Variable Density Tunnel
Encyclopedia
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.
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 of the tunnel must be increased 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 & 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 were 2⅛ inches (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 of up to 50 mph (80 km/h).
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The tank was partially destroyed by a fire in 1927. It was rebuilt and operational in 1930. It 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 twenty 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.
Wind tunnel
A wind tunnel is a research tool used in aerodynamic research to study the effects of air moving past solid objects.-Theory of operation:Wind tunnels were first proposed as a means of studying vehicles in free flight...
at NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
's Langley Research Center
Langley Research Center
Langley Research Center is the oldest of NASA's field centers, located in Hampton, Virginia, United States. It directly borders Poquoson, Virginia and Langley Air Force Base...
. It is a National Historic Landmark
National Historic Landmark
A National Historic Landmark is a building, site, structure, object, or district, that is officially recognized by the United States government for its historical significance...
. It was the world's first variable density wind tunnel that allowed accurate testing with small-scale models
Model aircraft
Model aircraft are flying or non-flying models of existing or imaginary aircraft using a variety of materials including plastic, diecast metal, polystyrene, balsa wood, foam and fibreglass...
. It was actively used as a wind tunnel from the early 1920s until the 1940s.
History
Accurate wind tunnelWind tunnel
A wind tunnel is a research tool used in aerodynamic research to study the effects of air moving past solid objects.-Theory of operation:Wind tunnels were first proposed as a means of studying vehicles in free flight...
testing requires matching the Reynolds number of the model with that of the actual aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...
. 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
Airfoil
An airfoil or aerofoil is the shape of a wing or blade or sail as seen in cross-section....
, 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 of the tunnel must be increased or the density of the air must be increased.
When National Advisory Committee for Aeronautics
National Advisory Committee for Aeronautics
The National Advisory Committee for Aeronautics was a U.S. federal agency founded on March 3, 1915 to undertake, promote, and institutionalize aeronautical research. On October 1, 1958 the agency was dissolved, and its assets and personnel transferred to the newly created National Aeronautics and...
started to build a modern wind tunnel, they got Max Munk
Michael Max Munk
Max Michael Munk was a German aerospace engineer who for National Advisory Committee for Aeronautics in the 1920s and made contributions to the design of airfoils....
, 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 & Dry Dock Company in Newport News, Virginia
Newport News, Virginia
Newport News is an independent city located in the Hampton Roads metropolitan area of Virginia. It is at the southeastern end of the Virginia Peninsula, on the north shore of the James River extending southeast from Skiffe's Creek along many miles of waterfront to the river's mouth at Newport News...
. The tank was 34.5 ft (10.5 m) long and 15 ft (4.6 m) in diameter. The tank's wall were 2⅛ inches (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 of up to 50 mph (80 km/h).
The tunnel was used for over twenty 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
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
airfoil designs.