The Power of Logical Fallacy

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The arguments used to motivate the Kutta-Zhukovsky circulation theory of lift of a wing (KZ-theory) represent at least two of Aristotle’s logical fallacies:

  1. Affirming the Consequent 
  2. Argument from Ignorance.

These fallacies are so common in science of today that they have come to be viewed not as weak unconvincing non-logic, but rather as strong convincing logic.

You may test your own perception of this phenomenon by reflecting over the logic of KZ-theory describing the flow around a 2d airfoil as potential flow modified by large scale circulation determined to satisfy the Kutta condition of zero flow velocity at the trailing edge. Compare with Why KZ is Incorrect.

1. Affirming the Consequent

KZ-theory uses the following logic:

  • If (A) there is large scale circulation around the wing section, then (B) lift is generated.
  • We observe (B) and conclude that (A) is true, that is that there is circulation.
  • We now have a theory explaining lift from circulation, which is also confirmed by experiment, without having to explain from where circulation is coming.

We may compare with the following “theory” which is also common, but less sophisticated:

  • If (A) there is lower pressure above than below the wing, then (B) lift is generated.,
  • We observe (B) and conclude (A), that is that there is lower pressure above.
  • We now have a theory explaining lift from pressure difference, which is also confirmed by experiment, without having to explain why the pressure is lower above, which is the real problem.

Or with the following:

  • If (A) men are smarter the women, the (B) there will be  more male than female professors.
  • We observe (B) and conclude that (A) men are smarter than women.

2. Arguing from Ignorance

The physical mechanism behind the satisfaction of the Kutta condition is presented to be viscosity, with the motivation that since no non-viscous cause to the this effect is known, it must be viscosity.

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