Trim tab location

[Jay]

I've noticed that a lot of aircraft put trim tabs on the trailing
edges of the normal movable control surfaces. Wouldn't this just
deflect the control surface to neutralize the forces and not affect
much change except drag? It seems like you'd want to put the trim tab
on the fixed part of the wing/stab/vert, that way its pushing the
airplane instead of the flight controls in your hands. I've noticed
some articulated mechanisims on some aircraft (anti-servo) but this
seems to be a complicated way to make up for the fact that the tab was
put in the wrong place to begin with. What am I missing?

[plasticguy]

"Jay" > wrote in message
om...
> I've noticed that a lot of aircraft put trim tabs on the trailing
> edges of the normal movable control surfaces. Wouldn't this just
> deflect the control surface to neutralize the forces and not affect
> much change except drag? It seems like you'd want to put the trim tab
> on the fixed part of the wing/stab/vert, that way its pushing the
> airplane instead of the flight controls in your hands. I've noticed
> some articulated mechanisims on some aircraft (anti-servo) but this
> seems to be a complicated way to make up for the fact that the tab was
> put in the wrong place to begin with. What am I missing?

I believe you are missing the point. The tab was not placed in the wrong
place.
The trim tab drives the surface. When you drive the surface to a new
position,
it creates forces in the same manner as if you had used the controls to
place it there.
Trims are made to unload the controls in the pilots hands, so not placing
them
on the surface would be futile. Scott.

[Bob Martin]

"Jay" > wrote in message
om...
> I've noticed that a lot of aircraft put trim tabs on the trailing
> edges of the normal movable control surfaces. Wouldn't this just
> deflect the control surface to neutralize the forces and not affect
> much change except drag?

You're confusing equal moments with equal forces. Putting the trim tab on
the trailing edge of the surface only creates a little force but has a
fairly large moment arm (about the control surface hinge line). The control
surface makes the same moment, but does it on a shorter arm (meaning the
total force is greater). The net effect on the entire airplane is that the
control surface's force vastly overcomes the trim tab's and puts a pitching
moment on the whole airplane.

[Jan Carlsson]

A small tab at the trailing edge is moving the control surface so the
control surface is doing the job (trimming) by a small change in the angle,
if the trim tab is sitting on a fixed part of the plane a bigger tab and or
angle is needed.

Jan

"Jay" > skrev i meddelandet
om...
> I've noticed that a lot of aircraft put trim tabs on the trailing
> edges of the normal movable control surfaces. Wouldn't this just
> deflect the control surface to neutralize the forces and not affect
> much change except drag? It seems like you'd want to put the trim tab
> on the fixed part of the wing/stab/vert, that way its pushing the
> airplane instead of the flight controls in your hands. I've noticed
> some articulated mechanisims on some aircraft (anti-servo) but this
> seems to be a complicated way to make up for the fact that the tab was
> put in the wrong place to begin with. What am I missing?

[sidk]

Bob,
Thanks for the great explanation.. I too have been puzzled by the trim
tab effect. I have read many "explanations" and yours is the first
that was articulated sufficently well to sink into my fuzzy brain.

Sid

"Bob Martin" > wrote in message >...
> "Jay" > wrote in message
> om...
> > I've noticed that a lot of aircraft put trim tabs on the trailing
> > edges of the normal movable control surfaces. Wouldn't this just
> > deflect the control surface to neutralize the forces and not affect
> > much change except drag?
>
> You're confusing equal moments with equal forces. Putting the trim tab on
> the trailing edge of the surface only creates a little force but has a
> fairly large moment arm (about the control surface hinge line). The control
> surface makes the same moment, but does it on a shorter arm (meaning the
> total force is greater). The net effect on the entire airplane is that the
> control surface's force vastly overcomes the trim tab's and puts a pitching
> moment on the whole airplane.

[Ray Toews]

I have wondered why some aircraft, use a small moveable control
surface seperate from the normal controls. I believe I have seen this
in the form of a small wing under the horizontal stab being used as
elevator trim.
I once installed a trim tab on the back of the elevator on an
Ultralight Challenger and induced severe flutter, something I did not
want to repeat and obviously it is more than just deflecting the
control.

Ray

On Thu, 24 Jun 2004 10:16:24 +0200, "Jan Carlsson"
> wrote:

>A small tab at the trailing edge is moving the control surface so the
>control surface is doing the job (trimming) by a small change in the angle,
>if the trim tab is sitting on a fixed part of the plane a bigger tab and or
>angle is needed.
>
>Jan
>
>"Jay" > skrev i meddelandet
om...
>> I've noticed that a lot of aircraft put trim tabs on the trailing
>> edges of the normal movable control surfaces. Wouldn't this just
>> deflect the control surface to neutralize the forces and not affect
>> much change except drag? It seems like you'd want to put the trim tab
>> on the fixed part of the wing/stab/vert, that way its pushing the
>> airplane instead of the flight controls in your hands. I've noticed
>> some articulated mechanisims on some aircraft (anti-servo) but this
>> seems to be a complicated way to make up for the fact that the tab was
>> put in the wrong place to begin with. What am I missing?
>
>

[Bob Martin]

<Ray Toews> wrote in message ...
> I have wondered why some aircraft, use a small moveable control
> surface seperate from the normal controls. I believe I have seen this
> in the form of a small wing under the horizontal stab being used as
> elevator trim.
> I once installed a trim tab on the back of the elevator on an
> Ultralight Challenger and induced severe flutter, something I did not
> want to repeat and obviously it is more than just deflecting the
> control.

Sometimes, a control surface will have "spades" extending either above or
below it. As I understand it, they serve to reduce the control loads on
that surface with high deflections. Usually, you'd see this on a very
high-maneuverability aircraft (like those used for competition aerobatics).
Adding such surfaces takes some knowledge though; you can't just slap one on
without careful study or you might rip the entire surface off.

Also, sometimes you see mass balances (usually like little teardrop shapes)
extending forward of the surface. The best example of this that I can think
of is on the P-38's elevator (though whether it was actually needed is still
up for debate). I don't quite understand what those do as much, though I
think they help with flutter.

[Big John]

Bob

They balance the weight of the control surface equal on each side of
the hinge.

A better wording might be static balance.

Am sure there will be others that add to this question.

Big John

On Sun, 27 Jun 2004 21:15:46 -0400, "Bob Martin"
> wrote:

><Ray Toews> wrote in message ...
>> I have wondered why some aircraft, use a small moveable control
>> surface seperate from the normal controls. I believe I have seen this
>> in the form of a small wing under the horizontal stab being used as
>> elevator trim.
>> I once installed a trim tab on the back of the elevator on an
>> Ultralight Challenger and induced severe flutter, something I did not
>> want to repeat and obviously it is more than just deflecting the
>> control.
>
>Sometimes, a control surface will have "spades" extending either above or
>below it. As I understand it, they serve to reduce the control loads on
>that surface with high deflections. Usually, you'd see this on a very
>high-maneuverability aircraft (like those used for competition aerobatics).
>Adding such surfaces takes some knowledge though; you can't just slap one on
>without careful study or you might rip the entire surface off.
>
>Also, sometimes you see mass balances (usually like little teardrop shapes)
>extending forward of the surface. The best example of this that I can think
>of is on the P-38's elevator (though whether it was actually needed is still
>up for debate). I don't quite understand what those do as much, though I
>think they help with flutter.
>