The Enigma of the Aerofoil

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The Enigma of the Aerofoil: Rival Theories in Aerodynamics 1909-1930, by David Bloor, 2011, describes the initial British criticism of the German circulation theory of lift developed by Kutta and Prandtl in the beginning of the 20th century, which resisted the attack in the absence of something better and became the leading theory propagated in text books still today. From the description of the book:

  • Why do aircraft fly? How do their wings support them? In the early years of aviation, there was an intense dispute between British and German experts over the question of why and how an aircraft wing provides lift. David Bloor … reveals the impact that the divergent mathematical traditions of Cambridge and Göttingen had on this great debate. 

It is illuminating to read the book from the perspective of the New Theory of Flight, which explains the real physics of generation of lift and drag of a wing resulting from a specific instability mechanism of potential flow, and in particular shows that the circulation theory is unphysical and thus incorrect.

The book describes that this fact did not prevent circulation theory for becoming the text book theory, because text books/scientists have to offer theory to earn their salary, and lacking a correct physical theory, an incorrect unphysical theory is better than no theory at all. This is well expressed in the concluding chapter Pessimism, Positivism and Relativism: Aerodynamic Knowledge in Content:

  • The progress in aerodynamics made in the Technische Hochschulen and the University of derived from the use of a theory of perfect fluids in potential motion. The theory dealt with an idealization and a simplification.
  • This theory was dismissed in Cambridge and London as physically false and logically self-defeating. It was false because it denied the viscosity of air and self-defeating because circulation was unchanging with respect to time, and its origin was beyond the reach of the theory.
  • The premises of the British objections were true and the reasoning based on them was sound, but the conclusion led to failure rather than success.The German advances in the understanding of lift and the properties of wings depended on the use of abstract and unreal concepts that were sometimes employed with questionable logic. Progress in aerodynamics thus depended on the triumph of falsity over truth. Everyone knows that false premises can sometimes lead to true conclusions and that evidence can sometimes support false theories, but the story of the aerofoil involved more than this.
  • The successful strategy involved the deliberate use of known falsehoods in artful balance with accepted truths. The supporters of circulation theory showed how simple falsehoods could yield dependable conclusions when dealing with a complex and otherwise intractable reality. This is the real enigma of the aerofoil.

In short: even an incorrect unphysical theory can be used as a scientific theory with some success, but it cannot compete with a correct physical theory.

1 Comment

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  1. Gunnar W Berman

    Why I Believe in You Claes is because I remember an ex student of the department of “Farkostteknik” who couldn’t get a satisfying explanation of the physics of flying. He after a short while changed goal of his education and became a machine engineer and nowadays sells Japanese heatpumps the last I saw of him.
    Kindest regards!
    Gunnar (W Bergman)
    technical journalist

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