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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
Chapter 6, "The Quest for Reduced Control Forces"
and NACA Rep. 794 (1944)
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, and also to increase the stability of that control surface. It is also used where the aircraft requires additional stability in that axis of movement.
Anti-servo tabs are particularly found on
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 {{Aircraft components Aircraft controls
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 servomechanism
In control engineering a servomechanism, usually shortened to servo, is an automatic device that uses error-sensing negative feedback to correct the action of a mechanism. On displacement-controlled applications, it usually includes a built-in ...
s, as they do not employ negative feedback
Negative feedback (or balancing feedback) occurs when some function of the output of a system, process, or mechanism is fed back in a manner that tends to reduce the fluctuations in the output, whether caused by changes in the input or by othe ...
to keep the control surfaces in a desired position; they only provide a mechanical advantage
Mechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device or machine system. The device trades off input forces against movement to obtain a desired amplification in the output force. The model for ...
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 theBristol Britannia
The Bristol Type 175 Britannia is a retired British medium-to-long-range airliner built by the Bristol Aeroplane Company in 1952 to fly across the Commonwealth. During development two prototypes were lost and the turboprop engines proved ...
and its Canadian
Canadians (french: Canadiens) are people identified with the country of Canada. This connection may be residential, legal, historical or cultural. For most Canadians, many (or all) of these connections exist and are collectively the source of ...
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. With the "geared spring tab" variant, a pilot is able "to maneuver a vehicle weighing as much as 300,000 pounds flying at an airspeed of 300 miles per hour or more".W.H. Phillips, ''A Career at NASA Langley Research Center''Chapter 6, "The Quest for Reduced Control Forces"
and NACA Rep. 794 (1944)
Anti-servo tabs
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, and also to increase the stability of that control surface. It is also used where the aircraft requires additional stability in that axis of movement.
Anti-servo tabs are particularly found on stabilator
A stabilator is a fully movable aircraft horizontal stabilizer. It serves the usual functions of longitudinal stability, control and stick force requirements otherwise performed by the separate parts of a conventional horizontal stabilizer and e ...
s, which must have some method of decreasing their sensitivity.
Incorporation of trim tab functionality
A servo or anti-servo tab may also function as atrim tab
Trim tabs are small surfaces connected to the trailing edge of a larger control surface on a boat or aircraft, used to control the trim of the controls, i.e. to counteract hydro- or aerodynamic forces and stabilise the boat or aircraft in a pa ...
, to relieve control pressure and maintain the control surface in the desired position. This obviates the need for a second device for trimming, which would increase drag.
Anti-servo trim tabs are particularly found on stabilators, but are also found on other control surfaces. The Beechcraft RC-12 Guardrail uses a rudder trim tab which incorporates anti-servo action, and the Beechcraft U-21 uses elevator and aileron trim tabs which incorporate anti-servo action.
See also
*Trim tab
Trim tabs are small surfaces connected to the trailing edge of a larger control surface on a boat or aircraft, used to control the trim of the controls, i.e. to counteract hydro- or aerodynamic forces and stabilise the boat or aircraft in a pa ...
* Nolder
* Balanced rudder ยง Aircraft
References
External links
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 {{Aircraft components Aircraft controls