# How do linear actuators for flight controls work in fighter jets?

I've built some rc planes and know that in order to control an aileron in a rc plane, that you need a servo, a control horn(sticking out of the aileron), and a pushrod. However, I was wondering how fighter jets control their ailerons. I was researching and I found that it control is via linear actuators. I was wondering how exactly it worked and tried to look for a video or an animation. However, I couldn't find anything that explains exactly how a linear actuator moves an aileron. Can you please help me understand this with a video or something like this. Thanks so much.

This is a picture of an f22 raptor someone sent me when I asked this question on Flite Test...they didnt respond after this this. You can see orange actuators in the picture but it doesn't really help me besides that. https://conceptbunny.com/lockheed-martin-f-22-raptor/

• Hey @Luke, can you give us an indication of your current level of understanding in regards to hydraulics. This might help structure answers better. You will need to understand general principles of hydraulics first in order to understand linear actuators. Also note there are a bunch of different designs of linear actuators. Jun 25, 2020 at 5:36
• I really dont unsterand hydraulics fully. But I undertsand the basics of how a linear actuator works. Could you give me a image or video of some of ailerons desigsn that incorporate linear actuator. Jun 25, 2020 at 22:12

The working principle is surprisingly similar between the RC as you've described and most actuator powered aircraft. The linear actuator extends/retracts a pushrod, which is connected to a horn attached to the pivot point on the control surface.

Below is the zoomed-in picture from the OP. I've taken the liberty of highlighting the two rudder actuators and the left stabilator actuator. The one attached to the stabilator is the most revealing: the pushrod essentially connects to a horn, which translates the linear motion to a rotational one.

Ref: https://conceptbunny.com/lockheed-martin-f-22-raptor/

Due to aerodynamic and stealth reasons, this mechanism is hidden under a smooth fairing for the F-22. But for other airplanes, you can see small blisters under their control surfaces that fair the actuator. Below is a picture of the Boeing 787 aileron. The blisters have been highlighted.

Ref: https://commons.wikimedia.org/wiki/File:Low-speed_aileron_of_Boeing_787.jpg

The linear actuator itself is almost always energized via hydraulic fluid, whether it comes from a central hydraulic system or an self-contained hydraulic systems (EHA). The following diagram illustrates the working principle:

Ref: https://www.machinedesign.com/mechanical-motion-systems/linear-motion/article/21832047/whats-the-difference-between-pneumatic-hydraulic-and-electrical-actuators

• Ok thank you so much. That really helped me. But are there any videos that show this system work without everything covering it so I can visually see how it works? Jun 19, 2020 at 21:15
• The last picture of the linear actuators are of the type normally used in landing gear, not flight controls. These are double acting unbalanced. The side with the rod isn't able to exert the same force as the other side without rod due to the reduced surface area of the piston (F=P*A) >>> reduced area means reduced force, therefore unbalanced Jun 25, 2020 at 5:50
• @Craig Your comments are accurate. The pictures are just for illustrative purposes. I also didn't mention all the closed loop controls required to achieve the continuous positioning.
– JZYL
Jun 25, 2020 at 11:04
• @JZYL I dont understand how the mechanisms works though. For example I dont understand how the linear acutoar and control horn translates to the movement of control surfaces. Bassicly im looking for a animation i guess of control surfaces moving but what is happening in the indside of the wing Jul 22, 2020 at 15:43
• @LukeJustin How to do think hydraulic diggers rotate their arms then? The transformation of linear to rotational movement is exactly the same. youtube.com/watch?v=X52fB45OQjU
– JZYL
Jul 22, 2020 at 15:59

It's really not so different from your models. Due to the strength of hydraulic actuators, those horns need not stick out into the airstream. Basically, if you move the hinge into one skin, you have most of the local thickness for hiding that horn.

Here is the interesting section from the linked picture. 125 is the control unit with the valves and 126 the actuator.

However, in order to keep forces down, a small bulge is needed on the lower wing to fair over the horn that would otherwise stick out of the surface (picture source). The near one is for the aileron and the far one for the wing flap.

Below is a diagram for the F-15: Here the hydraulic actuators are controlled via pushrods and pulleys. On more modern aircraft this will be an electric line and probably even an electro-hydraulic unit so no hydraulic lines need to be strung trough the wings (picture source).

The actuators themselves consist of a cylinder, a piston, a rod connected to the piston and sticking out of the cylinder on one end and continuing to the control horn, and a couple of valves. Hydraulic fluid is pressed into the cylinder on either end of the piston, depending on the desired piston movement. The rod then transfers the piston movement to the control surface.

The valves are controlled by cables or pushrods, as can be seen in the F-15 Lateral Controls diagram. On more recent designs, electrical control is used with a central computer calculating the best valve settings several times per second.

Why hydraulics are a good choice for such actuators can be read here.

• I think @Luke is trying to ask more how these actuators work internally (principles of hydraulics), rather than how these actuators sit in the aircraft. Jun 25, 2020 at 5:58
• @Craig And your interpretation of the question is enough justification for a downvote? Jun 25, 2020 at 7:46
• I removed the downvote just now because you have added the last section that explains how an unbalanced linear actuator works. Respectfully, before your edit, it was just some pictures that showed their location in the aircraft but not anything about how they worked. Jun 26, 2020 at 3:23
• @Craig I agree, downvoting random answers is much more helpful for the questioner than – beware! – adding a better answer yourself. I'm sure he will appreciate your invaluable service. Jun 26, 2020 at 4:59