Figure 5-16. The movement of the elevator is opposite to the  Figure 6-20.  The movement of the elevator is opposite to the  direction of movement of the elevator trim tab.  control pressures that may exist for that flight condition. As  power, pitch attitude, or configuration changes, retrimming  is necessary to relieve the control pressures for the new  flight condition.  The control forces may be excessively high in some aircraft,  and, in order to decrease them, the manufacturer may use  balance tabs. They look like trim tabs and are hinged in  approximately the same places as trim tabs. The essential  difference between the two is that the balancing tab is coupled  to the control surface rod so that when the primary control  surface is moved in any direction, the tab automatically  moves in the opposite direction. The airflow striking the tab  counterbalances some of the air pressure against the primary  control surface and enables the pilot to move the control more  easily and hold the control surface in position.  If the linkage between the balance tab and the fixed  surface is adjustable from the flight deck, the tab acts as a  combination trim and balance tab that can be adjusted to a  desired deflection.  Servo tabs are very similar in operation and appearance to the  trim tabs previously discussed. A servo tab is a small portion  of a flight control surface that deploys in such a way that it  helps to move the entire flight control surface in the direction  that the pilot wishes it to go. A servo tab is a dynamic device  that deploys to decrease the pilots work load and de-stabilize  the aircraft. Servo tabs are sometimes referred to as flight tabs  and are used primarily on large aircraft. They aid the pilot in  moving the control surface and in holding it in the desired  position. Only the servo tab moves in response to movement  of the pilot’s flight control, and the force of the airflow on  the servo tab then moves the primary control surface.  Antiservo tabs work in the same manner as balance tabs  except, instead of moving in the opposite direction, they move  in the same direction as the trailing edge of the stabilator.  In addition to decreasing the sensitivity of the stabilator, an  antiservo tab also functions as a trim device to relieve control  pressure and maintain the stabilator in the desired position.  The fixed end of the linkage is on the opposite side of the  surface from the horn on the tab; when the trailing edge of the  stabilator moves up, the linkage forces the trailing edge of the  tab up. When the stabilator moves down, the tab also moves  down. Conversely, trim tabs on elevators move opposite of  the control surface.  [Figure 6-21]  Many small aircraft have a nonmovable metal trim tab on the  rudder. This tab is bent in one direction or the other while on  the ground to apply a trim force to the rudder. The correct  displacement is determined by trial and error. Usually, small  Ground Adjustable Tabs  Antiservo Tabs  Balance Tabs  Servo Tabs  Figure 6-21.  An antiservo tab attempts to streamline the control  surface and is used to make the stabilator less sensitive by opposing  the force exerted by the pilot.  6-11  Nose-down trim  Nose-up trim  Trim tab  Elevator  Trim tab  Elevator  Tab up—elevator down  Tab up—elevator up  Pivot point  Stabilator  Antiservo tab 