Aerodynamics Text Book: Incorrect

· flying, theory of flight
Authors

The standard aerodynamics text book Aerodynamics for Engineering Students, 5th edition, by Houghton and Carpenter informs the students in Chapter 4:

  1. By the end of the 19th century the theory of potential flow was extremely well developed.
  2. But for the most important practical applications in aerodynamics potential flow was almost a complete failure.
  3. Flow separation and d’Alembert’s paradox both result from the subtle effects of viscosity on flows at high Reynolds number. 
  4. The great German aeronautical engineer Prandtl deserve most credit both for explaining these paradoxes and showing how potential flow can be modified to yield useful predictions of the flow around wings …and showed how skin-friction drag could be calculated.
  5. How can potential flow be adapted to provide a reasonable theoretical model for the flow around an aerofoil that generates lift?
  6. The answer lies in drawing an analogy between the flow around an aerofoil and that around a spinning cylinder. For the latter it can be shown that when a point vortex is superimposed, a lifting flow is generated.

Comments:

  • 1 and 2 are contradictory: potential flow extremely well developed and completely useless.
  • 3 is the central postulate of modern fluid mechanics: drag results form “subtle effects of viscosity”. We show that this is incorrect: Pressure drag as the main part of drag results from instability of potential flow at separation and not from any “subtle effects of viscosity”.
  • 4 gives Prandtl the role of Father of Modern Fluid Mechanics. Note the shift from the issue at hand of  drag dominated by pressure drag to skin-friction drag. It is thus effectively admitted that Prandtl did not resolve d’Alembert’s paradox  of zero pressure drag, but it is still suggested that Prandtl somehow solved the paradox by some reference to “subtle effects of viscosity”. This is giving the students deliberately  misleading information.
  • Both 4 and 5 indicate that the question considered by Prandtl and his many followers was how to “modify” and “adapt” the potential solution to give something not completely unreasonable. The issue was thus not real physics, but instead “trick physics” or pseudo-science.
  • 6 claims that the lift of  a spinning ball comes from circulation, which however is incorrect: It is well known by aerodynamicists that the lift effect comes from unsymmetric separation, and not from large scale circulation.
  • 6 exhibits the logical fallacy of confirming circulation by observing lift as an effect of circulation. The truth is that lift of spinning ball comes from unsymmetric separation and lift of a wing from 3d rotational separation.

The book is representative of text books in aerodynamics. Students of aerodynamics are subject to massive disinformation.

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