Category Archimedes

Philosophers of science traditionally construe observation as some unanalyzed and epistemologically unproblematic primitive used to test hypotheses. Philosophers have been largely oblivious to the facts that data are the point of observation, that collecting data involves instrumentation and modeling that can be very problematic, and that much experimental data collection has nothing to do with hypothesis testing or explanation. Just because they rarely concern hypothesis testing or explanation, flight – and ground-test are ideally pure case studies for the epistemology of instrumentation and data.

Examination of scientific practice suggests the following:

First Law of Scientific Data: No matter how much data you have, it is

never enough.

Second Law of Scientific Data: The data you have is never the data you

really want or need.51

The Second Law reflects the fact that the parameters of interest often are removed from what probes and transducers feasibly can measure. You must calculate derived measures from these direct measures via recourse to models.

Complicating such calculation are systematic errors – “black noise”, not Gaussian or “white noise” – associated with instrumentation. For example, mechanical pressure gauges are unreliable in their upper and lower quarter ranges. And pressure transducers have lag phenomena which create different distortions when pressure is increasing than when falling. Wind tunnels are confined spaces where wall-contact turbulence boundary effects affect the direct measurement values. All such effects must be corrected for in the data reduction process.52

These corrections are accomplished by the application of models predicting the contaminating influence of each boundary effect. Calibration curves based on measured variations in tunnel or instrument performance are another species of model used to make corrections. We see here how very model dependent ground and flight test data are. In each case the raw data are enhanced by the addition of a model that brings into relief the actual measured effects.

Here we encounter the First Law: The available data usually are insufficient for reliable interpretation. Data usually yield intelligible observations only by the addition of assumed models to “raw” data. Models range from simple choice of French curve for interpolating data to sophisticated mathematical structures. Sometimes the additions are empirically substantiated – as when there are systematic errors in the data due to instrument distortions or known chamber effects.

Frequently the additions are not substantiated, so we exploit the fact that at a certain level data and assumptions are interchangeable and make up for insufficient data by adding assumptions in lieu of more data.52. This is a general practice known

as structural escalation where the addition of, perhaps soft or unsubstantiated, assumptions to data produces a far more robust and stable structure that enables clear interpretation of the data.54 Here is where actual scientific practice parts company with standard philosophical wisdom asserting that assumptions or auxiliary hypotheses must be known or established before they can contribute to scientific knowledge.

When scientists model data via addition of unsubstantiated assumptions in lieu of additional data, the relevant epistemological question concerns not epistemic pedigree, but rather robustness: The significant question is whether intelligible structures revealed by addition of assumptions are real effects in the data or whether they are artifacts of assumptions added to the data. Computer modeling allows one to investigate whether the observed structures are real effects in the data or artifacts of the structurally-escalating assumptions.55

The basic strategy is to do variant or end-member modeling to see how robust the effects are: What happens to our structures if we vary the assumptions we add to the data? A robust effect in the data is one that is insensitive to the specific assumptions added to the data over the plausible range of assumptions. If an effect is robust, then it almost certainly is not an artifact of the added assumptions.

But flight test reveals that assessment of real vs. artifactual effects cannot be reduced to mere robustness. For example, many engine variables such as fuel-flow are collected via analog devices subject to considerable noise. A central part of data reduction (and a main GE EDP unit function) is filtering out noise to obtain real signal. Originally this was done simply by inserting various “plug-in” bandpass filters into the data transmission process. Such techniques work only when the signal-to-noise ratio is favorable.56 If it is not, but the signals are suitably regular, one can add together (“integrate”) various returns until one accumulates enough that the sum of their peaks rises above the noise level. While such techniques work well for radar astronomy, their applicability to flight test with its wildly varying test protocols is problematic.57

A more promising technique feeds noisy data into one input and a specified wave function into another input and performs a cross-correlational analysis. If the added wave function is a reasonable approximation to the true signal underlying the noise, there will be “considerable improvement in detection” of the true signal due to “the fact that we are putting more information into the system, and thus may expect to get more out.”58 The decisions as to which signals to add are based more on operator expertise than they are on established fact.

The correctness of such additive filtering cannot be assessed using robustness considerations. Indeed, the lack of robustness (the true signal quickly disappears if the added wave function is not close to right on) indicates the structurally escalated effects are real.

Whether robustness or sensitivity to parameter assumptions testifies to real vs. artifactual effects in the augmented data depends on available theories governing addition of assumptions to data. Good calibration data and corrections may settle things. For filtering, there is a well-established theory of noisy analog data underlying integration filtering techniques used in radar astronomy. (Our addition of a possible wave-form is a one-shot simulation of such integration techniques.) And given that established theory, if our additions are sensitive to the specific wave­forms added, the result is reliable.

These are theoretically driven evaluations of real effects. What if we have no theory – if the assumptions have no pedigree? Then we have real effects where robust parameter variation establishes stable effects. Robust effects in realistic parameter spanning spaces generally are real effects in data. But not all real effects are robust. Non-robust effects also can be real effects, but established knowledge or theory is required to make the case.

Data typically mix real effects with artifacts. We often throw away data to discover real effects. Thus noisy data are a mix of signal and noise, and we use filtering to throw away the noise, bringing the signal into relief. Knowledge about the measured parameter can reduce the amount of noise we collect. In flight test, most parameters are wave phenomena. Sampling at rates higher than the base frequencies yields mostly noise as data. That is why oscillographs don’t work in wind tunnels. Good instrumentation design reduces the amount of noise or artifact introduced into the raw data.

The notion of “raw data” used above is only heuristic. Every instrument design involves implementation of a model of the interaction of various physical parameters with instruments, of the systematic distortions such instruments undergo, and the correction of such instruments — all before we encounter the augmenting modeling assumptions involved in data reduction and analysis.

The points are: (i) all data are model-dependent, (ii) all data reduction and data analysis involves further modeling; (iii) thus there are no “raw data”; (iv) whether assumptions are previously established or unsubstantiated, standard techniques exist for evaluating whether effects revealed by the augmentation of data by assumptions are real effects or artifacts of the data; (v) both instrumentation and added assumptions can introduce artifacts into data; and (vi) the final assessment of artifacts vs. real effects requires recourse to theory or knowledge when effects are not robust.

The RPM numbers already given are enough to show the difficulty of the design problem. To compete with the CJ805-23, P&W wanted the same frontal area, and hence the same fan diameter, namely 53 inches. But their low-pressure design speed was 6500 rpm, 13 percent greater than GE’s fan RPM. This meant that P&W needed to live with a tip-speed over 1430 ft/sec, not only far above GE’s 1260 ft/sec, but also well above the 1400 ft/sec of NACA’s most successful supersonic design. The implied tip Mach number was totally out of the range of P&W’s compressor design technology. P&W had hired none of the central figures from NACA’s supersonic compressor research program. They had access to the NACA reports, but they had done nothing significant toward pursuing very high Mach number stage designs in house. Their compressor design technology was built around a huge data-base of two-dimensional airfoil performance, which they had developed through their own cascade wind-tunnel testing. They had extended this data-base to progressively higher increments in Mach number through testing double-circular-arc blade profiles. This had given them an empirical base for designing stages with pressure – ratios in the 1.25 range, but they had yet to utilize this capability in an engine, and it offered them no basis for designing highly loaded blades with tip Mach numbers above 1.2.

Exacerbating the problem was the fact that P&W had not developed any streamline-curvature computer programs. Their axial compressor computer program had been based on the non-iterative streamtube method that they had employed, in hand calculations, in designing the two spools of the J-57 compressor in the late 1940s. The great virtue of this computer program was that it incorporated their airfoil performance data-base, allowing them not only to identify preferred airfoils in the design phase, but also to predict compressor performance at off-design conditions. Radial equilibrium effects were taken into account in a rough way in the program by transferring flow radially from one geometrically pre­specified streamtube to the next in between blade rows. This approach had proved adequate for their designs in large part because neither of the spools of their compressors contained a great many stages, and hence the cumulative effects imposed by radial equilibrium were not that severe in the back stages of their spools. This streamtube method, however, offered no way for tailoring arbitrary blade contours in the way GE had. P&W was going to have to employ pre-defined airfoils in their fan.

Trans-Atlantic influences have always had an impact on American technology, beginning with the earliest European voyages to encounter the New World and the first permanent European settlers in America during the seventeenth century. Traditional symbols of the American frontier, like the ax, log cabin, and the Kentucky Rifle, all had European origins. Many American engineers learned their trade as apprentices to immigrant figures like the Englishman, Benjamin Latrobe (1764-1820). Born near Leeds, he was educated in Britain and Germany before emigrating to Virginia in 1796. Over the next quarter of a century, he engineered numerous major buildings, public waterworks, and influenced the design of the Capitol Building of the U. S. Congress in Washington, D. C. Latrobe’s career touched the lives of innumerable designers, engineers, and construction companies. Similar instances extended through the 20th century.

Americans may be vaguely aware of this scientific-technological legacy, but they also tend to regard certain modern technological phenomena as distinctively American: Henry Ford invented the automobile; the Wright brothers invented the airplane; and so on. While many aerospace engineers and historians have been aware of international influences in specific instances, the collective impact seems to have been far more pervasive than generally assumed.1 So that is the focus of this paper – the international impact on the history of American flight. There are three major themes of international influence: the influence of topical literature, the immigration of aerospace professionals, and the globalization of the aerospace industry since 1945. In this essay, I have focused on the first two factors: topical literature and aviation professionals.

In the United States, the Wright brothers’ powered flight on 17 December 1903 is generally regarded as a unique American triumph – a demonstration of traditional Yankee ingenuity. True, the Wrights made several significant contributions in control systems, airfoil theory, and propeller design. But they started by writing to the Smithsonian Institution, in Washington, D. C., for available literature on human flight. Consequently, they used a wind tunnel traceable from published work by Englishman Francis Wenham in 1871. Assorted aerodynamic theories came from a variety of sources, including the well known British engineer, John Smeaton. The Wrights learned a great deal from the published results of Otto Lilienthal’s gliding experiments in Germany, and studied data from other pioneering fliers like Percy Pilcher in Britain. Although the Wrights developed their own engine in 1903, the

207

P Galison and A. Roland (eds.), Atmospheric Flight in the Twentieth Century, 207-222 © 2000 Kluwer Academic Publishers.

internal combustion engine itself was largely the result of late nineteenth century refinements by Gottlieb Daimler and Karl Benz in Germany. The Wrights’ aerial achievement above a remote beach at Kitty Hawk, North Carolina, certainly owed much to non-American sources.2

In the years prior to World War I, aerial meets across the country and demonstrations at annual state and county fairs featured daring pilots from Europe as well as the U. S. These events helped the nation become “air-minded,” and often created a market for airplanes. For several years, one of the best selling aircraft in America was the French Bleriot, offered as a completely finished product or available in kit form. The Bleriot planes started dozens of Americans in aeronautical careers and became a recognized symbol of aviation progress on both sides of the Atlantic. About the time world War I broke out in Europe, there was an active aviation community in America, although it was recognized that the U. S. had fallen behind the Europeans in aeronautical research. The Europeans took a different view of aviation as a technological phenomenon, and their governments, as well as industrial firms, tended to be more supportive of what might be called “applied research.” As early as 1909, the internationally known British physicist, Lord Rayleigh, was appointed head of the British Advisory Committee for Aeronautics; in Germany, Ludwig Prandtl and others were beginning the sort of investigations that soon made the University of Gottingen a center of theoretical aerodynamics. Additional programs were soon under way in France and elsewhere on the continent. Similar progress in the United States remained slow. In fact, until 1915, most American-designed aircraft used airfoil sections from standardized tables issued by the Royal Air Force and by the French engineer, Alexander Eiffel, who pursued an ancillary career as an aerodynamacist in the years after completing his engineering masterpiece in Paris, the Eiffel Tower.

Proponents of an American research organization not only turned consistently to the example of the British Advisory Committee, but also used some of the language of its charter in framing a similar document for the United States. This early example of the impact of European literature on American efforts had evolved from first-hand observation and comparison on the eve of World War I. In 1914, acutely aware of European progress, Charles D. Walcott, secretary of the Smithsonian Institution, was able to find funds to dispatch two Americans on a fact-finding tour overseas. Dr. Albert F. Zahm taught physics and experimented in aeronautics at Catholic University in Washington, D. C.; Dr. Jerome C. Hunsaker, a graduate of the Massachusetts Institute of Technology, was developing a curriculum in aeronautical engineering at the MIT. Their report, issued in 1914, emphasized the galling disparity between European progress and American inertia. The visit also established European contacts that later proved valuable to the NACA.3

The outbreak of war in Europe in 1914 helped serve as a catalyst for the creation of an American agency. The use of German dirigibles for long-range bombing of British cities and the rapid evolution of airplanes for reconnaissance and for pursuit underscored the shortcomings of American aviation. Against this background,

Charles D. Walcott pushed for legislative action to provide for aeronautical research allowing the United States to match progress overseas. Walcott received support from Progressive-era leaders in the country, who viewed government agencies for research as consistent with Progressive ideals such as scientific inquiry and technological progress. By the spring of 1915, the drive for an aeronautical research organization finally succeeded and the National Advisory Committee for Aeronautics (NACA) was formally authorized.

The tour of Europe by Zahm and Hunsaker not only hastened the origins of the NACA, but also influenced the teaching of aeronautics in America, since both learned much from their visits to European research centers. In the process, Hunsaker became more familiar with the aerodynamic experiments of Alexander Eiffel, the renowned designer and constructor of the Eiffel Tower in Paris (1889). During the late 19th and early 20th centuries, Eiffel became immersed in aeronautical experiments including the construction of a wind tunnel of advanced design. Because there were so few reliable sources on aerodynamics, Hunsaker and his wife translated Eiffel’s book on the subject, and it became a key text for Hunsaker’s pioneering aeronautics courses at MIT.

After America entered World War I in 1917, a lack of adequate fighter planes forced combat pilots like Eddie Rickenbacker and others to rely on French and British equipment. The only U. S.-built plane to see extensive combat in the war, the de Haviland DH-4, was a direct copy of the British version. The DH-4 also served as the backbone of the pioneering U. S. Air Mail service during the 1920s.4

European influences also affected other prewar trends in America. On the invitation of Glenn Curtiss, Douglas Thomas left England in 1914 to join the young Curtiss airplane company. Thomas, an experienced engineer at the famed British firm of Sopwith, became a leading designer at Curtiss, where he played a central role in laying out the flying boat America and a trainer that evolved into the famous JN-4 “Jenny” series of World War I. Like the DH-4, the Jennies emerged as a major symbol of American aviation development in the postwar era. Thomas subsequently joined another company, Thomas-Morse and helped design the Thomas-Morse Scout, a significant biplane fighter design. The Thomas Morse organization had begun with two British brothers, William and Oliver Thomas (no relation to Douglas) who graduated from London’s Central Technical College in the early 1900s. They migrated to America, worked for Curtiss and various other engineering firms, and finally established themselves as the aeronautical engineering firm of Thomas Brothers in Bath, New York. World War I brought a volume of orders for planes and engines. Needing capital and room for expansion, Thomas Brothers merged with the Morse Chain Company in Ithaca, New York. As the Thomas-Morse Aircraft Corporation, the company employed more that 1,200 workers and became one of the leading aviation manufacturers of the World War II era. In the 1920s, Thomas-Morse eventually became part of Consolidated Aircraft, which evolved into the aerospace giant, General Dynamics. About the time of the 1920s merger, Oliver Thomas emigrated to Argentina to become a rancher. William remained in America and became fascinated with the pastime of building model airplanes. During the 1930s, he became prominent in promoting the hobby as a national phenomenon and was eventually named as president of the Academy of Model Aeronautics. Over the decades since, the Academy offered the creative framework that subsequently launched the careers of thousands of aeronautical engineers.5

In the immediate postwar era, America again drew on European expertise to develop the young National Advisory Committee for Aeronautics. With no firsthand experience, NACA planners built a conventional, open circuit tunnel based on a design proved at the British National Physical Laboratory. At the University of Gottingen in Germany the famous physicist Ludwig Prandtl and his staff had already built a closed circuit, return flow tunnel in 1908. Among other things, the closed circuit design required less power, boasted a more uniform airflow, and permitted pressurization as well as humidity control. The NACA engineers at Langley knew how to scale up data from the small models tested in their sea level, open circuit tunnels, but they soon realized that their estimates were often wide of the mark. For significant research, the NACA experimenters needed facilities like the tunnels in Gottingen. They also needed someone with experience in the design and operation of these more exotic tunnels. Both requirements were met in the person of Max Munk.

Munk had been one of Prandtl’s brightest lights at Gottingen. During World War I, many of Munk’s experiments in Germany were instantaneously tagged as military secrets (though they usually appeared in England, completely translated, within days of his completing them). After the war, Prandtl contacted his prewar acquaintance, Jerome Hunsaker, with the news that Munk wanted to settle in America. Officially, America still remained at war with Germany. For Munk to enter the United States in 1920, President Woodrow Wilson had to sign two special orders: one to countermand Munk’s status as an enemy alien, and another permitting him to hold a government job. In the spring of 1921, construction of a pressurized, or variable density tunnel, began at Langley. Under Munk’s supervision, the tunnel began operations in 1922 and proved highly successful in the theory of airfoils, contributing to the NACA’s growing reputation as a world center for airfoil research. Munk’s tenure at the NACA was a stormy one. He was brilliant, erratic, and an autocrat. After many confrontations with various bureaucrats and Langley engineers, Munk resigned from the NACA in 1929. But his style of imaginative research and sophisticated wind tunnel experimentation was a significant legacy to the young agency.6

The American aviation community continued to keep a close eye on European developments. While serving as NACA Langley’s chief physicist, Edward Pearson Warner was packed off to Europe in 1920 for an extensive tour designed to gain insights concerning research and development trends among Europe’s leading aviation centers. Soon after, the NACA established a permanent observation post in Paris. Headed by John J. Ide, this Continental venue maintained a steady flow of information to American civil and military authorities. The Paris office remained an important operation until World War II forced its closure.

Back in the U. S., the NACA continued to be influenced by Europeans on its staff as well as European theory imported to serve in NACA’s research projects. One of the principal figures to emerge in this era was Theodore Theodorsen, a Norwegian emigre and Chief Physicist at Langley in 1929. After graduating from the Technological Institute of Norway in 1921, he taught there and came to the U. S. three years later. He was an instructor at Johns Hopkins, 1924 to 1929, where he received his Ph. D. Steeped in mathematical research, he was a strong proponent of airfoil studies through theoretical analyses. In this respect, he proved a useful counterpart to experimental investigators like the American, Eastman Jacobs, who was pushing for a new variable density tunnel in the 1930s. Their exchanges helped shape research that led to laminar flow wings. While the NACA deserves credit for its eventual breakthrough in laminar flow wings, the resolution of the issue illustrates a fascinating degree of universality in aeronautical research. The NACA, born in response to European progress in aeronautics, benefited through the employment of Europeans like Munk and Theodorsen and profited from a continuous interaction with the European community – or at least in attempts to stay abreast.

In 1935, Jacobs traveled to Rome as the NACA representative to the Fifth Volta Congress on High-Speed Aeronautics. During the trip, he visited several European research facilities, comparing equipment and discussing the newest theoretical concepts. The United States, he concluded, held a leading position, but he asserted that “we certainly cannot keep it long if we rest on our laurels.” On his way home, Jacobs stopped off at Cambridge University in Great Britain for long visits with colleagues who were investigating the peculiarities of high-speed flow, including statistical theories of turbulence. These informal exchanges proved to be highly influential on Jacobs’ approach to the theory of laminar flow by focusing on the issue of pressure distribution over the airfoil. Working out the details of the idea took three years and engaged the energies of many individuals, including several on Theodorsen’s staff even though Theodorsen himself remained skeptical.

Once the theory appeared sound, Jacobs had a wind tunnel model of the wing rushed through the Langley shop and tested it in a new icing tunnel that could be used for some low-turbulence testing. The new airfoil showed a fifty percent decrease in drag. Jacobs was elated, not only because the project incorporated complex theoretical analysis, but also because the subsequent empirical tests justified a new variable density tunnel. Without diminishing the role of the NACA in laminar flow research, the British influence represented an essential catalyst in the story.7

Advances in aeronautical theory represented only one dimension of aeronautical progress in America; the European legacy embraced a variety of practical domains having a lasting influence on the American scene. During World War I, the Dutch designer Anthony Fokker gave his name to a series of German fighters that built a formidable reputation. He re-established his firm in Holland after the war, then moved to America in 1922, first as a consultant then as head of his own manufacturing company. The cachet of the Fokker name helped make his big, tri-motor airliners successful and materially promoted airline travel in the United States. The welded, tubular steel fuselage framework and cantilevered wings of Fokker transports represented a valuable example of design and construction during the pre-WWII era. Subsequent progress in modem, metal aircraft reflected a marked heritage from Germany in the person of Adolf Rohrbach, a pioneer in the art of stressed skin construction. Rohrbach delivered some highly publicized lectures in the United States during 1926 and published an influential article on this subject that appeared in the Society of Automotive Engineers Journal in 1927. Then there was Samuel Heron of Britain. Before settling in the United States in 1921, he had worked for Rolls Royce and other leading British engine manufacturers. In addition to his work in the technical center for the U. S. Air Corps at McCook Field, Heron worked for Wright Aeronautical, Ethyl Corporation, and other American companies. Heron proposed the sodium-cooled valve, a key component of high-powered radial engines that helped pave the way for the use of potent, high-octane fuels in modem aircraft powerplants. Charles Lindbergh’s non-stop flight across the Atlantic in 1927, in a plane powered by a Wright Aeronautical engine, owed a debt to several areas of Heron’s work in aircraft engines and fuels.8

A variety of additional practical issues needed resolution, and Europeans played a key role here as well. A catalyst in this respect was the Daniel Guggenheim Fund for the Promotion of Aeronautics. America lacked an aeronautical infrastructure. Commercial aviation in particular needed daily, reliable weather forecasts, a foundation of legal guidelines, and a nation-wide educational system for training aeronautical engineers and scientists. The Guggenheim Fund helped bridge these gaps, relying heavily on imported know-how and experts from overseas. Between 1926 and 1930, this private philanthropy supported a variety of programs that profoundly influenced the growth of American aviation. Since meteorology was necessary for accurate forecasting over airline routes, the Guggenheim Fund sponsored several research efforts and founded a department of meteorology at MIT. The expert who directed these Guggenheim efforts was Carl-Gustav Rossby, bom in Stockholm, and educated in Sweden, Norway, and Germany. After building the meteorology department at MIT, he went on to Chicago in 1941. Through his own research and through influence on a new generation of students, Rossby laid the foundations for aviation weather forecasting in the United States. The Guggenheims also promoted professional studies in aviation law, developing the Air Law Institute within Northwestern University. The American organization enjoyed immense benefits from an exchange of professors with the Air Law Institute of Konigsberg in Germany.9

As aviation in the U. S. progressed after World War I, the need for larger numbers of trained engineers became evident. Two of the pioneering American universities with major aeronautical training curricula had emigres as principal professors. At the University of Michigan it was Felix Pawlowski, trained in Germany and France before the war. In 1913, he began offering some of the first aeronautical engineering courses in America, worked for the U. S. Army War Department, and became head of Michigan’s Aeronautical Engineering Department in the postwar era. Pawlowski maintained close contacts with the aeronautical community overseas; Michigan’s curriculum was continuously enlivened by visiting European experts who fascinated students with discussions of advanced theoretical studies and research problems. Moreover, the aeronautical engineering curricula at universities across America relied heavily on British textbooks in advanced aerodynamics, structures, and related aviation topics. In addition to Pawlowski at Michigan, other schools also employed European professors.

At New York University, it was Alexander Klemin, who graduated from the University of London in 1909, and came to America in 1914. He took an MS degree at MIT and succeeded Hunsaker as director of its Aeronautics Department. In 1925, he became Guggenheim Professor of Aeronautics at New York University, where he enjoyed a long and distinguished career. In the 1930s, his interest in rotary wing flight made NYU a center of research in helicopters and autogiros. Klemin’s success in acquiring sophisticated wind tunnel facilities gave NYU an additional role as a center of productive testing for major northeastern manufacturers like Grumman, Seversky, Vought, and Sikorsky. Moreover, Klemin became a leading figure in the institutionalization of aeronautics in America. He was one of the people who helped create one of the early industry periodical magazines, Aviation, which gained strength through successive decades, and eventually became known as Aviation Week and Space Technology. In 1933, Klemin joined Jerome Hunsaker, Edward P. Warner, and others who desired a professional engineering focus apart from the Society of Automotive Engineers, the professional home of most of aviation’s practicing engineers. Like the founders of the NACA, the founders of the new American organization also looked to Europe for precedents and used the Royal Aeronautics Society as the model for the Institute of Aeronautical Sciences. In due time, the IAS evolved into the American Institute of Aeronautics and Astronautics, the premier aviation and aerospace organization in the United States.

These foreign influences received little or no acknowledgment in aviation circles, although there was one notable exception – the NACA cowling. Details of this important component, which enclosed radial engines in such a way that drag was notably reduced and cooling was enhanced, appeared in an NACA technical note in 1928. The NACA configuration unquestionably resolved many aerodynamic and practical problems. Nonetheless, the agency never took out a patent on the cowling, ostensibly because it was unwilling to joust with British experts over the relative merits of the “Townend ring” (after British researcher Hubert Townend) which predated the NACA design. As one veteran engineer, H. J.E. Reid observed in 1931, “It is regrettable that the [Langley] Laboratory, in its report on cowlings, did not mention the work of Townend and give him credit.” 10

But America still lagged in theoretical aerodynamics. In 1929, the Guggenheim Fund played a crucial role in luring the brilliant young scientist trained at Gottingen, Theodore von Karman, to the United States. Von Karman joined the faculty at the California Institute of Technology and helped transform the science of aeronautics especially in high-speed research. Within the decade, not only did the Institute’s research projects enrich the field of aerodynamic theory, its graduates began to dominate the discipline in colleges and universities across the nation. During and after World War II, Von Karman became a central figure in American jet propulsion and rocket research.11

The largest foreign group in American aeronautics was Russian – emigres who left their country in the wake of the Revolution of 1917 and the end of the Romanov dynasty. They occupied a variety of positions in academics and industry, and left an enduring legacy of progress. For example, Boris Alexander Bakhmeteff became Professor of Civil Engineering at Columbia University, where his work in hydraulics made him a recognized aviation consultant. He was bom in Tbilisi in 1880, educated in Russia and Switzerland, and taught hydraulics and theoretical mechanics at the Polytechnic Institute at St. Petersburg before coming to America in 1917. Alexander Nikolsky was bom in Kursk in 1902, and was educated at the Russian Naval Academy, 1919-1921. He did advanced studies in Paris in the mid – 1920s, coming to the U. S. in 1928. After further graduate study at MIT, he became a design chief at Vought-Sikorsky Division of the United Aircraft Corporation. Nicholas Alexander, bom in Russia in 1886, became a professor of aeronautical engineering at Rhode Island State College after World War I. There were many others who contributed to American progress as engineers and educators.12

Two Russian emigres became major figures in the American aviation manufacturing industry. Alexander Prokofieff de Seversky was bom in Tbilisi in 1894. After graduation from the Imperial Naval Academy in 1914, he had started post-graduate studies at the Military School of Aeronautics when World War I began. By the time of the Revolution in 1917, he had been shot down and lost a leg, although he returned to duty and shot down 13 German planes. He came to the United States in 1918 as part of a Russian air mission, but decided to remain, becoming a test pilot for the U. S. Army Air Service. His training eventually led to a post as consulting engineer, and he spent several years perfecting an improved bombsight with automatic adjustments. His patents on bomb sights earned money to start the Seversky Aero Corporation (later, the Republic Aircraft Corporation). Over the years, de Seversky invented several items: an improved wing flap; improved procedures in stmctural fabrication; turbo-superchargers for air-cooled engines. Seversky’s company designed and built the P-35 fighter in the 1930s, a plane with retractable landing gear and other features that represented an important transition to modem fighters in the U. S. Army Air Force. The chief engineer for the P-35, Alexander Kartveli, a fellow emigre from Russia, performed a critical role in the P-35 project, as well as its more famous successor, the legendary Republic P-47 Thunderbolt of World War II. Finally, de Seversky’s books and articles on aviation and aerial warfare were widely read in America and helped the country respond to the realities of the new air age as a result of World War II.13

Without a doubt, the best known Russian figure was Igor Sikorsky, bom in Kiev in 1889. Following his education at the Naval Academy of St. Petersburg, he took courses at the Polytechnic Institute of Kiev in 1907-1908. During those years, he began designing and building aircraft, leading to the first four-engined planes to fly.

After immigrating to the U. S. in 1919, Sikorsky developed a number of successful planes, and his company became a division of United Aircraft Corporation in 1929. A series of Sikorsky flying boats during the 1930s established important structural advances, set records, and helped the U. S. to establish pioneering overwater routes to Latin America and to the Orient. Sikorsky also spent considerable effort in perfecting helicopters, and his 1939 machine set the pattern for subsequent helicopter progress in America. As one knowledgeable engineer-historian wrote later, “Few men in aviation can match the span of personal participation and contribution that typify Igor Sikorsky’s active professional life.”14

Less well known, but significant nevertheless, were the contributions of a Sikorsky employee, also Russian, who started the United States on the road towards swept-wing aircraft in the postwar era. Additional emigres from other European countries also helped shape America’s research in high-speed aerodynamics and transonic analyses. Considerable influence emanated from Germany, a traditional leader in theoretical studies in the 1930s and through World War II. In many instances, personnel at the NACA’s Langley laboratories had made preliminary steps in the direction of advanced work, but the data gleaned later from captured German documents often served as catalytic elements in achieving postwar results. By the end of the war, American analysts were already unnerved by the success of Germany’s jet combat aircraft and missile technology, in addition to variable-sweep aircraft prototypes and seemingly bizarre advanced studies. Summing up these “shocking developments,” as NACA veteran John Becker remembered them, he also noted that NACA’s prestige with industry, Congress, and the scientific community had sunk to a new low.

Like several other chapters in the story of high speed flight, the story began in Europe, where an international conference on high speed flight – the Volta Congress – met in Rome during October 1935. Among the participants was Adolf Busemann, a young German aeronautical engineer from Lubeck, who proposed an airplane with swept wings. In the paper Busemann presented at the Rome Conference, he predicted that his “arrow wing” would have less drag than straight wings exposed to shock waves at supersonic speeds. There was polite discussion of Buseman’s paper, but little else, since propeller-driven aircraft of the 1930s lacked the performance to merit serious consideration of such a radical design. Within a decade, the evolution of the turbojet dramatically changed the picture. In 1942, designers for the Messerschmitt firm, builders of the remarkable ME-262 jet fighter, realized the potential of swept wing aircraft and studied Busemann’s paper more intently. Following promising wind tunnel tests, Messerschmitt had a swept-wing research plane under development as the war ended. The American chapter of the swept wing story originated with Michael Gluhareff, a graduate of the Imperial Military Engineering College in Russia during World War I. He fled the Russian revolution and gained aeronautical engineering experience in Scandinavia. Gluhareff arrived in the United States in 1924 and joined the company of his Russian compatriot, Igor Sikorsky. By 1935, he was chief of design for Sikorsky Aircraft and eventually became a major figure in developing the first practical helicopter.

In the meantime, Gluhareff became fascinated by the possibilities of low-aspect ratio tailless aircraft and built a series of flying models in the late 1930s. In a memo to Sikorsky in 1941, he described a possible pursuit-interceptor having a delta­shaped wing. Eventually, a wind tunnel model was built; initial tests were encouraging. Wartime exigencies derailed GluharefFs “Dart” configuration until 1944, when a balsa model of the Dart, along with some data, wound up on the desk of Robert T. Jones, a Langley aerodynamicist. Studying GluhareflPs model, Jones soon realized that the lift and drag figures for the Dart were based on outmoded calculations for wings of high-aspect ratio. Using more recent theory for low-aspect shapes, backed by some theoretical work published earlier by Max Munk, Jones suddenly had a breakthrough. He made his initial reports to NACA directors in early March, 1945. Within weeks, advancing American armies captured German scientists and test data that corroborated Jones’ assumptions. Utilization of theses collective legacies, as well as wartime studies on supersonic wind tunnels by Antonio Ferri, of Italy, all leavened successful postwar progress in high-speed research and aviation technology.15

World War II imparted additional aspects of international influence on American progress in aviation and air power. One example involved the famous Norden bombsight. Highly touted before and after the war as a top-secret, crucial American weapon, its originator and namesake was a Dutchman bom in the Dutch East Indies (1880), educated in Germany and Switzerland, an emigre to America in 1904, an entrepreneur during the 1920s and 1930s, and a well-to-do retiree in Zurich, Switzerland, where he died in 1965 as a non-U. S. citizen who still proudly held his Dutch citizenship. During the war, thousands of state-of-the-art, high precision aeronautical instruments in American aircraft came from the production facilities of the Kollsman Instrument Company. Paul Wilhelm Kollsman, bom in Germany, was educated in Munich and Stuttgart before immigrating to America in 1923; he founded the instmment company five years later. One of the most curious international episodes involved the celluloid femme fatale, Hedy Lamarr, the glamorous film star bom in Vienna, Austria, and George Antheil, the American-born composer. Based on Lamarr’s earlier marriage to an Austrian arms dealer and manufacturer, she picked up a workable understanding of electronic signals. With the assistance of the eclectic Antheil (and encouraged by Charles Kettering, the research director of General Motors), they patented a control device in 1942. Regrettably, their system for a jam-proof radio control system for aerial launched torpedoes was not fielded during the war. However, the principles in the Lamarr – Antheil patent became the basis for successful jamming systems that evolved in the 1960s. In a different context, chemical engineering research by the German – American firm of Rohm and Haas resulted in extremely significant wartime advantages for the United States. “Plexiglas,” the material almost exclusively used in U. S. military aircraft of World War II, was basically developed by the German component of Rohm and Haas in the late 1930s. Politically and legally separated from its German counterpart during the war, the American constituency of the firm perfected the product and turned out prodigious quantities of Plexiglas for the

American war effort. The U. S. component also produced military grade hydraulic fluid that retained its functional properties in both high and low temperature extremes, making it an invaluable part of Allied air combat operations.

This international context of the U. S. aviation industry was even more manifest in aircraft production. Between 1938 and 1940, British and French orders totaled several hundred million dollars and over 20,000 aircraft, at a time when Congress had authorized a U. S. Air Corps strength of only 5,500 planes. Official U. S. Air Force histories later noted that the pre-war European orders had effectively advanced the American aircraft industry by one whole year.16 Additional overseas legacies were represented by the development of the P-51 fighter and the evolution of jet engines.

The P-51 developed a reputation as one of the best fighters of World War II. Ironically, its introduction into the Air Force occurred almost as an afterthought. The design had originated in the dark days of 1940, when the RAF placed an emergency order with North American Aviation in California. In a series of around-the-clock design conferences, North American’s engineers finalized a configuration and hand – built the first airplane in just 102 days. The principal project engineer for the P-51 was Ed Schmued. Bom and educated in Germany, Schmued worked with aviation firms in Europe and South America before arriving in the United States in 1930, when one of the companies who employed him wound up as part of the North American Corporation. During the gestation of the P-51 design, the NACA’s Eastman Jacobs happened by one day, and the North American design team pressed him for details of the NACA wing to be used on the airplane. Relying on laminar flow, this feature constituted yet another element of the European legacy to American aeronautics. The P-51 Mustang emerged from the drawing boards as a lean, lithe airplane. After flying an early export version powered by an Allison engine, a canny test pilot from Rolls Royce (Ronald W. Harker) realized that the more powerful Rolls Royce Merlin engine might give the Mustang a stunning increase in performance. He was right. With a top speed surpassing 440 MPH, the Mustang could outspeed and outmaneuver any comparable German fighter. Rolls Royce licensed the Merlin engine for manufacture in the United States, and the hybrid P-5 ID Mustang went into production for the U. S. Army Air Forces in 1943. From beginning to end, the P-51 reflected a consistent European heritage.17

In America, the idea of jet propulsion had surfaced as early as 1923, when an engineer at the Bureau of Standards wrote a paper on the subject which was published by the NACA. The paper came to a negative conclusion: fuel consumption would be excessive; compressor machinery would be too heavy; high temperatures and high pressures were major barriers. These were assumptions that subsequent studies and preliminary investigations seemed to substantiate into the 1930s. By the late 1930s, the Langley staff became interested in the idea of some form of jet propulsion to augment power for military planes for takeoff and during combat. In 1940, Eastman Jacobs and a small staff came up with a jet propulsion test bed they called the “Jeep.” By the summer, however, the Jeep had grown into something else – a research aircraft for transonic flight. With Eastman Jacobs again, a small team made design studies of a jet plane. Although work on the Jeep and the jet plane design continued into 1943, these projects had already been overtaken by European developments.

Frank Whittle, in England, had bench-tested a jet engine in 1937, and four years later, a plane was developed to demonstrate it in flight. During a tour to Britain in April 1941, General H. H. “Flap” Arnold, Chief of the U. S. Army Air Forces, was dumbfounded to learn about a British turbojet plane, the Gloster E28/39. The aircraft had already entered its final test phase and, in fact, made its first flight the following month. Fearing a German invasion, the British were willing to share the turbojet technology with America. That September, an Air Force Major, with a set of drawings manacled to his wrist, flew from London to Massachusetts, where General Electric went to work on an American copy of Whittle’s turbojet. An engine, along with Whittle himself, eventually followed. A special contract went to Bell Aircraft to design a suitable plane, designated as the XP-59A. Development of the engine and design of the Bell XP-59A was so cloaked in secrecy that the NACA learned nothing about them until the summer of 1943.

The XP-59A, equipped with Whittle engine, became the first American jet plane to fly, taking to the air on October 1, 1942. Subsequent prototypes used General Electric engines that had evolved from the original Whittle powerplant. Similarly, many of America’s first-generation military jet planes began their operational lives with British engines. The USAF’s first operational jet fighter, the Lockheed P-80 Shooting Star, was designed around the de Havilland Goblin jet engine. British influence remained strong through the mid-1950s. The Republic F-84F Thunderstreak had a Wright Aeronautical J-65 engine, built under license from the Sapphire powerplant of British Armstrong Siddeley. Grumman’s U. S. Navy jet fighter, the F9F Panther, also relied on versions of British jet engines: the F9F-2 had a Pratt & Whitney J-42 (licensed from the Rolls-Royce Nene design); the F9F-5 used a Pratt & Whitney J-48 (licensed from the Rolls-Royce Tay engine series).

Clearly, American jet engines in the early postwar era owed much to this British bequest, along with a catalog of technological legacies from German sources. “Project Paperclip” brought some 260 scientists and engineers to work in America at United States Air Force research and development centers. Along with leading aerodynamicists came gas turbine specialists like Hans von Ohain and Ernst Eckert. The first jet plane to fly (in 1939) used a jet engine designed by von Ohain, who spent his postwar career in development laboratories at Wright-Patterson Air Force Base. An expert in heat transfer, Eckert soon found himself at NACA’s Lewis Laboratory, where he helped lay the foundations for film cooling of turbine blades – a fundamental advance in gas turbine technology. Eckert’s work at Lewis sparked a continuing process of successful research in this field; he wrote basic reference works on the subject; his tenure at the University of Minnesota established heat transfer studies as an accepted subject that subsequently occupied researchers at America’s leading aeronautical engineering schools. 18

The European legacy was also evident in postwar flight research, such as the rocket-powered X-15 research planes of the late 1950s. The X-15 series were thoroughbreds, capable of speeds up to Mach 6.72 (4534 MPH) at altitudes up to 354,200 feet (67 miles). There was a familiar European thread in the design’s genesis. In the late 1930s and during World War II, German scientists Eugen Sanger and Irene Bredt developed studies for a rocket plane that could be boosted to an Earth orbit and then glide back to land. The idea reshaped American thinking about hypersonic vehicles. “Professor Sanger’s pioneering studies of long-range rocket – propelled aircraft had a strong influence on the thinking which led to initiation of the X-15 program.” NACA researcher John Becker wrote, “Until the Sanger and Bredt paper became available to us after the war we had thought of hypersonic flight only as a domain for missiles….” A series of subsequent studies in America “provided the background from which the X-15 proposal emerged.”19 During the Cold War era, when America and the Soviets began their ideological and technological race to land a man on the moon, the American space effort continued to draw from assorted international sources. As a group, the most significant “catch” of Operation Paperclip may have been Wemher von Braun and the German research team responsible for the remarkable V-2 missile technology. The von Braun team assisted American counterparts in developing a family of postwar military rockets and related space technology, fabricated the booster for America’s first artificial satellite, Explorer I (January 31, 1958), and played a central role in developing the Saturn launch vehicles used in America’s successful manned lunar landing in 1969. Nor were the German emigres with the von Braun contingent the only foreign team to impact the American space effort. In the early 1960s, following Canada’s cancellation of an advanced jet fighter/interceptor designed by the Canadian firm, AVRO, the National Aeronautics and Space Administration immediately sought out the project’s key engineers to work on the early phases of the Apollo project. Over two dozen AVRO veterans signed on, becoming key players in research and development of Apollo systems and operational technology.20

We traced the evolution of flight and ground test instrumentation and data from Stage 1 to Stage 3. With the XB-70 project the transition to automated flight test instrumentation, recording, and data analysis essentially was complete. Although there is much overlap between airframe and engine flight testing, engine test cells, and wind tunnels, each test medium imposes its own peculiar instrumentation demands; thus there are variations among them in instrumentation and recording devices utilized at a given stage.

Instrumentation advances between 1940-1969, including computerization, generally increased the quantity of data – the number of variables, parameters, and readings – collected and analyzed but did not increase the accuracy of measurements (see Table 2). Numbers of measurements grew roughly at the rate of increased computational capability.

Primary motivations for undertaking huge costs of automated data collection and processing were

• the need to monitor more channels of data as aircraft themselves became more complex and computer-controlled;

• advantages of being able to monitor data in real time and use it to modify test protocols mid-flight.

The latter can lead to enormous, cost-saving, and ultimately is the economic justification for costly automated data systems with telemetry. For example, when Grumman used a computerized telemetry system backed by a CDC 6400 in development of the F-14 Tomcat they experienced a time saving of 67% compared to prior Grumman flight test programs and performed 47% fewer test flights.59 In less demanding test situations where such cost-saving is not expected, oscillographs and other Stage 2 techniques continue to be used even today.

The First and Second Laws of Scientific Data continue to govern aircraft and engine testing. Thousands of data channels did not obviate the need for modeling data via structurally escalating assumptions. By adding model structures to the data we come to see clearly what is the actual performance of our aircraft and engines. The intelligibility of experimental data largely depends upon correcting for systematic errors, deriving the measures you really want via modeling, and separating real effects from artifacts. We see what otherwise would be lost in the noise of instrumentation and raw data.

Experimental data are not some epistemic “given.” Flight test, test cells, and wind tunnels are quintessentially experimental yet rarely involve hypothesis testing or theory confirmation. Thus they provide marvelous insight into the heart of experiment undistorted by standard yet questionable philosophical views about testing, confirmation, or observation.

ACKNOWLEDGMENTS

Precursor portions were presented in a Year of Data talk, University of Maryland, College Park, September 1992, and to Andrew Pickering’s University of Illinois Sociology of Science lecture series, spring 1995. The assistance of Dr. Jewel Barlow, Director of the Glenn L. Martin Wind Tunnel at University of Maryland is much appreciated. Comments on the draft by Dibner workshop participants, a UMCP audience, and especially Peter Galison were quite helpful.

The following people assisted in the collection of photographs and information: Cheryl A. Gumm, Don Thompson, Jim Young, USAF Flight Test Center, Edwards AFB; Don Haley, NASA Ames Dryden Flight Research Facility; Tom Crouch and Brian Nicklas, National Air and Space Museum; Richard P. Hallion, Office of the

Frequency Modulation 3-10%

Pulse-Duration Modulation 1 -2%

Pulse Coded Modulation __________________________ 1%___________________________________

NOTES:

1 Most data from Kerr 1961.

2 Varies with type of data analysis. See Bethwaite 1963, p. 237, for estimates.

3 Accuracy varies with the quantities measured: Noise and vibrations: 5-10%

Most flight test channels 1-2%

A few selected channels, achievable only by using digital recording: 0.1-0.5%

It is very difficult to achieve 0.1% accuracy in flight test. Ground facilities such as wind tunnels may achieve accuracies of 0.001% at perhaps 8 measurements per second, during this period.

4 Higher frequency measures (e. g., vibrations) tend to have higher errors and require FM recording. For most other signals, PCM is more accurate.

Chief Historian, USAF. Part of the research presented here was supported by an NSF SSTS Award.

Sources for previously unpublished pictures are identified as follows:

Suppe Collection: photographs in my personal collection.

Young Collection: photographs in Jim Young’s collection, USAF Flight

Test Center.

GLMWT: photographs from the Archives of the Glenn L. Martin

Wind Tunnel, University of Maryland; used with the kind permission of Dr. Jewel Barlow, Director.

NASA/NACA pictures, which are in the public domain are identified by NASA photo number or source. Other pictures are reprinted from identified published sources and are used with permission of the publishers.

P&W’s solution to the design problem they found themselves in involved three elements. First, they had to reduce the Mach number at the leading edge of the fan rotor. They were already employing inlet guide vanes ahead of the J-57 and JT3C-6 low-pressure compressor. Inlet guide vanes are used to turn the flow ahead of the rotor, giving it a tangential or circumferential component. This lowers the relative velocity of the flow incident on the rotor blades. The inlet guide vanes for the fan had to provide additional turning of the flow toward the tip, but this was feasible. Thus, in spite of its high tip-speed, the outer portion of the fan blades would not have to be designed for Mach numbers far above P&W’s range of experience.75 The tip Mach number would still have to push beyond anything P&W had done before, but only incrementally beyond.

Second, P&W employed a two-stage fan, replacing the first three stages of the JT3C-6 low-compressor. The design pressure-ratio of the fan was 1.66, or an average of almost 1.29 per stage. While this was well above anything P&W had put in flight before, and hence demanded a significant reach, it was still modest compared with GE’s 1.655 pressure-ratio in its single stage fan, or even the 1.35 average stage pressure-ratio achieved in the NACA 5-stage transonic fan. The stage pressure-ratio, too, required only an incremental step, and not a quantum jump, beyond P&W’s existing technology. The inner portion of the two stages of the fan had to do the work that was originally done by three stages of the low-pressure compressor, requiring higher work-per-stage airfoils. But the 1.66 pressure-ratio of the two stages corresponded to the pressure-ratio across the first three stages of the original compressor. The fan design problem was further simplified by having the fan stream discharge immediately behind the second stage stator vanes, rather than ducting the flow all the way to the rear to join the core engine discharge, as in the Conway. This saved weight by eliminating a long, large radius duct, and it eliminated any need to match the fan discharge velocity with that of the core engine. The long, slender fan blades required part-span shrouds to prevent blade flutter, but P&W already knew how to do this from their experience with long, short-chord blades in their nuclear engine. Thus, while the two stage fan posed a challenge to P&W’s compressor designers, it did not require anything revolutionary in its design.

Third, P&W had to do something about weight. The JT3C-6 turbojet was excep­tionally heavy to begin with, weighing in at more than 4200 pounds, with a thrust-to-weight ratio of only 3.03.76 Although GE’s CJ805 turbojet produced only 11,000 pounds of take-off thrust, compared with the 13,000 pounds of the JT3C-6, it weighed but 2800 pounds, and the CJ805-23 turbofan engine weighed in at only 3800 pounds. Because the two stages of P&W’s fan were replacing three stages in the low-pressure compressor, the difference in weight at the front end of the engine was not so great, provided the fan was designed for low weight. But the added work being done in the bypass stream demanded that a fourth stage be added to the three stage low-pressure turbine. This threatened to push the weight of P&W’s turbofan engine to a point where it would have trouble competing with the CJ805- 23. P&W took several actions in response to this problem. They re-rated the low-pressure and high-pressure spools to operate at slightly different speeds, the low-pressure spool at 6560 RPM and the high-pressure spool at 9800 RPM.77 Instead of simply adding a stage to the low-pressure turbine, they replaced the existing third stage with two new stages, reducing some of the excessive mechanical safety margin in order to save weight.

The most important action P&W took to keep the weight of their turbofan engine down was to switch to titanium in the low-pressure compressor. They had already introduced titanium rotor blades and disks in advanced military versions of the subsonic J-57 – i. e. the J-57 without afterburner. Partly in response to complaints about the weight of the initial version of the JT3C-6, they were in the process of flight qualifying an advanced version, the JT3C-7, using titanium blades and disks in the low-pressure compressor to replace the steel blades and disks of the JT3C-6, reducing the weight by roughly 700 pounds. Because of the high tip-speed and the absence of an established track record in using titanium in commercial engines, conservatism appropriate to a commercial design dictated that the fan blades and disks be made of steel. By shifting to titanium elsewhere in the low-pressure compressor, however, the weight of P&W’s turbofan engine would be no greater than the weight of the JT3C-6.78 The conversion of the JT3C to a fan engine could thus be achieved with no penalty in weight at all.

The Same Point by a Different Route – the JT3D P&W designated their new turbofan engine the JT3D (see Figure 17). Its bypass ratio was 1.4, a little less than GE’s 1.56 owing to the somewhat larger size of the JT3C gas generator, compared with the CJ805; the total air flow of the engine was 450 lbs/sec, 30 lbs/sec more than the CJ805-23, yielding a take-off thrust of 17,000 pounds, compared with 16,100 pounds for GE’s engine. More important, its overall performance parameters were entirely competitive with those of GE’s engine: a specific fuel consumption around 0.55 and a thrust-to-weight ratio a little over 4.2. The virtues of the turbofan are most apparent when the JT3D is compared with the JT3C-6: a 4000 pound thrust increase, consuming as much as 500 pounds less fuel per hour in an engine of essentially the same weight.79 As Table 2 makes clear, the JT3D was no less a quantum jump over the JT3C than the GE CJ805-23 was over the CJ805-3.

P&W designed and flight qualified both a military and a commercial turbofan engine in a remarkably short time. The military version, designated the TF-33, was to replace the non-afterburning J-57 in the B-52 and the КС-135. The commercial version was to replace the JT3C on the Boeing 707 and the Douglas DC-8. Because the engine weight did not increase, the new engines could replace the old without any significant modification of the airframe. Most striking of all, these engine replacements did not necessitate scrapping of the original engines. A JT3C-7 could be converted into a JT3D in the overhaul shop by substituting the two-stage fan and its casing for the first three low-compressor stages, substituting a new third stage and adding a fourth in the low-pressure turbine, and a few other minor changes.80

From the user’s point of view, it seemed as if the JT3C had evolved into the JT3D, in the process yielding a quantum jump in performance.

Nevertheless, while the JT3D was a breakthrough in overall engine performance, it did not require any revolutionary breakthrough in component aerodynamic design. In this respect it was markedly different from the CJ805-23. The fan design required an advance in stage pressure-ratio and tip Mach number beyond P&W’s existing compressor design technology, but only an incremental advance, not a jump to an entirely new design regime. The use of titanium in a conservative commercial engine was new, but it was well on the way to occurring independently of the turbofan, and titanium had already been in use in military engines. Other improvements in performance in the JT3D gas generator were already on the way, motivated by the very high conservatism P&W had exercised in the design of their first generation commercial turbojet.

Precisely because P&W was already employing two-spool engines, they had been in a position to consider a bypass engine along the lines of the Conway as early as 1953 or 1954, either as a possible advance on the military J-57 or as an economically superior first generation commercial engine. The steps from bypassing part of the flow from forward stages in the low-pressure compressor to the fan design of the JT3D were merely incremental. Equally, P&W was in a position to develop the JT3D directly, without inducement from GE, in 1956, when GE was just starting the design of its aft fan. Undoubtedly, a fan version of the JT3 designed at that time would have had a smaller initial advance in performance, but it could easily have matured into the JT3D just from normal incremental advances in design technology within P&W. GE’s J-79 could not have evolved into the CJ805-23, but P&W’s J-57 could have evolved into the JT3D if P&W had been looking toward bypass engines.

Why then was P&W not the first to come up with a superior turbofan engine? Perhaps P&W had no influential in-house proponent of turbofan engines, comparable to Peter Kappus at GE. But this can at most be part of the answer, for the potential of bypass engines to realize high propulsion efficiency in the high subsonic flight speed range had been known for years, and Wislicenus had called attention to it prominently once again in 1955. So, the answer must also include aspects of P&W’s engineering style and orientation.

After initially developing the J-57 in the late 1940s, P&W had maintained a dis­tinctly conservative design approach, deriving its other principal engines from this one and upgrading them more through advances in materials, including alloys that allowed increases in turbine inlet temperature, rather than through advances in compressor aerodynamic design. This conservatism notwithstanding, in the early 1950s they had achieved total dominance in the high subsonic flight regime in military aviation, where GE was offering no competition at all, and from this they had taken a huge lead in the first generation of commercial transports that were under development in the U. S. Largely because of the extraordinary success of the J-57 two-spool compressor, they had become wedded to the comparatively un­sophisticated design methods used for it, choosing not to switch to more advanced methods when they began using digital computers. Given their analytical tools and their approach to advancing compressor design, P&W probably had difficulty envisaging how much of a jump in performance could be achieved in a front fan version of the JT3. An incremental step in tip-speed and pressure-ratio would permit a turbofan engine with a low bypass ratio like the Conway’s, but the gain from this was not dramatic. From the point of view of their compressor designers, a bypass ratio that might offer clear advantages would require a sequence of incremental advances in stage design – a sequence which looks much more tractable when necessitated by competition. Finally, P&W may have been thinking that the real future of commercial aviation lay not in the high subsonic flight regime, but in the supersonic regime. If they thought that the first generation commercial transports were merely stepping stones to supersonic transports, they had little reason to invest in the pursuit of more economically attractive engines for high subsonic flight.81

Whatever the reasons for P&W’s prior lack of interest in turbofans, and however much less of a design breakthrough the JT3D fan was than that of the CJ805-23, the rapidity with which they managed to come up with a folly competitive alternative to GE’s engine was an extraordinary feat unto itself. The first flight test of the engine took place in July 1959, seven months before the first flight test of GE’s CJ805-23 (delayed eight months by engine installation problems).

Without diminishing the original contribution of many figures who were bom and trained in America, the pervasive influence of international factors in the evolution of American aviation has been significant. Prior to World War I, European experience often provided the starting points for successful aeronautical investigations and served as the model for research institutions like the National Advisory Committee for Aeronautics. During and after the war, a considerable number of European emigres brought knowledge and entrepreneurial skills, providing a distinct legacy in both theoretical and applied aeronautics. There were degree programs at a handful of universities, but hardly a nucleus large enough to train hundreds of aero engineers needed to sustain a major aviation industry. Despite production of the DH-4 and biplane trainers during the war, there was still no comprehensive infrastructure to serve the requirements of aeronautics. During the 1920s and 1930s the Europeans helped fill these gaps. They were the theoreticians for the NACA; educators in universities; organizers of professional societies; leaders in industry.

During the decades between World War I and World War II, it might have been possible for Americans themselves to fill in the gaps in the aeronautical infrastructure. But it would have required many additional years, and America may not have been prepared for World War II. America’s postwar success in jet engines and high-speed flight technology likewise received invaluable momentum from foreign legacies. It might have been possible for the United States to develop large rockets for space exploration without the contributions of the von Braun team, but the lunar landing would probably have occurred in the 1970s, not the 1960s. Through professional literature, individuals, and hardware, the European influence on American aviation and aerospace history has been profound. Minus that influence, the record of American achievements in flight would have been dramatically diminished.

If you have looked out the window of an airplane lately, you may have noticed that jet engines are gradually getting shorter and fatter. You will see 737s, the most common airliner in service, with two types of engines of distinctly different shapes. The older models have long, stovepipe-shaped engines under the wings, where the newer ones (or older ones which have been retrofitted with new engines) have rounder, shorter powerplants, with a large shell or nacelle around the outside and a smaller cylinder protruding from the rear. Boeing’s latest, the 777, has relatively short but immense engines – each with diameter equivalent to the fuselage of the 737. This change represents the maturing of the turbofan engine, which in the early 1960s superseded the older turbojet engine. Strictly speaking, for the past thirty-five years we have been living in the fan age more than the jet age.

Turbofans have a number of advantages over turbojets, particularly lower noise and higher efficiency – both key factors in making commercial jet air travel socially acceptable and economically feasible. Yet they appeared relatively late: no aircraft was powered by a turbofan engine until after 1960. Flight Magazine, in its 1957 prediction of aero engines ten years in the future, did not even mention the fan engine.1 As late as 1959, after airlines had begun to contract for turbofan engines, at least one expert was still expressing skepticism about their practicality.2 Once they appeared, however, turbofans almost immediately became the dominant engine for high-subsonic flight – the regime in which commercial airliners fly. In 1960, Flight Magazine declared that engineers had agreed that all high-subsonic engines would be fans.3

Today, turbofans power virtually all large commercial transports, as well as most large military transports and many business jets, and afterburning turbofans power most military supersonic aircraft. While the technology has certainly evolved in the last thirty-five years, the original turbofan configuration nevertheless stabilized quite quickly. Pratt and Whitney introduced the JT8D in 1963, and it remains the single most common jet engine in commercial service – with more than 13,000 sold.

The rapidity, scope, and permanence of the turbofan’s proliferation suggests a new technology with such obvious advantages that it met no resistance and spread rapidly – a veritable “turbofan revolution,” to modify Edward Constant’s phrase.4 But the obviousness argument, that hallmark of corporate histories and trope of technological progress, breaks down upon closer analysis. For the advantages of the turbofan engine, or more generically of the bypass engine, were recognized almost

107

P Galison and A. Roland (eds.), Atmospheric Flight in the Twentieth Century, 107—155 © 2000 Kluwer Academic Publishers.

as early as those of the turbojet itself – Frank Whittle patented the idea in 1936, and a number of bypass engines were designed in the mid 1940s. Thus, nearly a quarter of a century elapsed between when fan engines were considered a good idea and when they actually became good enough to put them into service on airplanes. This paper explores this odd historical trajectory by asking two questions. First, if they took over so quickly, why did it take so long for turbofan engines to enter flight? And second, why did turbofan engines emerge when they did?

The answers to these questions include new engineering techniques, government – funded research, military requirements, and corporate competition. The story has a broad historical significance because the turbofan depended on and contributed to a stable configuration for commercial jet air travel at high-subsonic speeds5 – a major feature of today’s technological life. We are also interested, however, in questions of engineering epistemology – i. e. what knowledge do engineers use in design? how is this knowlege developed? and how precisely is it utilized? Walter Vincenti began to address these questions with his series of case studies in aeronautics, and we build on his work, especially regarding the role of uncertainty in design.6

Examining epistemological issues in the design of turbofans sheds light on other questions as well. For example, why, in 1997, might you be likely to fly on an aircraft with engines designed more than thirty years ago? Why do technologies experience periods of rapid change, followed by long periods of stability and incremental change? What follows, we argue, is fundamentally a story of radical and incremental change, but one that ends in a counterintuitive way. Rather than a radical innovation winning out over incremental improvements, we find a radical design that spurred incremental innovation in a competitor. The latter succeeded commercially and established the turbofan as an accepted technology.

Although GE’s CJ805-23 was the first flight-qualified turbofan engine, it was not the first to enter commercial service. Because it weighed 1000 pounds more than the CJ805 turbojet, it could not be installed on the Convair 880. It did fit both the 707 and the DC-8, but P&W’s rapid response pre-empted any chance for its replacing the JT3C on either of these aircraft. The CJ805-23 thus had to await the development of a new aircraft, the Convair 990, to enter service. First flight was scheduled for Fall of 1960, with production deliveries scheduled for March, 1961. Aerodynamic performance problems with the aircraft ended up moving the latter date back to September, 1962. Ultimately only 37 Convair 990s were sold. GE attempted to have the CJ805-23 introduced on the Caravelle, replacing the Rolls – Royce Avon, but this too fell through. The breakthrough turbofan engine ended up without an aircraft to fly on.82

The CJ805-23 had some problems in the field. Leakage from the hot turbine stream to the cold fan stream proved more of a problem on production engines than it had on the prototype, necessitating some minor redesign. More seriously, the turbofan bluckets began suffering thermal fatigue cracks, owing to the combination of transient thermal stresses (during start-up and shutdown) and the opposite camber of the fan and turbine blading. For a while the blucket thermal fatigue problem looked like it might be a fundamental fact of bluckets and hence not solvable at all, threatening to create a small financial disaster for GE.83 The problem was solved, but it surely did not help GE convince anyone to consider the engine on other aircraft. The last CJ805-23 was shipped in 1962. Its great engineering achievement notwithstanding, it was by all standards a commercial failure. The contrast between this outcome and the commercial success of P&W’s JT3D led Jack Parker, the head of GE Aerospace and Defense, to remark, “We converted the heathen but the competitor sold the bibles.”84 The fan design of the CJ805-23, however, had a more illustrious history. A scaled-down version of it was installed behind GE’s small J-85 engine to form the CF-700, a 4000 pound thrust engine. This engine flew on business jets into the 1990s, most notably the Falcon 20F and the Sabre 75A. The commercial failure of the CJ805-23 was not the fault of the fan design.

P&W’s JT3D entered service on the Boeing 707 in July, 1960, more than two years before the CJ805-23. Shortly thereafter it began powering Boeing 720B’s and DC-8’s, and the TF-33 entered service on the KC-135 and the eight-engine B-52H bomber, of which the military had ordered 102 in September 1959, and a few years later on the Lockheed C-141. JT3D-powered 707s were still in service into the 1990s, and the TF-33-powered B-52 served in the Persian Gulf War. P&W had delivered 8550 JT3D’s, including JT3C conversions, by 1983. Its success was outdone only by P&W’s JT8D, designed in 1959 on largely the same basis as the JT3D, with more than 13,000 delivered.

INTRODUCTION

The wind tunnel has been an essential instrument for the development of the airplane. From the time of the Wright brothers to the present, it has served aeronautical investigators as an indispensable tool for the improvement of aerodynamic performance. With the emergence of practical aviation on the eve of World War I, European and American countries set up their research programs and built laboratories with wind tunnels to conduct their investigations.

The wind tunnel is a relatively simple instrument, making air flow in a tunnel and measuring the force or moment exerted by wind on a body placed in it. As the theoretical treatment of aerodynamic flow is so difficult and complex, the wind tunnel serves as a useful device to gather empirical data in realms not predicted by theory. And yet, the measured data does not necessarily represent the aerodynamic performance of a real airplane in the sky. The theory of fluid dynamics tells us that the difference between the dimensions of the model and those of a full-scale aircraft would cause scale effect, a phenomenon measured by the dimensionless Reynolds number. Besides scale effect, wind tunnel data could be compromised by errors inherent in experimental procedures and wind tunnel structures such as the aerodynamic effect from the walls of a closed tunnel.

This chapter explores the early use of the wind tunnel by British aeronautical researchers and the controversy over the validity of its use. The main character is Leonard Bairstow, an aerodynamic experimenter who worked on the stability of the airplane through wind tunnel experiments, and who argued for the usefulness of such model experiments. Bairstow and his colleagues at the National Physical Laboratory (NPL) conducted aerodynamic experiments beginning in 1904. While their research produced useful data for airplane designers, investigators became increasingly aware of the discrepancies between the data from model experiments and those from full-scale experiments, as well as discrepancies between the data from different wind tunnels. Those discrepancies form one major thread in this story.

This paper also compares the activities inside and outside the laboratory setting, and the interrelations between these two realms. In his Science in Action, Bruno Latour presented a model to explain the process by which research results are generated from the inside of a laboratory and applied to the outside world, making the laboratory in the end an Archimedean point to move the world.1 The Aeronautics

223

P Galison and A. Roland (eds.), Atmospheric Flight in the Twentieth Century, 223—239 © 2000 Kluwer Academic Publishers.

Division of the NPL can be considered as such a laboratory. Its history reveals it to be a typical case of Latour’s laboratory, though its story differed from that of the ideal laboratory recounted in Science in Action.

In what follows, I will first briefly explain Bairstow’s stability research at the NPL, and the worldwide appreciation of its aeronautical significance. I will then present two episodes in which Bairstow rather coercively argued for the validity of model experiments and the postwar continuation of stability research. After describing how Bairstow became an influential leader of the British aeronautical community, I will explain how he came to be criticized for his insistent stand. The controversy illuminates not only the strengths and limitations of wind tunnel research but also differing perceptions of research inside and outside the laboratory.

An aircraft gas turbine engine takes in air through an inlet, increases its pressure in a compressor, adds fuel to the high-pressure air and bums the mixture in a combustion chamber, and then exhausts the heated air and combustion products, expanding first through a turbine, where energy is extracted from it to drive the compressor, and finally through a nozzle. Schematics of the three principal types of aircraft gas turbines are shown in Figure 1. In a turboprop engine, the turbine also drives a propeller, connected to the rotor shaft through gears, and it supplies the thrust required by the aircraft; the gas turbine in this case is just an alternative to a piston engine, converting chemical energy into mechanical energy. In a turbojet, by contrast, the thrust comes from the energized flow exiting the nozzle, literally the “jet” of exhaust. In bypass engines a significant portion of the thrust comes from exhaust air that bypasses the combustor and turbine. The bypass air must receive energy from one source or another in order to supply thrust. In the case of a turbo­fan engine the bypass air is pressurized by a fan. The ratio of the bypass air to the air that passes through the combustor and turbine is called the bypass ratio. Bypass engines are typically classified as low or high bypass in accord with this ratio. One

of the two engines on the 737 mentioned above, for example, is an older low-bypass engine, Pratt & Whitney’s JT8D, with a bypass ratio of 1.1 to 1; the other is a more recent high-bypass engine, General Electric’s CF6, with a bypass ratio of 5 to 1 – i. e. less than 17 percent of the total airflow goes through the combustor and turbine. (Hence the shorter, fatter appearance of the larger fan.) Figure 2 displays cutaways of these two engines.

In both turbojet and turbofan or bypass engines, indeed in aircraft engines generally, the magnitude of the thrust is the product of the exhaust mass-flow rate and the difference between the exhaust velocity and the flight speed. Turbojets typically achieve their thrust from a comparatively small mass-flow exiting at a comparatively high velocity. Bypass engines can achieve the same thrust from more mass-flow exiting at a lower velocity. One advantage this gives them is lower

Figure 3. Typical propulsion efficiency ranges for turboprop, turbofan, and turbojet engines. [L. C. Wright and R. A. Novak, “ Aerodynamic Design and Development of the General Electric CJ805-23 Aft Fan Component,” ASME Paper 60-WA-270, 1060.]

exhaust noise, for exhaust noise is a function of exhaust velocity to the 7th power. Their more important advantage, however, is that they offer higher propulsion efficiency in the range from around 450 to 750 miles per hour. By definition, propulsion efficiency is the fraction of the mechanical energy of the exhaust flow that is realized in propulsion of the vehicle. After a little algebraic manipulation, it turns out that:

2 Vflight

propulsion efficiency = ~——— ———–

‘exhaust ‘flight

Thus, the nearer the exhaust velocity is to the flight velocity, the higher the propulsion efficiency. So long as the components of the engine themselves perform at high thermodynamic efficiency, high propulsion efficiency can be turned into fuel savings.

Figure 3, taken from the technical paper describing the design of General Electric’s first successful turbofan engine7, indicates how propulsion efficiency varies with flight speed for turboprops, turbofans, and turbojets. The propulsion efficiency of turboprops drops rapidly above Mach 0.5, roughly 300 miles per hour, because of increasingly severe aerodynamic losses at the tips of propellers of that era.8 Because of their high exhaust velocities, turbojets do not match the maximum propulsion efficiency of turboprops until they reach flight speeds above Mach 1.

Turbofans are, in effect, hybrids, filling the propulsion efficiency gap between turboprops and turbojets. Just as in a turboprop, the flow leaving the combustor is used in part to drive a fan that supplies thrust from a high mass-flow air stream; the ducting leading into the fan controls the air flow entering it, enabling much higher speeds without the tip losses experienced in propellers. Just as in a turbojet, the thrust is coming from ducted flow exiting a nozzle; but the exhaust velocities are comparatively low in the colder fan stream, resulting in higher propulsion efficiency and hence more thrust for the same fuel consumption.

The emergence of the turbofan engine involved four partly overlapping steps: (1) advances in the turbojet by the engine companies in the late 1940s and early 1950s, leading to a new generation of military jet engines with increased power that later provided core gas turbines for bypass engines (including Rolls-Royce’s Conway, the earliest bypass engine to enter flight service); (2) major breakthroughs in axial compressor aerodynamic design by the NACA in the early 1950s which, though intended primarily for supersonic flight, ended up providing the separate technological bases for the contrasting fan designs of General Electric’s and Pratt & Whitney’s first turbofan engines; (3) GE covertly developing a turbofan engine in 1957 that achieved a quantum jump in flight performance by employing an aerody­namically very advanced single-stage fan, located aft of the core engine; and (4) P&W, in response to GE, rapidly developing in 1958 what proved to be the commercially more successful turbofan engine, with performance comparable to GE’s even though it employed less advanced aerodynamics in a two-stage fan at the front of the core engine.

The fact that these four steps do not form a single, simple evolutionary pathway demonstrates, among other things, the futility of attempting to tell the story of the “first” turbofan. Debates over firsts usually degenerate into questions of definition, and here such an approach would miss much of what is instructive in the episode. The critical historical and epistemological points surface from a number of separate threads of development (several of them not specifically aimed at turbofans), as well as from the interaction of several development projects, particularly those at GE and P&W. In place of the notion of “first,” we deploy and expand ideas of “normal” and “radical” design, which Vincenti proposes based on a schema set forth by Edward Constant.9 Vincenti closely examines normal design, where engineers work to improve performance of technologies whose fundamental layout and principles are established. He has little to say, however, about radical design, in which a new technology’s basic arrangement and function are yet to be determined (he believes, perhaps correctly, radical design to have received undue attention from historians). In the following history of the turbofan, however, we show that normal and radical design can interact, even when producing a final result that is in many respects incremental. The normal versus radical distinction remains clear, but less clear is whether the two must, or can, exist as separate trajectories. To trace their interactions, we shall describe the four steps listed above after briefly reviewing early efforts on turbofan engines and the reasons they did not displace the then existing turbojets.