My question is how does an aileron work considering it rotates around a fixed point and the actuator arm moves in a straight line. If what is shown in the first picture I have provided is true, then how can the actuator arm move the lever that moves the aileron considering the fact that the lever moves in a circular direction and the actuator arm only moves in a linear direction? If the picture is right doesn't that mean the lever that moves the aileron would have to change length to be able to move in a circular direction since the actuator arm moves in a straight linear direction? Please let me know if you need clarification.

First Picture Second Picture

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    $\begingroup$ Does this help: aviation.stackexchange.com/questions/78967/… $\endgroup$ Nov 20, 2022 at 3:56
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    $\begingroup$ Maybe if you see it it's easier (it's a spoiler actuator but it works just in the same way) $\endgroup$
    – sophit
    Nov 20, 2022 at 10:23
  • $\begingroup$ There are many, many places where machines have to convert "linear" motion to circular motion. The connecting rods of a steam locomotive or an automobile engine are examples of the "dogbone link" mentioned in an answer below. The rear shock absorbers of a typical motorcycle work like the actuator show in your figure. $\endgroup$
    – David K
    Nov 20, 2022 at 23:44

2 Answers 2


You will see two methods of providing articulation for the hydraulic cylinder to accommodate the vertical shift of the attachment at the aileron as the aileron moves.

In one method, one end of the actuator has a pivoting attachment to the structure and the other end a pivoting attachment to the control surface. The entire actuator moves in a small arc defined by the relative up and down shift of the attachment at the aileron as the aileron moves.

An alternate method is used if you need the actuator to be rigidly attached to the structure (usually because of space considerations that make it undesirable to have an actuator that can articulate up and down itself, or the design of the input linkage requires the actuator to be stationary).

The rigidly attached actuator is connected to the control surface by an articulating link, sometimes called a "Dogbone Link". This actuator operates on a fixed axis, and all of the geometric compliance is accommodated by the two pivot bearings in the dogbone itself. enter image description here


enter image description here

The actuator extends linearly indeed, and is mounted on both sides with bushings that allow for rotation. As can be seen in above picture, from this site.

So when the actuator extends:

  • the end at the actuator arm follows a circular curve;
  • the other end stays fixed in one point;
  • both ends rotate around their bushings.
  • $\begingroup$ What about the actuator on the top, the 2 bushings aren't parallel. Was this used for some other flight actuator? $\endgroup$
    – John Smith
    Nov 20, 2022 at 3:41
  • $\begingroup$ @JohnSmith That aileron actuator is not clamped in on the left hand side, it is mounted such that rotations about the left hand side attachment point are allowed. $\endgroup$
    – Koyovis
    Nov 20, 2022 at 3:44
  • $\begingroup$ But it couldn't be used for an aileron because the right hand side of the top actuator needs to rotate in a parallel direction to the left bushing, right? $\endgroup$
    – John Smith
    Nov 20, 2022 at 4:29
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    $\begingroup$ @JohnSmith I think what Koyovis is saying is that the bushing on the left can rotate so that it is parallel to the one on the right. (I'm not completely sure I'm understanding correctly, though.) $\endgroup$ Nov 20, 2022 at 11:11

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