Best place for CG along roll axis

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.

Regards,
Bud


Mxsmanic wrote:
writes:

Trimming doesn't change the ability of the horizontal stabilizer to
correct for CG location, etc. It merely adds in an initial deflection
of the elevator so as to make the effort needed by the pilot to move it
is small. Imagine having to hold a constant twenty pounds of elevator
on a cross country .

But doesn't trim in most aircraft involve moving the elevator with a
trim tab? That is, if the elevator is moved up by the trim, then
there's that much less travel remaining in the elevator in that
direction. So if you have quite a bit of trim, your safety margin for
additional elevator movement is reduced. Right?

In an aircraft in which the entire stabilizer moves for trim, I
suppose you could set any amount of trim and still have full travel in
both directions (doesn't the 737 work this way?).

--
Transpose mxsmanic and gmail to reach me by e-mail.

Neil Gould writes:

Yes, however I think that relationship would only confuse Mxsmanic, who
seems to be of the notion that somehow your control movement is limited or
"safety margin" decreased (whatever that might be) by using trim tabs. The
point I was making is that the range of elevator movement is largely
unaffected by trim settings.

If you set trim such that it moves the elevator up by one degree, you
have one degree less of upward travel remaining for the elevator. If
trim is set such that most of the travel of the elevator in one
direction is already used, there may not be enough left if you decide
you need more.

--
Transpose mxsmanic and gmail to reach me by e-mail.

writes:

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.

Yes, that was my concern.

--
Transpose mxsmanic and gmail to reach me by e-mail.

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, 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.

* 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.

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.

* 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.

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

"Neil Gould" wrote in message
t...
But doesn't trim in most aircraft involve moving the elevator with a
trim tab?

On many aircraft, the trim tab moves independently of the elevator, and on
others the elevator is adjusted.

Just so. And on still others, a separate flight control surface is
deflected. On even still others, the entire horizontal stabilizer is moved.
There are numerous means for adjusting trim, making it unwise to generalize
about the effects of trim on other aspects of flight control.

Trimming the elevator reduces the amount of effort required by the pilot
to hold altitude. If one is carrying a lot of "up" trim, for example, then
the aircraft is likely to be near critical AOA. The last thing you'd need
is have a lot more "up" elevator movement available.

This depends on the trim design. If it affects the overall angle of attack
of the horizontal stabilizer (either directly or by deflecting the
elevator), then the statement is true. But if it uses an independent
airfoil to create an up- or down-force separately, then the angle of attack
of the primary stabilizer/elevator surfaces is unchanged and is not
necessarily near the critical AOA (and in fact in that design would not be
even close, assuming the elevator control is in the neutral position).

If you're talking about the main wing of the airplane being near the
critical AOA, then that's only true if "one is carrying a lot of 'up' trim"
for the purpose of dealing with a heavy load, slow flight, or both. Nose-up
trim to compensate for excessively forward CG would not result in the main
wing's AOA necessarily being near the critical AOA and so more nose-up
elevator would not necessarily be a problem.

In an aircraft in which the entire stabilizer moves for trim, I
suppose you could set any amount of trim and still have full travel in
both directions (doesn't the 737 work this way?).

The overall range of a stabilizer's movement is usually the same
regardelss of trim settings. IOW, you don't usually get more "up" than
full "up".

I'm not exactly sure what you mean here. But it is certainly true that with
certain kinds of trim designs, one can get more nose-up force using nose-up
trim than using neutral trim.

Pete

"Neil Gould" wrote in message
t...
[...] The
point I was making is that the range of elevator movement is largely
unaffected by trim settings.

I think it's safe to say that the full *range* of elevator movement is
largely (or entirely) unaffected by trim settings, yes. In every trim
design that comes to mind at the moment, this is true.

But it is also true that in certain designs, the trim shifts the entire
range of full-up and full-down deflection (allowing the same amount of
deflection in each direction, but providing for a greater aerodynamic force
in one direction and less in the other), while in other designs, the trim
simply shifts the neutral point (resulting in more deflection available in
one direction and less in the other).

Pete

Recently, Mxsmanic posted:

Neil Gould writes:

Yes, however I think that relationship would only confuse Mxsmanic,
who seems to be of the notion that somehow your control movement is
limited or "safety margin" decreased (whatever that might be) by
using trim tabs. The point I was making is that the range of
elevator movement is largely unaffected by trim settings.

If you set trim such that it moves the elevator up by one degree, you
have one degree less of upward travel remaining for the elevator. If
trim is set such that most of the travel of the elevator in one
direction is already used, there may not be enough left if you decide
you need more.

There are several basic aspects of this that are eluding you. First and
foremost, one is flying, and therefore the principles of flight are fairly
important. As you trim upward, for example, your AOA changes. There is no
problem exceeding the critical AOA even with full up trim, so there is no
point in having *more* up elevator. One would not be any safer, or have
any more control over flying the plane. Secondly, the elevator will not go
more *or less* "up" or "down" than its travel limits without trim. IOW,
the elevator travel is not essentially affected by trim.

I think you could learn a lot and quickly dispell your misconceptions by
taking a couple of lessons.

Neil

Recently, Peter Duniho posted:

"Neil Gould" wrote in message
t...
[...] The
point I was making is that the range of elevator movement is largely
unaffected by trim settings.

I think it's safe to say that the full *range* of elevator movement is
largely (or entirely) unaffected by trim settings, yes. In every trim
design that comes to mind at the moment, this is true.

But it is also true that in certain designs, the trim shifts the
entire range of full-up and full-down deflection (allowing the same
amount of deflection in each direction, but providing for a greater
aerodynamic force in one direction and less in the other), while in
other designs, the trim simply shifts the neutral point (resulting in
more deflection available in one direction and less in the other).

Exactly. However, to Mxsmanic's notion, in either case, one still has more
control than needed to stall the aircraft or pitch the nose down, so the
idea that there is some reduced "margin of safety" due to using trim is
odd.

Neil

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

wrote in message
ups.com...
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.

I believe I did address that question.

[...]
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.

Because maximum force from the horizontal stabilizer (including the
elevator) comes when the elevator itself is fully deflected *and* the trim
tab is fully deflected *in the same direction* (which would mean the trim
control would be set opposite the elevator deflection being used...yes, it's
going to take a pretty hefty push or pull on the elevator to get it in this
position).

Like I said, inputting trim reduces available control force. But this
is a very nit-picky point, one that I would not make.

You said it, but it's not a point you would make? Intriguing.

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?

IMHO, any knowledge, however seemingly esoteric, it potentially useful to a
pilot. I can't say that I know of any specific situation in which this
exact information was applied, but I can definitely hypothesize a situation
in which it might be useful.

Note that the same understanding of how the trim works leads to a correct
understanding of what to do if the elevator control itself becomes jammed.
Different trim designs require different uses of trim to compensate for
this, and knowing the details is quite important for a pilot in this
particular not-rare situation.

This sort of knowledge is not so useless as you appear to think.

* 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 follows the entire empennage. How is it that you propose the
elevator obeys different aerodynamics?

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.

It is at the same starting point relative to the horizontal stabilizer, more
or less (ignoring for the moment the slight variation that might be caused
by airflow deflecting the elevator slightly from the perfectly-aligned
position).

This results in
more control in one direction (higher angle of attack), but less in the
other (Lower AOA).

How so? You seem to be assuming that the slight deflection I mention above
is in exactly the same magnitude, and in the opposite direction, of the
change in angle of the stabilizer. I see no reason to simply assume this to
be true. The elevator will trail in the airflow, but the airflow is being
deflected by the horizontal stabilizer. The elevator isn't going to trail
parallel to the average relative wind; it will trail parallel to the local,
deflected wind.

That said, regardless of where the exact neutral position for the elevator
is, you still get the same degree of deflection up or down relative to the
perfectly-aligned neutral position (that is, relative to what would be
perfectly neutral ignoring any deflection due to the airflow). Even if you
assume that the elevator's aerodynamically neutral position is significantly
altered, the actual control surface deflection possible relative to the
perfectly-aligned position remains the same regardless of trim setting.

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.

I don't see how. An elevator with such a trim system that has 25 degrees up
and down deflection with the trim in the neutral position will still have 25
degrees up and down deflection with the trim in any other position, relative
to the perfectly-aligned elevator position.

[...] 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.

But that's the point I'm making. Not all trim systems actually use any of
the control available from the elevator to effect their change. In fact, of
the three types I specifically mentioned, only one does.

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.

Um...one hopes that any relevant design aspects are taken into account in
the W&B charts. I don't see how that is relevant here.

* 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.

Oh but what does?

As before, it may not affect the mechanical movement of
the surface,

The trim does not affect the mechanical movement of the elevator control
surface. Is that what you mean to say?

but it sure affects the aerodynamics,

What sure affects the aerodynamics? The trim affects the aerodynamics? The
trim on a split-elevator system does not affect the aerodynamics of the
elevator.

and the aerodynamics
is what we should be interested in anyway.

Since the discussion is about elevator travel limits, I don't see why you
say that the aerodynamics are what we should be interested in anyway. If
that's the discussion you want to have, you are free to start it. But this
particular discussion is about elevator travel limits.

The trim system on the Lake
or any small GA aircraft is set to a pre-determined amount.

Pre-determined? What do you mean by that? Me, I set the trim at various
points of flight, to whatever amount is required at that particular time. I
have no "pre-determined amount" for my trim settings. Even when an airplane
has a marked takeoff, landing, etc. setting I use that only for a general
guide. As soon as the airplane is actually flying, the trim gets readjusted
to the needed 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.

But that doesn't change the elevator deflection limits. The elevator can be
moved up or down the same amount as it could at any other trim setting.
That's my point, and that's what I wrote.

But as we have seen, it most certainly affects how much control force
you have remaining,

But it doesn't. Not in all cases. In a C172, it does. The elevator is
deflected directly by the trim, and thus using the trim affects the
remaining range of elevator travel (and thus control force) remaining.

In other airplanes, the elevator is left with just as much deflection
remaining as it had at the neutral trim position. In those cases, the trim
contributes to the elevator control force, without any sacrifice in the
*additional* elevator force that can be commanded by the pilot.

That's the whole point.

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.

That wasn't the question. I don't doubt that when the trim contributes to
the total elevator force, this is taken into account with respect to
airplane design. If that's true, however, that means that the pilot MUST
understand this and be prepared to use the necessary trim deflection in
order to obtain maximum performance from the elevator when necessary.
Otherwise, full deflection of the elevator at less-than-full-deflection of
the trim will result in less than the full design capability of the pitch
control system.

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.

Weight & balance diagrams are driven by the design envelope of the airplane
and the lowest common denominator in that envelope. That lowest common
denominator may or may not be the trim system. You cannot infer anything
about trim systems generally from any one weight & balance diagram, or even
any collection of weight & balance diagrams.

Pete