lead with the earlier ice tunnel and the new LTPT, they developed a whole new series of laminar-flow airfoils. These, when translated into practical wings, had the potential for greatly reduced drag compared to the old wings with fully turbulent boundary layers. Ames aerodynamicists used their 1 x 3.5-foot tunnel to refine...
...airfoil contours and establish performance characteristics in the transonic range. The best of the new laminar-flow, low-drag airfoils (called the "6-series") was quickly adopted by the designers of advanced World War II fighters and bombers. This airfoil family still contributes to wing design in today's subsonic jets and propeller-driven aircraft.
