The awakening interest in the hypersonic speed range, with its exotic new facility concepts, tended to mask a quiet revolution that was taking place in facility designs at more modest speeds-the subsonic/transonic flight regime. Aerodynamicists had long been concerned that the flow within the confines of the test section walls might not represent the actual conditions of flight in free air-where the real aircraft disturbs the surrounding air to distances several times the dimensions of the plane. If wind tunnel walls are introduced a shorter distance away, the natural streamlines of flow near the test vehicle are strongly modified-possibly producing misleading test results. Furthermore, tunnel wall interference effects would...
....be expected to become more severe as test speeds approach Mach 1, where the tunnel choking phenomenon dominates the flow picture.
Early wind tunnel experimenters recognized that solid test section walls created unwanted interference, and they tried to circumvent it by making the models very small-typically 1 percent of the test section area near Mach 1. However, the smaller the model, the lower the Reynolds number and the less these tests simulated true flight conditions. A second approach eliminated the wind tunnel walls altogether and utilized an unconfined, open jet of air. Unfortunately, as the jet emerged from the tunnel, the streamlines began to diverge and once again the tests were compromised. Of course, wind tunnels continued to be built and used despite the disturbing effects of the walls. Everyone knew that wall interference was there, but they also knew that it became most serious when the airflow began to choke in the transonic range.
Since wind tunnel walls unduly strangled the flow streamlines around a model and the complete absence of walls (the open-jet idea) distorted the streamlines in the other direction, perhaps some sort of "partial wall" would more effectively simulate free-air conditions. In 1946 Ray Wright at Langley analyzed the potentialities of a partially open or slotted wind tunnel wall. His results suggested that...
...slots occupying only about 6 percent of the wall would be the happy compromise that would closely duplicate free-air conditions. Strictly speaking, Wright's analysis was applicable only to low-speed flows, but Langley aerodynamicists, led by John Stack, immediately recognized in this simple proposal the possibility of solving the serious problems they had been having with wind tunnel testing near Mach 1.
All supersonic aircraft would have to fly through the transonic range-at least briefly. Knowing what happened in this transition zone was critical to the supersonic fighters and bombers being planned in  the postwar era. The slotted wall concept therefore was quickly wrapped in military security.
The immediate question was whether the analytical promise of the slotted wall would be realized in practice. Langley promptly built a 12-inch slottedwall test section to test the concept. The pilot tunnel was a success, demonstrating much less wall interference and reduced choking effects. Wind tunnel airspeed could be increased continuously through Mach 1 by merely increasing the fan speed, a desirable but hitherto unattainable goal.
The benefits of the slotted wall were not without cost-an unfortunate fact of life in engineering. The price was measured in terms of additional fan power and it was high-about twice as much as for a tunnel with solid walls. No one quibbled about the price at NACA, for a good transonic tunnel was worth far more. During this period, Langley happened to be in the process of repowering its huge 16-foot high-speed wind tunnel in order to boost airspeeds into the low supersonic range. The opportunity to convert it into a transonic tunnel was seized immediately. Langley also went ahead with plans to build a new 8-foot slottedwall tunnel (later known as the 8- foot transonic pressure tunnel) designed from its inception around the new concept. However, the quickest way to apply the slotted-wall concept was by modifying the operational 8-foot high-speed tunnel. In February 1950 this tunnel was shut down, and slotted walls were installed in the amazingly short period of 21 days.
Nowhere in the annals of aeronautical history can one find a more convincing argument supporting fundamental research than in the success story of the slotted-wall tunnel. A serendipitous chain of events led from low-speed aerodynamic analysis to a breakthrough idea, and then by intuitive extrapolation to transonic speeds-a long-sought technical prize. No amount of long-range planning could have devised this scenario. The revolutionary slotted-wall invention ultimately led directly to the discovery of the famous Area Rule, which in turn spawned a whole new generation of aircraft. So important was the slotted wall in aviation research that in 1951 John Stack and his associates at Langley received the coveted Collier Trophy for their work.