Trim Tabs

Trim tabs trims the aircraft in flight. To trim means to correct any tendency of the aircraft to move towards an undesirable flight attitude. Trim tabs control the balance of an aircraft so that it maintains straight and level flight without pressure an the control column, control wheel, or rudder pedal. Note that the tab has a variable linkage which is adjustable from cockpit. Movement of the tab in one direction causes a deflection of the control surface in the opposite direction. Most of the trim tabs installed on an aircraft are mechanically operated from the cockpit trough an individual cable system. However, some aircraft have trim tabs that are operated by an electrical actuator. Trim tabs are either controlled from the cockpit or adjusted on the ground before taking off. Trim tabs are installed on elevators, rudder and ailerons.

Spring Tabs

Spring tabs are similar in appearance to trim tabs, but serve an entirely different purpose. Spring tabs are used for the same purpose as hydraulic actuators, that is to aid in moving a primary control surface. There are various spring arrangements used in the linkage of the spring tabs. On some aircraft, a spring tab is hinged to the trailing edge of each aileron and is actuated by spring loaded push-pull rod assembly which is also linked to the aileron control linkage. The linkage is connected in such a way that movement of the aileron in one direction causes the spring tab to be deflected in the opposite direction. This provides a balanced condition, thus reducing the amount of force required to move the ailerons. The deflection of the spring tabs is directly proportional to the aerodynamic load imposed upon the aileron, therefore, at low speeds the spring tab remains in a neutral position and the aileron is a direct manually controlled surface. At high speeds, however, where the aerodynamic load is great, the tab functions as an aid in moving the primary control surface.