A Public Confrontation

In March 1915, Lanchester gave an exposition of his theory at the Institution of Automobile Engineers in London.62 In the audience of over 150 members and guests was a fellow member of the Advisory Committee, Mervin O’Gorman, as well as Leonard Bairstow of the National Physical Laboratory. Lanchester devoted the first part of the lecture to the theory of lift or “sustentation.”63 The presentation started from the observed differences in pressure be­tween the upper and lower surfaces of an aircraft wing. For maximum effi­ciency, argued Lanchester, the flow of air over the wing must conform closely to the surface of the wing. Conformability, rather than the separation charac­teristic of Kirchhoff-Rayleigh flow, was the central assumption. At the tip of the wing, however, complications enter into the story. The higher pressures on the lower surface cause the air to move around the tip from the lower to the upper surface. When combined with the motion of translation of the wing through the air, the circulating motion at the tips has two consequences. First, it gives the flow over the top of the wing an inwardly directed compo­nent, toward the center line, but an outwardly directed component on the lower surface. Second, at the tips themselves, the circulation is swept back­ward to form two trailing vortices coming away from the ends of the wings. To complete the dynamical system, argued Lanchester, the two trailing vorti­ces must be joined, along the length of the wing, by a vortex that has the wing itself as its solid core. The vortex provided the circulatory component of the flow around the wing and accounts for the velocity difference between the flow over the upper and lower surfaces. This in turn accounts for the pressure difference, and hence the lift.64

Lanchester combined his exposition with some methodological observa­tions. He began by distinguishing the theoretical approach to aerodynamics from the purely empirical approach and noted that the two methods can, to a great extent, be followed independent of one another. Nevertheless, he insisted that engineering needed theory and that experiment without theory was “inefficient.” When variables were effectively independent, simple em­pirical methods of keeping everything constant except one variable might suffice; when variables were dependent on one another, this method ob­scured the crucial connections. At the conclusion of his lecture he returned to these methodological points, saying, “It has not been found possible in the present paper to do more than give an outline of the theory of sustenta – tion, with sufficient examples and references to practice and experiment to illustrate the importance of the theoretical aspect of the subject as bearing on the experimental treatment; the latter has hitherto been dealt with almost without considerations of theory, and has degenerated into empiricism pure and simple” (207). Although Lanchester was making a general claim about the guiding role of theory, there can be little doubt that he had the neglect of his own theory in mind. This was certainly how he was understood by some of his audience.

Lanchester’s lecture impressed at least some of the practical men, and it was greeted by an enthusiastic editorial in Flight.65 The immediate reception by the audience was, however, mixed. Mervin O’Gorman began the discus­sion after the lecture by congratulating Lanchester on his freshness of outlook and went on to offer empirical support for Lanchester’s theory. Experiments had been done on full-sized wings at the Royal Aircraft Factory that demon­strated the predicted inward and outward flow on the respective upper and lower wing surfaces.

We fastened pieces of tape at one end of the upper surfaces of the leading edge of the tips of an aeroplane wing, and arranged a camera, worked by a Bowden wire, to photograph them in flight; they were not put there for the purpose indicated by the author, but we got exactly what he says we should get, and I am glad to confirm him so far. (228)

Leonard Bairstow (fig. 4.10) then rose and adopted a different tone. He an­nounced to the audience that he was not convinced by Lanchester’s ideas.

I quite agree with Mr. O’Gorman that the paper is extremely interesting, but I also find it extremely controversial, and I disagree with his final conclusions.

(229)

By “final conclusions” Bairstow was referring to Lanchester’s suggestion that aerodynamics had degenerated into pure empiricism. Bairstow took it personally:

Many references have been made in the paper to experimental work at the National Physical Laboratory, which work is generally under my charge, and the author has done his best to put the N. P.L. on its defence for not making practical application of his theory. (229)

Given that much of Bairstow’s work had been on stability, and had been guided by the theory developed by G. H. Bryan, it is easy to understand why the general criticism might have struck Bairstow as unjust. The work on sta­bility was certainly not mere empiricism. But Lanchester was talking about lift. Here the charge of empiricism was more plausible. For example, Joseph Petavel, a fellow member of the Advisory Committee and the future direc­tor of the National Physical Laboratory, had given the Howard Lectures in March and April of 1913 at the Royal Society of Arts. He had devoted them to aeronautics, but his treatment had been purely empirical.66 He simply pre­sented his audience with a stream of graphs and empirical coefficients. There was no mention of either the discontinuity theory or the theory of circula­tion. And had not Bairstow himself admitted the resort to empiricism when he had addressed the Aeronautical Society that same year?67

This was true, but all that Bairstow needed to claim to rationalize his po­sition was that Lanchester’s theory was not acceptable because it was a bad theory. He was saying, in effect, show me an adequate theory and I shall use it to guide my experiments, but as yet no such theory is on offer. Bairstow’s objection was that Lanchester’s theory covered some, but not all, of the facts that were of interest to the aeronautical engineer. Bairstow had come pre­pared to prove his point: “I will not pretend to follow the analytical steps between the author’s statements of the vortex theory and his applications, but I will deal with two experiments made at the N. P.L.” (229). With this heavy hint that Lanchester’s position lacked logical clarity, Bairstow proceeded to show the audience two photographs. They depicted a square, flat plate set at an angle of 40° to a stream of water. The water was injected with ink to make the flow visible. Both photographs were taken from above, the first being at a slow speed of flow, the second at a faster speed. Referring to the first pic­ture, Bairstow conceded that it looked to him like the flow that Lanchester had described and as it had been presented in a line drawing (called figure 6) in Lanchester’s talk. Two trailing vortices could be seen coming from the sides of the plate (which Bairstow described as a low-aspect-ratio wing). The higher speed flow, however, presented a very different appearance. If one

figure 4.10. Leonard Bairstow (1880-1963). Bairstow was the principle of the Aerodynamics Division at the National Physical Laboratory, where he did extensive testing and development of G. H. Bryan’s work on stability. Bairstow was skeptical of the circulatory theory of lift and of any approach that ignored the viscosity of air. As a young man he had a reputation for intellectual pugnacity. (By permission of the Royal Society of London)

photograph fitted the theory, the other certainly didn’t. Introducing the first photograph Bairstow said: “The resemblance of this photograph to Fig.6 of the paper is very marked, and up to this point I am thoroughly in accord with the author as to the probable, and in fact almost certain, existence of the type of flow postulated in the early part of the paper” (230).

Moving on to the second picture with the more rapid flow, he added: “The type of flow is now very different from that to which the author’s theory applies. The fluid round the model aerofoil leaves it periodically in spinning loops. The spiral showing the spin inside the arch of one of the loops is very distinct” (230). He conceded that Lanchester’s theory might fit “the very best aerofoil that can be designed at its very best angle of incidence” (230), but the theory said nothing about the full range of significant flow patterns. The word “stall” was not used, but Bairstow’s argument was that Lanchester could not explain what happens when a wing stalls: “There appear, then, to be ex­ceptions to the author’s theory, or rather, there are cases of fluid motion of interest to aeronautical engineers which do not satisfy the conditions that the surface shall be conformable to the streams” (230).

Lanchester gave a robust reply. First, he put Bairstow in his place by re­minding him of their relative positions in the hierarchy of command. While Bairstow was in charge of much of the experimental work on aerodynamics at the NPL, he, Lanchester, was on the Advisory Committee for Aeronautics, which controlled that work. Would he, Lanchester, be denigrating the very institution for which he had responsibility?

Mr Bairstow has suggested that my paper is in some degree an attack on the National Physical Laboratory, or at least he states that I have done my best to put the Laboratory on its defence. I will say at the outset that the National Physical Laboratory is an institution for which I have the greatest possible respect, and I am happy to count amongst my friends members of the Labora­tory staff, whose work and whose capacity are too well known to be injured by friendly criticism. Beyond this, any criticism which is to be incidentally inferred as implied by my remarks is not only criticism of our own National Laboratory, but equally of every aerodynamic laboratory with whose records I happen to be acquainted. Finally, on this point, any destructive or detrimental criticism of the work being done in the aeronautical department of the N. P.L. must reflect adversely on myself, since I am a member of the Committee whose duty it is to direct or control the particular work in question. (241)

Having sorted out the status question, Lanchester turned to Bairstow’s photographs and the accusation that the circulation theory would only apply to a good aerofoil at the best angle of incidence. Is this really a fault asked Lanchester?

Put bluntly, my answer to this is that it is equivalent or analogous to saying that the theory of low speed ship resistance as based on streamline form, and skin friction, is invalid because it does not apply to a rectangular vessel such as a packing-case, and is only true if applied to the very best design of hull with the finest possible lines. (242)

If the theory applied to a few important facts that was triumph enough. All Bairstow’s photographs, Lanchester went on, dealt with flows outside the scope of his theory.

I consider it quite preposterous to suggest that my theory should be tested by its applicability to the case of a square plane at 40 degrees angle as to test the theory of streamline ships’ forms by tank experiments on a coffin or a cask of beer. (243)

Bairstow claimed that theories of wide scope served the interests of aero­nautical engineers, but Lanchester argued that they cut across, rather than expressed, the engineer’s pragmatic standards. Most practical solutions, said Lanchester, were narrow in scope. No one would expect to compute the “re­sistance of a ship in sidelong or diagonal motion through the water” by the same methods and equations “as those applicable in the ordinary way” (251).