In article ,
Steve Hix wrote:

In article
,
Alan Baker wrote:

In article ,
Steve Hix wrote:

In article
,
Alan Baker wrote:

In article ,
Steve Hix wrote:

In article
,
Alan Baker wrote:

Pusher or puller doesn't affect pitch forces. What affects pitch
forces
is the length of the moment arm between the centre of mass and the
thrust line.

Or center of drag vs thrust line?

The problem with that is "centre of drag" changes.

And, as it does, so would pitch resultants.

Better to take all the separate moments for the various components
about
the centre of mass.

Which will change as airspeed/AOA/etc. change, right?

No. The centre of mass *never* changes (in the context of this
discussion.

No, mass (other than fuel onboard) doesn't. But drag components can
change with changes in speed, AOA, flap positions, landing gear, cooling
flaps, etc.

Right. So take moments about the thing that doesn't change.


Come to think on it, the total moments of drag don't care about any
masses, just the shape(s) of the aircraft exterior.

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

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.

Right. But trying to take moments about a centre of drag that is
changing because of the very thing causing you to take the moments in
the first place is a recipe for madness.

Just take them about the centre of mass!

--
Alan Baker
Vancouver, British Columbia
http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg