tisdag 18 februari 2020

Fundamentals of Aerodynamics by John D Anderson as Old Theory of Flight

The book Fundamentals of Aerodynamics by John D Anderson describes the standard theory of flight as Old Theory of Flight with basic ingredients expressed in Chapter 4:
  • The purpose of this chapter is to present theoretical methods for the calculation of airfoil properties.
  •  In most of this chapter we will deal with inviscid flow, which does not lead to predictions of airfoil drag; indeed, d’Alembert’s paradox says that the drag on an airfoil is zeroclearly not a realistic answer.
  • However, if we lived in a perfectly inviscid world, an airfoil could not produce lift. 
  • Indeed, the presence of friction is the very reason why we have lift. These sound like strange, even contradictory statements to our discussion in the preceding paragraph. 
  • What is going on here? The answer is that in real life, the way that nature insures that the flow will leave smoothly at the trailing edge, that is, the mechanism that nature uses to choose the flow shown in Figure 4.18c, is that the viscous boundary layer remains attached to the surface all the way to the trailing edge. 
  • Nature enforces the Kutta condition by means of friction. If there were no boundary layer (i.e., no friction), there would be no physical mechanism in the real world to achieve the Kutta condition. 
  • So we are led to the most ironic situation that lift, which is created by the surface pressure distribution—an inviscid phenomenon, would not exist in a frictionless (inviscid) world. In this regard, we can say that without friction we could not have lift. 
We read that the Old Theory is strange, contradictory, not realistic and ironic. The New Theory of Flight presented on Secret of Flight shows that this characterisation is correct. It is now time to allow the Old Theory to retire since it is physically incorrect and no longer is needed as a facade when there is a physically correct theory.     

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