On the trailing edge of most primary control surfaces there is a small adjustable cutout or extended tab. These mini-controls are called "Trim Tabs". An example best described their use: Suppose you found it necessary to keep maintaining pressure on the elevator control to hold a desired pitch attitude, as in take off, climbing, and especially in the Private check-ride steep turn. You do not want to hold back pressure on the yoke, as the constant exercise would become very tedious. The purpose of the Trim tab is to hold the primary control surface in the deflected position aerodynamically instead of through control pressure. In other words Trim tabs allows you to maintain the position of a primary control surface without keeping your hands on the control wheel or stick. I am very guilty of flying with my hand on the trim wheel instead of the yoke, power trim tabs are most common.

Use the primary control surface to position the aircraft in the proper attitude. As in the 150 0r 172 as you get to altitude you level off first, then you decrease throttle to cruising RPM's, and then adjust the Trim tab to maintain straight and level flight so that it will maintain altitude without pressure or hands off. Anytime the aircraft is changing speed the control pressure will be constantly changing, so hold a constant altitude long enough for the speed to stabilize before adjusting the trim.

Gary

Aeronautical Engineer/Pilot/Boeing Defense