The Slotted Tunnel and the Area Rule


[311] Many of our greatest technological artifacts are themselves fine art: the Great Pyramid at Giza the Chartres cathedral, the Brooklyn Bridge. The engineers who designed these structures applied scientific principles, but they also used practical skills, cleverness in contrivance, and an innate sense of aesthetics. Until the Industrial Revolution, western societies recognized and appreciated the vital role oh art in their technologies. With the arrival and widespread use of awesome new machines like the steam engine in the eighteenth century, however, more and more people in Europe and America began to assume, incorrectly, that the only thing that was going into engineering design and invention was science - a type of knowledge theretofore considered too complex, abstract, and even dangerous for the average person to comprehend, let alone command. Although science and mathematics together played an increasingly important role in engineering from the Italian Renaissance its the fifteenth century on S truth artistic creativity continued to fix many of the outlines and fill in many of the details of our material surroundings through the industrial Revolution.1 It was just harder to spot amidst all the operations of modern applied science. Today the mind's eye remains one of the most essential organs of technological creation. Visual conception and imagination help to shape everything from the next model Buick Skylark to the next generation IBM personal computer.2

Aerodynamic research also involves artistry. The mind's eye made important contributions to the success of various major. programs at the NACA laboratory, particularly the design of the airfoil and cording families in the late 1920s and 1930s. After World War II, the most outstanding examples of artistry at Langley involved the design of the first transonic tunnel, whose key component was slotted-throat test action, and the discovery of the area rule, a new concept in the shaping of high-speed [312] aircraft. These two achievements by Langley researchers were products of intelligent guesswork, reasoning by intuition, and cut-and-try testing as much as product of numerical systems analysis, parameter variation, or theory. Both the slotted tunnel and the area rule derived largely from pictures in the mid. In a book about engineers in charge, this chapter will explore how visual. images charged engineers.


Model Support


After calling for a transonic research aircraft in the early 1940s, Langley researchers had continued to grapple doggedly with the choking problem of their wind tunnels. They persisted even after procurement of the XS-1 and D-558-1 was assured in 1945. It would have been folly for them to have done otherwise, since there was no assurance that the research airplane program was going to provide the unique kind of new data about transonic aerodynamics that the military services, the aircraft manufacturers, and the NACA itself required. Moreover, John Stack and his associates were die-hard wind tunnel advocates anyway, by nature predisposed to go after the choking problem of the conventional closed-throat tunnel, the problem that had led to the concept of the "sound barrier" in the first place. In the minds of Stack's team, the research airplane was a stopgap superior to drop bodies and rocket models, but a stopgap nonetheless; they would have preferred a Solution to the transonic impasse involving some discovery about the imperfect nature of their own precious ground-based type of facility.3

The first way that Langley researchers discovered to minimize the tunnel choking problem was the small-model technique.* By early 1944 choking data correlated from hundreds of previous tests in the lab's various high-speed tunnel: made it clear that the range of choked-out airflow speeds was primarily a unction of the ratio of the cross-sectional area of the test model to that of the tunnel. Experiments demonstrated that if the lab reduced the size of its models correctly to one-tenth of one percent of the tunnel throat area, its high-speed tunnels would still choke, but at approximately Mach 0.95 instead of 0.80. The range of speeds unobtainable in wind tunnels would be substantially narrower.4



Strut-supported model of Bell XS-2 in 7 x 10-Foot Tunnel, 1946.

The strut system used conventionally into the 1940s to support models in large wind tunnels disturbed the airflow so much. that many test results were questionable, especially at higher Mach numbers. In this photograph from August 1946, a strut-supported model of the Bell XS-2 is being tested in the 7 x 10-Foot Tunnel.


Langley's experts new that it was necessary to come up with a correct model support system if the choking range of tunnels was to be narrowed in actual practice with the small-model technique, since, when a smaller model was used, the struts used conventionally to support a model in a test section would contribute more to the choking of the airstream than would the small model itself. These struts were large, asymmetric, and usually attached directly to the forward part of the model surface; they caused local accelerations and changes in the alignment of the flow relative to the model that could not e corrected by any known method of determining support interference. I sum, this meant that test data at the higher Mach numbers were questionable.

Even before the advantages of the small-model technique were verified, experimentally and expressed in an NACA report, John Becker, head of the 8-Foot High-Speed Tunnel section, was working to develop a new model support system that would eliminate the interference effects and thus permit wind tunnel testing at higher Mach numbers. In 1943, Becker's division chief, John Stack, had gotten the NACA's approval to repower the 8-Foot HST from 8000-horsepower to 16,000-horsepower drive for operation at higher subsonic speeds (Langley had designed the tunnel in 1934 for Mach numbers approaching 0.8.) Becker knew that the conventional strut support....



Center-plate-supported model of Douglas D-558 in 8-Foot HST, 1947.

Center-plate support for a model of the Douglas D-558 reduced airflow blockage during this test in the 8-Foot HST, June 1947.


.....system would no work in the repowered tunnel because of its choking limitation. After considering a number of alternative types of new support arrangements, Becker thought to suggest symmetry as the key to a practical solution. In the summer of 1944, he went to Stack and told him about his idea for a center-plate support. This support would consist simply of a long thin vertical plate mounted across the tunnel diameter and attached to the floor and ceiling of the test section, Becker said. Wing models would be mounted in the plate's plane of symmetry, half spans protruding from each side, to reduce blockage of the airflow.

Stack decide to have the new type of model support installed in the 8-Foot HST while it was shut down for repowering. When this tunnel began operations with its new 16,000-horsepower drive in the spring of 1945, it had a center plate. Langley now had a ground-based facility that provided reliable data to above Mach 0.9. The first models tested on the center plate of this facility represented wing and tail configurations under consideration by the Army Air Forces as design components for its first high-speed jet bombers.5

The center-plate support proved particularly useful for studying the high-speed aerodynamic forces and pressures affecting isolated wings; it proved unadaptable, however, for investigating the performance of wing and body combination and complete aircraft configurations. What was needed [315] to investigate the performance of these more detailed shapes was a sting support system. With this system there was less interference: the model was supported from behind by a rod protruding forward from a vertical strut downstream of the test section, instead of from below by a strut intruding in the airstream of the test section.

Langley had tried stings before 1944, but it had done so for reasons other than to increase the Mach number at which a wind tunnel choked. But these stings contribute just as much to flow blockage as the conventional strut supports did, if for a different reason. Beginning in late 1944, a group of engineers in the 8-Foot HST led by Eugene C. Draley (Becker had since become head of the 16-Foot HST) began designing a new sting support system. Their specific Intention was to eliminate the source of the choking problem of the earlier stings: the large strut extending to the tunnel walls downstream of the model. After intensive study and several false starts, Draley's group arrived to a solution: move the strut farther downstream into the diffuser section an install a specially contoured insert or liner within the tunnel's existing walls to create a new closed-throat section ahead of the strut. These two changes compensated for blockage and resulted in the production of a more uniform flow. Langley used an early version of its new sting support system in the spring of 1946 to test models of the XS-1 and D-558-1 in the 8-Foot HST, thus enabling the NACA to provide extremely important and reliable performance data for speeds up to about Mach 0,92 a year before flight testing of the research airplanes began at Muroc.6

Langley's small-model technique and its center-plate and sting support systems were only two episodes in the NACA's movement during the period 1942 to 1947 toward bridging the transonic gap in ground-based research capabilities, here were others. In late 1944 Langley engineer Coleman duPont Donaldson invented the Annular Transonic Tunnel, a ring-shaped passage with a single-bladed axial fan that was driven to very high speed by a series of electric motors - in actuality, more of a whirling arm than a tunnel. This facility began operation in early 1947, and, though serious questions soon rose about the quality of its test results, it made an immediate impact by providing the first pressure distributions ever measured on an airfoil at Mach 1.7

A few months before Donaldson 's invention, another group of Langley engineers was exploring the utility of a crude but remarkable tunnel modification known as the transonic bump in the 300-MPH 7 x 10-Foot Tunnel. In truth, the bump was used in a way similar to Gilruth's wing-flow technique, the controversial free-flight test method that some of Langley's more die-hard wind tunnel personnel had rejected for so long as unscientific: a carefully shaped wooden bump or wave about a foot high was placed on....



Sting-supported model of Bell XS-2 in 9-Inch Supersonic Tunnel, 1947.

A sting-supported model of the Bell XS-2 was tested in the 9-Inch Supersonic Tunnel in July 1947.


....the floor of a wind tunnel and the test model mounted in the region of the bump predicted to experience supercritical flow, just as in Gilruth's method a model surface was mounted in a precise location for airflow on an actual aircraft wing. As air flowed over the bump, it accelerated to transonic speeds even though the speed of the main airflow remained subsonic. Results thus gave "a qualitative indication of the type of effects encountered at transonic speed, and fairly reliable indications of trends."8 The principal disadvantage of the bump test, like that of the wing-flow technique, was its low Reynolds number. Nevertheless, NACA researchers used the method until a better ground-based method was devised. Most configurations of the early X-series of aircraft, as well as of the D-558, went through tests on the bump.


Achieving a Transonic Tunnel


In 1946 Langley physicist Ray H. Wright conceived a way to do transonic research effectively in a wind tunnel by placing slots in the throat of the test section. The concept for what became known as the slotted-throat or slotted-wall tunnel came to Wright not as a solution to the chronic transonic problem, but as a way to get rid of wall interference (i.e., the.....



Transonic bump on floor of 300-miles-per-hour 7 x 10-Foot Tunnel, 1947.

Airflow over the bump in the 300-MPH 7 x 10--Foot Tunnel reached the speed of sound.


...mutual effect of two or more meeting waves or vibrations of any kind caused by solid boundaries) at subsonic speeds.

For most of the year before Wright came up with this idea, he had been trying to develop a theoretical understanding of wall interference in the 8-Foot HST, which was then being repowered for Mach I capability. Wright had received this special individual assignment because as a member of John Stack's research division, which was "populated almost entirely by engineers," he had proved himself "an indispensable consultant on matters mathematical and theoretical."9 In 1939 and 1940, for example, Wright had determined the critical speeds of a large number of existing airfoils and bodies from their low-peed pressure distributions.10 This determination helped Stack's group contribute to the design of the 16-series, a new family of soon-to-be-celebrate NACA airfoils with higher critical speeds.**

The problem of wall interference facing Wright in 1945 was as old as wind tunnel technology itself. From the time Francis Wenham had built the first primitive tunnel in 1870, aerodynamicists had questioned exactly how airflow confined within solid wooden or metal walls could be [318] simulating the actual conditions of flight in free air. The distance between these walls and the scale-model aircraft under investigation was at most only a few feet. Real aircraft disturbed the surrounding air to distances several times the scale of that dimension. Soon experts had discovered that it was impossible, because of the proximity of the solid walls, for airflow to stream naturally over and near the models. The walls strangled the flow streamlines, producing misleading aerodynamic results. Some experimenters had tried to prevent wall interference effects by making the test models smaller, reducing them from five percent to one percent of the test section area. But as with later use of the small-model technique at Langley, reduction in model size often raised the choking speed but also lowered the Reynolds number, thereby actually increasing the discrepancy between the environments of simulated and real flight. Some had also tried getting rid of the walls altogether-as in the small open jets devised in the 1920s by Briggs and Dryden at the Edgewood Arsenal (mentioned in chapter 9). But not having wails just distorted the streamlines in other ways.11

In attacking the wall interference problem, Wright benefited not only from the collective knowledge and experience of the engineers working around him, but also from his own hard work, good intuitions, and artistic perspective. This combination caused Wright to wonder whether "since the interference velocities due to - walls are of opposite signs with free and solid boundaries, opposite effects might be so combined in a slotted tunnel as to produce zero blockage."12 Theoretical methods were available for making wind tunnel wall corrections at Mach numbers well below the choking value. These methods were available for both closed- and open-throat tunnels. Wright's contribution would thus be in combining the corrections for the different types of throats in such a way as to eliminate the need for any correction at all.

Such an idea dated back to theoretical papers by Prandtl and Glauert in Germany during the 1920s. Stack and Jacobs had tested it at Langley in 1929 and 1930 - by partially blocking an open throat with large models to reduce airstream choking - on the way to their final closed-throat configuration of the 11-Inch High-Speed Tunnel. Considerable work on the problem was done by the British, Italians, Japanese, and Germans during World War II. Most noteworthy was the work by Carl Wieselberger in Germany. In 1942, Wieselberger suggested a specific configuration with 46 percent of the perimeter open (via two wide longitudinal slots) as a means to reduce the blockage effect in certain German high-speed tunnels.13

NACA researchers did not find out about this work until Maj. Antonio Ferri arrived at Langley in September 1944 from the Italian aeronautical....



Antonio Ferri, 1944.

After fighting the Nazis as chief of a partisan brigade, Antonio Ferri brought important new information to Langley in 1944 about current German and Italian research in high-speed aerodynamics.


...research center at Guldonia, where, until the fall of Mussolini's government one year earlier, the young doctor of engineering had been in charge of the Galleria Ultrasonora (supersonic tunnel).*** Besides reporting on Wieselberger's studies, Ferri brought papers to America covering recent tests he had conducted in a tunnel whose sides were 43 percent open. Together, this information showed that "the Italians had already succeeded in obtaining airfoil force data [in this semi-open tunnel]- up to about Mach 0.94, and the Germans to about 0.92."14

Ferri's first job at Langley was to complete his tabulation of all the relevant Italian airfoil tests at speeds approaching Mach 1. When he finished in 1945, the NACA published his analysis as Wartime Report L-143; it demonstrated for the first time in America that "partly open arrangements could be used effectively."15 In the following months, Langley tried to apply the Italian's semi-open principle, but the experimental configuration [320] experienced large pulsations. The lab would achieve a successful version of Ferri's arrangement in 1948, but in 1945 and 1946, when Ray Wright was working to achieve zero wall interference in the 8-Foot HST, none of Langley's high-speed tunnel experts were yet sure that the concept of the semi-open tunnel was valid.16

Besides understanding the problem of flow pulsation, Wright knew that Ferri's and Wieselberger's semi-open schemes could not work for the 8-Foot HST for at least two other reasons: (1) the 8-Foot HST was a much larger facility - Ferri's tunnel was only 1.31 x 1.74 feet - and, if semi-open, would require considerably more power than was available; and (2) the test section of the 8-Foot HST was circular, not rectangular as were Ferri's and Wieselberger's. Finding the degree of openness and exact slot design required by the circular 8-Foot HST for zero blockage would take a completely different solution.


Mathematical Analysis and First Test Programs


Knowing that the excess power required by slots tended to be proportional to the open area, Wright specified for analysis a configuration with ten narrow slots instead of the two wide slots of both the Wieselberger and Ferri configurations. It is important to note that in attacking the problem, his main weapon was applied mathematics - the same tool used by Theodorsen in the 1930s to lift the cowling program beyond its experimental impasse. Much later Wright would concede, during a conversation with colleague John Becker, that "a systematic experimental attack [i.e., the method of parameter variation] might have been equally effective."17 However, considering the key role of Theodorsen's applied mathematics when parameter variation had stalled in the cowling development, Wright's use of the term might should be underscored.

As a result of a long series of tedious calculations, Wright discovered the optimum peripheral openness of the 8-Foot HST to be about 12 percent, or some 30 percent less open than the schemes of Wieselberger and Ferri. This delighted him because it meant that less additional power would be required. Wright reported his findings to Eugene Draley, his section head, in the late summer of 1946. Draley encouraged Wright to test his theory experimentally - the response Wright expected, as he was accustomed to pleasing engineers who wanted things demonstrated empirically. First, however, he was to report his findings to the division chief, John Stack.

Stack received Wright's report enthusiastically. Starting from his experience in 1929 and 1930 working with different configurations of the [321] 11-Inch High-Speed Tunnel, he had been aware that closed-throat and open-throat tunnels had opposite characteristics in proceeding up toward Mach 1. But he had not since considered seriously how to design a high-speed tunnel half-open or half-closed and really make it work. Now, Wright was giving him good mathematical reasons to think that there was some way to do it.

Stack possessed an open mind toward innovation; his attitude was usually "Let's try the damn thing and see if we can make it work." According to teammate Mark Nichols, Stack


had an intuitive feeling for things that were important, and he was always very interested in all of the technical matters. He'd have some young engineer come and give him a story and, no matter how scrambled the story, he'd look for a gem of new information or a new idea. And he had the faculty for picking these things out of nowhere and then pushing them. He'd say, "This is important, now, what can we do to develop this idea?"


Stack's enthusiasm would then infect those around him. He would lead, but those who followed would be made to feel confident that they were just as vital to ultimate success as he was.18

After hearing Wright's concept, Stack worked up a full head of steam. He informed NACA research director George Lewis of the development, discussed its major implications with him, and then proceeded to build a test program. There is no documentary evidence, however, that Stack thought of slots in the wall of a tunnel's test section in 1946 or early 1947 as a solution to the transonic problem. After all, Wright did not suggest it, or apparently even consider it, as such a solution; for him slots were just a means by which to get zero wall interference at high subsonic speeds. The same seems to have been true for Stack.

The first team of researchers to get involved in testing a slotted-throat tunnel configuration worked not in the 8-Foot HST but in the 16-Foot HST section. "It was quite easy for us to add a test program for Wright's circular 10-slotted arrangement," recalls John Becker, head of the 16-Foot HST, because "for some time we had been investigating blocking corrections" in small circular "parasite" test sections operated off the 16-Foot HST. (These parasite sections operated at speeds up to Mach 1.6 by sucking outside air through a long diffuser into the low-pressure test chamber of the 16-Foot HST. For details, see Becker's High-Speed Frontier, pp. 76-78 and 100-101.) Vernon G. Ward, the man who had been conducting the blockage-correction study for the 16-Foot HST, was assigned as project engineer for the experiments.


[322] Accidental Discovery, Deliberate Debate


In the first test runs, which took place in early 1947, the slotted tunnel operating off the 16-Foot HST achieved a maximum speed, before choking, of Mach 0.97. Then, one day, one of the engineers wondered what would happen if he took the model out of the parasite test section and turned up the power of the driving fan. What happened excited this curious engineer and then excited everyone else at Langley who found out about it: the small experimental tunnel went up to and through the speed of sound just as beautifully as anyone could have imagined. (Which member of Stack's staff turned up the power is still uncertain, but Richard Whitcomb relates that it was definitely not Ray Wright.)19

At a meeting of Langley's General Aerodynamics Committee on 25 July 1947, Stack reported the unexpected success, discussed its implications, but mentioned no specific plans to install a slotted throat in any research tunnel. (The General Aerodynamics Committee was for the most part an informal discussion group of Langley physicists and engineers. It met once a month to take up major aerodynamic issues.) At that moment Stack was in fact leading his men in the 8-Foot HST section (Draley, Wright, Axel Mattson, Richard Whitcomb, and others) through the design of a 12-inch slotted-wall section to test Wright's concept, but he was keeping plans to himself until the right time-when slotted-throat designs were proven effective beyond a reasonable doubt and funds were available for converting both the 8-Foot HST (maximum speed Mach 0.75 before the repowering) and the 16-Foot (maximum speed Mach 0.70 before repowering) into transonic tunnels.

Stack faced "a very strong current of disbelief" at Langley about the efficacy of using slotted throats. The new tunnel design was known to involve problems for which no one yet had answers: "power requirements, the details of slot shaping,- the quality of slotted tunnel flow, model size limitations, possible combinations of wall divergence and slots, shock reflection problems above Mach 1, slots versus porous walls, etc."20 The strongest expressions of disbelief came from two of Langley's purer theorists, Antonio Ferri and Adolf Busemann, both of whom had arrived only recently from Europe. (Ferri's arrival from Italy and its impact on NACA history have already been mentioned. Busemann, whose "arrow-wing" theory was discussed in chapter 10 in relation to R. T. Jones's concept of a swept wing, came to work at Langley in early 1947 after having worked at the Luftwaffe laboratory near Brunswick, Germany. He was brought to this country soon after the end of the war as part of Operation Paperclip.)21



Exterior view of 16-Foot HST, 1940s.


Turning vanes of 16-Foot HST, 1940s.


The 16-Foot HST began operations on 5 December 1941, two days before the Japanese attack at Pearl Harbor. During the war, the tunnel tested various air-cooled aircraft engines, cooling systems, high-speed propellers, and even the shapes of the first atomic bombs. This was the first tunnel to receive the NACA's authorization for installation of a slotted-throat transonic test section (in 1947), but the second actually to get it done (in 1950). The picture to the left shows the vanes that turn the airflow around one of the tunnel's corners.


[324] Busemann and, in particular, Ferri felt sure that Wright's theory was wrong and that, if Stack continued charging forward with plans to convert the test sections of both the 8-Foot and 16-Foot HSTs into slotted throats, both he and the NACA would end up appearing foolish. Ferri took his case to at least one colleague, John Becker, who recalls that Ferri


conceded that slots could be used to reduce the blockage effect, but to have zero blockage at Mach 1 was physically unlikely except for very small models. He felt that many mathematicians and physicists who had an understanding of transonic theory would regard any NACA claim of valid data at Mach 1 for sizable models as absurd.


Unless someone could persuade Stack to at least use "some words of qualification when discussing slotted tunnels," Ferri advised, the NACA's international reputation would be permanently blemished. Becker's own opinion as head of the 16-Foot HST section was that more definitive answers to outstanding questions should be pursued in the model tunnel program "before any commitment was made to incorporate slots" in either the 16- or 8-Foot HST. 22

The slotted tunnel was the only item on the agenda for the September 1947 meeting of the General Aerodynamics Committee. Ray Wright and Vernon Ward had been asked by Samuel Katzoff, committee chairman, to begin the meeting by presenting their most recent results briefly, which they did "in rather modest terms." Stack, who had only grudgingly agreed to attend,


made a late entrance and sat down at the head of the table with a belligerent look on his face. Clearly it said, "Anyone who wants to argue about the slotted tunnel will have to take me on."23


After Wright and Ward finished their report, Busemann and Ferri had their chance to comment. Busemann reiterated his earlier conclusion that from the standpoint of theory an approach better than Wright's ten discrete slots was one involving a "homogeneous boundary" in which the slots were uniformly distributed about the periphery.24 Ferri's point, that slots could be used to achieve zero blockage but only with very small models, was lost on many participants "through a combination of poor English [whether he said 'subsonic' or 'supersonic,' people heard 'soup-sonic'] and extreme politeness."25



John Stack, 1946.

John Stack was a hard-charging, persuasive man whose attitude toward unproven technology was usually "Let's try the damn thing and see if we can make it work."


It is not clear whether Stack totally understood both points, because when both theoreticians had finished, Stack said in essence:


OK, I get your point. So, there is a unique situation exactly at Mach 1. We'll make this thing work at Mach .995. We'll make it work at Mach 1.005. And we won't give a damn about that little thing in between.26


Those who knew him best readily attest to the fact that Stack could be stubborn, and in this instance he had definitely already made up his mind. He was going to have his engineers roll up their sleeves and club away at the problem until it was solved. They were not to let the infinitesimal point in the middle stand in their way. A slotted tunnel would be built.

As head of the Compressibility Research Division, Stack was in a position to block internal opposition from those who knew the most about high-speed aerodynamics. His division had grown tremendously in size and importance in the early postwar period as a result of the shifting emphasis from subsonic to supersonic flight. Three major new facilities- the 4-Foot Supersonic Pressure Tunnel, the Gas Dynamics Laboratory, and the Induction Aerodynamics Laboratory - had been added to those [326] already under his supervision. Considering his experience, management responsibility, and great personal dynamism (some have compared him to a bull in a china shop, few subordinates risked opposing Stack once he had made up his mind.

Moreover, Stack's prestige and influence within the NACA were now approaching a zenith. In one month the XS-1 would break the sound barrier, an achievement for which he would share a Collier Trophy. George Lewis, who had only just retired as director of research, remained on with the NACA as "research consultant"; Stack was one of his favorite boys. Hugh Dryden, Lewis's successor, would not have the same paternal feelings for Stack, but as slotted-throat tunnel development was unfolding, the two men were enjoying a honeymoon period.****

In the fall of 1947 Stack had to decide which of the big tunnels to convert to a slotted throat, and then sell the idea to NACA headquarters. Only a hard-charging, persuasive man like Stack, willing to keep an idea alive at a time when most other experts would have preferred to kill it, could have accomplished this as quickly as he did. He decided to convert the 16-Foot HST first. This decision paved the way for quick approval by headquarters, which had just approved funding for a 60,000-horsepower repowering project that could be broadened to include conversion of the walls of the test section. (In fiscal year 1947 Langley had requested and gotten approval for 35,000-horsepower repowering. See appendix C. It should be noted that repowering was doubly relevant to this slotted-throat conversion, not only in terms of budgetary scheduling convenience, but because even a tunnel whose walls were 12 percent open required about twice as much fan power as one with solid walls.) Total conversion would require a special assignment of additional funds, however. On 10 January 1948, Langley submitted a formal "Description and Justification for Slotted Test Section" prepared by a member of the 16-Foot HST staff for consideration as part of the NACA's fiscal year 1949 budget request. Stack defended the justification vigorously and in person before top management in Washington. Management soon approved, but in doing so made clear that it could take two to three years to procure all the money needed for total conversion.27

[327] The entire 16-Foot HST staff was now "under heavy pressure to come up with the additional data" needed for an effective design. Stack assumed "personal supervision on a daily basis for the many interrelated slotted tunnel activities, ranging from expediting work on models in the shops, to working with the detail designers of the 16-foot section, and dealing as always with funding and approval problems."ë By holding frequent meetings, he not only made sure that researchers abided strictly by his schedules but also that they maintained as much enthusiasm for the project as he did himself.28

A few researchers came cautiously to Stack with objections and alternatives to the slotted throat. Before a million dollars or more was spent modifying and perhaps ruining a proven research facility, they wanted him to make sure that the slot design was perfected. Antonio Ferri seems not to have been among them, however. Alter Stack, his division chief, had made the decision to convert the 16-Foot HST, Ferri went to him and asked if there was anything he could do to help. Stack admired this loyalty; the Italian soon became one of the most trusted members of the team and a close personal friend.


Surprising Announcement


In the spring of 1948 Stack announced at Langley that the test section of the 8-Foot HST would also be converted to a slotted throat. This news stunned critics and defenders of the slotted throat alike. Researchers in the 8-Foot HST had been focusing their attention recently on using the new closed throat together with the new sting support system for research at Mach 1.2 and "had given little thought to the next step." Stack was discouraged, however, with preliminary results in the reconfigured facility: the Reynolds numbers of the tests were lower than desired. Moreover, he was "very impatient at the prospect of two or three years of procurement time before the 16-foot tunnel would be operable." Convinced that alteration of the 8-Foot HST would be cheaper and quicker because less complex technically, especially with all fabrication and installation being done inhouse, he waved off all protests from his men for more time to study the problem. Before long it was clear to everyone inside the NACA that Stack had in fact transferred top priority for a slotted throat from the 16-Foot to the 8-Foot HST.

Sometime in late 1948, the 8-Foot HST went through the speed of sound with a slotted throat, but the flow was awfully rough and uneven. Now engineers had to get down to the nitty-gritty and come up with the exact slot configuration for smooth transonic flow. Wright's theory guided their...



Ray H. Wright in diving suit preparing to enter transonic tunnel, ca. 1950.

Because the slots he was designing opened directly into the 8-Foot HST s hazardous igloo-shaped test chamber, where high levels of pressure, temperature, and noise would be encountered, Ray H. Wright had to don a diving suit before venturing into the test section.


...pursuit, as did design data from tests in the 12-inch model slotted section operated off the 16-Foot HST, but neither source of information sufficed. What was needed was creative use of the mind's eye and the touch of a sculptor. This artistry was provided by physicist Ray Wright and engineers Virgil S. Ritchie and Richard Whitcomb, By shaping the slots meticulously and continually by hand over a span of seven months, this trio refined the details of the slotted throat until smooth transonic flow distributions were finally achieved.29

The 8-Foot HST began regular transonic operation for research purposes on 6 October 1950. Just three months later, the 16-Foot HST also became operational with a slotted throat for transonic research. What made this short turnover time possible, according to Becker, head of the tunnel section, was not only the immediate exchange of critical knowledge about the proper shape of slots from the 8-Foot to the 16-Foot HST design [329] groups, but also the 16-Foot group's separate pursuit of improved slot technology, which had continued even after Stack had given conversion of the 8-Foot HST priority.

Despite limitations, the slotted tunnels became "best practice" in transonic research almost immediately. By the end of 1950, in fact, Langley engineers were busy planning a completely new slotted tunnel. The design of this facility, which became operational in early 1953 as the 8-Foot Transonic Pressure Tunnel, remedied three of the problems that had been plaguing the operation of the converted tunnels: (1) high humidity and fog, caused by the need to draw outside air into the main airstream for cooling purposes, (2) high turbulence, and (3) low Reynolds numbers.30

A problem left unremedied in Langley's slotted-throat transonic tunnels was "the inability of the slots to alleviate significantly the reflection of pressure disturbances from the solid regions of the walls." The tunnels did not choke going through Mach 1, but test data "often exhibited significant discrepancies when compared to free air."31 In the early 1950s, engineers at NACA Ames Laboratory in California designed a new type of transonic tunnel section to alleviate this reflection problem. Instead of slots, they incorporated a "mesh of holes" in the test section wall. Placed inside Ames's repowered 16-Foot HST (which had been built, like its twin at Langley, in 1941), this ventilated or porous tunnel began routine operations in late 1955 as the 14-Foot Transonic Tunnel. Tests in it helped solve the transonic stability problems of various missiles.32 For all practical purposes, however, the porous wall tunnel at Ames did not solve the problem which had been plaguing the operation of the slotted tunnels at Langley - for it eliminated the reflection at only one specific Mach number.

The military services and their contractors had been following Langley's slotted-throat developments closely since at the latest the fall of 1948, when they received the NACA's first confidential report on the transonic tunnel test sections.33 In December 1948, for example, Air Materiel Command headquarters sent several representatives to the laboratory to discuss the possibility of using a slotted throat in a ten-foot wind tunnel at Wright-Patterson AFB near Dayton, Ohio (formerly McCook Field, and later Wright Field). After meeting with Stack, Draley, Wright, and Ward, however, Bernhard Goethert, the air force's leading scientific brain at the meeting, concluded that because the NACA "was embarking on a program to obtain power measurements of large slotted throats," it would be unwise for military engineers to embark "on a systematic series of investigations on slotted-throat power considerations themselves." Rather, they should wait until the NACA's power requirements were available.34 This conclusion, which Air Materiel Command seems to have endorsed, pleased everyone.....



Original test section, 1945  of 16-Foot HST.

Slotted-throat test section, 1951, of 16-Foot HST.

The test section of the 16-Foot HST before (left, 1945) and after (right, 1951) installation of slotted walls. The 1945 photo shows the test section as seen through a window from the outside. (For another view of the 16-Foot HST's slotted test section, see the last picture in the Introduction.)


.....at Langley. After all, though Stack had committed both the 8-Foot and the 16-Foot HST staffs to the slotted-wall concept, a successful design for research use had not yet been achieved.

By the spring of 1949 rumors about Langley's transonic tunnel developments had circulated widely among those in the American aeronautics community who had not even been entrusted with the NACA's first confidential report. At its annual inspection held that May at Langley, the NACA had tried to divert attention from the slotted throat by having Stack deliver a talk in which he emphasized the Annular Transonic Tunnel. But the camouflage did not fool anyone, especially when, in the following month, Hugh L. Dryden, the NACA's new director of research, "took the unusual step of requesting other organizations to follow the Committee's policy of assigning a confidential classification to all information relating to the development of transonic wind tunnels."35 Clearly Dryden did this in deference to the military, which was then planning new supersonic fighters and bombers that would have to fly through the mysterious transonic region.

Classification meant that the NACA researchers responsible for the slotted throat had to sacrifice the personal advantages of quick and open publication. In 1950 when researchers at the University of Southern California reported work on their own slotted test section, Langley engineers reacted defensively. Eugene Draley complained in a memo that


[331] this work, to date, is apparently being done without any security classification, which is in violation of the security principles established by the NACA on this type of work. It is therefore recommended that steps be taken to correct this situation - not only because of the primary considerations associated with basic security requirements, but also because these results are being reported without any reference to any work by the NACA. The inference created by such work is that the whole subject and idea of slotted throats has been originated at the University of Southern California, which is very far from the truth.


In accusing USC researchers of misappropriating credit, Draley failed to mention that security regulations would have in fact prohibited the academics from making reference to anything they might have known about Langley's slotted-throat work. (Apparently USC did not know anything about it anyway.) He closed the memo by recommending that NACA headquarters "check into this matter and call the [USC] group's attention to the existence" of the NACA classified reports.36

Though outsiders continued to report independent work with slotted and porous test sections, Langley researchers soon got all the credit due them-Stack and his associates won the Collier Trophy for 1951 (Stack's second in four years) for developing the slotted wall. Two years later, with the principles of transonic wind tunnel design "widely known through independent research" both inside and outside the United States, Dryden withdrew his request "for special treatment for information relating to transonic wind tunnels." All thirteen previously published NACA reports on the slotted throat were subsequently declassified and announced in the usual manner.37 In its Annual Report for 1954, the NACA admitted that advertising the potential of the Annular Transonic Tunnel at the 1949 inspection had been subterfuge.

In contemporary press releases, the NACA claimed that Langley's development of slotted-throat transonic tunnels gave the nation a two-year lead over all other nations in the design of supersonic fighters and bombers. This bold claim, as Becker points out in The High-Speed Frontier (p.117), was based on projected good use of the area rule, a new concept in the shaping of high-speed aircraft. Ironically, considering his vivacity in the slotted-throat program, Stack would not be one of the area rule's staunch defenders.


Whitcomb's First Clue


Ever since his arrival at Langley in 1943, Richard T. Whitcomb, a 1943 graduate in mechanical engineering from Worcester Polytechnic Institute in Massachusetts, had worked under Stack in the 8-Foot HST [332] section. Management had tried to place him in the Instrument Research Division, but the young engineer made it clear that he wanted to work in aerodynamics. The building and testing of model airplanes, the mania of his boyhood, still fascinated him. The sandy-haired, blue-eyed engineer quickly established a reputation as a wunderkind, the rare engineer who was not only quite capable mathematically but also possessed a powerful intuition and unusual artistic talent for cut-and-try techniques. Though the 8-Foot HST did not go transonic until equipped with the slotted throat in 1950, it had been able to get up close to the speed of sound (up to Mach 0.95) after its repowering in 1945. Thus Whitcomb started to get a feel for transonic aerodynamics at least five years before starting the research investigation that led directly to his conception of the area rule.

During this seminal period Whitcomb conducted research on the biggest problem facing the designers of supersonic aircraft-the large increase in drag (associated with the formation of shock waves) that occurred at transonic speeds. He knew, first from laboratory wind tunnel tests and then from the flight of the Bell X-1 (no longer called XS-1), that small, lightweight rocket-powered configurations with limited missions could overcome the transonic drag problem; he knew also, however, that for operational turbojets, which would have to be considerably heavier than the X-1, the problem would be critical. If flying up to and through Mach 1 took gradual acceleration because of high drag, there would be insufficient fuel left for the aircraft to sustain supersonic flight for long after achieving it.

In July 1948, after analyzing all available transonic information from NACA ground facility and free-flight (including Wallops Island rocket-model) tests, Whitcomb submitted a proposal for wind tunnel tests of a swept wing and fuselage combination. Fairly substantial progress in reducing transonic drag rise had been achieved by using sweepback and optimizing the shapes of fuselages, and he felt that with proper arrangement and shaping, the drag-producing disturbances caused by the wing and fuselage might be made to counteract each other.38

In late 1949 and early 1950 Langley tested models incorporating Whitcomb's sweptback wing and body combination at high subsonic (Mach 0.95) and low supersonic (Mach 1.2) speeds. Results indicated very little favorable effect in reducing the drag. In fact, the results showed significantly higher total drag than transonic theory predicted for the drag of the wing and the drag of the body combined. Stymied, Whitcomb decided that he needed to know more about the fundamental nature of flow at transonic speeds before truly fruitful work on the major design problem of supersonic aircraft could begin.39

[333] As soon as the slotted-throat section was placed inside the 8-Foot HST, Whitcomb and his colleagues employed "every available tool" to study in detail what happened in the flow field around wing and body combinations at transonic speeds. These tools included (1) the tunnel balance, which had been used by wind tunnel researchers for years as the standard means of measuring the aerodynamic forces on a model (i.e., lift, drag, and pitching moments); (2) orifices sensitive to pressures at various points on the surface of a model from whose measurements one could calculate local velocities; (3) tuft surveys, involving little pieces of cloth attached in various places on a model surface, by which observers could tell if the flow was smooth or disturbed; and (4) schlieren photographs, a method for seeing shock waves (discussed earlier in chapter 9). None of these four methods were new; for instance, researchers had used tufts on some of the earliest airplanes flown at Langley. By using these available tools together, however, the 8-Foot HST group began to understand that drag patterns at transonic speeds were "completely different than anything that anybody had ever predicted" theoretically.40

The schlieren photographs were most startling. Besides showing the well-known shock wave that formed where air was pushed aside to make way for the nose of a high-speed projectile, the photos indicated two "fascinating new types" of shocks - one that had apparently built up as the fuselage and wings began pushing more air out of the way, and another near the trailing edge of the wing. In comparison with the size of the wing and body combination being studied, the disturbed area of air was now understood to be much larger than previously conceived. Whitcomb wondered if the sharp rise in drag occurring in transonic flight was caused by losses from the strong extra shocks. After all, this was the first time that these particular disturbances in the transonic flow field had been observed.

Whitcomb had his first clue to the area rule, but he did not yet know what to do with it. The conventional way to design high-speed aircraft was to follow Ernst Mach's advice. In the late nineteenth century Mach had shown that bullet-like shapes produced less drag in flight than any other known shape. Although no controlled, manned aircraft would attain that streamlined ideal - they required wings and a tail - designers of the first generation of supersonic aircraft still tried to mimic that shape as much as possible. As Richard P. Hallion, historian of supersonic aircraft, has explained: "They gave the fuselage a pointed nose, then gradually thickened the body - that is, increased the cross-sectional area - until the fuselage reached its maximum diameter near the middle." Only at the tail end did the designers begin to decrease the diameter of the fuselage.41 This was the rule of thumb.

[334] In November 1951 Langley put a systematic series of wing and body combinations through tests in the 8-Foot HST; models included swept, unswept, and delta wings, and bodies with various amounts of curvature in the region of the wing. The goal of the program was to evaluate the magnitude of the drag caused by the interference of the two shapes at transonic speeds. Results led Whitcomb to two important new ideas: (1) that variations in the shape of the fuselage, even small ones, could lead to pronounced changes in the drag of the wing, and (2) that in determining transonic drag, the drag of the wing and the drag of the body could not be considered separately; rather, the combination had to be considered as a whole, as a mutually interactive aerodynamic system.42




Beginning in college Whitcomb had made it his practice to leave some time each day just for thinking. As has often been the case in discovery of the unknown, it was precisely this type of freewheeling, looking-out-the-window contemplation that. led him to the area rule. One day late in 1951, while sitting at his desk trying to figure out why the shock waves were so different than anyone had expected, suddenly in his mind's eye he "saw" air passing over a body at transonic speed in a different way. A moment more of this creative visualization and - "Eureka, I've got it!" ***** He perceived that the ideal streamlined body for supersonic flight was not a function of the diameter of the fuselage alone, as the old rule of thumb had it; what really caused transonic drag rise was the total cross-sectional area of the fuselage, wings, and tail. Since wings added most to this area, designers could reduce drag significantly by tucking in or narrowing the fuselage where the wings attached and then expanding the fuselage at their trailing edges.43

What opened his mind's eye was a physical analogy made a few weeks earlier by Adolf Busemann during an in-house technical symposium. Busemann, who had been working on theoretical aspects of transonic flow for some time, had told the Langley crowd to work "as pipe-fitters." Aerodynamicists were accustomed to working with streamlines and streamtubes, the German scientist had reminded his audience. A common approach to theoretical analysis of airflow problems was to isolate the streamtube (i.e.,....



Richard T. Whitcomb, 1955.

Richard T. Whitcomb examines a model based on his transonic area rule in the 8-Foot HST, April 1955.


...bundle of streamlines) containing the object under study. Wind tunnel researchers should also visualize transonic problems in this way, Busemann said. Picture a problem in transonic aerodynamics, he urged, as "uniform pipes going over the surface of the configuration" being studied.44

That is exactly what Whitcomb was doing when he exclaimed "Eureka!"' - visualizing all the pipe lines affecting his swept wing and body combination. His mental plumbing job involved tinkering inside a transonic streamtube having a diameter greater than the wing span of the aircraft model. While imagining how the streamlines deviated as they passed across the nose, along the body, and finally up over the wings before reverting back to normal paths downstream, he got the idea that if air could be displaced less violently, the waves and drag would diminish, enabling his plane to pass more easily through the transonic zone. Specifically, he thought to pinch the waist of the fuselage so that streamlines which otherwise would be brushed aside sharply would have more room. The same amount of air had to get out of the way to make room for the plane, he knew, but if the [336] plane had a trimmed-down "wasp waist," the air would not be displaced in such violent shock patterns.

After jumping up from his desk, Whitcomb took Busemann's concept of what was happening aerodynamically at high speeds, as well as "every available bit of [transonic] data that [had] ever been gotten before, by drop tests and so forth," and compared the two sources of information with the pile of unexplained results from the slotted tunnel. As Whitcomb recalls, "It's been said the proof of a new theory is whether it will [explain] all of those pieces of information that you've been trying to fit together, and it did."45

His colleagues, in particular Stack, his boss, were not so sure. Still, Stack allowed him to present his area rule at the next meeting of Langley's elite technical seminar - perhaps, as one NACA veteran believes, so Busemann and the lab's other great mathematicians could prove it wrong.46 Busemann, however, did just the reverse:


At the end of [Whitcomb's 20-minute) presentation there was silence. Finally, Adolf Busemann stood up. Turning to his colleagues, the pioneer of sweptwing technology remarked, "Some people come up with half-baked ideas and call them theories. Whitcomb comes up with a brilliant idea and calls it a rule of thumb."47


With Busemann defending the rule, the skeptics retreated at least temporarily. Stack reacted characteristically: he told Whitcomb to go prove it.


Verification and Application


The basis of the area rule concerned the cross-sectional areas of a wing and body combination. If these areas obeyed the rule by having the proper relationship to each other, the resulting shape should enjoy minimum transonic drag. To verify the rule experimentally, Whitcomb designed models with variously pinched waists and tested them in the 8-Foot HST. By the end of April 1952, enough data indicated that "very significant reductions in drag could be obtained by contouring the fuselage" for him to start writing a formal report confirming the theory.48 The NACA published this paper, "A Study of the Zero Lift Drag Characteristics of Wing-Body Combinations Near the Speed of Sound," as Research Memorandum L52H08 in September 1952, and made it available immediately to American industry on a secret basis. By that time, however, at least one aircraft manufacturer - Convair - already had heard something about the area rule.

[337] Recent data from Langley's now transonic 8-Foot HST had suggested that Convair's YF-102, a fighter-interceptor being readied for air force service in defense of the continental United States, could not fly supersonically as planned - the transonic drag was higher than expected. Naturally this information disturbed Convair greatly. The company's designers had given the plane a bullet-shaped fuselage, knife-edge delta wings, the most powerful jet engine in existence (the Pratt and Whitney J-57), and everything else that was currently thought essential for sustained supersonic flight. The company's production managers had set up an assembly line in San Diego for the manufacture of hundreds of F-102s. Now NACA test results indicated that Convair's best-laid plans for an honest-to-goodness supersonic fighter had been insufficient. The company had good reason to fear that the air force might cancel the contract.

In mid-August 1952, less than a month before the publication of Whitcomb's report, a visiting team of engineers from Convair witnessed the questionable performance of a scale model of the YF-102 in Langley's 8-Foot HST. Someone asked Whitcomb what might be done to reduce the air resistance, and in response be described his surprising discovery of a new rule of thumb concerning transonic drag.

The historical records do not make clear what if anything John Stack said to the Convair representatives about the area rule or anything else. However, a few subsequent memos from Stack's division suggest that Stack, either for fear of transonic theory or some personal reason, wanted the whole area rule business "put to bed."49 This suggestion is supported by the recollections of several Langley veterans who knew Stack quite well. The company men flew home to California, taking the scale model with them for study. However, they were not totally convinced either by the theory or by the NACA's interpretation of the tunnel data that the YF-102 as originally contoured could not go supersonic in level flight.

Over the next several months Whitcomb worked with Convair to apply the area rule to the YF-102 configuration-which apparently means that Stack's skepticism stopped short of obstructionism. New wind tunnel tests, which began in May 1953, indicated far less drag but left room for improvement. Three months later, after more area-rule-based modifications, Whitcomb traveled to San Diego to help the company's aerodynamic department finalize its recontouring of the airplane. In October the NACA reported that Convair's modified aircraft, later designated the YF-102A, met the air force specifications for supersonic flight.50

Still hoping that the YF-102 might fly supersonically, Convair had continued all the while producing the prototype. In late 1953 and early 1954 the plane was test flown, but its performance mostly confirmed the....



Area-rule-based model of Convair F-102, 1953.

An area-rule-based model of the Convair F-102 being readied for testing in the shop of the 8-Foot HST, November 1958.


....NACA's pessimistic wind tunnel evaluation. Hating to delay receipt of the new aircraft but wanting it really to be supersonic, the air force called a halt to Convair's assembly line and advised the company to retool immediately for manufacturing the YF-102A.

Because Convair's designers had, with Whitcomb's help, already redesigned a model according to the area rule, it took less than seven months for a new prototype to be built. Besides the wasp-waist, the YF-102A was given a sharper nose and canopy, tail fairings, and a more powerful version of the J-57 engine. During a flight test five days before Christmas 1954, the YF-102A "slipped easily past the sound barrier and kept right on going."51 The area rule had helped to increase the plane's top speed by an estimated 25 percent. Delighted with the superior performance, the air force eventually contracted with Convair for over 1000 F-l02As. The advanced version of this model, designated the F-106 Delta Dart, flew as a vital part of the continental defense arsenal into the early 19808.52

Whitcomb's discovery of the area rule was particularly timely. His eureka experience occurred at the very moment that virtually all military fighters aimed at sustained level supersonic flight seemed doomed to remain just below Mach 1 because of the incapability of the jet engines of the time to overcome the tremendous drag rise. No matter how skeptical Stack or anyone else might have been about the new theory, aircraft manufacturers stuck in this quandary had little choice but to explore the theory's potential....



Rocket-powered model of Convair F-102 before application of area rule, 1953.

Rocket-powered model of Convair F-102 after application of area rule, 1953.

At Wallops Island in 1958, Langley's Pilotless Aircraft Research Division (PARD) tested rocket-powered models of the delta-winged Convair F-104 before (left) and after (right) application of the area rule.


....applications. In large part their future depended upon it. Convair faced up to the problem, and so did Chance Vought (which redesigned its F-8U carrier-based interceptor according to the area rule), Grumman, and eventually Lockheed (in April 1956, its area-rule-based F-104 Starfighter was the first jet to exceed Mach 2 in level flight).

Convair may have heard about the concept first, but Grumman built the first area-rule-based aircraft to fly supersonically. Just two weeks after receiving a copy of Whitcomb's September 1952 report, Grumman, under contract to the navy for a supersonic carrier-based fighter, sent a delegation to obtain further information. In February 1953, five months before his trip to Convair, Whitcomb visited the Grumman plant at the company's request to see the final design layout of the area-rule-based F9F-9 Tiger before slotted-tunnel and rocket-model tests at transonic speeds. By the end of the summer, results confirmed that this layout would have low enough transonic drag for supersonic speeds in level flight. So Grumman built the plane. On 16 August 1954, the flashy white F9F-9 Tiger (later to be called the F11F-1) "breezed through sonic speed in level flight without the use of [340] an afterburner, the first time this had been done."53 Convair's improved F-102, the F-102A, took four more months to match the achievement.

One month later, a story on the successful application of the area rule to the Grumman Tiger appeared in Aero Digest. In the eyes of the military and the NACA, the editor was in violation of that journal's written commitment to withhold publishing anything about this work until the veil of secrecy had been lifted officially. The editor had upset what had theretofore been considered "an object lesson in how a military scientific secret can be kept effectively to gain a time advantage over international competitors."54 This transgression would have been serious even if the age had not been that of Joseph McCarthy. The war might have been cold, but the United States was then engaged in hot technological competition with the Soviet Union for supremacy in the air.

One year later, in September 1955, the NACA released news of the area rule. The basic idea had been applied so widely among manufacturers and others that it made no sense to persist with secrecy. Articles praising the new aircraft design, dubbed for visible reasons the "Coke Bottle" and the "Marilyn Monroe," soon flooded the aviation journals and newspapers. Bold-faced headlines read: "NACA Formula Eases Supersonic Flight" (Aviation Week, 12 Sept. 1955); "Idea Called 'Major Key' To Supersonic Flights" (Newport News, Va., Daily Press, 12 Sept. 1955); "25 Per Cent Increase Made in Plane's Speed Beyond the Sonic Barrier" (Daily Press, 12 Sept. 1955); "The Area Rule: A Universally Applicable, Rule-of-Thumb Law for Transonic Design" (Flight, 30 Sept. 1955); and "How We're Beating the Russians through the Sound Barrier" (Look, 13 Dec. 1955).

Five weeks after the public announcement, the National Aeronautic Association awarded Whitcomb its coveted Collier Trophy for the greatest achievement in aviation in 1955. The citation read: "Whitcomb's area rule is a powerful, simple, and useful method of reducing greatly the sharp increase in wing drag heretofore associated with transonic flight, and which constituted a major factor requiring great reserves of power to attain supersonic speeds." As evidence of the value of the area rule, the citation asserted that the concept was being used currently in the design "of all transonic and supersonic aircraft in the United States."

Whitcomb continued to refine and extend his basic concept, not only for the design of supersonic bombers but also hopefully for future commercial jets. For several years he worked full bore, first under Stack and then under Laurence K. Loftin, Jr., on the problems of designing a supersonic transport to fly beyond Mach 2-that is, an SST. Eventually he left the frustrating new field and returned to transonics, where he knew he could make things pay off. Shortly thereafter, at Loftin's instigation, he began a new research project [341] on the airfoil characteristics of a new vertical takeoff (VTO) design being studied by the Ling-Temco-Vought Company. In 1965, as a result of this investigation, Whitcomb conceived the supercritical wing, an airfoil shape whose primary attribute was improved performance at high subsonic speeds but which also proved to have excellent high-lift characteristics because of its rounded leading edge and sharply down-curved trailing edge.55


The Engineer as Artist


Some analysts qualify Whitcomb's discovery of the area rule by mentioning that the concept was implicit in earlier theoretical works. Alex Roland, for example, in Model Research asserts that Whitcomb merely "provided the engineering data that turned [the rule] into useful applications and pointed out the adjustments needed to get the [Convair F-102] through the sound barrier."56

It is true that flow studies over the wing root of a sweptback wing and fuselage combination had led the German aerodynamicist Kuchemann to design a fighter plane with a tapered fuselage as early as 1944. The American intelligence teams that discovered this development tagged it the "Kuchemann Coke Bottle." Also, beginning in 1946, two British researchers, G. N. Ward of the University of Manchester and W. T. Lord of the Royal Aeronautical Establishment, had taken a mathematical approach to the transonic drag problem that, seen in retrospect, could have provided clues to the area rule. In the late 1940s, so did a doctoral thesis by Wallace D. Hayes at Caltech.57 But none of the forerunners recognized the potential of what they had. Perhaps because they reduced everything mathematically-which involves thinking with symbols-Ward, Lord, and Hayes had failed to see, as Whitcomb would, how to bring the physical elements together in a new aerodynamic combination. Whitcomb did not conceive the area rule by reading Hayes's theory, even if (despite his denials) he had already read the young man's Ph.D. thesis. He conceived it independently, thanks to a highly individual nonverbal process of thinking that involved seeing shapes and changing them, not interpreting symbols.

Whitcomb's introspective style of creativity was uncommon at Langley. Though he had a conservative, shy personality, he was a radical in the laboratory. In some respects, management did not know exactly how to deal with him. The best idea any of his supervisors came up with was to leave him alone except to help him through those administrative duties distracting him from what he really wanted to be doing. The best thing for a freewheeling mind, after all, was an open road.

* Langley veteran John V. Becker recalls the evolution of the small-model technique and other major innovations in transonic wind tunnel technology during the 1940s as part of The High Speed Frontier: Case Histories of Four NACA Programs (Washington, D.C.: NASA SP-445, 1980), pp. 98-118. This chapter extends Becker's highly technical story, based on oral testimony of other key participants in the developments and on further research into the archival record.

** The 16-series airfoil sections were actually derived from the 1-series low-drag sections, which were developed by the Eastman Jacobs team and first described by Jacobs in his 1939 Advance Confidential Report on laminar-flow airfoils (also published as Wartime Report L345).

*** Ferri held a doctorate in engineering from the University of Rome (1936). After the collapse of Mussolini's government in September 1943, Ferri had organized a band of partisans which fought the Nazis and Italian Fascists. In July 1944, when Allied forces took control of the Macerata province in which his partisan brigade (the "Spartaco") was operating, Ferri was contacted by an agent of the U.S. Army's Office of Strategic Services (OSS). He signed an agreement to work for the U.S. and to put all information in his possession at the country's service. Among the documents he gave to the OSS were numerous top secret technical reports, both Italian and German, which he had taken from Guidonia before it was seized by the Germans. Soon after his arrival in the U.S., the War Department assigned Ferri to act as an aeronautical consultant for the NACA at Langley Field, where he stayed (at the engineer-in-charge's special request) until 1950, when as an American citizen he chose to begin a teaching career at the Polytechnical Institute of Brooklyn.

**** Stack and Dryden were individuals of conflicting backgrounds and personalities - Dryden, a scientist, an introvert from a proper New England Protestant family; Stack, an engineer, the extroverted son of a first-generation Irish Catholic immigrant who had settled as a carpenter in the factory town of Lowell, Massachusetts. In October 1957, the differences between the two men came to a head: at an NACA dinner party hosted by NACA chairman James H. "Jimmy" Doolittle, Stack called Dryden an old fogey - loud enough that most everyone in the room could hear; Dryden never forgave him for that. See Roland, Model Research, p. 292, and Walter A. McDougall, - The Heavens and the Earth: A Political History of the Space Age (New York: Basic Books, Inc., 1985), p. 165.

***** The eureka phenomenon derives its name from an exclamation (heurika, meaning "I have found [it]") attributed to the ancient Greek engineer Archimedes (ca. 287-212 B.C.) on his discovery, while in his bath, of the method of determining the relation of weight to volume. Comic strips depict the eureka experience as a light bulb turning on over an inventor's head.

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