In article ,
"Morgans" wrote:

"Alan Baker" wrote

First: yes, any *fixed* point will do, which the centre of drag is not.

Second, the math is easiest when you pick the point that is actually the
one about which the body will rotate.

I think I get your point, about the plane rotating about the center of
mass, but I do not believe that a change in the amount of thrust and its
location in relation to the center of mass is what is relevant, in this
issue.

For a plane to not require a change in trim with a change in power, and not
to rotate, the thrust line must be in line with the center of drag in level
flight, no?

No.

Thrust, drag, lift weight are the way we normally coin the forces on an
aircraft, but the reality isn't quite that.

That formalism has both drag and thrust operating in the horizontal
axis, but we know that the engine's thrust will only actually be
horizontal at one particular angle of attack. Hence, only then will the
thrust line be aligned with the drag line.

That's why I keeps saying take all the moments about a fixed point to
really have a clear picture.


The downthrust or upthrust is added to partially negate the rotation caused
due to the distance the thrust is away from the center of drag.

Forget all about drag for a moment: you can boil down all the forces of
lift and drag -- the aerodynamic forces -- to a single force through the
aerodynamic centre of pressure. Now imagine an aircraft with the engine
off and the prop feathered and in a trimmed out glide.

Got that pictured in your mind? Good.

Now: when you start the engine and add its thrust into the equation,
what is the one direction in which you can apply that thrust and not
cause the aircraft to pitch.

Where must the axis of the thrust vector be?

Through the centre of mass. Period.

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