auxiliary control surfaces used to reduce the forces in the control systems of airplanes, helicopters, and rockets. The auxiliary surface has a relatively small area. It is usually mounted on the trailing edge of the main aerodynamic control surface and is deflected in a direction opposite to the direction of deflection of the main surface.
There are three types of servo controls: balancing tabs, servo tabs, and trimming tabs. A balancing tab is connected to the fixed surface of the wing or stabilizer by means of a link or spring mechanism; its deflection is determined by the rotation of the main control surface. A servo tab is the actuating member of a control system; the main control surface, which is freely suspended on an axle, is deflected by the action of the aerodynamic forces that arise when the servo tab is deflected. A trimming tab, like a control surface, has an independent control system. Unlike a balancing tab and a servo tab, which are designed to reduce the forces when turning a main control surface, a trimming tab is used under steady flying conditions to hold the main control surfaces in a deflected state over an extended period of time.