page_title: Variable Density Tunnel
section_title: Origin
hierarchical_section_title: Variable Density Tunnel / History / Origin
caption_attribution_description: English: The Variable Density Tunnel photographed arriving at the Langley Research Center by rail on February 3, 1922 from the Newport News Shipbuilding and Dry Dock Company. The closed-circuit tunnel, which could run at speeds up to 50 mph (80 km/h), was enclosed in an 80-ton steel enclosure that permitted operations at high pressure (up to 21 atmospheres). Maintaining high pressure permitted realistic testing conditions for airfoils while maintaining relatively low operating speeds, according to the Reynolds Number similarity parameter.
context_page_description: The Variable Density Tunnel was the second wind tunnel at the National Advisory Committee for Aeronautics Langley Research Center. Proposed by German aerospace engineer, Max Munk in May, 1921, it was the world's first variable density wind tunnel and allowed for more accurate testing of small-scale models than could be obtained with atmospheric wind tunnels. It was actively used as a wind tunnel from 1923 until its retirement in the 1940s. Langley Research Center historian, James R. Hansen, wrote that the VDT provided results superior to the atmospheric wind tunnels used at the time and was responsible for making NACA, the precursor to NASA, "a world leader in aerodynamic research". It is now on display on the Langley grounds, near the old Reid Conference Center and is a National Historic Landmark.
context_section_description: In 1920, the National Advisory Committee for Aeronautics brought Max Munk, a German aerospace engineer and student of Ludwig Prandtl at University of Göttingen, to work for them in America. After the end of the First World War, Munk required two presidential orders to work at NACA, and reportedly had difficulty adjusting to NACA's organizational structure, which was not as strictly defined as those in Germany. Regardless, he proposed his revolutionary Variable Density Tunnel design in the same year.
  The large, steel pressure tank of the VDT was designed with a working pressure up to 20 atmospheres and was constructed by Newport News Shipbuilding and Dry Dock Company in Newport News, Virginia. The tank was 34.5 ft (10.5 m) long and 15 ft (4.6 m) in diameter. The tank's wall was 2 ¹⁄₈ in (54 mm) thick. The tank required 85 tons (77.3 tonnes) of steel. The test section was 5 ft (1.5 m) 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 51 mph (82 km/h).
  During a fire in 1927, the internal wooden test section of the VDT was destroyed and was rebuilt with an open-throat design before complications required it be rebuilt again with a closed-throat design. The VDT re-entered service in 1930 and continued to aid the Langley staff in measuring the aerodynamic qualities of airfoils until it was deemed obsolete in the 1940s and converted into a pressure tank for other wind tunnels. The VDT was decommissioned in 1978 and was declared a National Historic Landmark in 1985 because of its historical influence setting the foundation for the beginning of space flight.
caption: The tank of the Variable Density Tunnel arriving in 1922
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