EASA / GCAA Part-66 · Module 11 · Aeroplane Systems · 11.9
Objective: the three primary controls each rotate the aeroplane about one axis. The tabs on their trailing edges do something different. They change how heavy the control feels, and depending on which way the tab moves relative to the surface, they can lighten it, deliberately weight it, or fly the surface on their own.
The three primary controls each work about one axis: ailerons roll the aeroplane about the longitudinal axis, the elevator pitches it about the lateral axis, and the rudder yaws it about the normal axis. Each is simply a hinged section of trailing edge, and deflecting it changes the camber of that surface so it produces a force in the opposite direction to the deflection.
The airflow pushes back. The force on a deflected surface acts behind its hinge line, so it produces a hinge moment that tries to return the surface to neutral. The pilot has to hold against that moment, and it is felt as stick force. On a large or fast aeroplane that force becomes unmanageable, which is the problem tabs and powered controls exist to solve.
Which way the tab goes decides everything. A balance tab is linked to move opposite to the surface, so its own small force helps hold the surface deflected and the stick becomes lighter. An anti-balance tab moves in the same direction as the surface, making the stick heavier, which sounds perverse until you remember that a control which is too light is dangerous because the pilot can overstress the aeroplane without noticing. A servo tab is the extreme case: the pilot's controls connect to the tab only, and the tab's aerodynamic force flies the surface, which is how large aeroplanes were controlled before hydraulic boost. A trim tab is different again. It is set by the pilot rather than linked to the surface, and it holds the surface where it is wanted so that the stick force is zero at the chosen condition.