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.