relative angle of attack

[John Denker <jsd@MONMOUTH.COM>]

Hi --

A few people poked fun when at 12:29 PM 8/12/99 -0700, William Beaty suggested:
> Perhaps we should think in terms of an
> *effective* angle of attack, rather than determining the AOA of the wing
> through geometry and then fighting over the fact that this determination
> of attack-angle allows cambered airfoils to generate lift at zero AOA.

....and even Brother Bill reconsidered his suggestion.

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.