A Servo tab (sometimes called a Flettner tab after its inventor Anton Flettner) is a small hinged device installed on an aircraft control surface to assist the movement of the control surface.
Servo tabs move in the opposite direction of the control surface. The tab has a leverage advantage, being located closer to the trailing edge
of the surface and thus can lever the control surface in the opposite direction. This has the effect of reducing the control force required by the pilot to move the controls.
In the case of some aircraft the servo tab is the only control that is connected to the pilot's stick or wheel. The pilot moves the wheel which moves the servo tab and then the servo tab, using its mechanical advantage, moves the elevator or aileron, which is otherwise free-floating.
An anti-servo tab works in the opposite way to a servo tab. It deploys in the same direction as the control surface, making the movement of the control surface more difficult and requires more force applied to the controls by the pilot. This may seem counter-productive, but it is commonly used on aircraft where the controls are too light or the aircraft requires additional stability
in that axis of movement. The anti-servo tab serves to artificially increase stability and also make the controls heavier in feel to the pilot.
On some aircraft with all-flying stabilators an anti-servo tab acts as a trimming device. In this use some manufacturers term it a "balance tab" or "anti-balance tab".
- From the Ground Up, 27th Revised Edition, Aviation Publishers, Ottawa, Ontario, Canada, 1996, pg 14.
- The Quest for Reduced Control Forces (NASA) — Monographs in Aerospace History: William Hewitt Phillips. A thorough dissertation on the reduction of control forces in high speed and large aircraft in the '40s, with excellent links to NACA reports of the time, holding present day validity