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Trim tabs are used on the tail's horizontal elevators to adjust its neutral position. What is the working principle behind this? How does this affect the sensitivity (the feel) of the pitch to the pilot in a plane with no fly by wire?

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The trim tab is like a control surface for the control surface. By changing camber at the rear end of the control surface, it creates a local force which changes the balance of forces such that the force-free deflection angle (sometimes called auswehwinkel after the German word for this angle) is shifted in a direction opposite to the deflection of the tab.

You could do the same with springs along the control rods, or a mass somewhere which pulls the control surface to the same force-free angle. But when speed changes, so do the aerodynamic forces, and your force-free angle will change. The forces caused by the tab change with airspeed, so the force-free angle set by a tab is unaffected by speed.

Don't confuse a trim tab with a Flettner tab: The trim tab stays at the same angle with varying control deflection, while the Flettner tab will move in opposite direction to reduce the stick forces caused by the deflection of the control surface. By setting the zero-deflection position of a Flettner tab, it can in parallel work like a trim tab and shift the zero-force deflection angle to the desired point.

Small aircraft like the Diamond Katana even have an anti-Flettner, where the tab is moving in the same direction as the control surface in order to increase stick forces. This is needed to fulfill certification requirements.

In practice, a combination of both springs and tabs is used to set the desired control forces and their gradient over speed.

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  • $\begingroup$ Does this in anyway effect the sensitivity of the pitch control? $\endgroup$
    – user8823
    Jun 11, 2015 at 7:01
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    $\begingroup$ @Rambo_412: Trim doesn't affect the sensitivity, it affects the neutral position (that is, the position of the control surface when you let go of the stick). As such it affects the "feel" in the sense of the normal pressure your hand pushes on the stick when your're not doing anything. Most people prefer to trim the plane so that it flies in a straight line when you let go of the stick - which means that when you hold the stick and the plane is not doing anything your hand feel no pressure. Some people (aerobatic pilots) prefer to fly with a tiny bit of pull on the stick. $\endgroup$
    – slebetman
    Jun 11, 2015 at 7:54
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    $\begingroup$ @Rambo_412: A Flettner tab reduces control authority, but a regular trim tab doesn't. It does, however, slightly shift the range of control moments which can be generated. $\endgroup$
    – Peter Kämpf
    Jun 11, 2015 at 10:11

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