PPRuNe Forums - View Single Post - Which one to believe; which version of lift is taught for the ATPL theory exams?
Old 17th Jan 2010, 18:45
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Islander2
 
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ft, thank you for your response. Might I ask, do you have to work hard at being so patronising, or does it come naturally? It hardly flatters you.

None of the authors I quoted are claiming to have a force without an equal and opposite reaction, so your condescending 2nd and 3rd paragraphs are wasted words. The authors are all well-regarded industry professionals ... and I was merely taking you to task over your 'general consensus' comment.

Also, thank you, I'm not given to quoting out of context, and I didn't. And, since it was me that was providing the quotations, I should have thought it self evident that I had already read the books!

Also be very vary of any reference selling by being devoid (or almost devoid) of mathematical formulae, such as your first reference.
Which is utter nonsense!

There is a small number of truly excellent, substantially non-mathematical texts that do a fine job of explaining the mechanics of flight, and there numerous theory texts where even the mathematically-literate will be left wrestling with the Navier-Stokes partial differential equations without having gleaned any basic insight into the relevant principles.

But if you want to base your consensus argument on mathematical texts, I've just picked two of the all-time classics off my bookshelf: von Mises and Glauert. A quick review reveals that, in both cases, their entire analysis of the lifting force is in terms of circulation and the pressure differentials around an aerofoil. Both texts are devoid of any explanation or analysis based on changes of momentum or equal and opposite reactions.

Which, of course, is not to say that Newton's theories do not apply to the generation of lift. Just that, for all the professionals that I interfaced with during my aeronautics career, those theories weren't a particularly useful way of either thinking about the mechanism for generating lift or for quantifying the size of the lifting force. Since you say you have immense respect for John Anderson, I'll leave my last word on the subject to a quote from him: "the net rate of change of downward momentum created in the airflow because of the presence of the wing can be thought of as an effect due to the surface pressure distribution; the pressure distribution by itself is the fundamental cause of lift."

So given your view that:
Even initiating the discussion about which one is "the strongest" implies having taken a wrong turn somewhere in the maze of theories, I'm afraid. It's a chicken or egg discussion ...
perhaps you'd better let Mr Anderson know that he's lost his way! Oh, while I'm at it, let me add Chris Carpenter (one time head of aerodynamics at the RAF College, Cranwell) to the list of people you should also contact. He makes the identical point to that in my quote from John Anderson above.

Last edited by Islander2; 17th Jan 2010 at 23:51.
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