Re: relative angle of attack

["JACK L. URETSKY (C)1998; HEP DIVISION, ARGONNE NATIONAL LAB ARGONNE, IL 60439" <JLU@HEP.ANL.GOV>]

Hi all-
        John Denker writes:
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But actually it was an excellent suggestion.  In the fluid dynamics
business, there is a quantity called "relative angle of attack" which is
*defined* so that the lift is zero when the (relative) angle of attack is
zero.  Compared to other choices of how to measure the angle, this choice is
  *) Less arbitrary
  *) More physical
  *) More well-defined.

For example, using the chord line as your reference (another common choice)
is not well defined if the wing has washout (which it almost always does),
or if it has flaps that can be extended.  Physically speaking, there is
*nothing* special about the chord line (except for a symmetric wing in
which case it coincides with the zero-lift direction).

The question "why does this airfoil generate nonzero lift at zero angle of
attack" only comes up if people mistakenly think there is something special
about the chord line.

Using the zero-lift direction as your reference works fine.  It is suitable
for high schools students, for pilots, and for researchers. It ensures that
certain meaningless questions never arise.
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        Also, there is the question of the angle between, say, the chord line of
the wing and the orientation, the thrust line, for example, of the rest of
the airplane. This angle is adapted to such things as the landing configuration
of the airplane, the length of the landing gear, and other practical
considerations.  If an aircraft has guns mounted in the wings, roughly
parallel to the chord line, then the wing placement determines the pointing
direction of the guns during a firing run.
        All of these considerations become irrelevant when angle of attack
is measured from the zero-lift angle.
        I have just looked at chapt. III of John Denker's book, and it
looks fine to me.  All that is missing, perhaps, is a one-page summary
of the chapter for distribution to the list each time this subject comes
up.
                                        Regards,


        "I scored the next great triumph for science myself,
to wit, how the milk gets into the cow.  Both of us
had marveled over that mystery a long time.  We had
followed the cows around for years - that is, in the
daytime - but had never caught them drinking fluid of
that color."
                Mark Twain, Extract from Eve's
                Autobiography