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Old 02-21-2004 | 04:02 PM
  #61  
Tall Paul
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From: Palmdale, CA
Default RE: A bone of contention =

Actually, Lou, it's in my nature as an engineering pedant to prefer to see terms which are defined as specific things to be used correctly, not modified without notice of the modification.
The Bernoulli equation:
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H=p + v^2*rho/2
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H is the total pressure.
It consists of the static pressure, which can not be changed for our purposes, and the dynamic pressure, which does change as a function of velocity, either over the airframe or in the tunnel, and the air density rho.
For level flight, rho is usually considered a constant.
p is the static pressure.It is independent of the existence or effect of the airframe.
v^2*rho/2 is the dynamic pressure. That is what works on the airframe.
It can be fairly stated that static pressure changes on the upper surface of an airfoil due to airflow.. But then it is no longer -the- static pressure, and there is a corresponding change to the pressure on the lower surface, which is also not -the- static pressure once it's changed, but must be considered -with- the pressure change on the upper surface as they act simultaneously.
As JohnG points out, such changes can both be "negative" relative to the ambient static pressure but one can be " more negative" than the other, and the sum of the two, not each seperately is what the airframe responds to.
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It's the non-rigorous use of specific terms which gets my interest, either when they're used too loosely, or just plain wrong.
When used loosely, they can lead to misperceptions, and when used wrong, they can generate serious problems if the bad definition is used to try something which is in itself not capable of working because the concept it is designed to is wrong.
Some of the "technical" explanations seen here appear to be more a subset of the "infinite number of monkeys".. typing Shakespeare.. with the more limited vocabulary of engineering making it easier to assemble technical terms at random and construct sentences which obey rules of English grammar, but in actuality have little information content, and what content there is can be totally backwards to the real world, and as mentioned can cause real harm if followed.
This is why I continually refer to other sources than just what is presented here. Sometimes what is seen hear is pure fantasy. Other times it's just fine, but there's enough fantasy presented as fact to create a "caveat emptor" attitude... Free advice is worth what you paid for it.