Testing of a Northrop XB-35 flying wing model in the 20-Foot Spin Tunnel . This picture shows the test section of the tunnel which has 12 sides. The distance across is 20 feet; the vertical length of test section is 25 feet. The 20-foot spin tunnel was authorized in 1939 and began operations in 1941.>From "Characteristics of Nine Research Wind Tunnels of the Langley Aeronautical Laboratory": "The Langley 20-foot free-spinning tunnel has been in operation since 1941. ... The tunnel is vertical with a propeller at the top, which draws air vertically upward, through turning vanes, into an annular return passage. The choice made of the return passage and the vanes, for the height and diameter selected for the tunnel, is such as to provide an energy ratio low enough to allow for natural rapid deceleration of the air and yet rapid enough to permit acceleration of the air without excessive power requirements. The fan is driven by a direct-current motor, which generally supplies 400 horsepower and, for maximum tunnel speeds, supplies over 1,300 horsepower. Motion-picture cameras are mounted at the side and bottom of the tunnel." 20 foot Spin Tunnel: A model of Northrop's XB-35 flying wing bomber was tested in Langley's 20 foot Spin Tunnel, 1943. The tunnel became operational in March 1941. Published in U.S., National Advisory Committee for Aeronautics, Characteristics of Nine Research Wind Tunnels of the Langley Aeronautical Laboratory, (Washington: Government Printing Office, 1957), p. 85; James R. Hansen, Engineer In Charge, NASA SP-4305, p. 463.
