"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?

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.
--
Jim in NC