Those are some finer points you and Jose have made. They started me to
thinking and I have some follow up questions and comments. I think you
have the tail waging the dog here, as you missed that the mechanical
movement of the elevator is not the primary issue. It is........how
much elevator control force is available with and without trim being
set.

Peter Duniho wrote:
wrote in message
ups.com...
Perhaps I misinterpreted what you said. Let's start with the example
that a certain plane has a usable elevator range of +/- 25 deg. from
level. If after loading the plane and taking off, it turns out that 5
deg of nose up trim(5 deg of elevator movement) is required for the
plane to be trimmed out in flight, that means that only 20 deg of up
elevator remains available to the pilot.

Note that the above description is only accurate for certain types of
airplane trim, and even then is only an approximation.

Specifically:

* In the case of a trim tab on the elevator, it's roughly correct. Moving
the trim tab results in a direct movement of the elevator, caused by the
trim tab's opposite aerodynamic force pushing the elevator in the desired
direction. In this case, if the trim is adjusted to deflect the elevator up
5 degrees, there is indeed only 20 degrees of travel left until the
full-deflection stop.

However! Note that the trim tab itself is a tiny elevator, offsetting the
force the elevator provides. So with the trim tab in a full-deflection
position,

Why would it need to be in the full trim position? It is true for any
amount of trim tab movement.

the total aerodynamic force available by moving the elevator the
remainder of the distance to its full-stop (in the trimmed direction) is
less than one would get moving the elevator without the aid of the trim tab.
As Jose points out, moving the elevator in the opposite direction as trimmed
would provide the maximum elevator force, as the trim tab would in that case
be working with the elevator, rather than against it.

Like I said, inputting trim reduces available control force. But this
is a very nit-picky point, one that I would not make.
And are you saying that you or any pilot has ever deliberately either
not used trim, or actually set in opposite trim in order to gain more
elevator control? Is this what you would consider necessary knowledge
for a pilot?

* In the case of a moveable empennage (eg Mooney), elevator deflection
travel limits are not necessarily affected by the trim setting. That is,
since the trim is affecting the angle of attack of the entire horizontal
stabilizer, the elevator may still keep it's entire range of motion relative
to the horizontal stabilizer.

That may be true for the mechanical system that moves the elevator, I
am not totally familiar with all trim systems available, but it is not
true for the aerodynamics of the elevator. The elevator may still be
able to move the same number of deg up and down as before trim was set,
but that motion is not from the same starting point. This results in
more control in one direction (higher angle of attack), but less in the
other (Lower AOA). This is reverse to the previous system, but it still
reduces control in one direction. Could be a big issue during landing
during gusty crosswind conditions, or during takeoff when unexpected
turbulence is encountered.

AFAIK, most jets use a similar trim mechanism, but they also generally have
much more complicated flight control systems. It's hard to generalize in
that case, as it's possible they include some kind of interconnection
between the trim setting and the elevator limits. But then they may have
other things affecting the elevator travel limits as well according to the
dynamics of the current flight situation.

I can imagine that a 777 has a complex elevator and trim system. I'm
sure not an expert on it, and I won't waste much time on it is this
forum. But the amount of control any elevator system has is a function
of span of the surface and surface area, airfoil, AOA, and such. Once
that is built into the aircraft, that determines how much elevator
control is available. Once you use some of that control, the amount
remanining to you is less. If there is a separate control surface that
is used for trim, the added capability that it provides is almost
certainly taken into account in the weight and balance charts.

* In the case of a split-elevator setup (eg Lake amphibian), elevator
deflection travel limits are definitely NOT affected by the trim setting.
The trim control operates what is effectively a secondary elevator,
separately from and independent of the main elevator.

Oh but it does. As before, it may not affect the mechanical movement of
the surface, but it sure affects the aerodynamics, and the aerodynamics
is what we should be interested in anyway. The trim system on the Lake
or any small GA aircraft is set to a pre-determined amount. Once the
trim is set, it continues to do what it was designed to do until it is
changed. If you set in nose down trim on such a system, it continues to
try to move the nose down even when the elevator is trying to move it
up. This means that you have less up control than if no trim was set.

The bottom line: it is incorrect to make an assumption about the effect of
trim on the use of the elevator. Each and every airplane needs to be
considered individually, taking into account the actual trim mechanism being
used and whether it affects the elevator travel (whether directly or
indirectly). In many airplanes, the trim setting has absolutely no effect
on the remaining elevator travel.

Pete

But as we have seen, it most certainly affects how much control force
you have remaining, which is the question that the OP asked. The bottom
line is, the elevator limit is usually set by the designers so that the
control surface never stalls, and so the elevator force is limited by
AOA, not mechanical movement. You can move the elevator 90 deg if you
want, but it will do you no good, as it has long ago stalled. The
horizontal stabilizer has just so much control capability due to it's
design factors. Once you use a portion of that capability, there is
that much less that you have remaining.

And one final question. Do you know of any weight and balance diagrams
that are a function of trim setting? I don't. Weight and balance
diagrams are driven by the ability of the control system to counteract
any imbalance in the CG vs aerodynamic center. Trim uses part of this
capability.

Bud