# Why (and how) do aircraft rudders center themselves without input?

Why (and how) do almost all aircraft rudders have some sort of way of re-centering the rudder when there's no input being made? For example, if you wanted to move the flight controls (fly by wire aside) you would move it to either the left or right and most of the time it will stay in that position.

Is this in order to prevent Dutch roll and other oscillations?

Edit- what ways can be implemented to provide this centering? For example I know on the Boeing 737 there is a self-centering mechanism that centers the flight controls. Do all planes have some sort of centering for the rudder's, alierons, and the elevators?

The reason rudder surfaces are self-centering is to provide hands-off stability, where the natural tendency for the control surface is to adopt a zero-deflection state in the absence of inputs from the pilot. This prevents the plane from flying itself suddenly out-of-control (which is called divergent response) if the pilot has to momentarily let go of the yoke or rudder pedals for any reason.

The rudder is made self-centering by design during flight by placing its hinge axis ahead of its aerodynamic center of pressure.

• Is this a by-design feature or just a by-nature aspect of the control surface? Your response implies that there's a mechanism or something about the design that causes this, while Chris' answer says that it's the nature of control surfaces. Commented May 22, 2023 at 18:53
• @William don't they design the control surfaces to be this way by nature? Commented May 22, 2023 at 19:20
• In flight, I would expect it to self-center due to the airflow, but not on the ground. If there is no self-centering device, maybe it's just the orientation of the rudder, plus gravity? Commented May 22, 2023 at 21:25
• @William, will edit. Commented May 23, 2023 at 3:58
• @Steve, will edit. Commented May 23, 2023 at 3:58

The flight controls are connected directly to the flight surfaces. The flight surface will naturally settle to a point of least resistance- i.e. whatever they are trimmed to. The pilot has to actively apply pressure in order to hold the flight surfaces away from the trim point, and when the pressure is released they will naturally return to the trim points.

This applies to all the flight controls- if you pull back on the yoke and then let go, it will definitely not stay in the same place.

A backward swept hinge line on the vertical tail means that the center of gravity of the rudder will be lowest in the center position, so the rudder will self-center on the ground by gravity alone. Still, it is desirable to tailor the force gradient of any control surface such that there is a detent at the center position to give force-feedback to the pilot when the control surface is in its center position. This is achieved in flight sometimes by a thick or blunt trailing edge. The fixed trim tabs on the rudder of a Klemm 35 below (own work) serve the same purpose.

Mechanical detents are normally found on trottle or flap levers. Primary control surfaces are mostly free-floating, with the exception of the control column in Bombardier aircraft where there is a noticeable detent in pitch direction. Bombardier calls this the "sneeze protection".

Ailerons need mass balancing to prevent flutter. On the ground, an overbalanced aileron with differential linkage (more trailing-edge up movement than down) will always fall into the fully-deflected position due to gravity. The balancing mass on the trailing-up side will have a larger lever arm and pull the opposite side with it.

In flight the aerodynamic force will center the control surface at the trim position (which might not be the center) - that is what trim tabs or trim springs are for. But this only means that there is a zero-moment point in an otherwise smooth moment gradient over deflection angle. Some aerodynamic tricks help to increase the gradient locally at the zero point (see the Klemm rudder above), but those are not generally adopted. The smooth gradient, on the other hand, is a certification requirement.

• So it's more of an aerodynamic reason vs having an actual self centering mechanism? Commented May 25, 2023 at 12:46
• @Boeing787 In flight: Yes. but it need not settle in the center position. In a sideslip it will settle at the floating angle. Commented May 25, 2023 at 17:31