At the Eleventh Hour

At eleven o’clock, on November 11, 1918, a cease-fire was declared on the west­ern front. The “war to end all wars” was over. European civilization would never be the same again, nor would aerodynamics. The military situation did not take Prandtl wholly by surprise, and he had already begun to explore the possibilities for a program of peacetime research.111 The comprehensive collapse of the German military effort meant that financial support for the Gottingen institute would all but disappear. At the height of the war Prandtl’s institute had employed around fifty people, but now some 60 percent of its staff had to be dismissed.112 At the very moment when the institutional and financial arrangements that had sustained German aerodynamic work were crumbling, the full scope of the Gottingen achievement was coming into view.

Toward the end of the war Prandtl finally brought together the overall theoretical picture that had been hinted at, and promised, but never pro­duced, for the readers of the Zeitschrift fur Flugtechnik. Even then it was not the readers of the Zeitschrift who would be the immediate beneficiaries. On April 18, 1918, Prandtl gave a comprehensive, and confidential, lecture on the theory of the lift and drag of an aircraft wing to the annual meeting of the Wisssenschaftlichen Gesellschaft fur Luftfahrt in Hamburg.113 A few months later, on July 26, 1918, Prandtl presented the first part of his classic paper “Tragflugeltheorie” to the Gottingen Academy of Science.114 It is the theory of the wing as laid out in these papers that I have described in this chapter.

In presenting his theory to the Academy, Prandtl prefaced the aerody­namic work with a highly abstract sketch of fluid-dynamic principles. Per­haps in deference to Hilbert and the Gottingen fashion for formal axiom systems he even offered two new “axioms” to be added to classical hydro­dynamics. (Axiom I stated that vortex layers can arise at lines of confluence. Axiom II was that infinite speeds cannot arise at protruding sharp edges of the body, or, if they do, only in the most limited way possible.) It is difficult to avoid the suspicion that these embellishments were added because Prandtl was conscious of addressing a high-status, scientific audience rather than an audience of engineers. Moritz Epple points out that the “axioms” Prandtl introduced do not justify the approximation processes that he used, nor do they operate as axioms in the way that Hilbert would understand them.115 The reference to “axioms” appears to have more to do with style than sub­stance. Furthermore, the general principles of fluid mechanics presented at the outset needed the help of drastic approximations before the theory of the wing could be presented in a recognizable and useful manner. As Prandtl introduced these approximations into his exposition, so the tone of the talk to the Academy changed. There was a shift from abstract principle to con­crete practice; from science to engineering; and from classical mechanics to technische Mechanik.116

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