Re: Neutral Point

[Eung Tai Kim <eungkim@kari.re.kr>]

At 05:59 ¿ÀÈÄ 98-06-30 +0100, you wrote:
>Tai Kim:
>
>(I assume that is the part of your name that we westerners would call the
>first name?)
>
>I am not an aircraft engineer like you, just a simple physicist. So, I may
>be excused for not knowing what a neutral point is. The "center of lift"?
>In that case, I thought it is not fixed but varies with speed (angle of
>attack) and elevator position.
>
>I had assumed that the "mean" center of lift must be in the middle of the
>c.g. box, but then I found that Velocity publishes exactly the same c.g.
>box for the standard wing and the 173 wing, which is obviously not correct.
>
>
>So, I agree: they should tell us more about these things (assuming they
know!)
>Thanks for any clarification!
>
>Best
>Simon
>
>Simon Aegerter, Winterthur, Switzerland

The mean aerodynamic center is not same as the center of lift.

The pithing moment about the center of lift is zero, however, as you
mentioned, the center of lift changes as the angle of attack changes.
So the aerodynamic specialist has introduced a concept of MAC(mean
aerodynamic center) which does not change.

The MAC is defined as the point about which the pitching moment does not
change as the angle of attack changes.
(This concept was very hard for me to understand when I first learn about
aerodynamics in school.)

For example, the MAC of the ordinary wing airfoil section locates near 25%
of the chord.
All the books about wing section carry the informations of MAC location and
the pitching moment coefficient about MAC for each airfoil.