(Community work) Image shows tab deflection direction in relation to an upward deflected elevator.
How do elevator servo and anti-servo (geared) tabs differ in handling? How to decide on which tab to use?
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3$\begingroup$ I'm not sure the image helps... $\endgroup$– Notts90Commented Mar 20, 2017 at 12:41
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$\begingroup$ BD-5 and Piper's used anti servo tabs, to add resistance to the controls . KC-135 uses servo tabs on the controls to allow manual reversion if the hydraulic boosting systems fail. $\endgroup$– strato manCommented Mar 20, 2017 at 20:42
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2 Answers
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There is some confusion on the different types of tabs that can be found on aircraft: servo tabs, geared tabs, trim tabs.
The servo tab is an aerodynamic lever, it is connected directly to the pilot flying controls. When they move the controls, they deflect the servo tab only, and feel aeroforces acting upon the servo tab only. When the servo tab deflects, it creates a force moment on the control surface ( elevator, aileron, rudder) which then deflects the control surface until aerodynamic moments are in equilibrium. If there is no airspeed, deflecting the servo tab has no effect on the control surface.
The geared tab changes the flight control forces to either increase them (usually in small aircraft) or reduce them (an arrangement used in the pre-hydraulic past of airliners). The control surface is directly connected to the stick/pedals - deflecting them causes the control surfaces to move. In order to alter control forces, a geared tab moves relative to the control surface:
For increasing stick forces, in the same direction as the control surface. Wiki and others mention anti-servo tabs, a less confusing name would be positively geared tabs.
For decreasing stick forces, in opposite direction of the control surface. Whether there is airspeed or not, the geared tab always deflects by a mechanical linkage when the control surface is deflected.
The trim tab has a constant angle relative to the control surface. The angle can be changed by extending/retracting a mechanical link, via a trim wheel or an electric motor with a screw jack.
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1$\begingroup$ Usually (and on the image in question), anti-servo tab is a force amplifier. It moves in the same direction as the surface and increases the force. It is used when the controls would otherwise feel to light for precise control. $\endgroup$ Commented May 7, 2017 at 9:27
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$\begingroup$ And how does it move in the same direction as the comtrol surface? By a geared mechanism? $\endgroup$– KoyovisCommented May 7, 2017 at 13:43
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$\begingroup$ Anti-servo == geared, so, yes. (servo and anti-servo work very differently) $\endgroup$ Commented May 7, 2017 at 13:52
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$\begingroup$ Point being, if we strictly follow the nomenclature, anti-servo tab would be a misnomer necause the tab does not have a motion amplification servo function..The fixed gearing, whether it be a force reduction or increase, would make it a member of the Geared Tab family.. But perhaps I'n nitpicking, or not aware of a past marketing campaign. $\endgroup$– KoyovisCommented May 7, 2017 at 13:53
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1$\begingroup$ Agree, it is a misnomer. But my point is that geared, while it can be both reducer or amplifier, is more often amplifier than reducer. $\endgroup$ Commented May 7, 2017 at 13:54
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Servo tabs assist the pilot (like trim). Anti-servo act against the pilot.
The all flying stabilator on a Piper Cherokee, as I recall, uses anti-servo. Since part of the stabilator is ahead of the hinge point, the anti-servo tab on the back aids in both control feel (making it not too light) and in return to center characteristics.
