Servo vs. Anti-Servo

Servo tab on a GlaStar built by Dave Ammenti. Photo Marc Cook.

When we see something written in a particular way enough times, we take it for granted that it’s correctly spelled or named.

Especially when the manufacturer calls it one thing and it’s really another. Take for instance the servo tab that helps reduce aileron forces on the GlaStar and Sportsman. Pete Todebush called me recently about a builder’s tip (Quick Notes, Flyer, 4th Quarter 2013, p.22) to right a wrong. He said we’re incorrectly calling it an “anti-servo” tab, and he is correct.

Advertisement defines a servo as “noting the action of certain mechanisms, as brakes, that are set in operation by other mechanisms but which themselves augment the force of that action by the way in which they operate.” has this to say about servo tabs: “Servo tabs move in the opposite direction of the control surface. The tab has a leverage advantage, being located well aft of the surface hinge line and thus can deflect the control surface in the opposite direction. This has the effect of reducing the control force required by the pilot to move the controls.”

On the other hand, 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.”

In its parts catalog, Glasair Aviation offers an Aileron Anti-Servo Kit, so you see how the incorrect term gets perpetuated, no?

Henceforth we pledge to use the correct term and call it what it is: a servo tab.

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Omar Filipovic
Omar Filipovic is president of the Glasair Aircraft Owners Association as well as the chief tinkerer and content editor for this website. He is also the web editor for Kitplanes Magazine. Omar is building a GlaStar in Portland, Oregon.