Best place for CG along roll axis

Crash Lander writes:

Mxsmanic, ask these questions over there. I guarantee you'll get the answers
you're looking for over there.

I'm getting good answers here.

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"Capt. Geoffrey Thorpe" The Sea Hawk at wow way d0t com writes:

Also, in real life there is a difference between having the CG too far
forward and not using the trim properly.

I had the weight far enough off that trimming didn't leave me much
elevator travel for maneuvering. That's what made me realize that
something was wrong.

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Mxsmanic wrote:

If I'm alone in the aircraft, in the left
seat at the front, can I put an counterbalancing weight in the rear
seat on the right, or would putting weight in a diagonally opposed
position make the aircraft unstable?


Yes, it would be slightly better to do it that way, since it moves the
CG to the center of the A/C. It would only cause instabilty at
EXTREMELY high roll rates. I ran one of my tanks dry once, when I
needed to change a leaky drain. The other tank was nearly full. It took
a fair amount of aileron to overcome the weight difference, but not
even close to the maximum. CG's off the centerline have only minimal
effect on pitch, and should not have a noticable effect on elevator
position.

Bud

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 .

Bud

Mxsmanic wrote:
"Capt. Geoffrey Thorpe" The Sea Hawk at wow way d0t com writes:

Also, in real life there is a difference between having the CG too far
forward and not using the trim properly.

I had the weight far enough off that trimming didn't leave me much
elevator travel for maneuvering. That's what made me realize that
something was wrong.

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"Mxsmanic" wrote in message
...
Steve Foley writes:

If you mean do they always do this, I can only speak for myself. I have a
pre-calculated one for myself with full fuel, myself with fuel to tabs
(partial tanks) and myself and a 170lb person in the front right seat,
both
with full fuel and tabs. Anything else I need to recalculate.

So if someone tosses a bag in the back, or if the person next to you
weighs more or less than 170 lbs, you redo the calculations?




Nobody 'tosses a bag in the back' of my plane. I take it, weight it, stow
it, and re-do the weight and balance.

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?).

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Recently, Mxsmanic posted:

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?

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

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?

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.

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".

Neil

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".

Actually, in the case of an elevator with a conventional trim tab,
trimming for "up" moves the trim tab =down=, which forces the back of
the elevator up (a little), which pushes the tail down (a little) which
pushes the nose up (a little), giving the aircraft a more nose-up
"neutral yoke" attitude.

You'll actually get a tad more up elevator authority (though at the
expense of a lot more control =force= required) if the nose is trimmed
down and you pull all the way on the yoke. That way the trim tab is
positioned up, which helps push the tail down (if the control is held in
position), rather than helping the pilot push the tail down.

Jose
--
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it keeps its brain." (chapter 10 of book 3 - Harry Potter).
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Mxsmanic wrote:

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?

Yes, that's what I tried to say.



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?).

This contradicts what you said in the first paragraph. No, the 737 nor
any other plane could possibly have this ability. This is like saying
that the 737 can be loaded out of the allowable CG range, or the
elevator (or stabilator) can be moved past the stall angle of attack
(which is what limits it's travel in the first place). The elevator has
only so many degrees it can rotate up and down before it stalls. Trim
doesn't change this.

Bud

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Recently, Jose posted:

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".

Actually, in the case of an elevator with a conventional trim tab,
trimming for "up" moves the trim tab =down=, which forces the back of
the elevator up (a little), which pushes the tail down (a little)
which pushes the nose up (a little), giving the aircraft a more
nose-up "neutral yoke" attitude.

You'll actually get a tad more up elevator authority (though at the
expense of a lot more control =force= required) if the nose is trimmed
down and you pull all the way on the yoke. That way the trim tab is
positioned up, which helps push the tail down (if the control is held
in position), rather than helping the pilot push the tail down.

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.

Neil