"Steve Hix" wrote in message
...
In article
,
Alan Baker wrote:

You have to take moments about something that isn't going to shift,
Steve. Centre of mass.


Steve replied ...

I know that that doesn't change (ignoring fuel burn), but things like
flaps' contribution to drag moments changes with changes in
configuration.

Else we'd never see pitch changes as we raise or lower the flaps, or
changes in speed as landing gear are retracted or extended.

Semantics can really make things confused. In aerodynamics and aircraft
design we simplify things to make the
math a bit easier. Instead of dealing with forces that move all over the
place and make it difficult to superimpose
mathematically to calculate their resolution and the resultant vectors which
would get relatively intractable, we
pick a point more or less at random and say "The force acts HERE!" Then we
figure out the moments about that
point contributed by our other factors. Then we merely sum up the moments,
using the appropriate arithmetic for
summing up vectors of course, and determine the resultant forces relatively
easily. To make this simplification
work we need to pick arbitrary points that don't move around. Things like
the "center of mass", or the "center of
lateral area", or the "center of gravity", or the "quarter chord line of the
lifting surface" and then we find how the
moments change about that point. We can even play additional simplification
games and normalize the actual moments
into a dimensionless coeficcient. If you look at a plot of the forces on a
wing you will see alongside the lift/drag
polar, another line labeled "moment coefficient." This moment coefficient
tells you how the center of lift moves about
relative to the quarter chord line of the wing. The neat thing about the
NACA 23012 airfoil section is the moment
coefficient is approximately zero throughout the normal angle of attack
range. This means the center of lift doesn't move
around very much on the wing. This is what made possible the monospar wing
on the DC-3! :-)

Highflyer
Highflight Aviation Services
Pinckneyville Airport ( PJY )