SERVO TABS

When the pilot’s control force acts only to deflect the tab, and not the main surface, it is designated a servo tab. This result is attained if the torsion spring of Fig. 6.27 is replaced by a free hinge. The control lever then becomes an idler and the force in the control rod is simply the reaction to the tab hinge moment, which is of course relatively small. The angle through which the control surface deflects is then governed by the kinematics of the linkage,

and the equilibrium of aerodynamic and control rod moments about the main surface hinge.

Both spring tabs and servo tabs are effective devices for reducing control forces on large high-speed airplanes. However, both add an additional degree of freedom to the control system dynamics, and this is a potential source of trouble due to vibration or flutter.