The book Aircraft Systems uses the following diagram for a mechanically controlled hydraulic linear actuator (for moving a control surface e.g. an aileron):

enter image description here

The following explanation is given about its operation:

As the pilot feeds a mechanical input to the flight control actuator, the summing link will rotate about the bottom pivot, thus applying an input to the servo valve. Hydraulic fluid will then flow into one side of the ram while exiting the opposite side resulting in movement of the ram in a direction dependent upon the direction of the pilot’s command. As the ram moves, the feedback link will rotate the summing link about the upper pivot returning the servo valve input to the null position as the commanded position is achieved.

However, I fail to understand this explanation given the diagram. If the mechanical signal is to the right, the summing link will slant to the right (i.e. /). I assume this is a command for the cylinder to move to the right. When the cylinder moves to the right, the feedback link causes the summing link to eventually return to the upright position (i.e. |). But when this position is achieved, the Servo Valve (SV) input is now more extended than it was before (the whole summing link is now upright but with a net shift to the right). How is this the null position that would cause the SV to close?

In search for an answer, I came across a better diagram which makes sense with a sliding sleeve of the SV (but this diagram is in my opinion not what the first figure illustrates):

enter image description here]]

Is the first figure simply a poor explanation and the second figure is more representative of how these hydraulic linear actuators work?


2 Answers 2


The actuator in the first picture is a position follower system and only works as intended if the servo valve is closed as a consequence of the resulting piston displacement. The system in the picture works if the cylinder end is connected to the flight control surface (the left eyelet with the cross in it) and the piston end is connected to the structure frame. The key for understanding is the shift in the centre of rotation:

enter image description here

  • The servo valve is closed when the summing bar is vertical.
  • At first, the cylinder is stationary and the pilot deflects the stick, rotating the summing bar about the lower hinge and opening the servo valve. (Black dotted line).
  • While the pilot keeps the stick stationary, the cylinder moves in response of the servo valve opening. If the initial deflection was to the right, the piston moves to the right. The centre of rotation is now the top hinge (mechanical input).
  • When the summing bar becomes vertical again at the point where the pilot now holds the stick, the servo valve closes. (Red dotted line).

The second diagram does not show how the cylinder is attached and where the mechanical input is connceted to at the botttom. It could be a position follower system or a simple velocity output where input deflects the servo valve, oil starts to flow as long as the input is open, and the output end moves with a speed proportional to the input deflection. If the lower "First Mechanical Connection" is attached to the moving surface and the cylinder is free to move, we get a mechanical position follower again.

The mechanical feedback provided by the summing link was how the first generations of hydraulically operated systems worked. Nowadays controllers are electronic, not mechanical: a position transducer measures cylinder output, and adjusts servo valve input in response to the control laws programmed in the controller.

  • $\begingroup$ "When the summing bar becomes vertical again at the point where the pilot now holds the stick, the servo valve closes". Why would it close at that point? If the black line going from the centre circle on the summing link to the SV represents some mechanical link, would the SV not be more open now than it was before the pilot input? Unless the black line is just an indicator of "some" kind of mechanical connection between the SV and the summing link, which in reality may be e.g. rotary... $\endgroup$ Dec 26, 2017 at 17:40
  • $\begingroup$ The black line is a mechanical connection. The whole actuator housing (marked Hydraulic Piston Actuator, with the servo valves mounted on top) moves towards the new position of the end of the link marked Mechanical Signalling. The elevator is connected to the moving housing bit, the piston is connected to the aircraft frame. $\endgroup$
    – Koyovis
    Dec 26, 2017 at 20:24
  • $\begingroup$ Sorry but is it not typically the piston that moves, and the housing (cylinder) that is stationary? I don't see clearly the relative movement of the housing, the cylinder and the piston $\endgroup$ Dec 27, 2017 at 12:31
  • 1
    $\begingroup$ The cylinder and the piston move relative to each other. The arrangement can be made such that the cylinder is fixed and the piston extends, which can be beneficial for the oil pressure and return connections feeding the servo valve, but in that case the feedback system looks different, the one drawn is the most straightforward to explain. The aircraft I've worked on and wrote simulation models for did have a moving cylinder and a fixed piston. $\endgroup$
    – Koyovis
    Dec 27, 2017 at 14:35

"I assume this is a command for the cylinder to move to the right." If you assume this is a command for the cylinder to move to the left then the feedback is negative and the mechanism is homeostatic. It's a poor design because the summing link is pushed to awkward angles at the extremes of its motion, and it's a poor teaching example because the mechanical signalling is in the opposite direction from the powered action.

The second diagram has the feedback motion moving in the correct direction, assuming that you understand that the valve casing (gray) is free to move. However the valve is so carelessly drawn that it will not work. If the control shaft moves rightward far enough to admit pressurized oil into the power piston at P1, it completely blocks P2 and nothing happens, and there's no room for it to move leftward at all.

This sort of thing is very hard to get completely right. Hydraulics can be difficult and feedback usually is confusing; put them together and you are navigating a landscape littered with embarrassing and expensive surprises. Only the most senior engineers dare to actually build hydraulic boosters for aircraft controls and they do not sleep well at night.

So it should be no surprise to find that the diagrams in some textbooks are unclear, unhelpful, or just plain wrong.

Note: Homeostatic, said of a system, means that it has a preferred state and tends to return to that state when perturbed.

In this case the preferred state is where [SV] is centered and not admitting pressure to either side of the HPA. If you were to watch this mechanism in action, you would see that it only stops trying to move when the center pivot of the Summing Link is in a certain position. You could say that the device's preferred state is any in which this center pivot is at home, or you could say that it is this center pivot which is truly homeostatic.

  • $\begingroup$ That was my thinking as well. What do you define as "homeostatic"? Stable? $\endgroup$ Dec 25, 2017 at 20:27
  • $\begingroup$ I've worked on design and operation of hydraulic actuators for aircraft controls and I usually sleep very well at night. Except tonight, after the heavy christmas lunch. $\endgroup$
    – Koyovis
    Dec 25, 2017 at 21:17
  • $\begingroup$ @Koyovis: I was thinking of en.wikipedia.org/wiki/Boeing_737_rudder_issues, see also b737.org.uk/rudder.htm. $\endgroup$ Dec 25, 2017 at 22:22

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