tisdag 7 augusti 2012

Physics of Flight Reviewed by Klaus Weltner

In Physics of Flight - reviewed by Klaus Weltner, we read in the Introduction:
  • The explanation of the aerodynamic lift has a long history, but there is controversy regarding the fundamental physics and their relation to Newton’s mechanics to date. 
  • This topic could be one of the most interesting and motivating in physics education. But the physics of flight nearly disappeared from the curriculum in schools and basic physics courses in most European countries. 
  • One reason, why teachers despite of students interest neglect this topic might be the fact that the conventional explanation of the aerodynamic lift based on Bernoulli’s Law has serious drawbacks and is partly erroneous. 
  • ... explanations based on Bernoulli’s law are dominating since the 1920s. 
  • This situation is changing recently. A growing number of authors question the conventional explanation and replace it by an explanation based on fundamental mechanics. 
Weltner eliminates Bernoulli because it requires the velocity field to be known and also circulation theory as a mathematical trick without physics:
  • The concept of circulation is a sophisticated mathematical discription of the velocity distribution but not the cause of the latter. 
Weltner instead advocates a general view - Aerodynamic lifting force as reaction force while air is accelerated downwards by the airplane:
  • The explanation based on the relation between aerodynamic lift and the acceleration of a downward air flow prevailed in textbooks in this simple form until 1920 without having been elaborated further. 
  • By approximately the year 1920, when aviation gained much interest in science and public, the explanation based on Bernoulli’s law appeared and displaced the explanation based on reaction forces. 
  • In any case it was necessary that the explanation of lift using Bernoulli’s law had to be complemented by giving a cause for the higher streaming velocity of the wing’s upper surface. 
But Weltner's explanation of lift simply as a reaction, is as empty of content today as it was when it was discarded in 1920. Weltner thus effectively reduces the aerodynamics of flight of the 20th century to zero and gives the reason why the physics of flight has disappeared from the curriculum.

6 kommentarer:

  1. Claes,
    What are your comments on Martin Ingelman-Sundbergs idea of lift generation?
    It seems perfectly credible to me, being an engineer.


  2. It is the same as Weltner's that is lift as reaction, which explains nothing since the cause of the reaction, push of air downwards, is not explained. It is like saying that there is an upward force on the wing from the air because there is a downward force from the wing on the air. This is self-evident or trivial and does not explain the miracle of flight.

  3. I was referring to the idea of air having to move in a curve over the upper surface and due to inertia will move away from said surface creating a reduction in air pressure.
    There is, demonstrably, lower air pressure on the top of the wing, and this is a nice explanation.


  4. Yes, but it remains to understand that it is the slip b c which prevents separation, and not viscosity as in std attempts of explanation such as Coanda.

  5. "slip b c" do you mean spanwise and chordwise?


  6. Full slip in all directions, reflecting small skin friction.