Variable Density Tunnel

The Variable Density Tunnel was a wind tunnel at NASA's Langley Research Center. It is a National Historic Landmark. It was the world's first variable density wind tunnel that allowed accurate testing with small-scale models. It was actively used as a wind tunnel from the early 1920s until the 1940s.

History
Accurate wind tunnel testing requires matching the Reynolds number of the model with that of the actual aircraft. The Reynolds number is the ratio of inertia forces to the viscous forces in the flow. It is computed as the product of the air density, ρ, the speed of the air relative to the aircraft, V, and the characteristic length scale, L, divided by the viscosity of air, μ. For an airfoil, the characteristic length is usually the chord length. If a full-scale airfoil is to be simulated in a wind tunnel by a small-scale model, the Reynolds number can only be matched by increasing the velocity or its density or by decreasing its viscosity. If air is used in the wind tunnel with a small-scale model, either the speed or the density of the air must be increased.

When National Advisory Committee for Aeronautics started to build a modern wind tunnel, they got Max Munk, who had studied at University of Göttingen, to design a wind tunnel using air under pressure. A large, welded-steel tank was designed with a working pressure up to 21 atmospheres. The tank was constructed by Newport News Shipbuilding and Dry Dock Company in Newport News, Virginia. The tank was 34.5 ft long and 15 ft in diameter. The tank's wall was 2+1/8 in thick. The tank required 85 tons (77.3 tonnes) of steel. The test section was 5 ft in diameter to match an existing NACA Wind Tunnel No. 1, which was an open-circuit tunnel operating at atmospheric pressure. The variable-density wind tunnel had a closed-circuit design with an annular return flow to minimize the volume of the tank. A fan powered by a 250-hp motor could produce an air speed up to 50 mph.

The tank was partially destroyed by a fire in 1927. It was rebuilt and operational in 1930, and was used up to the 1940s. Then the tank was used as a pressure tank to support other wind-tunnel activities at Langley. In 1978, the tank was taken out of service; it survives on display.

The tunnel was used for over 20 years. It produced the data for 78 classical airfoil shapes that were published in 1933 in "The Characteristics of 78 Related Airfoil Sections from Tests in the Variable-Density Wind Tunnel," NACA Technical Report 460. These data formed the basis for many World War II airfoil designs.