The development of large supersonic wind tunnels was accelerated by the emergence of the swept wing as a means of reducing supersonic drag. In 1945 Robert T. Jones of Langley (independent of the earlier work of Adolph Buseman) proposed that the sound barrier would be pierced more easily if an aircraft's wings were swept back. Initial NACA validation of the swept-wing concept near Mach 1 was carried out at Langley by free-fall tests of a systematic series of wing-body models dropped from high-flying aircraft. Drag was determined from measurements of the model acceleration during free fall. The first supersonic test in the United States of Jones' suggestion was made in the Langley 9- inch supersonic tunnel by Macon C. Ellis and Clinton Brown. Tests with a streamlined section of wire at a large angle of sweep indicated a dramatic reduction of drag in the supersonic range. Later tests with a slender delta wing at Mach 1.75 fully verified Jones' theory. Thus the race for supersonic aircraft supremacy began.
In February 1945 NACA began designing its first large supersonic wind tunnel at Langley. The war was still on and an accelerated construction schedule called for the tunnel to be in operation by the end of 1945- and on a budget of only $900 000! Langley aerodynamic engineers had been operating their tiny 9-inch experimental supersonic tunnel for 3 years. They now faced the Herculean task of building a vastly more powerful 4 x 4-foot tunnel in just 10 months.
Mach 2 was the goal. To attain this in a 4 x 4-foot test section, given the limited electrical power then available (6000 horsepower), the tunnel engineers had to cut the tunnel operating pressure back to 1/4 atmosphere. Even so, the air compressor had to handle 860 000 cubic feet of air per minute at a compression ratio of 2. The compressor was the key to the whole design, and, as it turned out, the key to the schedule. A seven-stage axial-flow compressor, one of the largest contemplated up to 1945, was aerodynamically designed by Langley. To help make the tough schedule, however, NACA assigned the mechanical design and actual fabrication to an industrial contractor. In the fall of 1945, the electrical drive motor and sections of the steel shell began arriving at Langley, but just when the scheduled goal seemed within reach, the construction of the compressor was halted by a 2-year strike. The tunnel did not begin operation until May 20, 1948.
Finally on the line, the 4 x 4-foot supersonic tunnel made up for lost time. Many well-known military aircraft and space vehicles were tested through the years: the famous Century Series fighters (F-102, F-105, etc.), the B-58 supersonic bomber, the X-2 research aircraft, and so on. So valuable was the tunnel that new drive motors were installed in 195O, bringing the power up from 6000 horsepower to 45 000 horsepower (continuous) and 60000 horsepower (for half an hour). The uprated tunnel remained in service until 1977, when it was dismantled. Its drive motors, cooling towers, and some support facilities were incorporated into the new National Transonic Facility (NTF) being built on the same site.
On the west coast in 1945, Ames was also exploring the supersonic range with an experimental tunnel. Their pioneer facility, in fact, had a 1 x 3-foot test section-considerably larger than the 9- inch tunnel at Langley. A 10 OOO-horsepower array of compressors permitted operation at Mach 2.2 at 4 atmospheres...
...pressure. A second supersonic tunnel of the same size operated intermittently off the tremendous amount of energy stored in the compressed air of the nearby 12-foot pressure tunnel. Ames was also ahead in the matter of funding. The U. S. Navy provided $4. 5 million to NACA to build a 6 x 6- foot supersonic facility for use in developing future naval aircraft and missiles. Not only was the resulting tunnel bigger than Langley's but it also incorporated two important new features that vastly increased productivity.
Conventional supersonic tunnels of the day had to shut down and change nozzle contours every time tests were to be run at different Mach numbers. H. Julian (Harvey) Allen, known for his low- drag airfoil research and later Director of the Ames Research Center, conceived an ingenious way to modify the nozzle contour in a continuous way while the tunnel was still in operation. Basically, one wall of the nozzle is kept fixed while the opposite wall slides axially, presenting a changing contour. Thus the tunnel nozzle is asymmetric but variable. The key to the whole idea was the recognition that unique contours could be found, using one fixed wall and one moving wall, that would provide uniform supersonic air velocities over a range of Mach numbers. In the 6 x 6-foot tunnel, the range was from Mach 1.3 to Mach 1.8. Later wind tunnels, notably the Ames 9 x 7-foot Unitary Plan Wind Tunnel (Mach 1.55 to Mach 2) and the Langley Unitary Plan Tunnel (Mach 1.5 to Mach 4. 6), employed this novel concept.
A visually arresting feature of the Ames 6 x 6-foot supersonic tunnel was its futuristic 50-inch- diameter glass windows through which an observer viewed the gleaming, mirror-surface, stainless steel walls of the test section. The huge glass disks, though, were not made to impress visitors. They were ground almost perfectly flat (i.e., optically flat) and had negligible internal flaws. The windows were larger than the 40-inch Yerkes refracting telescope lens, which until then was the largest optically perfect piece of cast glass. Through these glass ports passed the schlieren light beams that helped researchers visualize the flow of the supersonic air around the models in the tunnel. Any optical flaws in the windows would, of course, have distorted the pictures.
The Ames 6 x 6-foot supersonic tunnel did much to solve the mysteries of flight beyond Mach 1. Not only were new wing shapes developed for efficient...
....supersonic flight, but pioneering work was carried out in the areas of supersonic dynamic stability, aircraft control, panel flutter, and air inlet design. This tunnel, perhaps more than any other NACA wind tunnel, removed the label terra incognito from the supersonic map.
In this time period, at the Lewis Flight Propulsion Laboratory, adjacent to the Cleveland Municipal Airport, a supersonic propulsion tunnel was taking form on the drawing boards that some characterized as "an 87 000-horsepower bugle aimed at the heart of Cleveland!" This was the largest (8 x 6 feet) and most complex of the postwar Big Three supersonic tunnels. The "bugle" label was not too far from the truth because it had been decided to make this an open (nonreturn) tunnel; that is, with no recirculation of the air. Tunnel air, engine exhaust gases, and a great deal of noise would be vented into the Ohio countryside.
The mission at Lewis was the testing of aircraft power plants. An engine exhaust catcher had sufficed in the earlier altitude wind tunnel, but the advent of big turbojets and ramjets spewing huge amounts of hot gases made this device impracticable for full-scale engine testing. The following open circuit was devised instead. A seven-stage, 18-foot axial-flow compressor upstream of the test section duplicated engine inlet conditions at Mach 2, 35 000 feet altitude. Aft of the 8 x 6-foot test section, the hot air, filled with combustion products, was vented. The bugle, however, was muted by massive concrete walls and elaborate acoustical mufflers (called Helmholtz resonators) in the diffuser walls. It was not exactly quiet, but downtown Cleveland remained acoustically unruffled.
Clever solutions sometimes create unexpected new problems, and so it was with the Lewis open- circuit tunnel. The major problem was the moisture in the 150 000 pounds of air drawn in from the outside every minute. The air had to be dried in huge beds of activated alumina so that moisture would not condense in the tunnel. After each run, the alumina beds had to be heated for several hours to reactivate them.
Eventually the 8 x 6-foot supersonic tunnel was converted into a closed circuit by adding a return leg. For some engine tests, recirculation of the air was acceptable-and certainly much quieter. However, special doors allowed easy conversion to open-circuit operation when the engine exhaust gases were inimical to the test at hand. The return leg carrying subsonic air was later exploited by inserting a 9 x 15-foot subsonic test section for studying VTOL/ STOL aircraft configurations.
Commencing in 1948, numerous turbojet and ramjet engines passed through the Lewis tunnel. Also, models of supersonic fighters were mounted in the test section with complete simulation of engine inlet and exit airflows. As America entered the Space Age, rocket-powered vehicles were also tested in model form to determine such parameters as nozzle efficiency, controllability, and heating problems during flight.