’Tis evident, that all the sciences have a relation, greater or less, to human nature; and that however wide any of them may seem to run from it, they still return back by one passage or another.
david hume, A Treatise of Human Nature (1739-40)1
Why do aircraft fly? How do the wings support the weight of the machine and its occupants? Even the most jaded passengers in the overcrowded airliners of the present day may experience some moments of wonder—or doubt—as the machine that is to transport them lifts itself off the runway. Because the action of the air on the wing cannot be seen, it is not easy to form an idea of what is happening. Some physical processes are at work that must generate powerful forces, but the nature of these processes, and the laws they obey, are not open to casual inspection. If the passengers looking out of the window really want an explanation of how a wing works, they must do what any lay person has to do and ask the experts. Unfortunately the answers that the experts will give are likely to be highly technical. It will take patience by both parties if communication is not to break down. But given goodwill on both sides, the experts should be able to find some simplified formulations that will be useful to the nonexperts, and the nonexperts should be able to deepen their grasp of the problem.
In this book I discuss the question of why airplanes fly, but I approach the problem in a slightly unusual way. I describe the history behind the technical answer to the question about the cause of “lift,” that is, the lifting force on the wing. I analyze the path by which the experts, after much disagreement, arrived at the account they would now give. I am therefore not simply asserting that airplanes fly for this or that reason; I am asserting that they were understood to fly for this or that reason. I am interested in the fact that different and rival understandings were developed by different persons and in different places. I cannot speak as a professional in the field of aerodynamics; nor is my position exactly that of a layperson. I speak as a historian and sociologist of science who is poised between these categories.2
What are the specific questions that I am addressing and to which I hope to offer convincing answers? To identify them I first need to give some background. The practical problem of building machines that can be flown, that is, the problem of “mechanical” or “artificial” flight, was solved in the final years of the nineteenth century and the early years of the twentieth century. In the 1890s Otto Lilienthal in Germany successfully built and flew what we today would call hang gliders. From 1903 to 1905 the Wright brothers in the United States showed that sustained and controlled powered flight was possible and practical. What had long been called the “secret” of flight was now no longer a secret. But not all of the secret was revealed. Some parts of it remained hidden, and indeed, some parts are still hidden today. The practical successes of the pioneer aviators still left unanswered the question of how a wing generated the lift forces that were necessary for flight. The pioneers mostly worked by trial and error. Some had experimented with models and taken measurements of lift and drag (the air resistance opposing the motion), but the measurements were sparse and unreliable.3 No deeper theoretical understanding had prompted or significantly informed the early successes of the pioneers, nor had theory kept pace with the growth of practical understanding. The action of the air on the wing remained an enigma.
A division of labor quickly established itself. Practical constructors continued with their trial-and-error methods, while scientists and engineers began to study the nature of the airflow and the relation between the flow and the forces that it would generate. For this purpose the scientists and engineers did not just perform experiments and build the requisite pieces of apparatus, such as wind channels. They also exploited the resources of a branch of applied mathematics that was usually called hydrodynamics. The name “hydrodynamics” makes it sound as if the theory was confined to the flow of water, but in reality it was a mathematical description that, with varying degrees of approximation, was applied to “fluids” in general, including air. Thus was born the new science of aerodynamics. The birth was accompanied by much travail. One problem was that the mathematical theory of fluid flow was immensely difficult. The need to work with this theory effectively excluded the participation of all but the most mathematically sophisticated persons, and this did not go down well with the practical constructors. The mathematical analysis also depended for its starting point on a range of assumptions and hypotheses, about both the nature of the air and the more or less invisible pattern of the flow of air over, under, and around the wing. Only when the flow was known and specified could the forces on the wing be calculated. Assumptions had to be made. The unavoidable need to base their investigations on a set of assumptions proved to be deeply divisive. Different groups of experts adopted different assumptions and, for reasons I explain, stuck to them.
The first part of this historical story, the practical achievement of controlled flight, has been extensively discussed by historians. Pioneers, such as the Wright brothers, have been well served, and the attention given to them is both proper and understandable.4 The second part of the history, the development of the science of aerodynamics, is somewhat less developed as a historical theme, though a number of outstanding works have been written and published on the subject in recent years.5 The present book is a contribution to this developing field in the history of science and technology.
In the early years of aviation there were two, rival theories that were intended to explain the origin and nature of the lift of a wing. They may be called, respectively, the discontinuity theory and the circulatory (or vortex) theory. The names derive from the particular character of the postulated flow of air around the wing. (I should mention that the circulatory theory is, in effect, the one that is accepted today.) My aim is to give a detailed account of how the advocates of the two theories developed their ideas and how they oriented themselves to, and engaged with, the empirical facts about flight. To do this I found that I also needed to understand how they oriented themselves to, and engaged with, one another. I show that these two dimensions cannot be kept separate. This is why I have prefaced the work with the quotation from the famous Edinburgh historian and sociologist David Hume. The more one studies the technical details of the scientific work, the more evident it becomes that the social dimension of the activity is deeply implicated in these details. The more closely one analyses the technical reasoning, the more evident it becomes that the force of reason is a social force. The historical story that I have to tell about the emerging understanding of lift is, therefore, at one and the same time both a scientific and a sociological story. To understand the course taken by the science it is necessary to understand the role played by the social context, and to appreciate the role played by the social context it is necessary to deconstruct the technical and mathematical arguments.
In principle none of this should occasion surprise. Scientists and engineers do not operate as independent agents but as members of a group. They cannot achieve their status as scientists and engineers without being educated, and education is the transmission of a body of culture through the exercise of authority. Education is socialization.6 Scientists and engineers see themselves as contributing to a certain discipline, as being members of certain institutions, as having loyalties to this laboratory or that tradition, as being students of A or rivals of B. Their activities would be impossible unless behavior were coordinated and concerted. For this the individuals concerned must be responsive to one another and in constant interaction. Their knowledge is necessarily shared knowledge, though, in its overall effects, the process of sharing can be divisive as well as unifying. The sharing is always what Hume would call a “confined” sharing.
All too frequently, when scientific and technical achievements become objects of commentary, analysis, or celebration, these simple truths are obscured. Academic culture is saturated with individualistic prejudices, which encourage us to trivialize the implications of the truth that science is a collective enterprise and that knowledge is a collective accomplishment. Philosophers of science actively encourage historians to distinguish between, on the one side, “cognitive,” “epistemic,” or “rational” factors and, on the other side, “social” factors. They enjoin the sociologist to “disentangle” scientific reasoning from “social influences” and to distinguish what is truly “internal” to science from what is truly “external.”7 These recommendations are treated as if they were preconditions of mental hygiene and based on self-evident truths. Historians and sociologists of science know better. They know that the problem of cognitive order is the problem of social order.8 These are not two things, even two things that are closely connected; they are one thing described from different points of view. The division of a historical narrative into “the cognitive” and “the social,” or “the rational” and “the social,” is wholly artificial. It is methodologically lazy and epistemologically naive.
I shall now briefly sketch the overall structure of the events I describe in this volume. Of the two theories of lift that I mentioned, one of them, the discontinuity theory, was mainly developed in Britain. It was based on work by the eminent mathematical physicist Lord Rayleigh. The other, the circulatory theory, was mainly developed in Germany. It is associated primarily with the German engineer Ludwig Prandtl, although it had originally been proposed by the English engineer Frederick Lanchester. It rapidly became clear that the discontinuity theory was badly flawed because it only predicted about half of the observed amount of lift. At this point, shortly before the outbreak of World War I (or what the British call the Great War) in 1914, the British awareness of failure might have reasonably led them to turn their attention to the other theory, the theory of circulation. They did not do this. They knew about the theory but they dismissed it. At Cambridge, G. I. Taylor, for example, treated the discontinuity theory as a mathematical curiosity, but he also found Lanchester’s theory of circulation equally unacceptable. The reasons he gave to support this judgment were important and widely shared. Meanwhile the Germans embraced the idea of circulation and developed it in mathematical detail. The British also knew of this German reaction but still did not take the theory of circulation seriously. It was not until after the war ended in 1918 that the British began to take note. They found that the Germans had developed a mathematically expressed, empirically supported, and practically useful account of lift. Even then the British had serious reservations. The negative response had nothing to do with mere anti-German feeling. The British scientific experts were patriots, but, unlike some in the world of aviation, they were not bigots. Why then were they so reluctant to take the theory of circulation seriously? This is the main question addressed in the book.9
There are already candidate answers to this question in the literature, but they are answers of a different kind to the one I offer. The neglect of Lan – chester’s work became something of a scandal in the 1920s and 1930s, so it was natural that explanations and justifications were manufactured to account for it. Sir Richard Glazebrook, the head of the National Physical Laboratory, played an important role in British aviation during these years and was the source of one of the standard excuses, namely, that Lanchester did not present his ideas with sufficient mathematical clarity. Well into the midcentury, British experts in aerodynamics, who, along with Glazebrook, shared responsibility for the neglect of Lanchester’s ideas, were scratching their heads and wondering how they could have allowed themselves to get into this position. Clarity or no clarity, they had turned their backs on the right theory of lift and had become bogged down with the wrong one.
The retrospective accounts and excuses that have been given have been both fragmentary and feeble, though Lanchester’s biographer, P. W. Kings – ford, writing in 1960, still went along with a version of Glazebrook’s excuse.10 Other existing accounts merely tend to embellish the basic excuse by invoking the personal idiosyncrasies of the leading actors. The problem is analyzed as a clash of personalities. It is true that some of those involved had strong characters as well as powerful intellects, and some of them could pass as colorful personalities. All this will become apparent in what follows. The psychology of those involved is clearly an integral part of the historical story, but such accounts miss the very thing that I want to emphasize and that I believe is essential for a proper analysis, namely, the interconnection of the sociological and technical dimensions. Only an account that is technically informed, and sensitive to the social processes built into the technical content of the aerodynamic work, will make sense of the history. I want to show that the real reasons for the resistance to the vortex or circulatory theory of lift were deep and interesting, but not really embarrassing at all.
Although I have posed the question of why the British resisted the theory of circulation, I do not believe it can be answered in isolation from the question of why the Germans embraced it. Both reactions should be seen as equally problematic. The historical record shows that the same type of causes were at work in both British and German aerodynamics. In both cases the actors drew on the resources of their local culture and elaborated them in ways that were typical of their milieu and were encouraged by the institutions of which they were active members. Of course, the cultures and the institutions were subtly different. My explanation of the German behavior is thus of the same kind as my explanation of the British. The same variables are involved, but the variables have different values. Seen in this way the explanation possesses a methodological characteristic that has been dubbed “symmetry.” Because the point continues to be misunderstood, I should perhaps emphasize the words “same kind.” I am not saying that the very same causes were at work but that the same kinds of cause were in operation. Symmetry, in this sense, is now widely (though not universally) accepted as a methodological virtue in much historical and sociological work. Conversely, it is widely rejected as an error, or treated as a triviality, by philosophers. I hope that seeing the symmetry principle in operation will help convey its meaning more effectively than merely trying to capture it in verbal formulas or justify it by abstract argument.
The overall plan of the book is as follows. In chapter 1 I start my account of the early British work in aerodynamics with the foundation of the controversial Advisory Committee for Aeronautics in 1909. The committee was presided over by Rayleigh. The frontispiece, taken from the Daily Graphic of May 13, 1909, shows some of the leading members of the committee striding purposefully into the War Office for their first meeting, and then emerging afterward looking somewhat more relaxed. The minutes of that important meeting are in the Public Record Office and reveal what they talked about in the interval between those two pictures.11 It is a matter of central concern throughout this book. Chapter 2 lays the foundation for understanding the two competing theories of lift by sketching the basic ideas of hydrodynamics and the idealized, mathematical apparatus that was used to describe the flow of air. A nontechnical summary is provided at the end of the chapter. In chapter 3, I introduce the discontinuity theory of lift and describe the British research program on lift and the frustrations that were encountered. Chapter 4 is devoted to the circulatory or vortex theory and describes the hostile reception accorded to Lanchester among British experts. I pay particular attention to the reasons that were advanced to justify the rejection. In chapter 5, I identify and contrast two different intellectual traditions that were brought to bear on the theory of lift. One of them was grounded in the mathematical physics cultivated in Britain and preeminently represented by the graduates of the Cambridge Mathematical Tripos. The other tradition, called technische Mechanik, or “technical mechanics,” was developed in the German technical colleges and was integral to Prandtl’s work on wing theory. Chapters 6 and 7 provide an account of the German development and extension of the circulation theory as worked out in Munich, Gottingen, Berlin, and Aachen. In chapters 8 and 9 there is a description of the British postwar response, which took the form of a period of intense experimentation; it also gave rise to some remarkable and revealing theoretical confrontations. What, exactly, did the experiments prove? The British did not find it easy to agree on the answer.
The divergence between British and German approaches was effectively ended in 1926 with the publication, by Cambridge University Press, of a textbook that became a classic statement of the circulation theory. The book was Hermann Glauert’s The Elements of Aerofoil and Airscrew Theory.12 Glauert, an Englishman of German extraction, was a brilliant Cambridge mathematician who, in the 1920s, broke ranks and became a determined advocate of the circulation theory. As the title of Glauert’s book indicates, he did not just work on the theory of the aircraft wing, but he also addressed the theory of the propeller. This is a natural generalization. The cross section of a propeller has the form of an aerofoil, and a propeller can be thought of as a rapidly rotating wing. The “lift” of this “wing” becomes the thrust of the propeller, which overcomes the air resistance, or “drag,” as the aircraft moves through the air. Glauert’s book also dealt with the theory of the flow of air in the wind channel itself, that is, the device used to test both wings and propellers. This aspect of the overall theory was needed to ensure that aerodynamic experiments and tests were correctly interpreted. As always in science, experiments are made to test theories, but theories are needed to understand the experi – ments.13 The discussions of propellers and wind channels in Glauert’s book are important and deserve further historical study, but, on grounds of practicality, I set aside both the aerodynamics of the propeller and the methodology of wind-channel tests in order to concentrate exclusively on the story of the wing itself.14
In the final chapter, chapter 10, I survey the course of the argument and consider objections to my analysis, particularly those that are bound to arise from its sociological character. I use the case study to challenge some of the negative and inaccurate stereotypes that still surround the sociology of scientific and technological knowledge. I also ask what lessons can be drawn from this episode in the history of aerodynamics. Does it carry a pessimistic message about British academic traditions and elitism? What does it tell us about the difference between Gottingen and Cambridge or between engineers and physicists? Finally, I ask what light the history of aerodynamics casts on the fraught arguments between historians, philosophers, and sociologists of science concerning relativism.15 Does the success of aviation show that relativism must be false? I believe that, by drawing on this case study, some clear answers can be given to these questions, and they are the opposite of what may be expected.
During the writing of this book I had the great advantage of being able to make use of Andrew Warwick’s Masters of Theory: Cambridge and the Rise of Mathematical Physics.16 Although historians of British science had previously accorded significance to the tradition of intense mathematical training that was characteristic of late Victorian and Edwardian Cambridge, Warwick took this argument to a new level. By adopting a fresh standpoint he compellingly demonstrated the constitutive and positive role played by this pedagogic tradition in electromagnetic theory and the fundamental physics of the ether in the early 1900s.17
For me, one of the intriguing things about Warwick’s book is that the actors in his story are, in a number of cases, also the actors in my story. What is more, his account of the resistance that some Cambridge mathematicians displayed to Einstein’s work runs in parallel with my story of the resistance to Prandtl’s work. Like Warwick I found that their mathematical training could exert a significant hold over the minds of Cambridge experts as they formulated their research problems. In many ways the study that I present here can be seen as corroborating the picture developed in Warwick’s book. Of course, shifting the area of investigation from the history of electromagnetism to the history of fluid mechanics throws up differences between the two studies, and not surprisingly there is some divergence in our conclusions. Whereas Warwick’s attention is mainly (though not exclusively) devoted to the British scene, my aim, from the outset, is that of comparing the British and German approaches to aerodynamics. Furthermore, on the British side, I follow the actors in my story as they move out of the cloisters of their Cambridge colleges into a wider world of politics, economics, aviation technology, and war. If Warwick studied Cambridge mathematicians as masters of theory, I ask how they acquitted themselves as servants of practice.