Longitudinal or pitch trim can be accomplished in several ways. On aircraft with a nonmoveable horizontal stabilizer, trim could be provided by a trim tab arrangement or deflection of the elevators in much the same manner as described for the lateral trim systems.

Aircraft with a movable horizontal stabilizer and elevators are longitudinally trimmed by changing the angle of incidence of the stabilizer. Moving the four-way trim control switch on the stick grip fore or aft will raise or lower the leading edge of the stabilizer to provide the angle of incidence necessary for balanced flight. An electric trim motor and actuator arrangement provides movement of the stabilizer. Aircraft that use a movable horizontal stabilizer for longitudinal control trim do so by varying the neutral position of the control linkage, which, in turn, moves the surface. For example, longitudinal trim is provided by varying the position of the artificial-feel bungee, repositioning the linkage, and setting up a new neutral position for the stabilizer linkage. Anytime a new neutral is introduced by the trim actuator, the power valve shuttle is displaced. The stabilizer assumes a new neutral location, changing

Figure 9-42.Aileron trim control system.
 

 the attitude of the aircraft. The trim inputs may be provided by the pilot or the automatic flight control system. The actuator has two operating speeds-high speed for manual trim and low speed for AFCS trim.

Directional trim is necessary to compensate for yaw of the aircraft. Rudder trim is basically similar to the aileron trim. When the momentary throw rudder trim switch moves left or right, the trim actuator energizes to move the load-feel bungee, repositioning the rudder power mechanism input crank. The rudder linkage and the rudder are repositioned accordingly to a new neutral position.

Most aircraft with power-controlled actuators work in a similar manner, using an electric trim actuator to change the neutral position of linkage, deflecting the rudder to maintain the desired directional stability. Like the lateral and longitudinal trim systems, rudder trim action can be accomplished manually or automatically. Trim position indicators provide a cockpit indication of the amount of trim or surface deflection required by each trim system.