Balancing or Geared Tabs
Control surface tabs affect the pressure distribution at the rear of control surfaces, where there is a large moment arm about the hinge line. A trailing-edge-up tab creates relative positive pressure on the control’s upper surface and a relative negative pressure peak over the tab-surface hinge line. Both pressure changes drive the control surface in the opposite direction to the tab, or trailing-edge-down.
When a tab is linked to the main wing so as to drive the tab in opposition to control surface motion, it is called a balancing or geared tab. Balancing tabs are used widely to reduce control forces due to control surface deflection. They have no effect on the hinge moments due to wing or tail surface angle of attack. Airplanes with balancing tabs include the Lockheed Jetstar rudder, the Bell P-39 ailerons (augmenting Frise ailerons), and the Convair 880M.