24-Inch High Speed Tunnel (24-Inch HST). This tunnel was erected to provide a means for investigation of airfoils at high speeds. In the test section a velocity approaching that of sound (approximately 765 miles per hour) may be reached. The tip sections of propellers on modern high power airplanes frequently reach an air speed somewhat exceeding that of sound. In NACA TR No. 646, John Stack, W.F. Lindsey, and Robert Littell describe the tunnel as follows: "The N.A.C.A. 24-inch high-speed wind tunnel...is an induction-type wind tunnel. Compressed air from the variable-density wind tunnel discharged through an annular nozzle located downstream from the test section induces a flow of air from the atmosphere through the test section. Velocities approaching the speed of sound may be obtained at the test section. The tunnel is located outside the building that houses the variable-density wind tunnel. Except for certain modifications arising from its outside location and its size, the methods of operation are the same as for the 11-inch high-speed wind tunnel." (p. 74)
