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Old 02-27-2004 | 11:23 AM
  #87  
KenLitko
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From: Crown Point, IN,
Default RE: one last reply, really

ORIGINAL: LouW
in a fluid there is simply no way to create a force on a body other than by accelerating a mass of the fluid. In the final analysis, when you cut through all the details, a wing is simply an air deflector.

Sure there is a way to create a force without moving mass.... F=ma can be re-written as F=PA... Force is equal to pressure times an area.... no mass movement necessary. At least not with that equation.

The difficult thing to understand is how you get a pressure field set up in a fluid without moving it. After all... a fluid cannot sustain a shear force... right? Any pressure field in a free fluid will quickly dissipate because there is no way to confine it by using the fluid itself. Think of an explosion.... a quick, intense, pressure field, then nothing.... because it dissapates.

Like i mentioned before, a wing does not simply move through the air... it is pushed through the air. This "pushing" on the air creates this pressure field.

Don't be so quick to dismiss the theory as just a bunch of equations. That bunch of equations is pretty good at predicting the performance of a real wing.

BTW... you still haven't argued anything about the upwash. In order for this "mass of air moving downward", as you say, to create lift... it has to overcome the "mass of air moving upward" in front of the wing.

Another BTW... i goofed that explanation of an infinite force created by a airfoil... it's not infinite because it occurs along the length of the airfoil... it's infinite because each molecule of air moving downwards behind an airfoil would need to be replaced by a molecule of air above it... in an infinite stack... this infinite stack of molecules accelerated downward by an airfoil is what would cause an infinite lifting force to be developed. This is simple continuity.

This web page has a surprisingly good explanation of the concepts i've described:
http://regenpress.com/circlatn.htm