# Why are landing gear retraction mechanisms mostly based on the four bar system?

I would like to know about the principles of landing gear retraction system and I struggle to find clear information about it.

It seems to be about a four bar systems in most cases. This seems more complicated than necessary at first glance, why is a design with a simple hinge with an actuator that pulls the strut in not sufficient?

Most retraction retraction mechanisms involve a lot of pieces with complicated motions. I would like to understand the kinematics that justify the use of more than one or two bars. Intuitively, one would only expect an actuator acting directly on the landing gear, holding it into position, but the reality always involves complex rotational kinematics.

This question would also apply to the kinematics of flap tracks, which seem to be fairly complex as well.

• That's a pretty broad question. While there is some commonality, landing gear systems can be vastly different. Some retract forward, some sideways, and some backward. Some rely on hydraulics, some on pneumatics, and some require manual force to actuate. And most have some sort of overcenter lock to prevent them from folding back up under weight. What specific "kinematic principle" are you interested in understanding? Feb 15, 2019 at 19:11
• This is a valid question that should not have been declared Too Broad. The undercarriage retraction mechanism is part of aircraft design. Aug 4, 2019 at 0:42
• You should provide examples so that we can answer for those specific examples of undercarriage retraction mechanism and extend to similar mechanism. Aug 4, 2019 at 11:44

The schematic above (with four arcs that are easy to trace) will hopefully serve as a starting point. This is a rather simple mechanism, say compared to the A380's body landing gear.

The principle can be explained by trying the proposed solution, and see what problems we run into. If you place only one actuator to rotate the landing gear at the attachment point* to the airplane (magenta colored circle), you run into many problems:

• No locking mechanism
• Small lever arm
• Bigger actuating force needed
• Limited movement (no full retraction/extension possible), unless
• Packaging the mechanism in a small volume is not required

End result: back to square one, the first 3 problems, and no space for it in the limited volume of wings/bellies.

* If you place the actuator in a more convenient place like near the wing root, then it won't be able to get out of the way to stow the landing gear, unless you have a very thick unrealistic wing.

Note from the above schematic that the hydraulic cylinder (1) needs to retract/expand only by a relatively short distance to accomplish the retraction/extension.

The flaps and slats are also complex, not only you need to turn something around an axis, you need to translate it as well, creating the gaps seen, and package it neatly in a small volume.

If this basic objective is clear now, the next step is to read more, and to build models and/or play with kinematic simulation software. Try to build the same mechanism with one link, and then try to solve the issues you run into.

• Does a fail-safe design also play into this? Presumably any commercial aircraft would want gravity to be able to deploy the gear in the event of a failure. Aug 5, 2019 at 13:23
• @zymhan: In the example given, a hand pump does the trick. Big jets simply release the uplocks and gravity does the rest. If you want mechanical details, then it's a good question worth asking.
– user14897
Aug 5, 2019 at 14:24

The main landing gear retraction system poses quite a few problems to solve:

• The retracting actuator must have a reasonable duty cycle during the retraction. Due to trigonomical effects, retraction force can become very large - 1/cos $$\phi$$ for $$\phi$$ approaching 90° for instance.
• The actuator stroke must remain within reasonable limits.
• The retraction actuator and linkage must fold itself out of the way. Or be mounted at the other side of the wheel well, causing greater loss of useable wing space for fuel etc.

From Torenbeek:

Most retraction mechanisms are derived from the four-bar linkage and the designer must have very good reasons to deviate from this. A suitable pivot point must be chosen for the leg which, at the same time, gives the required wheel positions and allows adequate length of the leg. A retraction mechanism, generally consisting of a folding stay member and a retraction jack, is then required.

A picture from this video, which shows how the actuator retains a more-or-less constant favourable position and moment arm during the retraction process. This video shows the retraction of the A380 landing gear.Video

Of course, the exact configuration of a retraction mechanism depends on many structural details of the aircraft, and there are many different lay-outs. But the underlying principle remains the four-bar system in the vast majority of cases.