By the late 1920s, aviation was definitely here to stay-not only militarily but commercially. Airmail service had begun, as had embryonic air travel. Then in 1927 Lindbergh flew solo across the Atlantic. The possibilities of flight mushroomed. Flying had commercial potential as well as unplumbed military possibilities. In consequence, NACA's research facility at Langley Field was so much in demand that NACA decided to scrap Wind Tunnel No. 1 and replace it with two new wind tunnels in the same building. These would be added to the now-famous variable density tunnel to form a tunnel complex superior to anything in Europe.
The first tunnel to be constructed in the old building was the same size as tunnel No. 1-5 feet in diameter. What made the new S-foot tunnel different? Its test section was tilted 90 degrees and was built vertically for detailed studies of aircraft spinning. Spinning was a poorly understood phenomenon in the 1920s. All too often when an aircraft lost speed and rolled off on one wing, it developed a spinning motion about a vertical axis from which recovery was difficult and sometimes impossible. The so-called "tail-spin" killed many unwary pilots in those barnstorming days; it is still a major design concern today.
By creating conditions that caused models to spin in the tunnel, spin-recovery procedures could be worked out on the ground without danger to pilots and planes. This simulation involved a special "spinning balance"-a vertical axis on which the aircraft model was mounted and by which forces and moments could be measured. Thus the conditions that forced the model to spin (autorotate) in the tunnel could be established. Of course, free-flying aircraft are not anchored to a vertical axis, and Langley engineers were already thinking about "free-spinning" tunnels in which the models were completely unattached.
The second tunnel replacing tunnel No. 1 was the 7 x 10-foot Atmospheric Wind Tunnel (AWT), operational in 1930. The AWT was designed as an aerodynamic research tool to study high-lift wings and general problems of stability and control. The choice of tunnel shape and dimensions showed rare technical foresight in regard to future aircraft sizes. So useful was the AWT that NACA added four tunnels of the same size in the years that followed. A unique feature of the AWT was a six-component, floating-frame balance that could measure each of the three forces and three moments exerted along and about the spatial axes of the tunnel airstream.
A critical problem tackled by the AWT was that of landing speed reduction. The airfoil shape desired for high-speed, low-drag flight is quite different from the high-lift profile required for landing at low speeds. The basic wing had several arrangements of flaps that created high lift when lowered for landing and takeoff but provided low drag when retracted at cruise speeds. Such flaps are still seen on today's aircraft.
A second objective of the AWT was measuring pressures at specific spots on the wings and flaps. These local pressures varied significantly from one area to another, especially during aircraft maneuvers. The readings from pressure detectors on the aircraft surfaces enabled the structures engineer to design the lightest wing to withstand aerodynamic loads.