Procedure for calculating weight and balance

All it does is relieve the pressure. If there were no trim at all the elevator would be in the exact same position, it would just suck to have to hold it there.

Well, not exactly, not on all aircraft. Consider a trim system which
uses a trim tab. The tab projects (say) up, causing the elevator to
trail down as its neutral position, pushing the tail up and thus the
nose down. Once equilbrium has been reached (no pressure), clamp the
elevator in that position, and trim in such a manner that the tab no
longer projects up. Now, the elevator is (slightly) more effective at
pushing the tail up, because the trim tab is no longer in the airflow in
the opposite direction. The nose will be pushed (slightly) further
down, and the clamp will be resisting pressure. If you release the
clamp, going to a new "no pressure" condition, the elevator =will= move.

Looked at another way, if you come from an untrimmed position (with this
same aircraft), and position the yoke wherever it needs to be in order
to maintain the condition you want (say, straight and level, FSOA), and
then clamp the yoke in that position, you will be at the chosen
condition. But if you trim, the movement of the trim tab =will= have a
(slight) aerodynamic effect. When you actually achieve "no pressure",
you will be (slightly) out of trim for the effect you want.

1: I understand the effect is slight, perhaps even not noticable, but
I'll bet it can be measured.

2: I understand that when a real pilot actually trims for "no
pressure", there is a feedback loop where "no change in flight
condition" also feeds into it, so the above discussion is academic.

3: I understand that this will not be true, or will not be true for the
same reasons, for other trim systems.

However, the quoted statement above is not true, and if Mx made that
statement, he would have been jumped on, just because it's Mx making
that statement.

Jose
--
He who laughs, lasts.
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