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 | | From: | Dennis Straussfogel | | Subject: | Center of pressure vs. aerodynamic center | | Date: | Wed, 29 Dec 2004 09:48:09 -0800 |
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> I have a small problem to understand the difference of center of pressure
> and aerodynamic center.
>
> I've read in several books, but I don't really understand.
> Does anybody have an easy explanation?
>
"Center of pressure (c.p)" is exactly what you'd think it is: the point
at which the pressure distribution can be considered to act--analogous
to the "center of gravity" as the point at which the force of gravity
can be considered to act.
The concept of the "aerodynamic center (a.c.)," on the other hand, is
not very intuitive, and is often misused and misunderstood. Because the
lift and location of the c.p. on an airfoil both vary linearly (more of
less) with angle of attack (AOA), at least within the unstalled range of
AOA, it works out that you can define a point on the chord of the
airfoil at which the pitching moment remains *constant*, regardless of
the AOA. That point is usually near the quarter-chord point (.25c) and
for a symmetric airfoil the constant pitching moment would be zero. For
a cambered airfoil the pitching moment about the a.c. would be non-zero,
but constant. The usefulness of the a.c. is in stability and control
work where the aircraft can be defined in terms of the wing and tail
a.c.'s and the required lift and moments calculated without worrying
about the shift in c.p. with AOA.
Hope that explanation is "easy" enough, though a.c. doesn't really lend
itself to one.
Dennis M. Straussfogel, Ph.D.
Aerospace Engineering Consultant
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