Chapter 4

The Transonic Wind Tunnel
and the NACA Technical Culture

by Steven T. Corneliussen

When nuclear physics emerged as a compelling field of fundamental scientific inquiry during the 1920s, it needed new research tools, especially the invention of accelerators for probing nuclei with artificially energized subatomic particles.1 Similarly, when the United States began expanding a national effort in applied aeronautical research during the 1920s, that too needed new research tools, especially improved wind tunnels for experiments using artificial airflows. Subsequent progress in both fields regularly resulted from research-tool advances—as subsequent Nobel Prizes regularly recognized, and as subsequent Collier Trophies did not.

By midcentury this contrast could be counted with the more obvious dissimilarities between the two fields. Though both nuclear physics and American aeronautics had continually required new empirical knowledge, their preeminent prizes since the 1920s had shown markedly differing esteem for advances in the means for generating it. In 1951, when particle-accelerator pioneers Sir John Cockroft and Ernest T. S. Walton won the Nobel Prize for physics, they joined previous laureates who had advanced nuclear science by inventing the cyclotron-type accelerator, the cloud chamber for making subatomic particle tracks visible, the magnetic resonance experiment method, further cloud chamber refinements, and a photographic technique for studying accelerator-generated nuclear processes. But until the Collier Trophy for that same year —save for the special case of 1947— the Collier's awarding committee had ignored research tools altogether, instead naming as the greatest advances in American aeronautics only aircraft equipment, air operations, heroic flights, and new airplanes. Yet aeronautical researchers with their continually improving research tools, especially the engineers and wind tunnels of the National Advisory Committee for Aeronautics, had contributed importantly to many of these advances. Thus the awarding committee for 1951 added importantly to the Collier's scope when it recognized the NACA's new transonic wind tunnels and the twenty NACA technical staff most closely associated with their advent.

1 . Lord Ernest Rutherford, discoverer of the atom's nucleus, described in his 1927 "Anniversary Address as President of the Royal Society" a long-standing "ambition to have available for study a copious supply of atoms and electrons which have an individual energy far transcending that of the alpha and beta particles" available from naturally occurring radioactive sources in order to "open up an extraordinarily interesting field of investigation." Quoted in Mark Oliphant, Rutherford: Recollections of the Cambridge Days (Amsterdam, NY. Elsevier Publishing Company, 1972), p. 82. Daniel .J. Kevles, The Physicists: The History of a Scientific Community in Modern America (New York, NY. Alfred A. Knopf, 1978), p. 227, cites Rutherford's desire during the 1920s for a "million volts in a soapbox." Concerning this and other topics, I am grateful for useful observations and information from historian of physics Catherine Westfall, whom I thank along with John V. Becker, Jay Benesch, Albert L. Braslow, H. Scott Butler, Francis J. Capone, Norman L. Crabill, James R. Hansen, J. D. Hunley, Peter Kloeppel, Richard T. Layman, Robert Riolo, Jim Spencer, Geoffrey Stapleton, and Walter G. Vincenti for reading this essay in manuscript form.



The Midcentury Need for Transonic Tunnels

In the 1947 special case, combat aviator Chuck Yeager flew manufacturer Lawrence Bell's new X-1 airplane faster than the speed of sound. Yeager thereby not only pierced the so-called sound barrier, but helped operate a transonic research tool conceived mainly by veteran NACA high-speed researcher and manager John Stack. The resulting Collier cited not only the heroic flyer and the airplane builder, but the NACA research-tool innovator as well.2 Stack himself was not present in the California desert below the X-1 in its transonic research flights, but some of his NACA colleagues were. A detachment of engineers from Langley Memorial Aeronautical Laboratory in Virginia masterminded the experimental airplane's operation.3 They instrumented it for data-gathering, planned and then observed each flight in detail, and assessed what was measured and recorded. They wanted new empirical knowledge of the bewilderingly complex, sometimes literally dangerous range of air speeds near the speed of sound, which varies with air temperature and can surpass 740 miles per hour.

Their NACA bosses at Langley Field and in Washington wanted transonic research advances too. Air speed had proven crucial in World War II, and jets were beginning to replace propeller-driven warplanes. In a high-profile 1946 assessment of the national defense program, Senator James M. Mead's special investigating committee had severely rebuked the NACA, charging past failures of "vision and imagination" concerning "revolutionary aeronautical developments" like Nazi Germany's missile technology and the jets that both Great Britain and Germany had developed in the 1930s, when the American aeronautical establishment still thought jets infeasible.4 Accordingly, the NACA's 1946 annual report to Congress stated a resolve "to face the urgent necessity for renewed emphasis on fundamental research," as the NACA customarily called its practical-minded but scientifically grounded engineering studies. "Without certain essential design data," the report continued, "the development of very high-speed aircraft and guided missiles cannot proceed. "5 That word urgent recurs concerning transonics throughout NACA documents of the early postwar era, when air-war memories were fresh, Cold War worries were intensifying, and NACA bureaucratic-war strategies were beginning to target the Army Air Forces. Like the NACA, the AAF —soon to become the Air Force—

2. Richard P. Hallion, Supersonic Flight: The Story of the Bell X-1 and Douglas D-558 (New York, NY., Macmillan Company, 1972), p. 176, notes that research airplanes like the X-1 were not "fabricated for setting records. Rather, they were designed as research tools. Though they set some spectacular records ... their main function remained unchanged: the acquisition via flight instrumentation of data on a variety of areas." The NACA's 1954 annual report, p. 4, says the research airplanes' "prime justification was as tools to be used in developing necessary transonic information." (NACA annual reports are cited hereafter in the form AR54.) The 1947 Collier, following the frequent practice of the day, cited engineer Stack as a "scientist." But Stack's 1928 MIT degree was in aeronautical engineering, as reported in James R. Hansen, Engineer in Charge: A History of the Langley Aeronautical Laboratory, 1917-1958 (Washington , DC: NASA SP-4305, 1987), appendix B. The influence of Hansen's engineering-centered interpretation of NACA research history pervades this essay.

3. To counter the notion of military control of "the research direction" of  the X-1 program, Richard P. Hallion emphasizes the NACA's "virtual total control" in his review of Walter A. McDougall's ... the Heavens and the Earth: A Political History of the Space Age, (New York, NY. Basic Books, 1985); see Technology and Culture 28 (January 1987): 130-32.

4. Excerpt from Mead committee report, "Miscellaneous" folder, John Stack collection, Langley Historical Archive-hereafter called LHA—NASA Langley Research Center library. For LHA access and much else, I thank Langley historical program coordinator Richard T. Layman.

5. AR46, p. 2.


aspired to create and control expensive new national aeronautical research tools: large wind tunnels for experiments using artificial transonic and supersonic airflows.6

As a motivation for high-speed research, the urgency of international military competition —though not that of Washington political competition— shows in separate, representative pronouncements by the NACA and AAF research directors in 1947. "The urgency of aeronautical research results from the relation of air power to national security," reported Hugh L. Dryden to the NACA's main committee a few days after the X-1's famous October flight. "Aircraft having the highest speed dominate the air," he noted, adding-in a complete reversal of the NACA's cautious prewar belief-that it was "clear that there is no upper limit to the possible speed of aircraft." Dryden declared that "the nation that makes the best research effort to develop the new power plants and explore the problems of high-speed flight can lead the world in air power. That nation must be the United States.... It is the duty of the NACA to provide for the military services and the industry the basic data on aerodynamics and propulsion to make piloted supersonic flight not only possible, but safe and reliable."7 In even more forceful terms, these themes had also appeared that April in a magazine editorial titled "We Must Furnish the Tools" by Maj. Gen. Curtis E. LeMay, the aggressive World War II strategic bombing leader who now headed the AAF Research and Development Agency. So emphatic was this two-page argument for new national high-speed wind tunnels that John Stack kept a photostat of it in personal papers now preserved in the NASA Langley Research Center historical archive. LeMay's editorial warned that for lack of proper research tools the United States risked losing the air-superiority race. In World War II, it said, the Nazis had been "at least five years ahead," though fortunately not in actually "applying the results of their technical superiority." In the postwar world, however, "even a one- or two-year lag" could probably ,, never be recovered."8 Similar arms-race language concerning wind tunnels also appeared a few years later when the Collier's magazine article announcing the 1951 Collier Trophy headlined the awarding committee's assertion: "Now the U.S. has a two-year lead on the Communists in perfecting vital faster-than-sound planes."9

Harder to see in the late 1940s were the urgent political and bureaucratic motivations involved in the high-visibility push for new national aeronautical research facilities. Dryden and LeMay wrote only about the cooperation, not the rivalry, between the NACA and the Army Air Forces. But NACA historian Alex Roland has described a postwar NACA "at its nadir in reputation and influence" struggling "in deep and surreptitious competition" with the AAF.10 Thus for Hugh Dryden in Washington and John Stack at Langley, the NACA's organizational self-interest must have accompanied the arms-race justification as a motivation to develop technology, and to seek construction funding, for new high-speed research tools.

6. Alex Roland, Model Research: The National Advisory Committee for Aeronautics, 1915-1958, 2 vols. (Washington, DC: NASA SP-4103, 1985), discusses the Mead committee and other postwar forces acting on the NACA; see chapters 9 and 10. The Stack collection, LHA, includes several folders of Stack's planning materials for postwar national wind tunnel facility construction. It Must be noted that in an April 3, 1996, telephone interview, NACA and NASA high-speed research veteran John V Becker recalled no particular urgency in the day-to-day postwar transonics work at Langley Field, whatever the outlook and motivations of the NACA itself might have been. I conducted all of the telephone interviews cited, retained electronic notes from each, sent a draft of this essay to every interviewee, and am indebted to all of them.

7. "Report of the Director of Aeronautical Research Submitted to the National Advisory Committee for Aeronautics at its annual meeting, October 23, 1947," reprinted Roland, Model Research, 2:713-16; quotations from p. 714.

8. Aero Digest, April 1947, pp. 14-15; photostat in "Miscellaneous" folder, Stack collection, LHA.

9. Collier's, December 20, 1952, pp. 24-25.

10. Roland, Model Research, 1:259 and 1:214.


In any case, wind tunnels were the desired tools. To most American aeronautical researchers it seemed clear that whatever the usefulness of research aircraft for transonics, truly comprehensive empirical knowledge in the long run would have to come mainly from these ground-test facilities with their convenient, versatile, relatively affordable, and safe. laboratory conditions.11 In the distinct NACA technical culture especially, airborne tests represented a component that could only complement, not replace, the wind-tunnel-test component. Although the airflow physics of a purely supersonic tunnel differs fundamentally from that of a subsonic tunnel, the NACA already had effective supersonic tunnels when the X-1 flew in 1947, and at Langley in the following month John V. Becker even began operating a small hypersonic tunnel that could reach speeds well beyond five times that of sound.12 But in the airflow of high-subsonic, or near- sonic, wind tunnels —tools for the main transonic parts of the work that research directors Dryden and LeMay were emphasizing— complex troublesome effects arose, hampering tunnel operation and polluting or even ruining experimental data. No tunnel had yet been invented for overcoming these vexing transonic effects, despite NACA efforts dating back to the 1920s, despite efforts elsewhere, and despite a long-standing intuition that Stack and others shared about how to solve the problem.

So during the X-1's research flights in 1947, Stack —a high-speed wind tunnel innovator since 1928, and now a research manager— was not present in the California desert. Instead he was back at Langley, encouraging, smoothing the way for, and cajoling others who were trying to synthesize years of NACA experience to capitalize on that intuition and develop that solution. "Aeronautical experts swore it couldn't be done," the Collier's headline would trumpet once they had succeeded. But in reality engineers had long suspected that it could indeed be done, and that the answer would lie in somehow partly opening up a wind tunnel's walls. just after the war Langley physicist Ray H. Wright, skilled in applied mathematics and widely knowledgeable concerning tunnel technology,13 had used subsonic aerodynamic theory to calculate a solution: a tunnel with ventilation slots in the walls of its test section, the experiment chamber where the tunnel's artificial airflow moves across an instrumented test subject such as a scale-model segment of a wing. These test-section slots had to be precisely placed, paralleling the airflow direction, in the tunnel's interior surfaces above, below, and beside the test subject, which might either span a roughly cylindrical test section or be held in place by an apparatus behind it downstream. Wright and Stack and their colleagues hoped that these longitudinal openings could manipulate the complexities of air flowing at up to sonic speed, channeling the air around the test subject in just such a way as to yield valuable transonic research data.

In 1947 Langley was already trying out the slotted-wall idea in the test section of a small pilot tunnel, and had learned, apparently serendipitotisly, that the slots enabled smooth operation not just at very high subsonic speeds, but at low supersonic speeds too. By the time of Yeager's famous research flight that October, Stack had long since begun considering how to apply the slotted-wall results in two full-size high-speed tunnels —industrial-scale facilities with huge powerful fans and test-section diameters of eight feet and sixteen feet, sizable by any era's standards. With Ray Wright's specific design concept, Stack's vision and leadership, engineer Vernon G. Ward's technology-development contributions, and the NACA Langley technical staff's wind tunnel expertise and experience, the research-and-development effort relatively soon led to the conversion of these two

11. Hallion, Supersonic Flight, p. 45, reiterates in ch. 2 what lie has made clear throughout ch. 1: "The principal reason" for transonic research aircraft "was the inability of existing wind tunnels to furnish satisfactory and reliable transonic aerodynamic data."

12. Hansen, Engineer in Charge, pp. 467, 471, and 344-47.

13. The end of this essay addresses conflicting interpretations of the breadth of Wright's technological awareness.


Photo of the NASA Langley 16 foot slotted wind-tunnel
The present-day slotted-wall test section of the NASA Langley 16-Foot Transonic Tunnel, which began operating as the NACA Langley 16-Foot High-Speed Tunnel two days before Pearl Harbor. The tunnel's name derives in part from its test sections approximate diameter. The slotted-wall configuration shown here descends directly from the one in this tunnel that helped win the 1951 Collier Trophy. (NASA L-90-04029).


national research facilities: the now-retired 8-Foot High-Speed Tunnel, designated a national landmark in 1985, and the 16-Foot High-Speed Tunnel, later called the 16-Foot Transonic Tunnel and still operational with slotted walls in 1998. The resulting Collier Trophy for Stack and nineteen of his colleagues was the first ever awarded outright for a research tool, and the only Collier ever awarded for a ground-based one-even though, as with particle accelerators and detectors for nuclear science, wind tunnels have been crucially important for American aeronautics.14

14. "From the time of the Wright brothers, the wind tunnel ... proved to be the essential piece of versatile experimental machinery on which much about the progressive evolution of aircraft depended," writes ,James R. Hansen in Spaceflight Revolution: NASA Langley Research Center from Sputnik to Apollo (Washington, DC: NASA SP-4308, 1995), p. 436, restating a main message of his earlier Engineer in Charge. "The wind tunnel dominates aeronautical research just as the microscope dominates biology, the telescope astronomy, and the particle accelerator nuclear physics," writes Roland in Model Research, 1:xiv. In this essay I do not address "tunnel vision" —Roland's name for a criticism of the NACA occasionally mentioned but seldom forthrightly leveled: that its engineers too often allowed research tools, especially wind tunnels, to dictate rather than merely serve research programs. In Model Research see especially 1:xiv-xv, but also 1:108, 220-21, and 309 and 2:507 and 520; see also Edward W. Constant, Isis, 73:4:269 (1982) 609-10.


Cutaway drawing of the 16 foot tunnel recirculating loop
Exterior photo of the 16 foot tunnel
The 1951 Collier Trophy recognized a transonic-research-technology advance first applied in the two NACA Langley wind tunnels shown in these midcentury views.15 Top: Air flows counterclockwise in the 16-Foot Transonic Tunnel, passing repeatedly through the test section linked to the topmost floors of the facility's brick office building. Bottom: In the 8-Foot High-Speed Tunnel's similar circut, a concrete igloo enclosed the test section. (NASA photos L-90-3752 and NACA 12000.1).

15. The photographed artist's drawing is from the early 1950s, the actual photograph from earlier still. The modern 16-foot tunnel circuit has an air-removal system for enhanced low-supersonic operation.


In fact, given the wind tunnel component in the NACA's overall contributions, a Collier Trophy for an NACA tunnel seems fitting, as three low-subsonic examples from the 1920s and early 1930s illustrate. Each was the first of its kind in the world, and was soon copied elsewhere.16 The Variable-Density Tunnel, or VDT, could, with fairly good success for the time, counteract scale effects —the skewing of test data inherent in testing scale models instead of full-scale aircraft or aircraft components. By the early 1930s, according to aeronautics historian Richard K. Smith, VDT-generated information published in formal NACA reports enabled aircraft designers to select a wing shape for a given application incisively, rationally, and conveniently.17 That the VDT became an official national landmark in 1985 may help validate its historical significance. The Propeller Research Tunnel, or PRT, circumvented scale effects and other technical difficulties simply by being powerful enough, and large enough in its test-section diameter of twenty feet, to test at full scale a propeller and engine mounted on an actual fuselage or on a portion of a full-size wing. Several observers have noted that the NACA's first Collier Trophy, the one for the speed-enhancing engine cowling discussed in chapter 1, might well have recognized instead the PRT, the research tool that enabled the cowling's development.18 The Full-Scale Tunnel, or EST, operational for nearly two-thirds of a century starting in the early 1930s, took the PRT's full-scale-testing principle one step further: in its thirty-by sixty-foot test section it could hold an entire small airplane. The EST was also designated a national landmark in 1985.

With a technical staff continually devising such tunnels and other research tools, the subsonic-era NACA became widely recognized for its applied aeronautical research. The organization became highly adaptable for fulfilling its statutory charge of finding practical solutions to the problems of flight-problems eventually defined as including the aerodynamics, and somewhat belatedly the aeropropulsion, of transonic and supersonic flight. In fact, during the 1920s and 1930s the NACA's earliest efforts in transonics began to grow out of its extensive subsonic efforts, and ultimately led to the transonic wind tunnel for which the 1951 Collier Trophy recognized 'John Stack and associates at the Langley Aeronautical Laboratory, NACA." So besides celebrating the slotted-wall transonic tunnel's promise for jets, and beyond finally recognizing one representative NACA wind tunnel, the Collier Trophy for that year illuminates the effectiveness of the research-tool-centered NACA technical culture.

16. Useful sources on NACA wind tunnel history include Hansen's Engineer in Charge and Donald D. Baals and William R. Corliss, Wind Tunnels of NASA (Washington, DC: NASA SP-440, 1981). Roland, Model Research, 2:508-14, lucidly explains wind tunnels and tunnel technology.

17. "Better: The Quest for Excellence," in Milestones of Aviation, p. 241, ed. John T. Greenwood (New York, NY. Hugh Lauter Levin, 1989), 222-95.

18. Roland, Model Research, 1: 117; Hansen, Engineer in Charge, p. 134; Hansen, chap. 1 in this volume; John V. Becker, The High-Speed-Frontier: Case Histories of Four NACA Programs, 1920-1950 (Washington, DC: NASA SP-445, 1980), p. 140. For the present essay and much else, Becker's book is centrally important as both a primary and a secondary source. In its introduction, Becker says he wrote it as a "participant-author" because the NACA's research solutions actually evolved as more than just "the inevitable result of wise management, inventive researchers, and unparalleled facilities," and because he believes that to "provide fundamental insights into the NACA's technical accomplishments the record should include the doubts and misconceptions that existed in the beginning of a project, the unproductive approaches that were tried and abandoned, the stimulating peer discussions that provided new insights, and the gradual evolution of the final solution. This kind of information is hard to find." Edward W. Constant, reviewing the book in Isis, 73:4:269 (1982) 609-10, calls it (p. 609) "an extraordinary glimpse into a whole category of technological knowledge not commonly covered either by the history of science or by the history of technology."


A Technological Organization's Group Achievement

Academic or Nobel Prize —like norms for assigning credit were only partly relevant in the Collier's recognition of the transonic tunnel achievement, for the cited triad of "conception, development, and practical application" of the slotted wall included effective work outside the purely intellectual realm. In fact, the Collier for 1951 required distinguishing among specific kinds of contributions as well as among contributors, including the technological organization itself—though the Collier committee at first adopted a simpler view. A look at how and where slotted-wall credit has been conferred, both by the Collier and by other means since, may show something about NACA-era views of the nature of technological achievement, and does show the central importance of a well-integrated technical culture in the NACA's work.

The slotted-wall achievement did have an important intellectual component, as Stack's technical peers have duly recognized in later citations and discussion in aeronautical publications. But Collier Trophy notwithstanding, they have not credited Stack. Although the Collier committee singled him out, and in fact originally intended the award for Stack alone, for over half a century Stack's professional peers have generally attributed the origin of slotted walls either by crediting the NACA generally or by citing the 1948 paper of Stack's Collier-winning "associates" Ray Wright and Vernon Ward, the engineer who spearheaded proof of the slotted-wall principle with the first small pilot tunnel.19 Technical authors have left Stack not only uncited but unmentioned, even in passages that summarize historical background. It must be noted that Stack's rise within Langley management during the 1940s meant fewer papers from him and, when he did write, a broad-overview approach not conducive to academic citation.20 And it must also be noted that Stack quite possibly intended not to take academic credit; Wind Tunnels of NASA author Donald D. Baals, one of Stack's Collier-sharing associates, said in 1996 that Stack might well have intended to send credit Wright's way.21 Another associate, veteran NACA and NASA high-speed researcher John V. Becker, emphasizes the distinction between kinds of contributions. His book The High-Speed Frontier.- Case Histories of Four NACA Programs, 1920 -1950 says unambiguously that the "first successful many-slotted transonic tunnel configuration was devised single-handedly by Ray H. Wright," that Wright was "the designer of the transonic tunnel," that "Wright's personal decision in 1945 to get down to cases" initiated the multiyear transonic tunnel effort, most of which "clearly bears the stamp of

19. NACA Research Memorandum L8J06, "NACA Transonic Wind-Tunnel Test Sections." The folder "Standardization of Wind Tunnels, October 13, 1948-Thru Feb. 1949" in the Research Authorization 70 file, LHA, contains this paper's approval and distribution paperwork as well as the October 6, 1948, final editorial copy. (Hansen, Engineer in Charge, pp. 572-74, explains the usefulness, and the use, of the LHA's research authorization files, hereafter cited in the form RA70. With two linear feet of documents, RA70 traces much of the evolution of wind tunnel technology from the early 1920s to the early 1950s.) The NACA republished the Wright-Ward paper in 1955 as Technical Report 1231, but changed it somewhat, mainly by deleting a paragraph near the end reporting lack of understanding of the low-supersonic capability and by slightly altering conclusions 4 and 6. The NASA Langley library holds the original 1948 RM version on microfiche. Key antecedents for the 1948 paper include Ray H. Wright, Physicist, and Vernon G. Ward, Aeronautical Engineer, to Compressibility Research Division Files, "Tunnel Wall Interference Effects in an Axially Slotted Test Section—Preliminary Tests," March 12, 1947 (Stack collection folder "New Types of Wind Tunnels, 1947," LHA) and Wright to Chief, Full-Scale Division, "Theoretical consideration of the use of axial slots to minimize windtunnel blockage," May 24, 1948 (Stack collection folder "Slotted-Throat Tests, 1946-48," LHA). The latter says the "theoretical investigation" it means "to record and preserve" may "later be combined and published with the results" of an experiment in progress, obviously the Wright-Ward pilot-tunnel experimentation —and indeed the eventual Wright-Ward paper reflects much from Wright's memo.

20. Becker, High-Speed Frontier, pp. 52, 53.

21. Telephone interview, April 7, 1996.


Wright's insights and personal integrity," but that 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" as soon as it did.22

The practice of excluding Stack from credit appears to have begun well before 1951, and it has continued for half a century. In October 1948, NACA research director Hugh Dryden began limited, high-priority circulation of the Wright-Ward paper. Within days, Clark Millikan of the Guggenheim Aeronautical Laboratory wrote to congratulate the NACA and to express hope for "following the lead given by Messrs. Wright and Ward." His letter does not mention Stack. Within weeks, Air Force wind tunnel expert Bernhard H. Goethert, formerly of the German aeronautical research establishment, visited Langley; Dryden had officially informed the military about the slotted wall's "revolutionary nature," and Goethert hoped to learn how to apply it. Wright, Ward, and Stack himself, together with engineer Eugene C. Draley, hosted Goethert's intensive visit and tour.23 Yet Goethert wrote in his 1961 book Transonic Wind Tunnel Testing that the "first really successful transonic wind tunnel was investigated in the United States in 1947 in tests at the NACA." The passage footnotes Wright and Ward and leaves Stack unmentioned. Moreover, Stack's name barely appears at all in Goethert's book, an exhaustive survey of a research technology that the 1951 Collier Trophy credits Stack above all others with founding.24 Similar attribution patterns appear in a 1955 NACA paper that in part reviews past NACA slotted-wall work, in a 1960 Air Force paper summarizing that service's wind-tunnel-development efforts, and in the 1965 textbook High-Speed Wind Tunnel Testing.25 Stack's exclusion persisted in the mid-1990s at NASA Langley Research Center, where two papers addressed the slotted-wall issues that Wright and Ward first discussed in print. Both explicitly attribute the technology's origin to Wright and Ward. Neither mentions Stack, though upon inquiry, each principal author readily confirms clear awareness of him. One of these papers surveys the characteristics and technical history of what is now called the 16-Foot Transonic Tunnel, one of the two large Langley facilities where "practical application" of the slotted wall helped earn the 1951 Collier Trophy for Stack and his associates.26

22. Becker, High-Speed Frontier, pp. 99, 112, 115. In a July 15, 1988, letter to historian Hansen (copy in my files), Ward asserted a credit-claiming version of "the true facts in regard to the elimination of choking in wind tunnels and the developmental and design research of the NACA Transonic Wind Tunnel." Certainly his pilot-tunnel efforts did contribute importantly, and apparently he did personally discover the unexpected low-supersonic capability. However, his recollections conflict with the documentary record, discussed later in the present essay, concerning the clarity, and thus the priority of Wright's 1946 expectations and intentions for the nearsonic significance of the theoretical work Wright began conducting before Ward became involved.

23. The RA70 folder "Standardization of Wind Tunnels, October 13, 1948-Thru Feb. 1949," LHA, contains a copy of Draley's December 21, 1948, memorandum reporting Goethert's visit in detail (clearer copy in folder "Special File, R.A. 70, April 1947 Thru Dec. 48") and a copy of Millikan to Dryden, October 19, 1948 —of which a signed copy is in the "Research Authorization 70" folder, together with copies of Dryden's October 8, 1948, "revolutionary nature" letters to military research flag officers.

24. Goethert, Transonic Wind Tunnel Testing (New York, NY. Pergamon Press, 1961), p. 23, but see also p. 61. (Publication of the North Atlantic Treaty Organization's Advisory Group for Aeronautical Research and Devel opment, edited by Wilbur C. Nelson, from a series by the NATO-AGARD Fluid Dynamics Panel, which Linder an earlier name had also published Goethert's important paper "Flow Establishment and Wall Interference in Transonic Wind Tunnels," AEDC-TR-54-44, pp. 247-292 in AGARD Memorandum: Presented at the Sixth Meeting of the Wind Tunnel and Model Testing Panel,, Paris, France, 2-6 November 1954, AG17/P7.)

25. B. H. Little, Jr., and James M. Cubbage, Jr., "The Development of an 8-Inch by 8-Inch Slotted Tunnel for Mach Numbers up to 1.28," NASA TN D-908, August 1961, originally published January 1955 as classified NACA RM L55B08; M. Pindzola and W. L. Chew, "A Summary of Perforated Wall Wind Tunnel Studies at the Arnold Engineering Development Center," AEDC TR-60-9, August 1960; and Alan Pope and Kennith L. Goin, High-Speed Wind Tunnel Testing (New York, NY. John Wiley & Sons, Inc., 1965), pp. 103, 104.

26. Joel L. Everhart and Percy J. Bobbitt, "Experimental Studies of Transonic Flow Field Near a Longitudinally Slotted Wind Tunnel Wall," NASA Technical Paper 3392, April 1994, and Francis J. Capone, Linda S. Bangert, Scott C. Asbury, Charles T. L. Mills, and E. Ann Bare, "The NASA Langley 16-Foot Transonic Tunnel: Historical Overview, Facility Description, Calibration, Flow Characteristics, and Test Capabilities," NASA Technical Paper 3521, September 1995.


So why did the Collier committee members plan originally to cite Stack alone? Possibly they simply wanted a heroic interpretation like that in James Michener's 1982 novel Space, which attributes the transonic tunnel solely to "a genius named John Stack" who had a "brilliant idea" that led to "airplanes that could break through the sound barrier almost as undisturbed as a horse-drawn carriage heading for a country picnic in 1903." Possibly the committee's initial plan reflected a view like that of Orville Wright, who —no doubt remembering what actually led up to 1903— had complained in 1944 that Colliers were going too often to aviation organizations instead of innovative individuals. Possibly the intention reflected public relations aims of the NACA, whose executive secretary and chief propagandist John F. Victory chaired the Collier committee for 1951. The NACA apparently had a long-standing involvement in the award selection, and in at least one case —1947, when it seemed certain the NACA would be among those recogized— had calculated possible combinations of recipients to promote .27

If the committee members did intend the heroic interpretation, probably they wanted to lend a bit of romantic appeal to an award for an unromantic, ground-based research tool. Historian John William Ward has analyzed an analogous and much better known instance of credit-assigning in American aeronautics: the case of Charles Lindbergh. Concerning the adulation of Lindbergh, Ward observes that it is "strange that the long-distance flight of an airplane, the achievement of a highly advanced and organized technology, should be the occasion of hymns of praise to the solitary, unaided man." He describes a tension inherent in Americans' understanding of the new phenomenon of aviation: their identification with pioneering, self-reliant, free individuals versus their lack of interest in the collectivized, organized industrial society such individuals often actually represented. Possibly the Collier committee saw and sought to avoid such a tension in the choice between the pioneering Stack and the technological organization he represented. After all, this was already going to be the only Collier ever given for something so likely to be seen as inherently boring: not a heroic flight, not a new airplane, not a successful aviation program, not an improvement in airplane equipment. just a wind tunnel, a noisy industrial plant for turning out research data. The NACA itself is the analog of the uninteresting and therefore uncredited collectivized industrial society in the Lindbergh achievement, but the analog of the lionized Lindbergh himself is John Stack, already identified by an earlier Collier as a pioneering individual for conceiving the plane that broke the sound barrier. A Washington Post article the week after that earlier award had said he didn't "look like a man of science" but was instead "a rather handsome fellow whom you'd take for a lawyer, a football coach, or even an actor."28

In any case, in public relations and other nontechnical realms the Stack-alone interpretation lived on even after the 1951 award actually did partly credit members of the technological organization that stood behind Stack. The 1954 NACA annual report tilts toward such a description, emphasizing Stack's primacy in the achievement. In a 1957 speech, NACA executive secretary Victory tilted all the way: he portrayed the accomplishment as an individual one, and flatly attributed it to Stack alone. At the 1962 ceremony

27. James A. Michener, Space (New York, NY-. Random House, 1982), p. 175. Model Research, 1:351 n. 6 discusses Wright's complaint. In "George William Lewis," Year Book of the American Philosophical Society, 1948, pp. 269-78, NACA chairman Jerome C. Hunsaker notes longtime NACA research director Lewis's National Aeronautic Association life membership and says that Lewis had served on the Collier committee. Model Research, 1:383 n. 56 cites an "adamant" 1948 note from Hugh L. Dryden to Victory asserting the NACA's interest in promoting a three-way joint award of the then-impending Collier for 1947.

28. Ward, "Charles A. Lindbergh: His Flight and the American Ideal," in Technology in America: A History of Individuals and Ideas, 2d ed., ed. Carroll W. Pursell, Jr. (Cambridge, MA: MIT Press, 1990) pp. 211-26 (originally in The American Quarterly, spring 1958, as "The Meaning of Lindbergh's Flight"). "Intuition Brought Supersonic Flight," Washington Post, December 21, 1948.


awarding the Wright Brothers Memorial Trophy to Stack, the printed program declared that Stack had won two Colliers: one jointly for the X-1, and another "singly ... for his development of the transonic wind tunnel." A 1993 history of the National Aeronautic Association, the organization that awards the Collier, mentions the associates and the teamwork, but names only Stack.29

But Stack himself knew better. When he learned of the Collier awarding committee's impending misassignment of credit, he took decisive steps to correct it. Recognition of the nineteen associates, a substantial partial cross section of the NACA technical culture, resulted from plain forthrightness in Stack, a product of that culture and in many ways an exemplar of its norms. High-Speed Frontier author John Becker, one of the nineteen himself, described Stack's reaction to word that he had won this second Collier to go with the one he had already shared with Yeager and Bell:

A few weeks before the second award was presented to him by President Harry S. Truman on December 17, 1952, Stack appeared unexpectedly in my office in a state of considerable agitation. He had just received notice of the award from J. F Victory, chairman of the committee for the Collier Trophy. Stack said he was reluctant to accept the award as the sole recipient because so many others at Langley had contributed importantly. He wondered how the others would react. I believed they would feel as I did that he 'richly deserved this recognition. Without his aggressive leadership and promotional efforts there would have been no large transonic tunnels at Langley at that time. But Stack was insistent that the other principals should be included and we worked up a list of some 19 names.

In the end Stack could not get his colleagues individually cited, but did manage to distribute some of the recognition by getting the words "and associates at Langley Aeronautical Laboratory, NACA" added to the formal citation. Before the award ceremony he issued a press release describing each person's participation and emphasizing the "teamwork, the pooling of scientific capacities in a research laboratory, that makes an idea successful." He also helped organize a dinner to recognize the nineteen. Even a decade later, Stack's official NASA biography sheet still made the point that in his 1952 acceptance of the Collier for 1951, he had "confirmed that NACA know-how and teamwork were largely responsible" for it.30

Like Stack in 1952, previous NACA individual Collier winners Lewis A. Rodert for 1946 and Stack himself for 1947 had also publicly declared NACA teamwork the real basis for their achievements. Rodert had said that his Collier was "awarded for the general work of all of us" and that he had been named "because only individuals [could] be so designated." Stack had emphasized a nearly identical sentiment.31 This focus on the effective team rather than on any individual was entirely consistent with both the official outlook and the actual practice of the NACA. Aerospace historian James R. Hansen says that

29. AR54, p. 13; "Current Status of Aeronautical Research and Trends Towards Tomorrow," June 8, 1957, p. 6, Milton Ames collection folder "Victory, John F.,'' LHA; program for "Wright Memorial Dinner, Aero Club of Washington, December 17, 1962, Sheraton Park Hotel, Washington, D.C.," Stack collection folder "Awards and Biographical Information," LHA; Bill Robie, For the Greatest Achievement: A History of the Aero Club of America and the National Aeronautic Association (Washington, DC: Smithsonian Institution Press, 1993).

30. Becker, High-Speed Frontier, pp. 61, 62; Stack's press release was reprinted in the Langley Air Scoop, December 19, 1952, available in the LHA; Stack biography sheet, Stack collection folder "Awards and Biographical Information," LHA. Becker noted in an April 3, 1996, telephone interview that Stack was "very ill at case" when he heard about the award, and that "it didn't cost him anything to add on" the associates, for he knew that in any case he would get most of the credit.

31. Langley Air Scoop, January 9, 1948, LHA; Hansen, Engineer in Charge, p. 304.


George Lewis, whose quarter-century tenure as the NACA's first research director lasted until after World War II, characteristically "emphasized teamwork over individual genius" and that Lewis believed in Thomas Edison's "nonheroic theory of invention and especially liked its emphasis on collective action." Lewis once asked that Langley frame and display a presidential tribute to Edison that he thought "aptly cover[ed] the aims and purposes" of the NACA. In the quotation, President Hoover —like Lewis, an engineer— had attributed "both scientific discovery and its practical application" to the "labor of a host of men" gradually "building up the structure of knowledge" in "great laboratories." Lewis's Successor Hugh Dryden, coauthor in the 1920s of NACA reports on wind-tunnel-like experiments with transonic jets of compressed air, held similar views. His two-sentence letter transmitting the 1948 Wright-Ward report to Clark Millikan ends with a forthright attribution of slotted-wall "development"—a term in the eventual Collier citation's triad of "conception, development and practical application "— not to the unmentioned Stack, and not even to authors Wright and Ward, but to "the Committee's Langley Laboratory,"32 where flourished what later came to be called the NACA technical Culture.

As a management cliché, teamwork can obviously evoke skepticism or even cynicism, but NACA veterans have confirmed that this officially declared teamwork actually did flourish at the level of hands-on routine, and not just in managers' imaginations or public pronouncements. Stanford aeronautical engineering professor emeritus Walter G. Vincenti, for instance, who helped comprehensively define the transonic wind tunnel problem as an NACA engineer in the 1940s, and who writes on NACA history and the epistemology of engineering, has described the group dynamics of some important NACA flight research of about 1940 as exemplifying "the kind of fruitful melding of personal and group ambition and interest that can arise when talented technical people join in what they see as a demanding and worthwhile task. The whole was more than the sum of the parts." Becker, who joined the NACA Langley staff in 1936, says that a consequence of daily group discussions in the mid-1930s Langley lunchroom was that "often no one originator of an important new research undertaking could be identified. The idea had gradually taken form from many discussions and in truth it was a product of the group." He reiterated in 1996 that "seldom was there one clear, unequivocal route to a solution" to be found by one person alone; more often, he said, things really did happen by way of the group's interactions over time. Concerning the overall assignment of slotted-wall credit, Becker, who avoids expansive phrases and carefully distinguishes NACA public relations pronouncements from technical facts, tends to view the achievement as an important subset of all the late-1940s NACA transonics work-and he calls that overall program "one of the most effective team efforts in the annals of aeronautics."33

Four decades after the Collier Trophy for 1951, this teamwork-oriented, sometimes underappreciated NACA technical culture became a topic of some interest concerning NASA, especially in public-policy discussions of NASA's future. "NASA did not rise like a new creation from the sands of time when the space race began in 1957," declared

32. Hansen, "George W. Lewis and the Management of Aeronautical Research," in Aviation's Golden Age: Portraits from the 1920s and 1930s, ed. William M. Leary (Iowa City, IA: University of Iowa Press, 1989), 93-112, quotation front p. 106. Concerning the Hoover quotation, see Hansen, Engineer in Charge, pp. 132, 133, Roland, Model Research, 1:105, and Lewis's original request letter in the Milton Ames collection folder "George Lewis," LHA. In "Fact Finding for Tomorrow's Planes," National Geographic, December 1953, pp. 757-80, Dryden attributed "aeronautical progress [to] the growing store of human knowledge that underlies and makes possible the practical accomplishments," p. 758. Dryden to Millikan, October 8, 1948, "Rescarch Authorization 70" folder in RA70 file, LHA.

33. Vincenti, What Engineers Know and How They Know It: Analytical Studies from Aeronautical History (Baltimore, MD: Johns Hopkins, 1990), p. 91; Becket, High-Speed Frontier, pp. 22, 23, 61; see also p. 74; telephone interview. April 3 1996.


Howard E. McCurdy in his 1989 article "The Decay of NASA's Technical Culture." There and in a 1993 book, McCurdy describes the technical culture of the NACA as both an antecedent and a standard for that of NASA, which came into being in 1958 to combine, replace, and extend the NACA and other federal organizations. An underused means for adding to understanding of this technical cultural heritage is historical study, and one useful topic for such study is the NACA's handling of the transonic wind tunnel problem over the course of the three decades leading up to 1951.

To identify characteristics of the NACA and early NASA cultures, McCurdy's book relies primarily on observations and impressions of NASA staff, drawing secondarily on several historians of the NACA. The result, says sociologist Diane Vaughan in her book on the 1986 Challenger disaster, is an "unparalleled history of organizational culture" that shows NASA able during the 1960s "to maintain the strong technical culture that preexisted Apollo." Vaughan's own extensively researched study cites few directly NACA-related historical sources.34 Other public discourse has also addressed NASA's NACA technical cultural heritage, sometimes with little reference to formal scholarship of any kind. In popular literature, Michener's Space, Tom Wolfe's The Right Stuff and Apollo: The Race to the Moon by Charles Murray and Catherine Bly Cox presume the importance of the technical cultural link.35 So do public-policy studies from Washington. A 1994 National Research Council report takes an explicitly historical approach involving the NACA to justify recommendations about NASA's building new national subsonic and transonic wind tunnels, but uses as its sole NACA source a self-serving, semiofficial historical summary ghostwritten in the 1950s for the NACA chairman by a public affairs officer. A 1994 Congressional Budget Office study of possible new NASA directions asserts that the agency's "organizational history is relevant to the criticism of its current conduct" and observes that among "NASA's institutional predecessors was the National Advisory Committee on [sic] Aeronautics. Its purpose was to develop useful aviation technology, a task that by most accounts it accomplished well." But beyond tying discussion of NASA's "original organizational culture" to McCurdy, the CBO study names no such accounts.36 So there may well be room in the conventional wisdom, and a use in public-policy discussions, for an enlarged historical perspective concerning the NACA technical culture. Useful materials are available for it. Historian James R. Hansen's work, especially Engineer in Charge: A History of the Langley Aeronautical Laboratory, 1917-1958 and Spaceflight Revolution: NASA Langley Research Center from Sputnik to Apollo, contributes substantially to elucidating the technical cultural link between the NACA and NASA. So does Alex Roland's Model Research: The National Advisory Committee for Aeronautics, 1915-1958.

Scholarly studies, both historical and sociological, occasionally attempt brief distillations concerning the NACA technical culture. Roland says that "the NACA by 1926 was committed to a research philosophy that valued process over prescience, the team over the individual, experiment over theory, engineering over science, incremental refinement

34. McCurdy, p. 304, "The Decay of NASA's Technical Culture," Space Policy (November 1989) 301-10; McCurdy, Inside NASA: High Technology and Organizational Change in the U.S. Space Program (Baltimore, MD: Johns Hopkins University Press, 1993); Vaughan, The Challenger Launch Decision: Rishy Technology, Culture, and Deviance at NASA (Chicago, IL: University of Chicago Press), p. 499 n. 17 and p. 210. See also p. 502 n. 88.

35. Wolfe, The Right Stuff (New York, NY: Farrar Strauss Giroux, 1979); Murray and Cox, Apollo: The Race to the Moon (New York, NY: Simon and Schuster, 1989).

36. National Research Council , Assessing the National Plan for Aeronautical Ground Test Facilities (Washington, DC: National Academy Press 1994). According to Roland (Model Research, 1:319), the historical summary document "Forty Years of Aeronautical Research," which NRC chap.1 cites from The Smithsonian Report for 1955, was written by Walter T. Bonney for Jerome C. Hunsaker and "sings the Committee's [i.e., the NACA's] praises and ignores its problems and shortcomings." Bonney's summary also appeared in AR58. Congressional Budget Office, Reinventing NASA, March 1994, pp. 21 and 17.


of the existing paradigm over revolutionary creation of new paradigms." He then distills his own summary to six words: "the triumph of engineering over science," a variation of the thought that Hansen distills even further in his book title Engineer in Charge, a phrase that McCurdy in turn has appropriated to name the NACA cultural tradition. The McCurdy distillation of the original NASA technical culture that Vaughan selects to quote is consistent with Roland's, Hansen's, and others' historical scholarship: it "consisted of a commitment to research, testing, and verification; to in-house technical capability; to hands-on activity; to the acceptance of risk and failure; to open communications; to a belief that NASA was staffed with exceptional people; to attention to detail; and to a 'frontiers of flight' mentality."37

The history of the NACA's handling of the transonic wind tunnel problem may contribute to revising or refining such distillations. When NASA's antecedent technical culture began taking shape around 1920, a new research problem had arisen: on aircraft with increasingly powerful engines, longer propeller blades were traveling through larger arcs, their tips in some cases reaching sonic speed. Since a propeller is an airfoil, a complex, precise aerodynamic shape like a wing, this made transonic aerodynamics a practical aeronautical research issue, even though transonic flight —and the word transonic, for that matter— were still some distance in the future. So the NACA effort that eventually led to the slotted-wall transonic tunnel began. From the 1920s until the advent of NASA, this effort paralleled, reflected, and sometimes even partly constituted the development of the NACA itself. The effort's history suggests a few candidate modifications to distillations summarizing the NACA's technical culture: Its members conceived research, researcher, and research tool as organically interconnected. With an externally compelled applied-research focus, they sought what Stack came to call "physical understanding without mathematical weakness," but they kept in view the additional practical goal of fundamental scientific understanding. By continually enlarging their corporate technical and scientific memory and by continually developing craftsmanship in the arts of aeronautical research, they learned to exercise technical intuition deftly, and to adapt flexibly to new problems though usually not until doing so accorded with the priorities of industry or the military.

Wind Tunnels, Transonics, and the NACA of the 1920s

The NACA's job was to supply American industry and the military with information for designing better airplanes. This information mainly took the form of more than 16,000 formal reports published and distributed during the research organization's forty-three years, an average of about one per day from 1915 to 1958. 38 Much of the NACA's information-generating research addressed the centrally important topic of aerodynamics, which means predicting the complex interactions between airplane and air, which in turn means understanding the nearly constantly changing flow field —the pressure, density, temperature, and relative velocity 39 at each point in the air affecting and affected by the airplane at each moment of flight. This predicting can be very hard. Flow fields differ for every contemplated aerodynamic configuration, and change with each airborne maneuver. Even at an airplane's slowest, its flow-field velocities match the wind speeds of a robust hurricane. The ideal form of flow-field understanding, using the mathematical language of the science of fluid dynamics, is a reliable theory— a comprehensive, systematized conceptual

37. Roland, Model Research, 1:98 and 99; McCurdy, Inside, NASA, pp. 12, 134; Vaughan, The Challenger Launch Decision, p. 209 quotes p. 302 in McCurdy, "The Decay of NASA's Technical Culture."

38. Model Research, 1:xiii and 2:556.

39. John D. Anderson, Jr., calls these the "four fundamental quantities in the language of aerodynamics," p. 36, Introduction to Flight: Its Engineering and History (New York, NY. McGraw-Hill, 1978).


model applicable to the task of making correct performance predictions about possible airplane and airplane-component designs. Unfortunately, this level of understanding is hard to attain, especially for transonic speeds. It is possible, though, to get empirical information applicable to design problems by conducting wind tunnel tests or flight tests that replicate, or at least approximate, flow fields of interest. By 1920, the NACA had begun conducting both kinds.

A wind tunnel test replicates a flow field by moving air across a test subject instrumented for data-gathering, usually a scale model but sometimes an actual airplane or a full-scale component of one. The method is functionally equivalent to flight, for as Leonardo da Vinci pointed out, "what an object does against the motionless air, the same does the air moving against the object at rest."40 Of course, da Vinci never tried establishing this functional equivalency in wind tunnel airflow near the speed of sound. A flight test, on the other hand, generates data by moving an instrumented test subject through the air. In 1919 the NACA began relatively low-speed flight experiments with ordinary biplanes. But to cite the more varied flight-testing examples from the NACA's 1940s-era efforts in transonics, a flight-test subject could be a piloted research airplane like the X-1, or it could be a scale-model airplane or wing shot skyward on a rocket, dropped from an airplane at high altitude, or fastened to the upper surface of, say, a P-51 Mustang's wing, where airflow could accelerate to sonic speed during steep subsonic power dives.

Both wind tunnel and flight tests generate useful information, but as the postwar NACA transonics effort illustrated, flight tests often require more time, effort, and resources, with each datum preciously won. A carefully crafted model dropped from altitude or launched on a rocket required an elaborate tracking system on the ground, had limited capacity to accommodate measuring and data-transmitting devices, and was expended in a single brief use. For wing-mounted models, the host airplane's own flow field often spoiled the smaller localized flow field under study. Transonic research airplanes, besides being expensive and requiring extensive support, also endangered their pilots: the NACA's Howard Lilly, third human to exceed the speed of sound, died in a May 1948 crash of the D-558-1 Skystreak, an aircraft comparable to the X-1. Although flight tests did contribute substantially in the midcentury attack on transonic aerodynamics, the postwar transonic-research-tool development goal was always to achieve the flexible, convenient, productive, and safe laboratory conditions of the wind tunnel. As the NACA had recognized even before 1920, in a tunnel's easily accessible test section, experiment setups are endlessly and comparatively cheaply reconfigurable, and results are comparatively easily observable and measurable. Of course, even if easily obtained, data from a tunnel's artificial conditions must still meet a verisimilitude criterion: they must correspond somehow with the actual flight conditions being replicated, either directly or by the application of reliable mathematical correction factors. Meeting this verisimilitude requirement was the central challenge of NACA wind tunnel history, and the NACA's best-known success in meeting it was the slotted-wall transonic tunnel.

Long before the 1951 Collier Trophy for that success, and long before there was an NACA, aeronautical researchers recognized the wind tunnel's advantages. The efforts of Orville and Wilbur Wright to engineer the first airplane included methodical studies of small aerodynamic shapes in artificial flow fields inside a six-foot-long wooden box with a fan at one end. By 1920, when the NACA began operating its first wind tunnel at Langley, several tunnels were in use in the United States, but the world standard was being set at Ludwig Prandtl's aeronautical laboratory in Göttingen, Germany, where the closed-circuit,

40. Quoted p. 91 in Hugh L. Dryden, "Aerodynamics—Theory, Experiment, Application," Aeronautical Engineering Review 12, No. 12 ( December 1953): 88-95.


return-flow tunnel had been invented and refined. Its airstream cycled repeatedly, with power-saving efficiency, around its return circuit and through its test section. Years later at Langley, the precise placement of carefully calculated ventilation slots in the test-section walls of two high-speed versions of such runnels made them the first capable of transonic testing. But in June 1920 at Langley, no world standard was set, or even met, by the NACA when its first wind tunnel started operating. Lacking a return circuit, it was "obsolete when it was built," according to Wind Tunnels of NASA author Baals. 41

However, in that same year of 1920 the NACA, through its executive committee chairman Joseph S. Ames, did at least take steps to learn more about wind tunnels worldwide. In his capacity as NACA aerodynamics committee vice-chairman, Ames wrote to several prominent figures in American aeronautics to ask for help outlining "a program of tests to be made in the, wind tunnels of this country and of Europe with a view to securing what one might call standardization, that, is, information which would enable one to connect the data published, as obtained in these different wind tunnels." The immediate motivation was calibration. Analyzing the divergence of results from research tools carrying out identical experiments can improve interpretation of the results; by calibrating tunnels against each other, researchers could better extrapolate likely flow-field behavior aloft from artificial flowfield behavior off the ground. W. F. Durand of Stanford University, for decades a major figure in American aeronautics, answered Ames with strong support for the standardization tests. He offered several suggestions and specifically mentioned the need to include French and British results. The NACA did discuss the idea with Europeans; Prandtl sent five specific cross-comparison-test ideas, and the British and Dutch also Sent Suggestions. 42 In 1922, with Ames's cross-calibration testing program begun, the NACA's annual report included a section called "International Standardization of Wind-Tunnel Results." The young research agency's early-1920s efforts to correlate research results also included data from flight testing, which had started at Langley in 1919 at least partly for tunnel-comparison purposes.43 Though the immediate motivation for all of this cross-comparison work was calibration to sharpen understanding of research results, the effort must also have calibrated and sharpened the NACA's understanding of its need for better research tools.

Already in 1920 that need had begun to extend to the transonic, as seen when Ames's letter elicited an expression of concern about wind tunnel results for the high-speed range that was not even yet called by that name. Elislia N. Fales of the Army's aeronautical laboratory at McCook Field near Dayton, Ohio —now Wright-Patterson Air Force Base— replied that standardization "has especial significance when, as in the McCook Field tunnel, speeds are attained which involve density changes."44 Fales was bringing up the fundamental problem

41. Baals, Wind Tunnels of NASA, pp. 9-14. This obsolescence assertion by wind tunnel expert Baals —himself notably loyal to the NACA for over half  a century— contrasts with NACA publicists and chronicler George W. Gray's claim that Tunnel No. 1 "followed the best engineering practice of its day," Frontiers of Flight: The Story of NACA Research (New York, DC: Alfred A. Knopf, 1948), p. 34. Although open-circuit tunnels were indeed widely used circa 1920, within a few years the NACA replaced Tunnel No. 1. Gray is better valued for crystalline technical explanations than for impartiality. Roland, Model Research, 1:319 calls his, book "as fine a summary of the NACA's claims for itself as is likely to be, prepared."

42. In the folder "R.A.'s—Standardization of Wind Tunnels 1920-1926," RA70, LHA, are Ames to A. F.Zahm, L. J. Briggs, E. B. Wilson,W F. Durand, E. N. Fales, .J. G. Coffin, H. Bateman, and F. H. Norton, August 23, 1920; Durand to Ames, September 24, 1920; a copy of W. Knight's May 3, 1920, letter thanking Prandtl; and a two-page document with August 12, 1920, receipt notation titled "Suggested Aerodynamical Comparative Tests," which includes Piandtl's actual suggestions. Langley engineer Elton W. Miller, rebutting Aero Digest criticisims of the NACA in 1930, apparently did not know about Ames's letter; see Model Research, 2:658, item 7.

43. AR22, p. 36; "Fifty Years of Flight Research: A Chronology of the Langley Research Center, 1917-1966" (NASA TM-X-59314, apparently a  republication of work by Michael D. Keller), p. 16.

44. Fales to Ames, August 31, 1920, in RA70 folder "R.A.'s—Standardization of Wind Tunnels 1920-1926," LHA.


of compressibility, a phenomenon already known in the field of fluid dynamics and beginning to require attention in the subfield of aerodynamics. Even at slow speeds, a flying object slightly compresses some of the air that it meets, raising that air's density and thus altering a key flow-field characteristic. At speeds approaching that of sound —that is, at transonic speeds much higher than those of the airplanes of 1920, but equal to those of some propeller tips of the day— this compressibility becomes significant and starts to degrade the performance of airfoils. For propellers, compressibility effects degrade the production of propulsive thrust. For airplanes themselves, compressibility effects call become disruptive and even dangerous, as indeed happened when airplanes began attaining much higher speeds in the late 1930s. Under NACA auspices, Fales in 1920 co-wrote "Wind Tunnel Studies in Aerodynamic Phenomena at High Speeds," a report on work that Stack later called "the earliest experimental investigation of airfoil characteristics as affected by compressibility," and that Becker says introduced two important compressibility terms: critical speed and burble. 45 At critical speed, some of the airflow accelerating across the airfoil surface reaches the speed of sound, creating a flow-field-disrupting compressibility burble, a discontinuity in the flow.

When Fales raised this high-speed research issue in answering Ames, little had yet been learned about how to study transonic phenomena. Becker notes, for instance, that Fales and report coauthor F. W. Caldwell did not even mention the centrally important ratio of flow-field speed to the local speed of sound —Mach number, as Swiss high-speed researcher Jakob Ackeret in 1929 proposed calling the ratio— even though the concept itself had been known to fluid dynamicists. for decades.46 Of course, Much was still to be learned about how to study aeronautical questions in general. Research tools were often quite rudimentary and unsophisticated. The Propeller Research Tunnel at Langley, for example, originally had plain commercial platform scales for aerodynamic measurements. With air flowing around all engine-and-propeller configuration mounted on a framework atop the scales, researchers simply weighed the thrust and drag.47

Research tools were also rudimentary for the transonics studies the NACA at first contracted out during the 1920s, as future NACA research director Hugh Dryden learned firsthand. Becker says that with the high-speed work of Caldwell and Fales the "seeds of interest had been sown" in both the NACA and the National Bureau of Standards, another government agency with aeronautics interests. Accordingly, new high-speed studies began under NACA auspices. The work involved NBS aerodynamics section head Dryden, a 1919 Johns Hopkins Ph.D. in physics and mathematics whom Ames, in his capacity as a Johns Hopkins physics professor, had originally recommended to NBS. Ames once described Dryden as "the brightest young man . . . without exception" that he ever encountered. Like the transonic wind tunnel effort itself, Dryden was to contribute substantially over the years to defining the NACA technical culture. In 1947, after serving since 1931 on the NACA's aerodynamics committee, he joined the NACA staff to replace aging research director George Lewis just when slotted walls were being developed at Langley. Thereafter, in numerous articles in both the professional and popular press,

45. F. W. Caldwell and E. N. Fales, NACA Report 83,1920; John Stack, NACA Report 463, "The N.A.C.A. High-Speed Wind Tunnel and Tests of Six Propeller Sections," 1933 (quotation on p. 416 in AR33); High-Speed Frontier, pp. 3-5.

46. Becker, High-Speed Frontier, p. 5. In Milton Ames collection folder "May 24, 1948, Transonic Wind Tunnels (Slotted Throats) Memo by Ray Wright," LHA, Hugh Dryden's June 25, 1948, lecture notes cite Ackeret's "'An Resistance at Very High Speeds' in Schweitzerische Bauzeitung 94:179, 1929." Edward W. Constant, The Origins of the Turbojet Revolution (Baltimore, MD: Johns Hopkins, 1980), p. 288 n. 8, says Ackeret introduced the term in 1935 at the Volta high-speed conference in Italy.

47. Gray, Frontiers of Flight, pp. 54,55. See also Roland, Model Research, 1:118 concerning the "primitiveness of early NACA research."


Dryden articulated the NACA's outlook on all aspects of aeronautical research. From 1958 until his death in 1965, he helped link the old NACA and the new NASA by serving as NASA's deputy administrator, bringing with him "the loyalty of the NACA's 8,000" employees, according to Richard K. Smith. It was Dryden's transonics experimentation of the 1920s that began these decades of contributions to the NACA and NASA technical cultural traditions. And that work involved rudimentary research tools, as Dryden recalled in an illustrative anecdote in a 1953 National Geographic article celebrating the research aspects of flight's first half-century:

As long ago as 1923 I was experimenting with propeller tip sections in a sonic-speed jet of air at General Electric's Lynn, Massachusetts, plant. Afterward when my colleagues and I walked out into the streets, we noticed that passers-by seemed unusually interested in our group. We later realized we had been unconsciously talking in very loud tones to compensate for the temporary deafness caused by working for several hours with our heads a few inches from a 12-inch sonic jet.48

Dryden, Army Lt. Col. G. F. Hull, and Dryden's NBS colleague Lyman J. Briggs —a recipient of Ames's 1920 tunnel-standardization proposal letter, and years later the NACA's wartime vice-chairman— had gone to Lynn to use General Electric's huge centrifugal compressor, which, in Becker's words, "provided them in effect with a ready-made free-jet wind tunnel." 49 It could eject a jet of air at transonic speed from a circular nozzle just over a foot in diameter. The researchers took with them six three-inch-wide steel models, each representing the aerodynamic shape of a standard Army propeller blade, and each over seventeen inches long so as to completely span the high-speed jet of air, extending beyond its boundaries. So important was the precise construction of such models that Langley, developing its own aeronautical research craftsmanship, later bought the machining equipment that these particular models' Massachusetts maker also used for fashioning test subjects for the twenty-atmosphere pressure of Langley's Variable- Density Tunnel. The experimenters also took a specially constructed wind tunnel balance, an instrument with which they could hold a model airfoil in the airstream, incrementally change the airfoil's angle with respect to the airstream, and measure the resulting lift and drag forces. Their 1925 NACA paper "Aerodynamic Characteristics of Airfoils at High Speeds" reports that the investigation, carried out to obtain propeller-design information, showed that "the use of tip speeds approaching the speed of sound for propellers of customary design involves a serious loss in efficiency." Becker believes this work confirmed and extended that of Caldwell and Fales, offered the first useful attempt at explaining compressibility phenomena, and provided "the first statement of the relation between the critical speed and the known low-speed velocity distribution about the airfoil" —a piece of fundamental understanding "resurrected and exploited" a decade later in Langley's efforts to improve high-speed airfoils by designing them to have higher critical speed and thus a delayed compressibility burble.50

48. Becker, High-Speed Frontier, p. 7. Richard K. Smith, The Hugh L. Dryden Papers 1898-1965 (Baltimore, MD: Milton S. Eisenhower Library, Johns Hopkins University, 1974), pp. 20-28. Elizabeth A. Muenger has also noted Dryden's public advocacy of NACA research; see p. 64, Searching the Horizon: A History of Ames Research Center, 1940-1976 (Washington, DC: NASA SP-4304, 1985). Dryden, p. 762, "Fact Finding for Tomorrow's Planes," National Geographic Magazine, December 1953, pp. 757-80.

49. Becker, High-Speed Frontier, p. 8.

50. Report 207 appears pp. 465-79 in AR25; p. 466 discusses W. H. Nichols, the purchase of whose equipment Hansen reports on p. 83, Engineer in Charge. Becker, High-Speed Frontier, pp. 8, 9, 20.


Still, the methods and tools were rudimentary. For example, the experimenters made some unquantified, purely qualitative observations based on airflow patterns that appeared in oil they had placed on the model airfoils to keep them from rusting in bad weather —an apparently serendipitous ad hoc technique in the wind tunnel art of flow visualization. More significantly, expert observers later noted several limitations in the open jet of air,51 some of which the experimenters themselves addressed in a section of their report called "Precision of Results":

The large power consumption of the compressor (5000 horsepower at high speeds) and the high cost of operation have made it impossible to repeat observations at will. In the interest of economy, many of the measurements were made while the [compressor equipment was] being put through shop tests. During such tests, the speed of the air stream was not under our control, and would often vary before a complete set of observations could be made. The noise of the airstream was so great that it was difficult for observers to communicate with each other while the compressor was running, so that modification of the program to meet changing conditions was difficult.

Besides these bothersome impediments to proper scientific procedure, the jet of air also imposed an important fundamental limitation-a version, in fact, of the problem that Wright, Stack, and their associates overcame years later at Langley: jet boundary effects, or, more simply, wall interference. An enclosed test section's walls can distort the artificial flow field and thereby also the test results, particularly at transonic speeds. Similarly, even though an open jet has no solid walls to degrade flow-field verisimilitude, distortions comparable to those in a closed test section nonetheless arise because of the de facto boundary between the open jet and the surrounding air it hurtles through. An open jet does not constrict its artificial flow field within actual walls, but it still introduces measurement-distorting boundary effects.

So complex are boundary effects in the transonic range, wrote Bernhard Goethert in Transonic Wind Tunnel Testing in 1961, that the late-1940s effort to invent slotted walls could not have succeeded based on experimentation alone, but required an "orientation of theoretical calculations." This notion too —like Fales's introduction of the compressibility issue— arose concerning tunnels in general in Ames's 1920 tunnel-standardization discussion. American wind tunnel pioneer Albert F. Zahm, replying to Ames's letter, suggested beginning the cross-calibration project by having "the ablest theoretical aerodynamicists," such as Prandtl, "discuss the mathematical theory of the flow in a wind tunnel." Without "adequate theory, furnished before hand," wrote Zahm, "it seems improbable that all the observations and precautions would be taken that are necessary to make wind tunnel data strictly comparable. "52 In contrasting the gathering of empirical information with the larger issue of erecting a comprehensive theoretical framework into which it can fit, Zahm raised a question that engaged members of the NACA technical culture throughout the forty-three years the agency existed. The question has also engaged observers, critics, and historians both during and after those years —especially Hansen, not only in the essay that opens this volume, but in other works including his NACA Langley history Engineer in Charge. Usually

51. Becker, High-Speed, Frontier, pp. 8, 11; Eastman Jacobs, p. 341, "Experimental Methods —Wind Tunnels: Part 2," in William E. Durand, ed., Aerodynamic Theory, Vol. 3 (New York, NY. Dover, 1963; republication of 1935 version), pp. 319-348.

52. Goethert, Transonic Wind Tunnel Testing, p. 236; Zahm to Ames, September 17, 1920, in the RA70 folder "R.A.'s—Standardization of Wind Tunnels 1920-1926," LHA. It must be noted that Becker says that Ray Wright "agree[d]" in a 1978 interview that systematic experiments might also have worked; see High-Speed Frontier, p. 100.


the question is seen in terms of the science and engineering of aircraft themselves, but as Zahm's letter shows, and as the NACA's transonic wind tunnel achievement highlights, it also applies to the science and engineering of the primary research tools of aeronautics. Fluid dynamics is as fundamental for wind tunnels as it is for airplanes. Thus it was that Ray Wright, a physicist and applied mathematician among NACA engineers, eventually used what Zahm in 1920 called "the mathematical theory of the flow in a wind tunnel" to provide for the accurate replication of transonic flow fields.

A tension between empiricism and theory existed from the start in the NACA. The agency's first annual report in 1915 lamented a general "distrust of mathematical formulae" and "a natural tendency on the part of designers and constructors to assume that mathematical theories are of use only to those who are mathematically inclined."53 Such distrust seems to have been more common in American aeronautics than in European. Theodore von Kármán, a longtime leader in American aeronautics trained by Prandtl at Göttingen, reminisced in the 1960s about the contrast of "the practical inventor vs. the theoretical mathematician" he had found "characteristic of American scientific life in the twenties," and about the need, as he had long seen it, "to draw mathematics and engineering closer together" in this country.54 The NACA's Max M. Munk, the former Prandtl student who proposed the Variable-Density Tunnel in the early 1920s, worried that those desiring efficient mathematical condensing of empirical experience would encounter not only a distrust of mathematical formulae but an even deeper antipathy to theoretical approaches and understanding in general. In an influential 1922 paper on airfoil design theory, Munk revealed acute defensiveness concerning the place of theory in aeronautics: "Is it really necessary to plead for the usefulness of theoretical work? This is nothing but systematical thinking and is not useless as sometimes supposed, but the difficulty of theoretical investigation makes many people dislike it." Ironically, the new theoretical ideas in Munk's paper led in the 1930s at Langley to Theodore Theodorsen's further theoretical work, and then to the theory-based, wind-tunnel-refined wing-design successes of Eastman Jacobs and others, including Stack —work that produced low-drag NACA laminar-flow airfoils, contributed to NACA advances in shaping airfoils for delaying to higher speed the onset of compressibility effects, and illustrated the utilitarian NACA's ever-present practical interest in enlarging fundamental understanding. Walter Vincenti has observed that complexity precluded experiment-based success in this wing-design work, just as Groethert has observed it did in the invention of slotted walls: both efforts required that orientation of theoretical calculations.55 By the late 1930s, the NACA commonly incorporated such an

53. AR 1915, p. 14 and p. 13.

54. Theodore von Kármán with Lee Edson, The Wind and Beyond: Theodore von Kármán, Pioneer in Aviation and Pathfinder in Space (Boston, MA and Toronto, Canada: Little, Brown and Co., 1967), p. 124. Longtime von Kármán Student, friend, and colleague William R. Sears, p. 36, "Von Kármán: Fluid Dynamics and Other Things," Physics Today , January 1986, pp. 34-39, wrote: I think von Kármán believed that any problem in engineering (and perhaps in a much broader category) could profitably be attacked mathematically."

55. Report 142, "General Theory of Thin Wing Sections"; Walter G. Vincenti, "The Davis Wing and the Problem of Airfoil Design: Uncertainty and Growth in Engineering Knowledge," Technology and Culture 27 (October 1986): 717-58, especially pp. 740-14, 749, arid 750; Hansen, Engineer in Charge, pp. 81 and 111-18. In an April 3, 1996, telephone interview, Becker confirmed that there was cross-pollination at Langley concerning theoretical understanding of laminar-flow airfoils and closely related high-speed airfoils. Vincenti calls for more scholarship on airfoil design on pp. 738 and 739 in his article, which also appears as a chapter in What Engineers Know and How They Know It.


orientation in much of its research. 56 Like other NACA work, NACA transonics efforts came to rely on empirical approaches mainly, but as Zahm had recommended for subsonic tunnels back in 1920, not exclusively.

Nonetheless, forceful criticisms of the NACA's general focus on applied research rather than on deeper scientific questions have appeared from time to time, and bear on the history of NACA transonics. For the early NACA, perhaps the best-known general statement of the charge came in 1930, when Aero Digest accused the agency of being far too narrowly and myopically empirical, never seeking to apply test results "to any logical system, to digest them, and to interpret their general significance in the sum of general knowledge . " 57 Among historians, perhaps the best-known leveling of this charge comes from Edward W. Constant in The Origins of the Turbojet Revolution, a 1980 analysis of the pre-World War II convergence of technological developments, comprehensive scientific understanding, and combined scientific and technological imagination that resulted in the first jet aircraft —in Britain and Germany, but notably not in the United States. Constant says that before World War II the U.S. aeronautical research establishment, including the NACA, "had no interest in fundamental aerodynamic science," as shown in part by the "unimaginative" George Lewis's lack of interest in Theodore von Kármán's recommendation that a large supersonic tunnel be built. Constant's overall formulation of the charge, however, specifies more than the mere malfeasance of dwelling on the production of engineering data for near-term application, and more than the mere nonfeasance of failing to seek comprehensive theoretical understanding. Beyond these sins of commission and omission, Constant believes, was a more fundamental failure, a utilitarianism so narrowly focused on existing technology and so unimaginative as to constitute a sort of tragic flaw in the character of American —and therefore NACA— science and technology. Unlike the British and the Germans, the fundamentally flawed prewar American aeronautical research establishment could not even see, and therefore could not act upon, the synthesis possibilities that had gradually become implicit for aeropropulsion in the areas of turbomachinery, aerodynamics, and aircraft streamlining and structures. Like von Kármán Constant sees differing "national patterns in the pursuit and utilization of aerodynamic science," and he observes that they "may reflect fundamentally differentiated cultural traditions. No later than 1900 Germany certainly had an unequalled tradition of mathematical and theoretical excellence in science and also had developed a deliberately close relationship between science and industry. Britain shared a similar if more empirical and less mathematically rigorous tradition in science. In contrast, the United States still was possessed of a scientific tradition extreme in its empiricism and utilitarianism."58

Whatever the validity of such criticisms, the early NACA did not employ its empiricism and utilitarianism unaware. In 1915, future NACA chairman (1941-1956) Jerome C. Hunsaker noted that experiments designed to answer current practical questions could also, over time, supply answers to deeper scientific questions, much as George Lewis believed. In Model Research, Roland says this principle became de facto NACA research policy by the late 1920s. In Engineer in Charge, Hansen shows how the principle applied in the matter of the cowling: the NACA first provided a quick practical solution and won the Collier Trophy, but in the longer term also worked for and achieved a genuine depth of theoretical understanding. In 1923, Joseph Ames used a courtroom simile to describe the principle: when the NACA conducted its practical tests, said Ames, it was "also doing

56. Hartley A. Soule, "Synopsis of the History of Langley Research Center, 1915-1939," p. 37 (item CN-141,573, Langley technical library); Becker, High-Speed Frontier, p. 23.

57. Frank A. Tichenor "Why the N.A.C.A.?" Aero Digest, December 1930, pp. 47ff.; reprinted in Model Research, 2:652-57; quotation on p. 657.

58. Constant, Turbojet Revolution; quotations from pp. 159 and 176.


fundamental scientific work continuously, exactly as a justice of a high court expresses his deepest thoughts as obiter dicta. "59

Certainly Ames's obiter dicta principle applied in the evolution of the NACA's understanding of the fluid dynamics of wind tunnels —the scientific component that supplemented engineering experience and technical craftsmanship in the overall wind tunnel expertise that began to grow in the NACA from about the time of Ames's 1920 initiative. The epistemological task of isolating and identifying this scientific component belongs to followers of Walter Vincenti, who has engaged similar questions about American aeronautical history. That such a component was indeed present, however, can be seen in Goethert's firsthand observation that the slotted-wall invention required an orientation of theoretical calculations. Possibly the scientific component was still small in 1922, when the NACA's annual report listed five technical papers on wind tunnels, one of them a Prandtl translation. Possibly it was small in 1925, when Joseph Ames told the NACA executive committee that Munk had developed a theory of tunnel wall interference. Possibly it was still small in 1930, when the available body of formal wind tunnel knowledge had grown large enough that an NACA report about correcting test data for subsonic open-jet boundary effects could cite four NACA and three European works on wind tunnel technology, along with one American and four European works on related aerodynamics topics— with only one source predating the 1920s. And certainly the scientific component was overrated in the NACA's 1934 annual report, which claimed that with the appearance of an NACA subsonic study called "Experimental Verification of the Theory of Wind-Tunnel Boundary Interference," the problem of fundamentally understanding wall interference could "for all practical purposes be considered solved." The problem had been solved "for all types of wind tunnels," the annual report said, even though the technical report in question carefully noted that only "conventional" and "ordinary" tunnels had been involved 60 —as well it should have noted, given that in that same year of 1934 Langley built its second small high-speed tunnel in part to investigate the far-from-conventional, far-from-ordinary transonic boundary effects that had been revealed in its first one, built in 1928.

That first high-speed tunnel had indeed raised lots of questions. The NACA built it to begin conducting "in-house" the kinds of studies Dryden and others had been conducting under NACA auspices elsewhere. It resembled a pipelike metal chimney, as for an open circular fireplace, with an eleven-inch-diameter test section about where such a chimney Would have a flue damper. Compressed air powered it, tapped from an ideal reservoir at twenty atmospheres of pressure: the much larger Variable-Density Tunnel, which had to be depressurized occasionally anyway. The small vertical tunnel used the induction-jet principle, suggested by George Lewis based on a cursory contemporary Langley study of thrust augmentation, an antecedent of jet propulsion. In a rush lasting just long enough to yield some test data, piped-in air entered the tunnel just above the test section from an opening that ringed the pipe's circumference. This motion entrained a more massive flow of air

59. Hunsaker as quoted by Hugh L. Dryden, p. 93 in "Aerodynamics—Theory, Experiment, Application," Aeronautical Engineering Review 12, No 12 (December 1953): 88-95; Roland, Model Research, 1:103; Hansen, Engineer in Charge, ch. 5, especially p. 137; Ames as quoted in Model Research, 1:349 n. 20. At least two NACA research engineers of the pre-World War II era have disagreed with the charge that engineering unduly triumphed over science in the NACA: Roland, reporting disagreement with his own interpretation in Model Research quotes Ira Abbott, 1:345, n. 60, and John Becker, 1:346, n. 65. Langley engineer Elton W. Miller, rebutting Aero Digest, restated the obiter dicta principle, though without Ames's figure of speech, in a memorandum to the engineer in charge, December 19, 1930, reprinted in Model Research, 2:657-59.

60. AR22, pp. viii and 45; "Fifty Years of Flight Research: A Chronology of the Langley Research Center, 1917-1966" (NASA TM-X-59314, apparently a republication of work by Michael D. Keller), p. 27; Montgomery Knight and Thomas A. Harris, Report 361, "Experimental Determination of Jet Boundary Corrections for Airfoil Tests in Four Open Wind Tunnel jets of Different Shapes"; AR34, p. 16; Theodore Theodorsen and Abe Silverstein, NACA Report 478.


upward from the room, generating a high-speed flow field around a small model facing downward in the test section. Both closed and open test sections were tried, giving Langley engineers a sense of the contrast between a walled-in jet of high-speed air and an open one. Despite some open-jet advantages, an enclosed test section was chosen for permanent use. This comparatively modest research tool, called the 11-Inch High-Speed Tunnel, began operation in mid-1928, about when John Stack completed his aeronautical engineering degree at Massachusetts Institute of Technology and, in Becker's words, arrived in Virginia "to dominate Langley high-speed aerodynamics for the next 30 years."61

A Measured Pace in the 1930s

"It is gratifying," the NACA modestly proposed in opening its 1933 annual report to Congress, "to report that the past year was notable as witnessing the greatest advance in airplane performance and efficiency accomplished in any single year since the Great War. This is largely the cumulative result of years of organized scientific research conducted by this Committee and of the practical application of the results by the Army, the Navy, and the aircraft industry." Apparently this expansive claim had substantial legitimacy. Richard K Smith has written that between 1928 and 1938 "no other institution in the world contributed more to the definition of the modern airplane" than the NACA.

Photo of Eastman Jacobs at the variable density tunnel an exterior of the building.
Both figuratively and literally, the low speed, pressure tank-enclosed Variable-Density Tunnel breathed life into early NACA high-speed research. With the leadership of Eastman Jacobs, far left, the VDT helped establish the NACA's wind tunnel credentials and its confidence for further innovation. It also provided blasts of high-pressure air to power the NACA's original 11-Inch High-Speed Tunnel and later the 24-Inch High-Speed Tannel shown here. The later vertical tunnel, with its twenty-four-inch-diameter test section, worked in the same way as the eleven-inch tunnel, but accepted larger models and had better data-gathenng instruments. (NASA photos NACA 3310 and NACA 11443).

61. Stack, NACA Report 463, "The N.A.C.A. High-Speed Wind Tunnel and Tests of Six Propeller Sections," 1933; Becker, High-Speed Frontier, p. 13.


Photo Left John Stack loads a model into the wind tunnel Right: stack sitting, examines a model plane.
John Stack. NACA research craftsman and research leader Left: Research craftsman Stack in the 1930s, reaching to help W. F. Lindsey adjust instrumentation inside Langley's briefly disassembled 24-lnch High-Speed Tunnel. When closed, the pipelike vertical apparatus could channel a many-hundred-mile-per-hour flow of ascending air through its twenty-four-inch-diameter test section and across a tiny downward-facing aerodynamic model linked to measuring and recording devices. (Photo courtesy John V. Becker) Right: Research leader Stack after World War II, when —in colleague John Becker's words— he was widely "recognized not only as the NACA's leading expert in aerodynamics, but also as an unusually colorful character" with "tough assertive characteristics" who "was at his best in the midst of conflict, crusading passionately for some cause such as a new wind tunnel. "For three decades Stack helped define the NACA technical culture, but unlike NACA director Hugh Dryden, he found himself excluded from helping to do the same in NASA after the NACA years ended in 1958. 62 (NASA photo 48,989).

Smith's aeronautical history colleagues Hallion, Hansen, and Roland, as well as physics historian Daniel J. Kevles, have made similar assessments. Even Constant, in Turbojet Revolution, mildly praises the interwar NACA for its subsonic work. Continuing the annual report's self-congratulation, however, the NACA entered a realm where gaining later endorsements for its work in the 1930s has been hard, but incurring criticism has been easy: speed. Calling speed "the most important single factor" for improving airplanes, the report proclaimed that "primarily as a direct result of the Committee's researches there have been great increases in speed and efficiency during the past year, which have opened a new era in the development of both military and commercial aircraft. "63

Of course, With no serious thought yet given in American aeronautics to Jets, the NACA merely meant that propeller-driven airplane speed would continue to be developed

62. High-Speed Frontier, pp. 34, 14, and 13. On p. 176 in The Birth of NASA: The Diary of T. Keith Glennan (Washington, DC: NASA SP-4105, 1993), NASA's first administrator, writing about a July 1960 visit from Stack, called him "an interesting character —almost ready for retirement, outspoken and somewhat lacking in common sense," adding that although he was "one of the very best men in the aeronautical field" it was "obvious" he should not become associate director of Langley.

63. AR33, p. 1; Smith, "Better: The Quest for Excellence," p. 240; Hallion, Test Pilots: The Frontiersmen of Flght (Garden City, NY Doubleday & Company, Inc., 1981), p. 50; Hansen, "George W. Lewis and the Management of Aeronautical Research," p. 94; Roland, Model Research, 1:xiii; Kevles, The Physicists: The History of a Scientific Community in Modern America (New Yoik. NY. Alfred A. Knopf, 1978), pp. 292-93: Constant, Turbojet Revolution, ch. 6, especially pp. 156, 159, and 175. A mildly positive assessment appears on p. 75 of Walter A. McDougall.... The Heavens and the Earth: A Political History of the Space Age (New York, NY: Basic Books, Inc., 1985).


in this "new era." So a better term for the NACA's 1930s now appears to be plateau, as used by NACA and NASA aeronautical engineer Laurence K. Loftin, Jr., in Quest for Performance: The Evolution of Modern Aircraft. Airplane development, he wrote in 1985, "has been characterized by a series of technological levels, or plateaus, that extend over a period of years. Each level has been exemplified by an aircraft configuration type that is gradually improved by a series of relatively small refinements, without any major conceptual change." The mid-1930s forerunner of the P-47 Thunderbolt fighter, for instance —with stressed-skin metal construction, low cantilever wing with trailing-edge landing flaps, fully cowled radial engine with controllable-pitch propeller and geared single-speed super-charger, enclosed cockpit, and retractable landing gear with wheel brakes— represented, along with the DC-3 and the B-17, "the definitive and final configuration of the propeller-driven aircraft concept." Room remained, of course, for additional smaller refinements, like improvements in propeller-blade design. The NACA contributed substantially to reaching this plateau, but a new era, Loftin wrote, would actually require a "revolutionary breakthrough or new concept."64

For American aeronautical researchers as opposed to certain imaginative technologists in Europe, then, the idea of a "new era" in aviation speed in the 1930s suggested differing sets of research questions: those for propeller planes and those for jets. And since the research question generally dictates the need for the research tool, this difference was reflected in the NACA's high-speed wind tunnel development during the 1930s. Marching in time with conventional technology, and a few but not too many steps ahead, it proceeded at a conservative, measured pace.

Long before 1933, in fact, some European technologists had begun considering the possibilities for breakthroughs leading to very high-speed aircraft, and the possibilities for corresponding high-speed wind tunnels as well. Constant, alert to instances of foresight concerning radical technology by change, closes Turbojet Revolution  by alluding to a 1922 discussion among French and English engineers concerning the possibility of flying "With incredible speed in the stratosphere."65 In 1924 in France, E. Huguenard's paper on high-speed wind tunnels predicted airplane speeds beyond 500 miles per hour, and conjectured that although speeds up to almost 750 miles per hour had formerly seemed "fabulous ... as in Jules Verne," they now appeared "realizable, not in a remote future, but immediately." This nearly quarter-century-early conjecture of almost sonic flight speed may suggest why Becker calls Huguenard "overly sanguine." Whatever the excesses of Huguenard's enthusiasm, though, it is plain that in 1924 he squarely addressed a future that actually started arriving in the late 1930s— and that by 1925 his paper and its ideas were noted in the United States. The NACA published a translation that year, well before the agency used versions of two preexisting tunnel-technology ideas that Huguenard discussed: a compressed-air reservoir for driving a high-speed tunnel, and, for observing high-speed phenomena, an optical technique based on the way light behaves in air of changing density. Also in 1925, Scientific American favorably summarized Huguenard, reporting his prediction of 500-mile-per-hour speeds, his speculation about the need for

64. Laurence K. Loftin, Jr. Quest for Performance: The Evolution of Modern Aircraft (Washington, DC: NASA SP-468, 1985), pp. ix, x, 95, and 96.

65. Constant, Turbojet Revolution, p. 246.


some form of reaction propulsion, and his emphasis on the coming importance of wind tunnels for high-speed flight.66

For any NACA high-speed researchers inclined to consider the possibilities Huguenard had proposed, however, the late 1920s and early 1930s, with their official focus on propeller-tip studies, would have presented a certain tension. A 1929 report of an NACA-sponsored study of tiny airfoil models in an open, two-inch-wide transonic jet of air provides a typical example of the focus: "If a propeller is mounted directly on the shaft of a modern highspeed airplane engine," wrote Lyman Briggs and Hugh Dryden, explaining the practical engineering design question motivating their study, "the outer airfoil sections of the propeller travel at speeds approaching the speed of sound. It is possible by the use of gearing and a somewhat larger propeller to reduce the speed of the propeller sections, but only at the expense of additional weight and some frictional loss of power. In order to determine whether gearing is desirable, it is necessary to know the loss of efficiency due to high tip speeds and to compare this loss with that due to gearing." In other words, in their tests at speeds involving compressibility, they merely sought airfoil performance data to use in determining the optimum tradeoff, or balance, between competing design choices. The report mentions nothing about applications of the work to wings for very high-speed flight.67

Even in the mid-1930s, in fact, a forward-looking NACA engineer would have been aware that the NACA officially believed the trend to higher flight speeds would level off not too far above 500 miles per hour. Among Huguenard's enthusiasms, on the other hand, had been a willingness to project continuation of the upward trend. Observing that aircraft speeds had regularly doubled nearly twice per decade, Huguenard criticized those who always found "formulas" to show that "each new performance" in this trend would be the last. He even gave these doubters a name that fit the official NACA: pessimistic calculators. For the NACA, research director George Lewis seems to have exemplified this restrained outlook, at least in his public statements. In 1932 he predicted that the impressive upward trend in flight speeds would end for "airplanes as they are now constructed" at about 500 miles per hour. "At that speed," Lewis added, "the resistance of the air against the plane becomes so great that it would be physically impossible to obtain an engine giving enough added horsepower to pull the plane through the air at a greater speed." Although Lewis did note, by way of qualification, that "no one knows what the airplane of the future will resemble," his 1932 emphasis corresponded entirely with Loftin's 1985 concept of the plateau. John Becker arrived at Langley in 1936; when asked in 1996 if Lewis and Stack in those days might have harbored some hidden belief in a sonic future, he responded with confidence that he believed they had not.68

66. NACA Technical Memorandum 318, June 1925, translation of E. Huguenard, "High-Velocity Wind Tunnels: Their Application to Ballistics, Aerodynamics and Aeronautics," from La Technique Aéronautique, November 15 and December 15, 1924; quotations on p. 28. Huguenard refers (p. 15) to "a report by the American Lieutenant Sewall, to the United States War Department (S. Sewall, 'Report on high-velocity wind tunnels,' November 12, 1918)." Becker, High-Speed Frontier, p. 12. Both William F. Durand and Hugh L. Dryden later cited Huguenard: Durand in a reference list recommended on p. 252 and appearing on p. 349 of Volume III of Aerodynamic Theory: A General Review of Progress, ed. Durand (Dover Publications Inc.: New York, 1935, 1963) and Dryden, pp. 2 and 3, lecture notes, "Sixty Years of Experimental Supersonic Research" in Milton Ames collection folder "May 24, 1948, Transonic Wind Tunnels (Slotted Throats) Memo by Ray H. Wright," LHA. "High-Speed Wind Tunnels," Scientific American 133 (October 1925): 275-77. It is interesting to note Scientific American's claim, in its 150th anniversary issue, September 1995 (p. 58), that "technology and the future have always been the province of this magazine." The issue boasts (p. 14) that the magazine covered the Wrights almost two years before Kitty Hawk, and quoted Robert Goddard saying in 1920 —six years before the first liquid-fueled rocket flight, the kitty Hawk of rocketry— that "a rocket capable of reaching the moon could be built."

67. NACA Report 319, "Aerodynamic Characteristics of Twenty-Four Airfoils at High Speeds," 1929.

68. Huguenard, TM 318, p. 28. "How Fast Can We Fly?" The Sunday Star, September 11, 1932, in Milton Ames collection folder "George Lewis," LHA. Telephone interview, July 18, 1996.


In any case it would be difficult to establish that in the 1930s the NACA could have pushed high-speed research or high-speed tunnel technology much faster than it did, even if it had wanted to. Industry and military energies compelled its focus on the technology of today and tomorrow but not the day after. Becker states flatly that even as late as 1940, the research-agenda-setting aircraft industry considered Mach 0.8 —roughly 600 miles per hour— "a rather optimistic upper limit for the future." He also says that most "NACA veterans believe that it would have been quite impossible in the prewar period to have obtained any major support from the military, industry, or Congress for research and development aimed at such radical concepts as the turbojet, the rocket engine, or transonic and supersonic aircraft." One such veteran, who helped build Langley's 24-Inch High-Speed Tunnel in the mid-1930s, believed it "certain that if the NACA had had the foresight to do research on the turbine engine in the decade before World War II, the agency would have met with such technical ridicule and criticism about wasting the taxpayers' money that it would either have had to drop it or have been eliminated." And indeed the prewar NACA did face political perils difficult enough to negotiate without the agency's also seeking to venture too boldly beyond or above the technology plateau of the day.69

It is worth noting, moreover, that the prewar NACA in many ways did plan for the future, within the limits of a political reality in which lend-lease had eventually to be concocted to help the British halt the Nazi onslaught. In the mid-1930s, for instance, the NACA —advancing at the steady, measured pace of the times in American aeronautics— built not only the twenty-four-inch tunnel but the 500-mile-per-hour 8-Foot High-Speed Tunnel, later repowered for still more speed. This strategic resource was to become in 1950 the first large facility to operate with slotted walls. In the late 1930s the NACA began planning Langley's 16-Foot High-Speed Tunnel, the other large facility later converted. Alarmed years in advance about war's likelihood-in part thanks to George Lewis's visits to Europe-the NACA also sought to build new research laboratories, and indeed had managed to get funding to start a pair by the time of Pearl Harbor. For two years in the late 1930s "after learning of the frantic pace of aeronautical research in Europe, especially in Germany," wrote Alex Roland, "the NACA was unable to convince the Congress or the Bureau of the Budget that a crisis was in the making, a crisis requiting a crash program in aeronautical research." Yet the postwar Mead committee charged that the prewar NACA knew "of the need for increased personnel and facilities to carry on its research work" but "did not request sufficient funds from Congress." However, more than three years before Pearl Harbor the NACA did include in its annual report to Congress a frank plea for expansion— a plea highlighted, analyzed, and endorsed by a January 1939 editorial in the New York Times.70 Thus for the prewar NACA and the country, an apt analogy might be that of the so-called next-quarter syndrome, in which a corporation's stockholders compel a shortsightedness that its critics contrast with the foreign competition's supposed longer view. It is true that the prewar American aeronautical establishment failed to invent jets and guided missiles. But it is also probable that the failure originated at a cultural level deeper than that of the scientific and technological choices actually available to American aeronautical researchers and their managers—as even one of their main critics, Constant himself, all but proposed in conjecturing about those "fundamentally differentiated cultural traditions" of Europe and America.

69. Becker, High-Speed Frontier, pp. 162 and 31 (see also p. 147); Hansen, Engineer in Charge, p. 184. Concerning the NACA's prewar travails, see Roland, Model Research, ch. 6 and 7.

70. Lewis's "Report on Trip to Germany and Russia, September-October, 1936" is in the Milton Ames collection folder "George Lewis," LHA. Roland, Model Research, 1: 147. Excerpt from Mead committee report, Stack collection folder "Miscellaneous," LHA. On January 10, 1939, the Times quoted and editorialized (p. 18) about statements in AR38, and on p. 8, along with articles about the "nation's rearming," featured an article headlined "Our Air Supremacy Is Held Endangered; National Advisory Committee Says Intensified Research Is Necessary to Retain It."


Therefore it is also worth noting, concerning researcher Stack and manager Lewis, that in the 1940s Stack sometimes implied or even claimed that Lewis and the NACA had actually shown substantial foresight early on concerning flight at very high speeds. A 1948 newspaper story quoted Stack claiming that the "NACA's supersonic flight project really [went] back 20 years" to when Lewis, "with his long nose for the future, put in the first high-speed wind tunnel."71 But if a supersonic-flight motivation for building the 11- Inch High-Speed Tunnel really did exist in 1928, it was apparently completely hidden. In 1945, Stack claimed in his formal paper for the prestigious Wright Brothers Lecture of 1944 that in the 1920s, when "a few foresighted aeronautical scientists" had planned ahead for very high-speed flight, Lewis had shown "great foresight" in sponsoring Langley's brief, cursory jet propulsion study. 72 Maybe such claims only represent what Constant has called the NACA's "habitual but mythic retrospective attribution of foresight to itself ." Certainly Stack understood the NACA public relations juggernaut and could often be part of it; Roland says that by the 1950s he became too much a part of it. In any case, a draft of the Wright Brothers paper shows that Stack also considered claiming that "probably the first practical application of jet propulsion in aeronautical work" was Langley's, and long-nosed Lewis's, adaptation of the cursory jet study's induction-jet principle for the eleven-inch tunnel. By permanently deleting that claim, Stack avoided its justifying any "long nose" descriptions of himself—not for prescience, but for exaggeration .73

By the time of the NACA's 1933 boasting about speed, Stack himself was calculating at least somewhat optimistically about future propeller-driven high-speed flight, but there is evidence he felt constrained from pressing even that topic too far. The tension shows in a pair of historically significant papers he wrote, early contributions in his substantial compressibility research output during the years before he rose high in management. One, published as an article in the January 1934 Journal of the Aeronautical Sciences, reflected mainly his own outlook. The other, published officially as NACA Report No. 463, reflected mainly the organization's outlook. The journal article described a possible high-speed airplane and addressed its high-speed-flight potential. The NACA report described the 11-Inch High-Speed Tunnel and emphasized its usefulness in propeller-tip studies.

The journal article, "Effects of Compressibility on High-Speed Flight," presented performance predictions Stack had computed for a highly aerodynamically refined propeller-driven airplane that he called "hypothetical" but "not beyond the limits of possibility." Stack's computations showed that speeds much higher "than those so far attained" were "possible and likely," in part by using wings of a compressibility-effects-delaying shape derived from experiments in the eleven-inch tunnel. Some of this new design information, Stack wrote, was "already available to designers." With compressibility ignored, Stack's computations predicted a top speed of 566 miles per hour for the hypothetical airplane.

71. "Intuition Brought Supersonic Flight," Washington Post, December 21, 1948.

72. Stack, p. 128, "Compressible Flows in Aeronautics: The Eighth Wright Brothers Lecture," Journal of the Aeronautical Sciences 12, No. 2 (April 1945): 127-48. See also pp. 2 and 3 of the hand-annotated 36-page double-spaced typescript, apparently, by Stack, titled "Report of the NACA Executive Committee: Supersonic Center Project," Stack collection folder "Revised Unitary Program, 1946-48," LHA. It claims that "sonic and supersonic tunnels" operated at Langley in the late 1920s.

73. Constant, reviewing High-Speed Frontier in Isis, 73:4:269 (1982): 609-10, says (p. 610) that Becker generally "debunks" the NACA's retrospective claims of foresight. Roland's strongest criticism of Stack's exaggerations appears on pp. 261-63 and in the accompanying n. 6 on p. 384, Model Research. Stack's crossed-out 1944 exaggeration is on p. 9 of the typed draft (corresponding to p. 128 in the article version), Stack collection folder "Wright Bros. 1944 Lecture," LHA. Study of the relation between transonic wind tunnel development and NACA public relations practices is mainly beyond the scope of the present work as it evolved, but is also the principal desideratum it generated. One example: the episode of the Annular Transonic tunnel, which the newly security-minded postwar NACA advertised in such a way as to confuse outsiders concerning NACA progress in transonics.


With compressibility considered, that would fall to 524, but the new wing shape, Stack computed, could raise the top speed to 544 miles per hour "due to the delayed compressibility burble." At one point the journal article enthused about long-standing NACA foresight and leadership in high-speed-flight studies, but cited as evidence only the 1925 report of Briggs, Hull, and Dryden —which solely addressed propellers, though its analysis could be transferred and applied also to wings and thus to Stack's optimistic subject. And he calculated even more optimistically in an early handwritten draft, where a line he ultimately did not publish went so far as to say —as Huguenard did say back in 1924, and as the NACA did not say until the postwar world was upon it —that it was "dangerous to predict a maximum speed beyond which increases may be impossible ."74

In NACA Report No. 463, "The N.A.C.A. High-Speed Wind Tunnel and Tests of Six Propeller Sections," Stack addressed high-speed technology with an entirely different slant, conservatively emphasizing propeller tips and not the airplane itself "Speeds common to most aircraft" were low compared to the speed of sound, the introduction admitted, but knowledge of compressibility was nonetheless "essential" because propeller tip speeds commonly did reach the "neighborhood" of sonic speed. The introduction mentioned that racing airplanes had been attaining speeds "as high as half the speed of sound," and that "even at ordinary airplane speeds, the effects of compressibility should not be disregarded if accurate measurements are desired." But the report did not squarely address compressribility's overall future implications for the entire airplane —the very subject of Stack's roughly concurrent journal articIe— until near its end, where the statement appears that compressibility "is of considerable importance in the structural design of fast-diving airplanes," affecting distribution of loads. One of the report's conclusions also made the qualitative prediction that "errors may be expected in the estimated design loads for airplanes which attain speeds such as those attained by diving bombers when in a dive if the effects of compressibility on the wing moment coefficient are neglected." Nothing in the report's title, its lengthy opening summary, or its introduction suggested the presence of this kind of information. Yet that kind of information was to become very important at about the time of Pearl Harbor, when Stack and others at Langley helped solve serious, sometimes fatal, structural problems compressibility was causing in warplanes.

But the sharp contrast between this pessimistically calculating official report and Stack's optimistically calculating journal article, though it illustrates the NACA's conservative early-1930s research priorities, shows only one of the ways in which the report is significant in the multidecade evolution of the transonic wind tunnel. There are others. In focusing far more on the research tool than on the data obtained with it, the report introduced to the aeronautical world the NACA's first high-speed wind tunnel, including the early test-section-development work that Becker says strongly influenced slotted-wall development years later. The report called for a larger wind tunnel, and then served throughout the 1930s as the standard reference to cite for describing how experiments were performed not only in the eleven-inch tunnel, but in the larger twenty-four-inch apparatus that indeed did ensue and that was operated in the same way. And the report correlated high-speed wind tunnel data with results from full-scale propellers operated at high tip speeds in the low-speed airflow of the Propeller Research Tunnel —a notable instance of technical cross-pollination between the NACA's subsonic and transonic research efforts.

In yet three more ways, three particularly important ones, Stack's 1933 report illuminates transonic wind tunnel evolution and its NACA technical cultural implications. First, it defined the engineering science of NACA transonics—"physical understanding without

74. Journal of the Aeronautical Sciences 1 (January 1934): 40-43. The October 1933 manuscript and preceding draft materials are in an untitled Folder, Stack collection, LHA; the paper's title is written on the front of the folder, not its tab.


mathematical weakness," to borrow a distillation Stack would use in 1942—by addressing the difficulties of attaining theoretical understanding of compressible flow, by claiming comprehensive accommodation of what little theoretical understanding was already available, and by showing Stack's acute determination to respect and use theory but not to let "mathematical complications" impede attainment of the physical kind of understanding an engineer often wants to visualize. Second, the report generally identified the vexing transonic tunnel issues that Langley later won the Collier Trophy for solving: "the effect of the tunnel walls," the test-data-skewing "constriction effect at the test section due to the presence of the model," the relation between model size and test-section size, and the question of a mathematical "constriction correction" to make transonic test results for artificial, ground-bound flow fields correspond with physical reality aloft. Third, as an approach for confronting these issues, it introduced as potentially useful what Goethert, looking back in 1961, called the indispensable "orientation of theoretical calculations." The report suggested conducting "a theoretical analysis of the flow in the tunnel with a view to determining the constriction correction," and added that the "analysis should include an examination of the effects of compressibility" —an important stipulation, the report said, but one that "because of the mathematical difficulty involved" seemed "improbable" in 1933. In 1944, however—when transonics had become a top research priority, thus making theoretical study of transonic tunnel flow a priority too— NACA research engineers H. Julian Allen and Walter Vincenti conducted just such a theoretical analysis at the new Ames Laboratory in California. Their report's title echoed Stack's 1933 language: "Wall Interference in a Two-Dimensional Flow Wind Tunnel, with Consideration of the Effect of Compressibility."75 But what they showed was that in fact there could be no correcting of test data for the worst conditions of transonic flow within solid-boundary tunnels. Although their report apparently did not directly influence Ray Wright's search for a way to circumvent any need for corrections, Stack's idea of addressing the transonic wind tunnel problem via theory obviously did. If the awarders of the 1951 Collier were right in their original intention to credit Stack alone, if they perhaps really just meant to take the longest view of Stack's overall contributions to transonic wind tunnel development, the justification might well start with "The N.A.C.A. High-Speed Wind Tunnel and Tests of Six Propeller Sections" of 1933.

In 1933 Langley Field's runways were not yet paved. The term sound barrier was not yet sensationalized; that happened in 1935 following a casual remark to a journalist from British high-speed researcher W. F. Hilton. In 1933, the NACA's newly updated compilation of standard aeronautical nomenclature still included lots of biplane terms, but not compressibility, Mach, or any word with the suffix sonic.76 Nonetheless, the NACA's high-speed research program, at its measured pace, continued advancing in sophistication during the mid-1930s, led by Eastman Jacobs and Stack.

Stack's papers trace the progress. In 1934, he and Albert E. von Doenhoff published an NACA report on airfoil research in the eleven-inch tunnel. The stated focus was still propellers, but wings and high-speed flight were now slightly more visible within the official field of view. According to Hilton's 1951 book High-Speed Aerodynamics, this was "Stack's classic paper, which exerted great influence by virtue of its early publication." But Stack and von Doenhoff had relied on experimental parameter variation, the systematic empirical method that James Hansen emphasizes as centrally important in the NACA's

75. Report 782.

76. Langley's runways were not paved until 1937, according to Robert I.Curtis, John Mitchell, and Martin Copp, Langley Field: The Early Years, 1916-1946 (Langley Air Force Base, Virginia, 1977), p. 101. Hansen, Engineer in Charge, p. 253, discusses the sensationalized remark. AR33 contains one of the periodic updates of the NACA's report on standard nomenclature.


engineering science. The compressibility burble itself remained mysterious. Stack's 1935 report "The Compressibility Burble" declared that although the eleven-inch tests had "yielded much valuable information for design problems," they had also shown the necessity of a "more fundamental investigation ."77

The 1935 report itself described early stages of such an investigation, conducted "to determine the physical nature of the compressibility burble." The experiments took place in the new twenty-four-inch tunnel, where improved instruments could simultaneously gather for correlation two kinds of data about transonic air interacting with a test model's surfaces: pressures and photographic images of the accompanying compressibility shock patterns. A schlieren optical system generated the photographable images by exploiting the behavior of light passing through air that is changing abruptly and radically in density. The report overlapped substantially with the paper famously presented by Jacobs that year at the international Volta conference on high-speed aeronautics in Italy. Later, in 1938, Stack W. F. Lindsey, and Robert E. Littell published a refined and extended version of Stack's 1935 report: "The Compressibility Burble and the Effect of Compressibility on Pressures and Forces Acting on an Airfoil." Becker says that together with Jacobs's Volta paper, "these publications proclaimed the first major contribution of NACA in-house high-speed research—the fundamental understanding of the burble phenomena derived in large part from the revelations of the schlieren photographs ."78

The 1938 report's research focus expressly included "future high-speed aircraft," and by this point in the prewar decade the research-methods focus had also widened: though still primarily empirical, it now included substantial overlap with airfoil theory, as Stack's 1939 "Tests of Airfoils Designed to Delay the Compressibility Burble" Shows.79 The 1939, report's antecedents included work by Langley theorist Theodore Theodorsen, which itself built in part on Max Munk's 1922 airfoil theory paper —the one in which Munk lamented the general distaste for theory he perceived in others. The overlapped work notably included Jacobs's new computational method for designing drag-reducing laminar-flow airfoils, for the physics involved in sustaining laminar flow is similar to that involved in delaying the compressibility burble: both require shaping the airfoil to control the way pressure changes in air flowing across its surface. To devise his computational design method, Jacobs had inverted Theodorsen's theoretical approach. The work Stack reported in his 1939 paper incorporated closely related analysis.

But even with its sophistication in high-speed research methods, Stack's 1939 report maintained the NACA's long-standing conservative outlook on high-speed research purposes. Its introduction, after noting that "high-speed aircraft" themselves needed "serious consideration," added that it was "important to realize, however, that the propeller will continue to offer the most serious compressibility problems." Of course, with the world war starting, this technology prediction had genuine merit within the context of continued refinements crucial for the conventional warplanes that would soon swarm from American factories. But overseas, jets were also in development. The NACA's high-speed research-and its high-speed research tools and methods-had advanced during the 1930s at a measured pace. Within a few years, Hugh Dryden and Curtis LeMay would be calling for an urgent one.

77. "Tests of 16 Related Airfoils at High Speeds," Report 492; W. F. Hilton, High-Speed Aerodynamics (New York, NY, London, England; Toronto, Canada: Longmans, Green and Co., 1951), p. 81; "The Compressibility Burble," Technical Note 543, October 1935.

78. Report 646; High-Speed Frontier, p. 19. The never-capitalized word schlieren has long vexed authors and editors. A. C. Kermode, Mechanics of Flight (London: Pitman Publishing 1972), p. 317, noted that "it is not the name of some German or Austrian scientist, but simply the German word for streaking or striation, which is descriptive of the method."

79. Technical Note 976, December 1944, reprint of ACR of June 1939.


The Pace Hastens

Before Pearl Harbor some warplanes could already dive fast enough to encounter dangerous compressibility effects —as roughly predicted by that brief, unemphasized conclusion in Stack's official NACA report of 1933, when propeller tips constituted the NACA's only official compressibility research focus.80 In transonic airflow, as Hugh Dryden explained in a 1948 Physics Today article, "disturbances known as shock waves" arise. These "abrupt changes in pressure and temperature" can lead to "a violently fluctuating motion shaking or buffeting the wing, and if the wake of the wing strikes the tail, the tail structure may be subjected to loads varying with violent irregularity sufficient to damage it."81 Vulnerable airplanes during the war included the Bell P-39 Airacobra, the Curtiss P-40 Warhawk, and the Republic P-47 Thunderbolt. 82 At about the time of Pearl Harbor, when the problem had just arisen, Stack and others used Langley's twenty-four-inch and eight-foot high-speed tunnels in a rush effort to learn how to counteract the disturbances and stop the Army's new P-38 Lightning from occasionally breaking up and crashing. They quickly showed that a special under-the-wing flap could be developed to do the job.83 The P-38, of which over 10,000 were ultimately built, went On to shoot down more Japanese aircraft than any other fighter.84 The NACA went on to encounter still more complex transonic research problems during the 1940s, and to invent research tools-including the slotted-wall tunnel-for solving them.

By October 1948, when Dryden explained transonic research problems to a broad audience with the Physics Today article and advertised a new transonic research tool to a tiny audience with the Wright-Ward paper, the NACA's compressibility research focus had long since expanded. Maybe Stack had been prudent in 1933 to delete front his high-speed-airplane journal article the claim that it was "dangerous to predict a maximum speed beyond which increases may be impossible," but now the NACA itself officially gloried in seeing no "definite limit to the speed that may be attainable."85 The late-1930s goal of refining airfoil shapes to delay the onset of compressibility had been replaced: "Regardless of how high the critical Mach number may be raised," asserted Stack in his 1944 Wright Brothers Lecture, "flight at supercritical speeds must eventually be solved."86, Devising airfoils suitable not just for delaying the burble but for negotiating the entire transonic range would only be part of the solution. Effective transonic aircraft would also have to stay stable and controllable in an aerodynamically complex environment. 87 Moreover, researchers since the 1930s had been aware that separate high-speed tests of individual components —a cowling and a wing both meant for the same fuselage, for instance— could not always predict the components' performance in use together. Therefore solving supercritical flight required seeing the "integrated whole," as NACA main committee member Edward P. Warner called the principle of conceiving transonic

80. Conclusion 6, "The N.A.C.A. High-Speed Wind Tunnel and Tests of Six Propeller Sections."

81. Dryden, "Faster Than Sound," Physics Today 1, No. 6 (October 1948): 6-10 (see p. 8).

82. Hallion, Test Pilots, p. 187.

83. John Stack, "Compressible Flows in Aeronautics: The Eighth Wright Brothers Lecture," Journal of the Aeronautical Sciences 12, No. 2 (April 1945): 127-48 (see pp. 141-42).

84. John D. Anderson, Jr., "Faster and Higher: The Quest for Speed and Power," Milestones of Aviation, ed. John T. Greenwood (New York, NY. Hugh Lauter Levin, 1989), 78-147 (see p. 127).

85. AR46, p. 2.

86. "Compressible Flows in Aeronautics," p. 140. Becker, High-Speed Frontier, p. 35, sees this December 17,1944, Washington lecture as marking expansion of the NACA's research focus in transonics.

87. Dryden, "Faster Than Sound," p. 8.


aircraft in an organic rather than a modular way.88 For example, jet engines needed to be integrated into airframes specifically designed for the task. Much later the area rule, the transonic design principle described in chapter 5, grew Out of NACA research engineer Richard Whitcomb's integrated view of the whole aircraft, and was nurtured by his experiments in one of the original slotted-wall transonic wind tunnels —a tunnel he helped to commission and refine.

In 1948, however, NACA transonic researchers' tools were mainly research aircraft, rocket- and airplane-borne models, and a few partly effective, sometimes even makeshift adaptations of high-speed tunnels. Sonic progress had been made in designing jet-propelled warplanes. Loftin says that the P-80 (later F-80) Shooting Star climbed and flew faster than the first U.S. jet, the P-59, thanks to "a careful synthesis of weight, size, and thrust parameters, as well as close attention to aerodynamic refinement." In April 1948, a swept-wing F-86 reached supersonic speed in a dive. jet-propelled bombers were being developed.89 But judging by the summer 1948 responses of thirteen aircraft manufacturers, the Air Force, and the Navy to an urgent NACA survey, these efforts only helped stimulate more desire for transonic data—as well as interest in the research tools with which the data would be obtained.

The NACA aerodynamics committee's survey asked the agency's industrial and military clients how the NACA could best use its research tools to aid transonic aircraft design. The answer: numerous practical-minded requests for empirical data on wing planforms, airfoils, controls, and complete three-dimensional —that is, integrated whole— models, with secondary interest in air inlets, buffeting effects, pilot escape, bomb bays, and aircraft stability. The Air Force and eleven of the thirteen companies also addressed research tools and methods. One consensus recommendation called for increasing rocket-borne model tests by a factor of three. Another pleaded that "the NACA continue under as high a priority as possible the study, development, and procurement of test facilities for obtaining [transonic data] in a manner equivalent to that followed in the best available low-speed wind tunnel testing" —that is, in convenient, versatile, relatively cheap, and completely safe laboratory conditions. Of the fifteen respondents, only three even mentioned theory; one of these few, Benedict Colin of Boeing, urged that the NACA "obtain very fundamental data on the aerodynamics of transonic flow rather than attempt solutions of small specific items." Although the survey-sponsoring NACA aerodynamics committee formally agreed with the respondents' decidedly empirical majority view, it pointedly emphasized as well that the "NACA should also continue to give careful consideration to results of theoretical work."90

Indeed the aircraft industry and the military in the pressure of 1948 may generally have had little interest in theory. Dryden apparently gauged the military that way concerning wind tunnel theory, in any case. Even though Wright and Ward had translated theoretical ideas into a useful research tool, Dryden's October 1948 letters transmitting their paper to military research authorities carefully cautioned against letting the "considerable amount of background theoretical material ... obscure the practical significance of the

88. W. S. Farren, "Research for Aeronautics-Its Planning and Application," Journal of the Aeronautical Sciences 11, No. 2 (April 1944): 95-109, addresses the "integrated whole" idea; the phrase itself-appears in Edward P. Warner's appended remarks, p. 108. See also Loftin, Quest for Performance, p. 248. Concerning the preliminary sense of the idea in the 1930s, see the last paragraph of Stack's "Effects of Compressibility on High-Speed Flight, p. 12 in AR39, and Becker, High-Speed Frontier, p. 26.

89. Loftin, Quest for Performance, pp. 288, 295, 357.

90. "Summary of Recommendations on Research Problems of Transonic Aircraft Design, Compiled by Aerodynamic Research Branch, NACA Headquarters, for the Special Subcommittee on Research Problems of Transonic Aircraft Design," July 1948, Stack collection, LHA —where the thicker copy's extra appendix contains all the individual letter responses including Colin to NACA, July 26, 1948. Both copies contain the NACA aerodynamics committee's formal answer to the survey responses and recommendations.


work."91 Nonetheless the postwar NACA itself, insofar as it could, sought to stay mindful of the benefits that improved research tools would represent for aerodynamic theory in general —the obiter dicta benefits, in Joseph Ames's 1923 courtroom simile—and in turn of theory's benefits for aeronautical engineering. Stack elucidated the NACA's wind-tunnel-centered version of this awareness the following June. Studies of transonic flows with models sent skyward on rockets or dropped from high altitude, he wrote, "have defined fundamental problems of fluid mechanics. Experimentation with standardized equipment, nonexpendable models, under closely controlled conditions permitting detailed measurements"—that is, in wind tunnels— "still appears to be a most important key to progress toward the attainment of the ultimate goal, that is, successful complete calculation of such flows."92

Not that Stack himself had never exhibited a decidedly empirical outlook. In 1942, soon after leading the somewhat dramatic applied-research solution of the P-38 problem, he taught a University of Virginia night school postgraduate course called "Compressibility Effects in Aeronautical Engineering," held for Langley staff only. Without the usual NACA public relations constraints, his opening lecture proclaimed that he would "exclude, insofar as possible, the mathematical exercises which though elegant are frequently so meaningless to the engineer," and that he would try instead "to adhere more closely to the discussion of physical concepts, introducing mathematical methods only as necessary to aid in understanding the physical concepts.... I think that it is well if we realize in the beginning that in this field the engineer is leading the mathematical scientist. The present state is such that the engineer is projecting himself perhaps to some extent blindly into difficulties, and by physical reasoning without mathematical weakness"—the phrase that distills Stack's approach to transonics— "arriving at the expedient solution of his difficulties." But the course syllabus somewhat belied this energetic introductory emphasis on empiricism, citing the objective of covering "the fundamentals of compressible flows, the status of present knowledge on the subject, and its application to engineering problems," and naming "summary of significant theories" the subject of six of thirty-two scheduled hours —three hours each for the "subcritical range" and the "supercritical range." And indeed the opening lecture, once past the introductory remarks, did immediately invoke in some detail compressibility's fundamental fluid dynamics context.93 Two years later in Washington, Stack's 1944 Wright Brothers Lecture on compressibility mainly addressed experimentation, but it too rested distinctly within a scientific, theoretical context. Here is what we have done, that lecture said, in circumstances where little prospect has existed for advancing theoretically. This leading NACA aeronautical research engineer primarily sought near-term physical understanding, but secondarily, and for practical ends in the longer term, he wanted to see it attained by "physical reasoning without mathematical weakness" within the formal scientific realm of fundamental understanding.

In 1951, W. E. Hilton reemphasized the long-standing common belief that whatever theory's long-term potential, it held little near-term prospect for advancing transonics.94 Hugh Dryden, however, maintained a formally scientific outlook about transonics in the

91. October 8, 1948; copies in RA70 folder "Research Authorization 70," LHA.

92. Typescript of "Methods for Investigation of Flows at Transonic Speeds: Paper for Presentation at the Naval Ordnance Laboratory Aero-Ballistics Research Facilities Dedication Symposia, June 27-July 1, 1949," pp. 13 and 14, in Stack collection folder of the same name, LHA. This passage reappears verbatim on p. 573 of Stack's 1951 revision of the 1949 paper, "Experimental Methods for Transonic Research," pp. 563-592j, Third Anglo-American Conference 1951, in a Stack collection folder labeled with the revised paper's title. The 1951 version was obviously published; the citation for reference 54 on p. 172 in Becker, High-Speed Frontier, says the 1951 conference took place September 7-11 in London.

93. See materials in the Stack collection folder "Defense School 1942," LHA. Quotations from pp. 1 and 2 of typescript "Introduction, Orientation, and Summary."

94. High-Speed Aerodynamics. See especially the preface and p. 9.


late 1940s anyway. As an employee of the National Bureau of Standards, he had served on and helped lead the NACA aerodynamics committee since 1931. During the war he managed a large guided-missile research and development project for the military. 95 After the war he was deeply involved in NACA transonics as high-speed aerodynamics committee chairman. In September 1947 he joined the NACA staff and took over from George Lewis as director of aeronautical research, a title shortened in 1950 to director.96 His scientific outlook on transonics was an extension of his general view that the "discovery of how to make better aircraft results from the discovery of rational theories firmly supported by experimental evidence." 97 At Langley in early 1947, he chaired a conference on high-speed, aerodynamic theory attended by luminaries including Theodore von Kármán, who briefly summarized the state of compressible flow theory, and Tsien Hsue-shen, known for later famously leaving the United States and leading China's development of missile technology After the theory conference Dryden reported, apparently with some disappointment, that despite worthwhile exchanges between theorists and experimentalists, "the hoped-for result of a rather concrete definition of the direction which future theoretical research in the field should take was not achieved."98 But this veteran of early experiments with open transonic jets of air could also adopt the outlook of Stack and other practical-solutions-seeking NACA experimentalists and say that "progress in those aspects of aeronautics for which a rational theory has not yet been developed proceeds by the recognition of the common features of complex flow patterns." 99

Thus it was that in engineer Stack at Langley and in physicist Dryden in Washington, the early postwar NACA had leadership well suited for fostering conception, development, and practical application of the slotted-wall transonic wind tunnel. Each had extensive personal experience in practical-solutions-oriented transonic experimentation, but each also understood and genuinely valued the formal fluid dynamics context. From long membership, each knew and had confidence in the NACA technical culture with its accumulated technological and scientific understanding and its highly developed tradition of aeronantical research craftsmanship. In such a setting Stack could follow his intuition concerning physicist and applied mathematician Ray Wright's theoretical ideas, and Wright, in the words of historian Hansen, could benefit "from the collective knowledge and experience of the engineers working around him" and from his own "good intuitions."100

Intuition was important. Hilton in 1951 called transonic aircraft design "more a product of trained intuition than the result of applying exact scientific principles."101 Instances of similar intuition pervade NACA research history, according to historians of the three laboratories existing or begun by the time of Pearl Harbor. Such instances also pervade NACA transonic research history. Hansen makes intuitive technological artistry the theme of "The Slotted Tunnel and Area Rule," chapter 11 in his Langley history.102 Becker

95. Richard K. Smith, The Hugh L. Dryden Papers: A Preliminary Catalogue of the Basic Collection (Baltimore, MD: Milton S. Eisenhower Library, Johns Hopkins University, 1974), p. 23.

96. Roland, Model Research, 1:247, 2:713, and 2:490.

97. See Dryden's addendum to W. S. Farren, "Research for Aeronautics-Its Planning and Application," Journal of the Aeronautical Sciences 11, No. 2 (April 1944): 95-109 (see p. 106).

98. Minutes, "Informal Conference on High-Speed Aerodynamic Theory," February 3, 1947, and p. 2, Minutes of Meeting Of Subcommittee on High-Speed Aerodynamics," April 29, 1947, in Stack collection folders "Conf. on High-Speed Aerodynamic Theory" and "Subcommittee on High-Speed Aerodynamics 1946-1948," LHA.

99. Remarks appended to Stack, "Compressible Flows in Aeronautics" (see p. 145).

100. Engineer in Charge, p. 318.

101. High-Speed Aerodynamics, p. 9.

102. Hansen, Engineer in Charge. See also Elizabeth A. Muenger, Searching the Horizon: A History of Ames Research Centey, 1940-1976 (Washington, DC: NASA SP-4304, 1985), p. 39, and Virginia P. Dawson, Engines and Innovation: Lewis Laboratory and American Propulsion Technology (Washington, DC: NASA SP-4306, 1991), p. 74.


believes that what motivated "initiation of in-house NACA research in high-speed aerodynamics" in the first place was intuition, not the "great foresight" Stack mentioned in his 1944 Wright Brothers Lecture. In that lecture Stack said that he and his colleagues devised the first NACA schlieren flow-visualization apparatus in early 1933 when they "had in the airfoil experiments temporarily exhausted [their] intuition as regards methods for improving aerodynamic shapes."103 Stack's 1952 Collier press release says that in large part his and his colleagues' "faith in the probability of a solution" in 1946 had rested in Wright's "Subsonic high-speed theoretical studies,"104 a statement about acting on scientifically trained intuitive faith that calls to mind Stack's 1942 classroom remark about an engineer's "projecting himself perhaps to some extent blindly into difficulties, and by physical reasoning without mathematical weakness arriving at the expedient solution of his difficulties. " In December 1948, following the award of the X-1 Collier to Stack, Bell, and Yeager, the opening lines of a Washington Post article set forth a version of Stack's philosophy on the relation between intuition and technological success:

"Intuitive research" brought flight many months ahead of schedule, states the man most responsible. John Stack, designer of the first plane to fly faster than sound, says that "believing what - you couldn't prove and trusting it " paid off by speeding up normal scientific processes. He puts it this way: "You say to yourself, if these things are true, then this must be true. You haven't an exact answer but you do have an intuitive answer. So if you want to make a big step forward, you take a chance of falling flat on your face and trust your intuition. " 105

Surely at that moment in late 1948 —two weeks after Bernhard Goethert's formal visit concerning Langley's modest initial proof of the slotted-wall principle— Stack must have had at least partly in mind the chance of falling flat on his face not with the X-1, already proven in the sky, but with the slotted wall, as yet proven only in miniature. He later said there had been "no turning back" once a construction contract had been signed earlier in 1948 for installing a slotted wall in Langley's huge 16-Foot High-Speed Tunnel.106 And surely a reason for trusting his intuition was the NACA itself, a technical culture with broad general experience building wind tunnels and long-standing specific experience replicating high-speed flow fields in some of them.

Precisely Defining the Transonic Tunnel Problem

Although the NACA had been accumulating understanding of the difficulties of replicating transonic flow fields since the 1920s, the overall problem was apparently not comprehensively defined anywhere until the mid-1940s. Even in 1947, the textbook Wind-Tunnel Testing could only note somewhat vaguely that the "proper procedure for testing and correcting the results of high-speed tests has not been completely established" and that it "appears that the accentuated blocking and the shock-wave reflection off the tunnel walls contribute to the uncertainty."107 NACA translations of European papers partially addressing the difficulties had been available since the mid-1930s to augment Langley's own growing understanding. In 1935, for instance, Swiss supersonics

103. Becker, High-Speed Frontier, p. 13; Stack, "Compressible Flows in Aeronautics," pp. 128 and 130.

104. Reprinted in the Langley Air Scoop, December 19, 1952, available LHA.

105. "Intuition Brought Supersonic Flight," Washington Post, December 21, 1948.

106. Langley Air Scoop, December 19, 1952, available LHA.

107. Alan Pope, Wind-Tunnel Testing (New York, NY: John Wiley & Sons, Inc., 1947), p. 207.


expert Jakob Ackeret discussed the blocking effects of test models on tunnel capabilities near Mach 1, contrasted the near-sonic-speed performance of open jets of air with that of airstreams enclosed within solid tunnel walls, and noted shock wave reflection in tests at low supersonic speed —problems addressed also in a 1938 paper by Italian aerodynamicist Antonio Ferri, whose own accumulated understanding about solving them was put to good use when the NACA managed to import him at the end of the war. 108 In November 1943 the Army formally requested that the NACA define the overall problem. A preliminary report of special work in the 24-Inch High-Speed Tunnel ensued in short order, for apparently the work had begun in advance; Stack had even discussed its main conclusions at an October meeting of the NACA aerodynamics committee in Washington. The Army asked for copies of the preliminary report to send to aircraft manufacturers including Douglas Aircraft Corporation, Curtiss-Wright Corporation, General Motors, and Northrop. Langley's Robert W. Byrne completed a full technical report during 1944, "Experimental Constriction Effects in High-Speed Wind Tunnels,"109 one of several NACA studies to define the problems of replicating transonic flow fields in a tunnel, and the one Stack customarily cited retrospectively in later years.

One such study was that 1944 theoretical one at Ames Laboratory by Allen and Vincenti: "Wall Interference in a Two-Dimensional Flow Wind Tunnel, with Consideration of the Effect of Compressibility," the kind of analysis Stack first called for in his 1933 NACA report about the eleven-inch tunnel. Allen and Vincenti may not have directly influenced Ray Wright, but fifty years later their report remained useful for technical study. 110 For its comprehensive explanation of the fundamental problems of closed-wall wind tunnel operation at transonic speeds, it also remained useful for historical study.

The paper begins —as Stack's 1933 report had begun— by alluding to two numerical indicators aerodynamicists use, among other purposes, to score the similarity of a wind tunnel's flow field to an actual flow field aloft: "The need for reliable wind tunnel data for the design of high-performance aircraft has led in recent years to attempts to make the conditions of tunnel tests conform more closely with the conditions prevailing in flight, especially with regard to the Reynolds and Mach numbers." Reynolds number combines measures of an aerodynamic object's size and of its flow field's density, speed, and viscosity into a simple ratio expressed as a whole number. Ideally in a test with a scale model, this score should be high enough to conform with that of the simulated full-size airplane or component in its actual flight conditions. But most wind tunnel tests mismatch the fullsize value of the Reynolds number by using a model of considerably reduced scale. The

108. NACA Technical Memorandum 808, "High-Speed Wind Tunnels," November 1936 translation of a paper Ackeret presented at the Fifth Convention of the Volta Congress, Italy, September 30 to October 6, 1935; see Part B, "Wind Tunnels for Subsonic Velocities." NACA Technical Memorandum 901, "Investigations and Experiments in the Guidonia Supersonic Wind Tunnel," July 1939 translation of a paper Ferri presented in Berlin in October 1938. Stack's early-1960s professional biography ("Awards and Biographical Information" folder, Stack Collection, LHA), p. 4, asserts that Stack "initiated action" that led to the postwar importation of both Ferri and Adolf Buscinann, the German credited with suggesting in the 1930s the benefits of swept wings; see also Hansen, Engineer in Charge, pp. 318-20.

109. "Minutes of Meeting of Committee on Aerodynamics, October 12, 1943," pp. 21 and 22, in Stack collection folder "Committee for Aerodynamics Minutes 1643," LHA. In the two folders constituting the RA1204 file, LHA, are "Preliminary Data for Army Air Forces, Material Command: Constriction Effects in High-Speed Tunnels," January 31, 1944, letters requesting copies of the preliminary data report for manufacturers, and the official January 8, 1944, research authorization document, which cites a November 16, 1943, Army request. Stack's October synopsis and the January preliminary report directly echo Byrne's Advance Confidential Report L4L07a, December 1944.

110. Joel L. Everhart and Percy J. Bobbitt, "Experimental Studies of Transonic Flow Field Near a Longitudinally Slotted Wind Tunnel Wall," NASA Technical Paper 3392, April 1994, cites in its introduction H. Julian Allen and Walter G. Vincenti, NACA Report 782.


low-speed Variable-Density Tunnel counteracted this mismatch by using pressurized air, which of course meant higher air density in the flow field, and therefore also a higher density term in the ratio—which in turn meant improved verisimilitude as indicated in the higher score. Another way to raise a test's Reynolds number is simply to diminish the mismatch by using a larger-scale model. In fact, in that way the old Propeller Research Tunnel and the Full-Scale Tunnel simply canceled the mismatch: they were large enough for tests not at reduced scale, but at full size. The second verisimilitude indicator, Mach number —a shorthand term not yet used in 1933 by Stack, who still called it compressibility factor— compares flow speed with the speed of sound for the given conditions. It leads to a simple ratio too: for example, Mach 0.8 for a speed eight-tenths that of sound. In subsonic tunnels, the fundamental physics of Englishman Osborne Reynolds had long framed the problem of achieving flow similarity; for tunnel airflows involving compressibility, the physics of Austrian Ernst Mach now required attention as well.111

However, because of "practical limitations in size and power," Allen and Vincenti continued, "most existing wind tunnels, whether high speed or low speed, are not capable of providing full-scale Reynolds numbers for all flight conditions." Their readers would not need reminding that to enlarge a tunnel's airflow channel size for larger models, and thus for higher Reynolds numbers, or to increase its airflow speed for higher Mach numbers, leads with exponential quickness to a prohibitively expensive power bill—assuming enough power is available at all. An obvious partial answer, the authors said, was to rise as large a model as possible in a given tunnel. But in the case of a high-subsonic-speed tunnel, the larger the model, the more magnified the problems of testing it. As Mach number rises, there is a "tendency of the [compressible] flow pattern .... if unrestrained, to expand." But since the tunnel walls indeed do restrain expansion of these streamlines of flowing air, the resulting test data need correcting—that is, need artificial adjustment by some formula or mathematical procedure—"if they are to be applied with confidence to the prediction of free-flight characteristics." This analysis led Allen and Vincenti to the centrally important issue of correcting results from solid-wall tunnel tests at the still higher subsonic Mach numbers where the complication known as choking arises—the problem that Aviation Week later reported "had effectively bottlenecked" transonic tunnels until NACA researchers "licked" it by inventing slotted walls.112

Concerning choking, Allen and Vincenti's readers would recall a fundamental airflow-physics principle: subsonic air moves faster when its channel constricts, but a supersonic airstream must expand to go faster. A test model, by constricting the channel, creates in effect the nozzle of a supersonic tunnel: a convergence of the flowing air followed by a divergence. The result is that "sonic velocity is reached at all points across a section of the tunnel at the position of the model, and the flow in the diverging region downstream of this section becomes supersonic. When this occurs, increased power input to the tunnel has no effect upon the velocity of the stream ahead of the model, the additional power serving merely to increase the extent of the supersonic region in the vicinity of the model. At this point the tunnel is said to be 'choked' and no further increase in the test Mach number can be obtained." That is, choking cannot be overcome by brute force, and for a

111. I am grateful to veteran NACA and NASA aeronautical engineer Albert L. Braslow for suggestions about this passage and much else in the essay. Concerning flow similarity, in 1934 in NACA Report 492, "Tests of 16 Related Airfoils at High Speeds" — a "classic paper which exerted great influence'' according to W F. Hilton (p. 81, High-Speed Aerodynamics)— John Stack and Albert E. von Doenhoff wrote: "It has been shown that the speed of flow expressed in terms of the speed of wave propagation, or the speed of sound, in the fluid is an index of the extent to which the flow is affected by compressibility. Thus, the ratio of the flow velocity to the velocity of sound, V/Vc, is a parameter indicative of the pattern similarity in relation to compressibility effects just as the Reynolds number is air index of the effects of viscosity."

112. "NACA Tunnels Bare Secrets of Transonic," Aviation Week, May 28, 1951, p. 13.


given model and solid-wall tunnel, the choking Mach number is the speed limit. Moreover, the authors' theoretical analysis confirmed what had been long suspected,113 that "at the choking Mach number, the flow at the airfoil in the tunnel cannot correspond to any flow in free air. It follows that, at choking, the influence of the tunnel walls cannot be corrected for. Further, in the range of Mach numbers close to choking where the flow is influenced to any extent by the incipient choking restriction, any correction for wall interference may be of doubtful validity." In other words, once very near or at choking speed in a solid- wall tunnel, there is no translating the test data into usefulness, for these results do not correlate with actual flight conditions, not even in some hidden way.

In the end, the point was that only very small models —with very low Reynolds numbers, and thus with little verisimilitude— could be tested at near-sonic speeds in enclosed, solid-wall tunnels. Bernhard Goethert once cited an illustrative case involving a complete three-dimensional model rather than the tunnel-spanning "two-dimensional" case Allen and Vincenti addressed. For test speeds up to Mach 0.95, the model could be large enough to block head-on only one-fifth of one percent of the tunnel's airflow. This meant it could have "a maximum diameter of no more than 5.5 inches in a 10-foot-diameter wind tunnel" —a relative size like that of a softball inside a transport airplane fuselage. "It is apparent," Goethert concluded, "that transonic testing in a closed wind tunnel is very impractical." 114

Ray Wright, Principal Agent of a Collective Solution

A 1994 NASA Langley technical paper identifies the ultimate source of the slotted-wall solution that NACA Langley devised for the transonic tunnel problem in the late 1940s: "The first 30 years of wind tunnel wall-interference research yielded an important fact for modern wind tunnels; that is, theoretically and experimentally, solid-wall corrections are opposite in sign front those of open-jet test sections. Thus, if a wall is partially open, an adjustment to the geometric openness should be possible to obtain a near-zero wall-interference correction and thereby allow a more realistic simulation of free-air conditions."115 In even plainer terms, ventilation openings placed in Just the right way in a tunnel's walls can cause the complex data-polluting effects of open-wall and closed-wall interference to cancel each other. The statement echoes similar ones by Becker, Stack, and Wright and Ward. It also echoes Goethert, who had served in the Nazi-era German aeronautical research establishment, and whose 1961 book asserted that Germany, Italy, and Japan "produced theoretical correction-free slot arrangements" but failed actually to build slotted tunnels for high-speed compressible flows only "because of the circumstances connected with and following World War II."116 In different circumstances possibly the NACA could have found the solution earlier itself, though certainly there was no prewar call for it from industry or the military. In any case, Becker says that early experience with open jets in the eleven-inch tunnel "more than any other single factor encouraged Stack and his cohorts 15 years later to embark on the further developments which produced the transonic slotted tunnels," and that "Stack often referred to this early work as the genesis of transonic facility development."117

113. Becker, High-Speed Frontier, p. 66 says that the NACA by 1938 had begun to see that "there was no hope of 'correcting' data taken in the choked condition."

114. Goethert, Transonic Wind Tunnel Testing, p. 49.

115. Everhart and Bobbitt, "Experimental Studies of Transonic Flow Field Near a Longitudinally Slotted Wind Tunnel Wall," p. 1.

116. Becker, High-Speed Frontier, pp. 38, 98, 100, 114; Stack, "Experimental Methods for Transonic Research," p. 592a; Wright and Ward, pp. 1, 2; Goethert, Transonic Wind Tunnel Testing, pp. 21, 22.

117. Becker, High-Speed Frontier, p.65.


At war's end two tunnel-technology studies in particular helped motivate Langley's translation of this open-closed idea into a specific proposal for longitudinally slotted walls: Antonio Ferri's high-speed tests in an Italian semi-open tunnel, presented in a report he wrote upon arriving at Langley, and Coleman duP. Donaldson's comparisons of open and closed high-subsonic-speed airflows, presented in a report Ray Wright wrote after Donaldson left for military service. Ferri investigated the performance of a rectangular test section of about sixteen inches by twenty-one inches, with solid side walls but no top or bottom to restrain the airstream. Becker calls the work "the first real demonstration that partly open arrangements could be used successfully" near Mach 1, and says it helped motivate the Donaldson study. Donaldson tested a postage-stamp-sized airfoil in both open and closed three-inch-wide jets of compressed air, much as Dryden and others had done with small open jets in the 1920s-only this time under genuine laboratory conditions, with good instrumentation for taking data. Donaldson's tests were intended generally "to show the nature of the jet-boundary interference" in both the open configuration up to Mach 1 and the closed configuration up to choking at just under Mach 0.8. Donaldson concluded that open jets "should be advantageous for tests at high Mach numbers." Becker later wrote that this study helped spur Langley's conversion of a small high-speed tunnel to the semi-open configuration. Stack later wrote that it served "to show, in principle, the possible difference in choking limitations for open- and closed-throat tunnels." Thus it was that Ray H. Wright, the man who committed Donaldson's study to paper, entered the year 1946 fully mindful of this crucial difference for the laboratory replication of compressible flow fields up to Mach 1. 118

The question of what Ray Wright was mindful of in 1946 is important for two reasons. The less important one has to do with proportioning credit for the slotted-wall transonic tunnel. The more important one has to do with assessing the effectiveness of the NACA technical culture.

Both Baals, in Wind Tunnels of NASA, and Hansen have portrayed Wright as having a solely subsonic and somewhat technically naive outlook in proposing the longitudinally slotted wall that year. "Strictly speaking, Wright's analysis was applicable only to low-speed flows," Baals wrote, "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." This interpretation conflicts not only with the story as Becker tells it, but with the record of Wright's activities up to 1946. Becker portrays Wright exercising both technological initiative and scientific imagination in an effort purposefully targeting the wind tunnel replication of transonic flows. That Wright's theoretical work happened to be subsonic, Becker says, simply derived from the constraints of the available mathematical techniques. 119

But it is Wright's formative activities at Langley during the decade leading up to 1946 that really matter for they show that Wright, like Stack, was a genuine product of the

118. See Becker, High-Speed Frontier, pp. 39,79, and 99, and Stack, "Experimental Methods for Transonic Research," p. 580, concerning Antonio Ferri, "Completed Tabulation in the United States of Tests of 24 Airfoils at High Mach Numbers (Derived from interrupted Work at Guidonia, Italy, in the 1.31- by 1.74-Foot High-Speed Tunnel)," ACR L5E21, June 1945 (also called WR L-143) and concerning Ray H. Wright and Coleman duP. Donaldson, NACA Technical Note 1055, "Comparison of Two- Dimensional Air Flows About an NACA 0012 Airfoil of 1-Inch Chord at Zero Lift in Open and Closed 3-Inch Jets and Corrections for Jet-Boundary Interference," May 1946 (but actually, and significantly for Ray Wright's education, completed in early January, according to p. 34). Becker, High-Speed, Frontier, pp. 79 and 99, treats Donaldson as the latter's main author, even though Wright's name appeared first in the heading. Donaldson described his and Wright's contributions in an April 11, 1996, telephone interview.

119. Baals, Wind Tunnels of NASA, p. 61; Hansen, Engineer in Charge, pp. 316, 317; Becker, High-Speed Frontier, chap. III, especially pp. 99-104; see p. 100 concerning the mathematics.


NACA technical culture, and in the case of the transonic wind tunnel, its important agent. Less importantly, these activities also demonstrate his entirely sophisticated awareness of his slotted-wall proposal's implications. In the late 1930s, he worked alongside 8-Foot High-Speed Tunnel designer and veteran high-speed research engineer Russell G. Robinson on the air foil design problem of delaying the compressibility burble, building on work going back to the 1920s. By the end of the war he was working on wall interference in the eight-foot tunnel, which was being repowered for sonic speed, and he helped establish a new, minimally flow-field-disrupting method for holding its test models in place —a system first used in 1946 tests of research airplane models including the X-1. By early 1946 he had written up Donaldson's comparative investigation of transonic open and closed boundary effects, which linked directly to what he was about to propose. Thus the pre-1946 activities of physicist and applied mathematician Ray H. Wright constituted something like an apprenticeship in the engineering art and science, such as they then stood, of transonic Wind tunnel testing. But by far the most revealing formative activity of this soon-to-be agent of accumulated NACA understanding took place in August 1946, when he wrote a memorandum. 120

Wright's lengthy, detailed memorandum to Langley's compressibility research chief advocated synthesizing what the NACA already knew about high-subsonic and near-sonic wind tunnel research. "As a result of work on wind-tunnel interference and of other experiences gained over the past several years," it began, "ideas and information have been accumulated for a number of useful report projects that could be carried out with a minimum of time and effort." The point was to assess the organization's corporate store of technical and scientific knowledge about transonics up to Mach 1, and to determine how to exploit it —at minimal expense, and with the practical goal of improved research capabilities. For each of sixteen possible report project topics, Wright wrote a paragraph-length synopsis drawing on his overall awareness of existing NACA work. The topics included "general consideration of the effect of compressibility on wind tunnel interference," "wind tunnel interference at Mach numbers greater than the critical," and "flow conditions and tunnel-wall interference near choking." To be based on these three in particular, together with another closely related three, he projected among the sixteen prospective projects a "general report on wind tunnel interference at high speeds," for which a "considerable amount of material [was] already in existence" that he said "should be compared, sifted, and collated."

In this transonics-focused memorandum Wright also suggested precisely the famous project in which he himself was apparently already engaged: "wind tunnels with zero or negligible interference." For this one the accompanying synopsis is the memorandum's lengthiest, amounting to a prospectus for the theoretical and experimental work that would lead to slotted-wall tunnels. Thus it also amounted to a plan for finally realizing Langley engineers' long-held intuitions about an open-closed solution to the transonic tunnel problem. To circumvent the difficulties of high-speed wall interference, it said, "as well as to prevent choking, the wind tunnel may be so designed as to minimize the interference. If the interference can be entirely prevented, the obtaining of model data can be simplified by abolishing the necessity for making tunnel-wall corrections." The tunnel would use "an automatically compensating method" of "multiple-sided open-closed test sections." Mathematical techniques, Wright wrote, were "available for investigating this problem," and if a "mathematical investigation indicated a probability of success," small-scale, principle-proving model wind tunnels "incorporating the automatically compensating features should be designed and tested. The possible usefulness of such an investigation,"

120. Becker, High-Speed Frontier, pp. 27, 74, 75, 99; "Memorandum for Chief of Compressibility Research Division: Possible Report Projects that Could be Completed with a Minimum of Time and Effort," August 27, 1946, signed "Ray H. Wright, Physicist," in Stack collection folder "Research Problems & Questions (Reid's trip to Europe) 44-46," LHA.


added the technologically sophisticated, NACA-engineer-trained physicist, "suggests that it should be carried out as soon as personnel can be spared. Only a bare start has been made on the calculations."

Stack's 1952 press release crediting the slotted-wall contributions of his nineteen associates begins by describing his "old written notes" from 1946 showing that "for some time" he and others had had a "faith in the probability" that a transonic tunnel solution was in hand, that "a good part" of this faith "rested in the subsonic high-speed theoretical studies of Ray H. Wright," and that "in the late summer of 1946" the arrangements began for the small proof-of-principle pilot project that Vernon G Ward spearheaded. 121 How or even whether these notes relate to Wright's August 1946 memorandum is not clear, but it is clear that Wright comprehensively understood the problems of replicating transonic flows up to Mach 1, and that much of his ability to contribute importantly to their solution derived directly from a formative decade of immersion in the technical culture around him.

Photo Physicist Ray Wright wears a diving suit to protect himself while taking readings next to the operating slotted-tunnel.
Physicist Ray Wrights decade-long immersion in the practical-solutions seeking NACA engineering culture prepared him to propose a workable theoretical solution for the transonic wind tunnel problem. Later, his participation in the solutions hands-on realization in the 8-Foot High-Speed Tunnel required immersion in the harsh conditions that slots caused in the chamber beside the formerly entirely enclosed test section. To withstand these conditions, Wright wore a diving suit. (NASA photo L-64110).

121. Langley Air Scoop, December 19, 1952, available LHA.


Relatively soon, NACA Langley developed slotted walls well enough to apply them in two national wind tunnel facilities, all under the general guidance and technopolitical shepherding of Stack, who according to Becker "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." The newly converted tunnels were valuable; Loftin says they "provided a new dimension in transonic testing."122 But like other useful research tools, they were imperfect too. NACA advertising notwithstanding, difficulties persisted between Mach 0.98 and Mach 1.05, part of the range from Mach 0.95 to Mach 1.2 that the NACA's 1948 survey participants had unanimously agreed was where "the real fundamental lack of information occurs." The difficulties remained in slotted-wall transonic tunnels even a half-century later. In the eight-foot tunnel in 1950, Langley engineers spent months making improvements to the initial slotted-wall installation.123 For example, Richard Whitcomb remembers coordinating directly with Langley woodworkers to devise an apparatus at the downstream end of the test section to reintroduce the air that had gone through the slots —an efficient, focused, red-tapeless way of working that he says became "totally verboten" before he retired. By 1953, Langley high-speed researchers had commissioned a new eight-foot tunnel, this time with slotted walls planned from the outset, and with other improvements including pressurization at two atmospheres for higher Reynolds numbers, a test section designed for easier data-gathering, and modifiable slot shapes.124 History, or public relations, might momentarily have highlighted the original two slotted-wall tunnels, but transonic research questions continued to arise, and NACA researchers like experimentalist Whitcomb continued devising research tools for answering them.

Over the years, though, NACA researchers tended not to advertise their research tools, possibly contributing to the Collier Trophy's tardiness in recognizing a wind tunnel. Roland says that even though research tools were among the NACA's chief accomplishments from the time of the Variable-Density Tunnel, NACA research director George Lewis feared sharing information about them with the NACA's competitors. Possibly this secrecy has exacted a cost in the understanding not just of the research tools, but of the technical culture from which they derived. To explain the secrecy, Lewis once compared NACA research tools to Stradivarius violins. "Antonio Stradivari," he wrote, "made a success by making the world's finest violins, and not by writing articles on how others could construct such instruments. "125 But Stradivari could only have learned to make such fine instruments where he did learn: among the Cremonese masters, a technical culture whose corporate technical memory, scientific understanding, and shared traditions of craftsmanship 126 enabled its members to build devices that move air in just such a way as to produce beautiful music. Much the same can be said for the technical culture of the NACA, where engineers —and engineering-minded physicists— learned to build devices that move air in just such a way as to produce useful knowledge.

122. Becker, High-Speed Frontier, p. 109; Loftin, Quest for Performance, p. 252.

123. Becker, High-Speed Frontier, p. 113; "Summary of Recommendations on Research Problems of Transonic Aircraft Design," p. 8 in "Report of Special Subcommittee" section; p. 18, Francis J. Capone, Linda S. Bangert, Scott C. Asbury, Charles T. L. Mills, and E. Ann Bare, "The NASA Langley 16-Foot Transonic Tunnel: Historical Overview, Facility Description, Calibration, Flow Characteristics, and Test Capabilities," NASA Technical Paper 3521, September 1995; Hansen, Engineer in Charge, p. 110.

124. Richard Whitcomb, telephone interviews, April 1 and 19, 1996.

125. Roland, Model Research, 1:246; Dawson, Engines and Innovation, p. 32.

126. Thomas Levenson, "How Not to Make a Stradivarius," The American Scholar 63, No. 3 (Summer 1994): 351-78, describes Stradivari as "essentially a craftsman of science, one with considerable, demonstrable knowledge of mathematics and acoustical physics," who attained his skills in an instrument-making culture of "old masters" with a science-based "accumulation of craft technique" in Cremona, Italy.

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