fredag 21 september 2012

Aircraft Design Engineers Fooled by AIAA

Let us continue our study how AIAA expresses its world-leading expertise in the book Aircraft Design: A Conceptual Approach, by D. Raymer, 1992, published in the AIAA Education Series described as "creating a comprehensive library of the established practices in aerospace design" with the following conclusion of the Foreword: "For many years Aircraft Design will be a valuable text book for all who struggle with the fundamentals and intricacies of aircraft design".  We read on p 35 under Airfoil Lift and Drag: 
  • An airfoil generates lift by changing the velocity of the air passing over and under itself. The airfoil angle of attack and/or camber causes the air over the top of the wing to travel faster than the air beneath the wing.
  • It can be seen that the effect of the airfoil is to introduce a change in airflow, which seems to circulate around the airfoil...
  • This "circulation" is the theoretical basis for the classical calculation of lift and drag-due-to-lift The the greater the circulation the greater the lift. 
  • Odd as it sounds, an airfoil in two-dimensional flow does not experience any drag due to the creation of lift. 
  • The figure below shows typical pressure distributions for the upper and lower surfaces of a lifting airfoil at subsonic speeds. Note that the upper surface of the wing contributes about two-thirds of the total lift. 

AIAA thus sells the 2d Kutta-Zhukovsky circulation theory of lift. AIAA claims that the theory offers a way to calculate lift and drag, but then directly admits that the drag comes out to be zero, which is described as "odd".

But AIAA does not admit that the pressure distribution in the presented figure with its high pressure at the trailing edge suggested by the theory, is not what is observed in reality. 

AIAA is thus fooling aircraft design engineers into relying on an unphysical theory which does not describe reality. AIAA continues along the same track by rejecting the New Theory of Flight without proper evaluation, which is also "odd".

AIAA further ruins the hope of any prospective CFD-user during any foreseeable future:
  • No current codes actually attempt to solve the true full Navier-Stokes equations due to the difficulty of mathematical analyzing turbulence. Turbulence occurs at the molecular level, which would probably require gridding with billions of molecular sized grids. 
  • CFD does not replace the wind tunnel. In fact , it really doesn't even reduce the number of wind-tunnel test hours.
But if this was true in 1992, it is not true any more in 2012: The computations behind the New Theory of Flight shows that it is indeed possible to solve the full 3d Navier-Stokes and thereby compute lift, drag and pressure distribution for a full aircraft using affordable millions of mesh points, thus basically eliminating the need of a wind tunnel. This possibility is today rejected by AIAA without evaluation, which is another way of fooling aircraft design engineers.

1 kommentar:

  1. I don’t think the aircraft design engineers will trust AIAA again.