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.