Confrontation at the Royal Aeronautical Society

From 1922 the Royal Aeronautical Society (RAeS) became the main public forum in London for informed, and sometimes sharp, debate over the merits of the Prandtl theory. This was a matter of deliberate policy. Bairstow had become president of the RAeS and, at an ordinary general meeting on No­vember 2, 1922, reported that the council of the society felt that there should be more opportunity for the expert discussion of technical subjects. Talks to the society need not be kept accessible to a general audience and could be prolonged over more than one session. An obvious topic for such treatment, said Bairstow, was Prandtl’s theory. To start the ball rolling Bairstow invited Major Low to give his “Review of Airscrew Theories.”76

Major Low was Archibald Low, the designer from Vickers, who has al­ready been mentioned in relation to the conflicts between the manufacturers and the Royal Aircraft Factory. Low’s role in that dispute showed that he was not a divisive man, but he always had definite opinions and was prepared to speak his mind. He belonged to the section of practical men inclined to be sympathetic rather than hostile to Lanchester. Having constantly defended the National Physical Laboratory and the Factory from its detractors, he said he now felt justified in offering some outside criticism. In the journal Aero­nautics Low had earlier expressed the view that “there was a tendency on the part of the official circle of aero-dynamic science in this country to think they were absolutely ‘it’ and that there was very little outside. . . . That was a dan­gerous attitude of mind to get into. He believed we no longer had anything like the supreme position of advantage.”77 Low had acquired his rank of ma­jor during the war and was now employed by the Air Ministry. He was based in the library of the ministry and was engaged on translation work. Low was later to become a member of the Fluid Motion Panel of the Aeronautical Re­search Committee. Although his mathematical expertise was not comparable to that of, say, Glauert, his contributions were deemed interesting by authori­ties such as Taylor and Southwell.78 As Bairstow said, when introducing Low, their speaker had earned the reputation of being “very interesting and very contentious.” Laughter greeted this remark, but it may have been nervous laughter.

Low used his talk as an excuse to lay out the basis of the circulation theory and Prandtl’s work. He had a command of the German literature and could not resist taking Bairstow to task for the inadequacy of the foreign references in the latter’s recently published Applied Aerodynamics. Low described for his audience some of the German papers that had been available for a number of years but had lain neglected. He described the basic geometry of confor­mal transformation and sketched the main results of the work on the infinite wing. He then gave a qualitative account of Prandtl’s theory of the finite wing and reported that the transformation formulas, linking wings of different as­pect ratio, had been confirmed experimentally. Here Low quoted the first volume of the Ergebnisse that Prandtl had mentioned in his exchange of let­ters with Southwell.

In the course of the talk it became clear that Low wanted to force Bairstow and others to acknowledge their culpability for neglecting the circulation theory. They had disregarded Lanchester and left it to the Germans to de­velop insights that Lanchester had published in 1907. Lanchester had shown “remarkable insight into the physics of a problem that had baffled scientists of the last century. Had our physicists followed up his ideas, this country might have shared in the work” (43). Low went on to make a comparative observation. He noted that Lanchester’s work on the theory of lift had been ignored in this country while being known in Germany. By contrast, G. H. Bryan’s work on the theory of stability had been fully appreciated in Britain but had made much less impact in Germany. To illustrate his claim Low cited Joukowsky’s acknowledgment of Lanchester in the Zeitschrift fur Flugtechnik and contrasted it with a reproach by Reissner, directed at his fellow country­men, for their neglect of Bryan. As Low put it:

Although not till recently honoured in his own country, Lanchester has had very full recognition in Germany, unlike Bryan, who is generally ignored. In Joukowsky’s words, “Lanchester’s distinguished service is the elucidation of the transition from plates of infinite span. . . to finite span in simply con­nected space” (Z. f.F. u.M., 1910, p. 282). Compare this with Reissner’s reproof to German writers, “Bryan’s highly distinguished service in first (1904) putting the problem of aeroplane stability in complete mathematical form should not be ignored in citing names” (Jahrbuch d. Wiss. Gesell. f. Luftfahrt, 1915-16, p. 141). (43 – 44)

Knowing how these two quoted sources should be interpreted is obviously no easy matter, but Low’s point is an interesting one. Perhaps the strengths and weaknesses of the two nations complemented each other. Any overall assessment of British and German aerodynamics should take this possibility into account.79

In the lengthy discussion after Low’s talk, Bairstow declared that he would speak “mainly as a critic of the Prandtl theory” (62). Bairstow admitted that he was impressed by the way Prandtl had brought experiments on aerofoils of different aspect ratio into agreement and by Betz’s success in bringing cal­culated and measured pressure distributions into alignment. Overall his po­sition was that Prandtl’s theory connected together a great number of facts. It was “a very good empirical theory,” but, he told his audience, they should not think of “scrapping all their previous work.” Prandtl’s theory “was not sufficiently well established” (62). Bairstow declared himself surprised that Low had got through the whole of his lecture “without mentioning a fun­damental property of air on which its motion depends, viz., its viscosity” (63). This brought Bairstow to what he called his fundamental objection to Prandtl’s theory: “They could have various theories which were good or de­fective in various proportions, but ultimately if they were going to deal with a real physical problem they must come back as the basis to physical ideas. They had in the equations given by Stokes, and the experiments of Poiseuille and Stanton, very strong experimental indications that these equations were sufficient to account for the phenomena, whether it was a steady flow or an eddying flow. These equations did not appear in the Prandtl theory” (63).

In what was presumably a reference to the boundary-layer equations, Bairstow said that Prandtl gave “other equations” but that nobody knew what relation they had to the Stokes equations. The Stokes equations were cur­rently the subject of research by a group at Imperial College. The members of this group “naturally looked for the source of the circulation of which the Prandtl theory makes use, [but] without finding it. In the solution of Stokes’ equations it appeared there was no circulation, i. e., the motion of a viscous fluid around a body moving in it was free of circulation. He knew of no natural mechanism which could produce circulation in a viscous fluid and that seemed to him to make a great difference to one’s appreciation of the Prandtl theory” (63). Prandtl’s theory, said Bairstow, apparently speak­ing of both the theory of the boundary layer and the aircraft wing, was not a “fundamental theory” in the way that Stokes’ equations were fundamental. He concluded by suggesting that both Lanchester and Prandtl were aware of these limitations and knew that they had not provided the last word in aero­foil or propeller theory. The “ultimate solution,” insisted Bairstow, must be along other lines.

There was no way in which Low could match the technical authority of this attack, but he was not lost for a tart rejoinder. Casting himself in the role of the “engineer,” responding to Bairstow the “pure scientist,” he said he had no objection to providing scientists with endowments and facilities to allow them to pursue their “strictly abstract studies.” But who knows when, if ever, these studies will bear fruit? As an engineer “he did not intend to wait for them on this occasion” (65). With the benefit of hindsight one cannot deny that Low had a point.80