THE HIGH SPEED FRONTIER
 
 
Chapter 3: Transonic Wind Tunnel Development (1940 -1950)
 
THE SLOTTED TRANSONIC TUNNEL
 
 
 
[98] The idea that the opposite effects of open and closed walls could be utilized in certain combinations to reduce or eliminate any net effect of the walls on wind-tunnel test results dates back to the classical Prandtl and Glauert work of the twenties. It was considered extensively by several other authors in the thirties. During the war, theoretical work on the problem was continued in England, Germany, Italy and Japan, and several investigators identified partly-open wall arrangements which theoretically eliminated the blockage effect on velocity at the tunnel axis (refs. 46, 94). Moreover, the general similarity rules showed that this result would continue to be valid at high subsonic speeds if the models were not too large (ref. 83). Reid mentioned the German activity to us [99] when he returned from his War Department Alsos Mission assignment in 1945. No actual construction of a multi-slotted tunnel had been started, however, because of war circumstances. Technical reports covering the foreign theoretical work did not become available to Langley until after the war. Ferri's successful use of the rectangular semi-open tunnel for body and airfoil testing up to Mach numbers near 1 with no apparent jet-boundary effects was the first real demonstration that partly-open arrangements could be used effectively. As explained previously, Langley developed an improved version of this approach for high-speed airfoil testing in 1948. However, it was not practicable to employ this scheme in very large facilities such as the 8-foot tunnel because of its excessive power requirements relative to closed throats and other problems.
 
The first successful many-slotted transonic tunnel configuration was devised single-handedly by Ray H. Wright. Wright had been hired in 1936 as a scientific aide at $1200 per annum following unhappy employment as an inspector in a whiskey distillery where his M.S. in physics from the University of Kentucky was largely wasted. (The distillery job had been especially distasteful to Wright, a teetotaler, because he came home every afternoon reeking of whiskey.) Expecting to be told what to do at Langley under the close supervision of some senior physicist, Wright was surprised to find his boss at the 8-foot tunnel, R. G. Robinson, to be an engineer who sought theoretical answers and advice from him in an area where he had little knowledge and no experience. He had a natural aptitude for applied mathematics but his training in the subject had been rather limited. He received permission to acquire the needed additional skills by studying on the job as time permitted. As a result, in a group populated almost entirely by engineers he became an indispensable consultant on matters mathematical and theoretical.
 
No one told Wright that the time had come to define a slotted tunnel. His assignment was very broad-to study the wall interference problem with reference to operations in the repowered 8-foot tunnel. He was, of course, familiar with previous research and he had aided Donaldson in a small preliminary study relating to airfoil blockage in semi-open and closed tunnels (ref. 53). He was aware in a vague way that Stack along with many others intuitively anticipated that a partly-open configuration could be found eventually, but had received no specific directive to work [100] on the problem. The goal that he chose to focus on was specifically related to the test section of the 8-foot tunnel-to determine a slot configuration for its circular test section which would produce zero axial velocity increment due to the blockage effect from a body of revolution. The semi-open rectangular tunnel solution of Weiselberger, which in effect was a slotted tunnel with two slots, was not applicable on three counts: it was not circular, it would have had a power requirement well in excess of what was available, and it was known from Ferri's application to have serious flow pulsations.
 
Wright attacked, the problem analytically because, as a specialist in applied mathematics, that was his established method of research. Experimental work at 8-foot had almost always been done by the engineers. He agrees that a systematic experimental attack on the problem might have been equally effective (ref. 117). A specific 10-slot configuration was selected for analysis, the object of the calculations being to find the slot width or degree of openness that would result in zero blockage. All such calculations, because of their difficulty, necessarily assumed low-subsonic or incompressible flow. If, however, zero net axial blockage could be achieved, the general similarity rules suggested this result would continue to be valid at transonic Mach numbers (refs. 46, 48). Wright regards the tedious mathematics he used as "sloppy" because of the lack of definite convergence of his solution. The results suggested that the tunnel should have about 12 percent of the periphery open in contrast to Weiselberger's two-slot value of 46 percent (ref. 118). This was most encouraging because the excess power required by slots tends to be proportional to the open area, and would be much less in the 10-slot circular tunnel.
 
In the late summer of 1946, Wright discussed his tentative results with his section head, E. C. Draley, emphasizing the dual questions of convergence and whether the result would hold good at high Mach numbers. They decided to try to answer these questions by experiments with a 10-slotted model. Wright approached Lindsey to learn whether the 9 x 9-inch jet equipment might be utilized, but primarily because of its circular shape the slotted test section could not readily be adapted. He came next to my office with his problem. For some time we had been investigating blockage corrections at the 16-foot tunnel using the [101] "parasite" technique previously described for our demonstration tunnels to power three circular test sections of varying size (fig. 23). Thus, it was quite easy for us to add a test program for Wright's circular 10-slotted arrangement. V. G. Ward, who had been working with C. H. McClellan on our blockage correction study, was assigned as project engineer for the experiments.
 
In the spring of 1947, Wright had an opportunity to discuss his work with Busemann who had recently been assigned to Langley as one of the foreign scientists acquired under the Navy's "Paperclip" program. Busemann suggested that a better theoretical approach would be to assume that the slot effect was uniformly distributed about the periphery rather than in discrete slots. He believed both lift interference and blockage effects could be treated from the standpoint of this homogeneous boundary. He also noted that the mathematics for the homogeneous wall promised to avoid the convergence problem. Unfortunately, much of....
 
photo of the
 
FIGURE 23--"Parasite" tunnel used to test the first successful slotted throat. The 12-inch diameter test section is at extreme left. Tunnel operates by suction of outside air into the test chamber of the 16-Foot High-Speed Tunnel.
 
[102] ....this advice was wasted, partly because Wright could not understand much of Busemann's English. Some four or five years later, Don Davis heard Busemann's arguments, presumably in improved English by that time, and applied the method successfully. His solution (ref. 119) is general in character but can be made to yield results comparable to any particular slot arrangement. When applied to Wright's particular case, satisfactory agreement was revealed between the two theories. Significant further improvements and extensions of the theory appeared later (ref. 94).
 
When Stack was told of Wright's theoretical results in the late summer of 1946, he sensed that the partly-open test section he had long anticipated had been found. He informed Dr. Lewis of the results and evidently mentioned their possible implications for the 8-foot and 16-foot tunnels (ref. 78). High priority in the Langley shops was provided for Ward's 12-inch diameter model slotted test section, and early in 1947 the experiments started. A key feature was tests of a body of revolution which would have caused choking in a 12-inch closed-throat tunnel at about Mach 0.70. In the first runs, the slotted tunnel speed could be increased to Mach 0.97 before choking occurred at the diffuser inlet, not in the test section- a problem which could be eliminated by relieving the diffuser contour. Unexpectedly, the Mach number without the model could be increased smoothly through Mach 1 up to about 1.15 as the diffuser pressure was reduced. Comparative pressure tests of the same small model in the 8-foot closed-throat tunnel showed good agreement with the small slotted tunnel up to the onset of choking at M = 0.96 in the 8-foot tunnel tests.
 
These early 1947 results were impressive, but there was no immediate acceptance of the slotted configuration and no immediate planning to incorporate it into either the 8-foot or 16-foot tunnels. Stack presented a summary of the situation as it existed in mid-summer of 1947 at a meeting of the General Aerodynamics Committee on July 25, 1947. He made no mention of any specific plans to install slotted throats in the large tunnels, although he did infer that the Wright/Ward work had important implications. According to the minutes, he told the group only that plans and funding had been approved to repower 16-foot with 60 000 hp (instead of the 40 000 hp originally requested and approved for the fiscal year 1947 budget) to produce Mach 1.3 in a closed-throat [103] supersonic nozzle. Similar performance was believed obtainable in the 8-foot tunnel with its 18 000 hp (ref. 81).
 
Privately, however, Stack had begun telling his associates in mid-1947 that the 16-foot tunnel should consider using a slotted throat. He described this in his later press release as the period when a "definite commitment" to this idea was made (ref. 78). At first, it was really only a commitment in his own mind. He went on in the press release to say that some of his colleagues considered such a move premature. In his words, he was conscious "of a very strong undercurrent of disbelief." And, indeed, there was good reason for disbelief. The many major unanswered questions at that time included: the power requirements, the details of slot shaping, especially near the entrance and diffuser regions, the quality of slotted tunnel flow, model size limitations, possible combinations of wall divergence and slots, shock reflection problems above Mach 1, slots versus porous walls, etc. My own opinion was that an orderly continuation of the model tunnel program for as long as needed to provide answers should be pursued before any commitment was made to incorporate slots in the 16-foot or 8-foot tunnels.
 
A day or so after the July 25 meetings, Ferri knocked on my door and sat down to discuss a new concern relating to the slotted tunnel program. He conceded that slots could be used to reduce the blockage effect, but to have zero blockage at Mach 1 was physically unlikely except for very small models. He felt that many mathematicians and physicists who had an understanding of transonic theory would regard any NACA claim of valid data at Mach 1 for sizable models as absurd. NACA's reputation would be blemished, he said, unless we could convince Stack to use some words of qualification when discussing slotted tunnels. Later discussions with Busemann revealed that he agreed with Ferri on this point. I suggested that the best way to make this important point clear to all concerned would be to air the subject at a meeting of the General Aerodynamics Committee, and I arranged with Sam Katzoff, Chairman of the Committee, to make the slotted tunnel problem a principal item on the agenda for the September 1947 meeting. Meanwhile, I told Stack of this special concern. He agreed to attend the meeting but was obviously irritated.
 
Ward and Wright presented their results in rather modest terms at the [104] September meeting. Stack made a late entrance and sat down at the head of the table with a belligerent look on his face. Clearly it said, "Anyone who wants to argue about the slotted tunnel will have to take me on. Ferri made his comment but the point was lost through a combination of poor English and extreme politeness, and the minutes of the meeting make no mention of it.
 
Actually, there was basic validity to Ferri's argument. In their report (ref. 118), Wright and Ward cautioned that the allowable model size for zero blockage "must decrease as the [subsonic] Mach number increases." This fact was strongly underscored in a much later very careful investigation (ref. 108) which found that, even with a model blockage ratio as small as 0.0003, significant interference effects in slotted tunnels occurred near Mach 1. Such a model would have a cross-section of only about nine square inches in the 16-foot tunnel, and this is more than an order of magnitude smaller than the previously considered "safe" size of about 144 square inches. For the larger size, the results appear interference-free up to about Mach 0.98, however, so that the extensive data obtained with large models through the fifties and sixties are suspect only in the range beyond about 0.98. (See fig. 24.)
 
There had been several ideas for possible closed-throat test section concepts for the repowered 16-foot tunnel which would have enabled it to cover the subsonic speed range up to M = 0.95 and the supersonic range from about Mach 1.1 to 1.3. On March 5, 1946, B. W. Corson, Jr., had suggested that trials be made of the use of air addition to form a "throat," or air removal to provide expansion similar to the diverging walls of a supersonic nozzle. (In later years, he successfully combined the removal idea with the slotted test section in the design now in use to vary the speed of the 16-foot tunnel between Mach 1 and 1.3.) The Langley engineering section had developed designs incorporating adjustable walls in a rectangular test section, and a "revolver" design using an assemblage of interchangeable fixed nozzles. At the time of Stack's decision to go with a slotted throat in 1947 the interchangeable nozzles were the favored scheme (ref. 120). Mechanically, this was a rather awesome arrangement of several 16-foot diameter nozzles carried on a rotating mechanism similar to the cylinder of a revolver.
 
Next to its elimination of choking the slotted tunnel was especially....
 
graph of data discrepancies
 
[105] FIGURE 24.-Comparison of flight (drop tests) and slotted-tunnel drag data showing discrepancies at Mach numbers above 0.98, 1973 data.
 
....attractive because it also eliminated the need for these complex and costly mechanically-variable test sections. I had given a good deal of thought to other possibilities for Mach number variation in fixed closed-throat test sections in the hope of finding a sound scheme that would be competitive with the slotted throat. In August 1947 I proposed that heat addition or removal be investigated as a means of Mach number variation (ref. 121). This idea grew out of an analysis I had made the year before of the possibility of using the ramjet principle to power high-speed wind tunnels, a suggestion offered by Vannevar Bush (ref. 122). Although sound in principle, the heat transfer schemes proved impractical for very large test sections.
 
The large postwar shift of research emphasis toward supersonic flight caused a major expansion of the Compressibility Division's facilities, including the addition of the 4-foot Supersonic Pressure Tunnel, the Gas Dynamics Laboratory, and the Induction Aerodynamics Laboratory (transferred from the Full-Scale Division). In order to achieve a better [106] balance in regard to size and scope of management responsibility, the 8-foot and 16-foot tunnels were transferred to the Full-Scale Division which operated the 19-foot and full-scale tunnels. The individuals now responsible for the slotted throat developments were not likely to offer much resistance to Stack's inclination to rush ahead, a situation which he undoubtedly considered satisfying. My doubts about the path he was taking were so strong, however, that I ignored the organizational changes and continued to plague him with criticism and suggestions.
 
During the fall of 1947, as Stack's plans to install a slotted throat in the 16-foot continued to solidify, I spent some time analyzing a new scheme whereby variable Mach number could be obtained simply and at low cost in a fixed closed-throat nozzle for the 16-foot (ref. 123). The basis of my idea was a 50-foot-long Mach 1.3 nozzle (fig. 25) which had such a gradual area expansion that quasi-uniform flow existed at each station, providing a continuous gradual Mach number increase from 1.0 to 1.3. A sting-supported model mounted through a swept-back strut on an external track positioned the model at any desired location. Mach 0. to 0.95 would be covered in the throat location and the low supersonic range from about 1. 10 to 1. 30 would be covered by moving the model and its support downstream. (Because of its high power requirements, the slotted throat would be limited to a maximum Mach number of about 1.1 for the same 60 000-hp input.) Subsonically, a model of smaller size than for the slotted throat would have to be used and the choked speed range between about 0.95 and 1.10 could not be covered. The test models would also be operating in a small pressure gradient; however, this effect was quite small, amounting to less than 3 percent correction in drag for 5-foot-long models (ref. 123). The scheme appeared to be much simpler and cheaper than the "revolver" idea for alternate nozzles. A recent demonstration of the practicality of changing model location in a fixed nozzle had been made in the 8-foot Mach 1.2 nozzle where the model had undergone subsonic testing in the throat and Mach 1.2 testing in the downstream position (ref. 124).
 
I presented this scheme to Stack in late December 1947, hoping that there might still be time to encourage what I believed to be a more rational sequence of events for developing the slotted concept. I emphasized how a slotted throat could readily be incorporated later in...
 
 
[107] FIGURE 25.-Fixed-geometry nozzle with variable model position for transonic testing up to Mach 0.95 and from 1.10 to 1.30. This was one of several schemes put forward in the 1946-1948 time Period as alternatives to the slotted throat.
 
 
[108] the long test section after the slot design had been properly developed. It was quite obvious, however, that by this time Stack had become so deeply and totally committed to the slotted throat that there was no turning back. It was apparent that he regarded my scheme more as a possible obstacle to gaining top-level final approvals for the slotted program than as an opportunity to pursue a more moderate course. He suggested that in case a major difficulty should be encountered in the slotted program the long test section should be considered as an alternative, and he requested that I record the idea in a memorandum (ref. 123).
 
The slotted throat for the 16-foot tunnel was handled as a part of the 60 000-hp repowering project. A formal description and justification was prepared by Corson, head of the 16-foot tunnel section, on January 10, 1948 (ref. 120). It is obvious now that the understanding of the slotted throat as evidenced in the Corson text was seriously deficient in at least two major areas, the power requirements and the problem of valid testing at low supersonic speeds. The fact that the slotted tunnel would not provide generally useful test capabilities at speeds for typical models in the range from about Mach 0.98 to 1.05 because of problems related to reflections of compression waves had not yet been learned (ref. 108).
 
Ward was under heavy pressure to come up with the additional data needed for the 16-foot design. The technique of operating the 16-foot tunnel itself in order to provide suction power for the 12-inch model slotted tunnel was proving too slow to meet the demand, and Stack asked me to consider using the blower equipment in our Induction Aerodynamics Laboratory to power Ward's tunnel. We assigned W. J. Nelson to work with Ward, and by early spring of 1948 a comprehensive program had been agreed upon (ref. 125) and tests were in progress. Nelson quickly developed a keen interest in the problem and initiated his own program of investigation of slotted and porous-wall configurations (ref. 126) using a small rectangular tunnel better suited to such work than Ward's scale-model of the 16-foot test section, which by now had become octagonal and 8-slotted. (This octagonal arrangement had been proposed by E. M. Gregory of the engineering group as a mechanically desirable approximation to Wright's original configuration.)
 
By early spring of 1948, Stack was providing his personal supervision [109] on a daily basis for the many interrelated slotted tunnel activities, ranging from expediting work on models in the shops, to working with the detail designers of the 16-foot test section, and dealing as always with funding and approval problems. He held frequent meetings of the key individuals involved at this time including E. Johnson, P. Crain, and E. Gregory of the engineering and shop groups, and E. Draley, B. Corson, A. Mattson, R. Wright, W. Ward, and W. Nelson of research. Stack was at his best in this kind of operation. He was adamant regarding schedules, at times ruthless in dealing with any interference, and always able to inspire, to make quick decisions, and to give effective orders.
 
One day, after the 16-foot tunnel project was well underway, he surprised everyone by announcing that the 8-foot tunnel should also be converted to a slotted throat. At that time, the plaster liner had just completed successful development and was starting to be used for research. The 8-foot group had given little thought to the next step beyond the plaster liner and protested that they would need time to study the possibilities. The plan to slot the 8-foot tunnel quickly took form under strong pressure from Stack. Since the necessary fabrication could be done in Langley's shops and the installation made by Langley labor, this relatively inexpensive alteration was not subject to the formal approval and procurement processes of a major new facility. Before long, it was apparent that it would precede the 16-foot project, becoming the first large slotted tunnel to be placed in operation. Stack's main motivation in adding the 8-foot tunnel slot development was probably concern over the low Reynolds numbers of the model throat tests, a concern which turned out to be well founded. He was also naturally very impatient at the prospect of two to three years of procurement time before the 16-foot tunnel would be operable.
 
It was a fairly straightforward matter to replace the old 8-foot test section with a 12-sided, 12-slotted version built in the Langley shops. Some use was made of Ward's model work with the 16-foot tunnel configuration, particularly for the diffuser entrance area requirements. Ward had also found that tapering of the slot width was desirable to prevent too rapid initial expansion at low supersonic speeds (ref. 127), but this feature was not used; the slots were rectangular and similar to those of Wright's analysis. The slots opened directly into the igloo-shaped [110] test chamber and it was obvious that hazardous pressure, temperature, and noise levels would be encountered (fig. 26). Diving suits were, therefore, worn by operators whose presence was required in the test chamber during the initial tests with the slots (fig. 27).
 
As in Ward's model tests, the simple rectangular slots provided reasonably uniform flow for choke-free model testing at speeds up to Mach 1. At supersonic speeds, however, intolerable large deviations occurred (ref. 128). On the tunnel axis for a nominal Mach number of 1.09, the speed varied between extremes of Mach 1.0 and 1.16. Large power losses occurred due to inefficient features of the flow at the downstream end where it entered the diffuser. Several months were devoted to correcting these difficulties. Valuable guidance and design data were provided by the work of Ward and Nelson with the model slotted test sections. But it is quite evident from a study of the final reports (refs. 128, 129) that by working directly with the 8-foot throat itself, a degree of important detail and refinement were attained well beyond anything that could have been done with the small models. Wright's principal co-workers in this effort were V. Ritchie and R. Whitcomb. Excellent supersonic tunnel-empty flow distributions were eventually achieved. An efficient flapped scoop-type entrance section for the diffuser entrance was devised by Whitcomb to reduce the power requirements, the flap being left open for subsonic operation and closed for supersonic (fig. 28). Research usage of the tunnel commenced on October 6, 1950, some seven months after the start of slot developmental testing.
 
The slot technology improvements from the 8-foot program were passed on to the 16-foot, 8-slot design. According to NACA claims (ref. 129) this made it possible for 16-foot to become operational after only 30 hours of shakedown in December 1950. Actually, along with the research operations a continuing program of slot development was pursued in both tunnels. The presence of the tunnel boundary layer was found to have an important influence on slot behavior, neglected in an of the theoretical studies. Furthermore, the slot widths for elimination of lift interference were shown to be much smaller than those for zero drag interference (ref. 130). Perhaps the most important limitation discovered in the early usage of the big tunnels, however, was the inability of the slots to alleviate significantly the reflection of pressure disturbances from ....
 
[111] FIGURE 26.-Slotted throat installation in the repowered 8-Foot High-Speed Tunnel, 1950.
 
test chamber operator putting on protective diving suit
 
[112] FIGURE 27.-Ray H. Wright, designer of the slotted throat, dons a diving suit for protection against noise and heat in early runs in the test chamber of the 8-foot slotted tunnel.
internal view of slotted throat wind tunnel
 
[113] FIGURE 28.-View of the 8-foot slotted throat showing diffuser-entrance flaps.
 
.....the solid regions of the walls. Thus, although there was no choking and although the speed could be increased continuously in the low supersonic region above Mach 1, the test data often exhibited significant discrepancies when compared with free air. For the selected cases considered in ref.129, the Mach range above about 1.02 showed such effects in the 8-foot tunnel; better agreement was shown for another selected model tested in 16-foot. The general experience in 16-foot, however, has revealed so many uncertainties in the range from about 0.98 to 1.05 that it is usually bypassed in setting up test programs (ref. 135). Similar low-supersonic data are also considered not valid in the present 8-foot tunnel operations (Whitcomb and Bielat interviews). The model sizes for valid operation in the range Mach 1.1 to 1.3 are no larger than for solid-wall tunnels.
 
Knowledge of the NACA programs of the 1946-1950 period was, of course, readily available to the military services and their contractors, and [114] this stimulated many activities outside NACA. Some of these are listed in the Appendix. Publications covering work with slotted test sections at Brown University (ref. 131), and with porous test sections at Cornell Aero Lab (ref. 132) appeared in the early fifties along with others. In the 1951 Annual Report of the NACA, J. C. Hunsaker's letter of transmittal to the Congress announced to all the world,
 
During the year the Committee completed the installation of a transonic ventilated throat in the 16-foot tunnel at the Langley Aeronautical Laboratory. This is of exceptional importance because it permits model airplane tests at transonic airspeeds in wind tunnels, hitherto impossible because of choking....
 
With this stimulus together with the advanced status of background technology on the subject, it was a foregone conclusion that transonic throats would quickly blossom throughout the world. By 1954, realizing that the technology already had become more or less universal, NACA dramatically removed the classified wraps from much of its work and announced in the annual report that through "intensive effort" starting prior to 1942 and a "calculated gamble of millions of dollars" NACA had won a "vital" two-year advantage for the United States in the "world race" to learn how best to fly at transonic speeds.
 
COMMENTARY
 
Looking back on the situation that existed in early 1945 when the 8-foot tunnel started operating with 18 000 hp, one sees a combination of favorable circumstances from which it was inevitable that some usable form of partly-open throat configuration would crystallize. Pertinent features of this environment included:
 
 
Wright's personal decision in 1945 to get down to cases and try to define analytically a multi-slotted circular configuration was the act that set in motion the events that led in about five years to the successful operation of the first large transonic tunnels. Most of the developmental testing in this period also clearly bears the stamp of Wright's insights and personal integrity. It is equally clear that without the enormous contributions of a quite different kind made by Stack the achievement of the large slotted tunnels would not have happened in 1950. To begin with, Stack had promoted almost single-handedly the projects to repower 8-foot and 16-foot. And, although he did not specifically initiate the slotted-tunnel studies, he had created a research environment in which an idea like Wright's could be freely explored and allowed to grow. Stack's principal personal contribution, however, was in promoting and implementing the plans for immediate application of the slotted throat in the two major facilities. He persevered in this against the conservative advice of senior staff members. What drove him with such zeal is not entirely clear. We had only begun to exploit the "small-model" technique and could have continued for years supplying much of the transonic data needed by designers at speeds up to Mach 0.95 and at Mach 1.1 or 1.2, with the rocket models providing additional high Reynolds number transonic data. In part, Stack's zeal grew from his ill-founded belief that the slots would permit interference-free testing of large models throughout the transonic zone-in the low supersonic as well as in the high subsonic portions. Perhaps he particularly wished to make good on his ambitious projections to G. W. Lewis in 1946. Undoubtedly, he also sensed the dramatic impact that the first large tunnel operating through Mach 1 with substantial models would have.
 
It is also evident now from experience with large slotted tunnels that no amount of preliminary testing in small model tunnels can eliminate the need for refined developmental testing in the full-scale facility itself. Thus, by proceeding immediately (and to all appearances in 1948, prematurely) with the 8-foot installation, the NACA slotted tunnel [116] developers came to grips at once with all of the real full-scale problems. The solutions found here had a convincing validity and value beyond anything that could have been done in the model tunnels.
 
It could hardly be expected that NACA's first public disclosures of the slotted tunnels would be modest and carefully qualified. The entire accomplishment dating back to the start of high-speed research in the early thirties was indicated to be exclusively NACA's (refs. 129, 133). Both of these documents emphasize that "large-scale aerodynamic research" can now be "conducted throughout the full transonic speed range." Ref. 133, the 1954 Annual Report of the NACA, gave the de tails later and mentioned some problems under the heading of "Fluid Mechanics."
 
Learning that NACA had declassified sufficient slotted-tunnel material to cover the 1954 disclosures, the Institute of the Aeronautical Sciences solicited a paper on the subject from Stack for its summer meeting in 1954. Stack relayed the preparation of this paper down to section head A. T. Mattson. With Stack breathing down his neck and the agency involved in glorification of a dramatic new accomplishment, Mattson was under great pressure to accent the positive aspects, and this explains the slanted quality of his paper. For reasons unknown, Stack told Mattson at the last minute that he would not attend the meeting, and Mattson had the unhappy task of presenting his glowing paper to an audience which included a number of outspoken eminent skeptics. Fortunately, the paper did admit the problem that had been found at low supersonic speeds, although it stated a bit too hopefully, "in most practical cases this is not a serious problem as even within this range the effects are not great and can be defined" (ref. 129). As we have seen, the problem still exists in the region from about Mach 0.98 to 1.05 in which valid testing is not ordinarily possible.
 
The 1954 NACA Annual Report heightened the drama by calling the entire enterprise "a calculated gamble" involving "millions of dollars and the future value of one of NACA's most valuable wind tunnels." Actually, the cost of the new throats was a minor part of the total costs of repowering. And, if the slots had failed to perform, they could have been simply covered over and both tunnels could have operated with the small-model technique.
 
[117] The general claim by NACA that the slotted tunnel provided America with a two-year lead time in aircraft development over her adversaries (ref. 78) is evidently based on Whitcomb's initial use of the 8-foot tunnel for the wing-body testing which led to his enunciation of the area rule (refs. 72, 134). General unsupported statements of this kind are hard to accept, but even so, few would argue with it if the claim were based on the massive total contribution of NACA's many-pronged attack on the transonic problem in the forties.
 
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