thumb|upright=1.14|Servo tab

A servo tab is a small hinged device installed on an aircraft control surface to assist the movement of the control surfaces. Introduced by the German firm Flettner, servo tabs were formerly known as Flettner tabs. Servo tabs are not true servomechanisms, as they do not employ negative feedback to keep the control surfaces in a desired position; they only provide a mechanical advantage to the pilot.

Servo tabs

A servo tab, or balance tab, moves in the direction opposite to the desired movement of the control surface. It deflects airflow, generating force on the whole control surface in the desired direction.

The tab has a leverage advantage, being located well aft of the control surface hinge line, and thus its airflow deflection moves the control surface in the opposite direction, overcoming the resistance generated by the airflow deflection of the control surface. This has the effect of reducing the control force required from the pilot to move the controls.

In some large aircraft, the servo tab is the only control that is connected to the pilot's stick or wheel, as in the Bristol Britannia and its Canadian derivatives. The pilot moves the wheel, which moves the servo tab; the servo tab with its mechanical advantage moves the elevator or aileron, which is otherwise free-floating.

Anti-servo tabs

thumb|right|An anti-servo tab on the elevator of an [[Grumman American AA-1|American Aviation AA-1 Yankee]]

An anti-servo tab, or anti-balance 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. The anti-servo tab serves to make the controls feel heavier to the pilot, This obviates the need for a second device for trimming, which would increase drag.

Anti-servo trim tabs are particularly found on stabilators, and the Beechcraft U-21 uses elevator and aileron trim tabs which incorporate anti-servo action.

See also

  • Trim tab
  • Nolder
  • Balanced rudder § Aircraft

References

  • 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 1940s, with excellent links to NACA reports of the time, holding present day validity