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While studying landing gear, I came across this term 'bungee spring' to which there was no definitive explanation in text with regards to its usage.

enter image description here

So in the image, you can see for the jury strut/ drag brace down lock strut there are 2 bungee springs used even after the presence of downlock actuators.

As per my assumptions they must be used to give the jury strut some extra kick while extension and gravity extension and improve redundancy or there's some other reason?

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    $\begingroup$ I don't see any bungee springs labelled on the image, and the jury strut springs are steel springs, not bungee ones (bungee spring means a rubber chord). $\endgroup$ – Jan Hudec Jul 5 '20 at 11:04
  • $\begingroup$ Yes, I understand, in a couple of images, the springs were referred to as bungee springs so I used the same term. $\endgroup$ – Salmonbeing00 Jul 5 '20 at 13:30
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    $\begingroup$ Are you referring to the blue/white bungee, or the jury strut springs, or the downlock springs? $\endgroup$ – JZYL Jul 5 '20 at 23:20
  • $\begingroup$ You'd have to do a detailed examination of the engineering of that landing gear design to really understand the purpose of the springs. Certainly there are many planes that don't use bungee cords in their gear. $\endgroup$ – jamesqf Jul 6 '20 at 4:37
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The key to this is the text in yellow in the bottom right-hand corner

Lower torsion link (Disassembled)

The bungee is simply being used to support part of the mechanism that is being worked on. It will be removed when the work is complete and the lower torsion link reassembled.

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    $\begingroup$ Are those even bungee chords and not just pieces of plain non-springy string? $\endgroup$ – Jan Hudec Jul 5 '20 at 20:36
  • $\begingroup$ It looks like bungee to me, @JanHudec, but you could be right. It doesn't affect the answer, though. $\endgroup$ – CatchAsCatchCan Jul 5 '20 at 20:47
  • $\begingroup$ Well, either way I understood the question to be about the two springs labelled jury strut springs, which are steel springs, and the ‘bungee’ being only used due to confusion with other gear assemblies that may actually use bungee chords in similar places. $\endgroup$ – Jan Hudec Jul 5 '20 at 20:50
  • $\begingroup$ Now for that blue and white striped string, must be there to hold something in place during repairs? $\endgroup$ – Robert DiGiovanni Jul 5 '20 at 22:32
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The purpose of the (bungee) cord was correctly explained by CatchAsCatchCan... however

If the question is about the springs, their purpose is to help pull and lock the drag strut into it's down position. Once the drag brace jury strut reaches the level position in picture, the spring load is enough to keep the landing gear from collapsing under any "normal" external load in any direction.

Only way to retract the gear is to release the drag brace jury strut from it's locked position by pulling it upwards with the drag brace strut actuator. Should the actuator fail, gear lock will be achieved and maintained by the springs alone.

Please note there is a similar arrangement in the strut seen in the behind. The spring is brought outside the strut via an extension to the jury strut, and is therefore pulling upwards to lock the jury strut.

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The drag brace strut is an "overcenter" locking strut. It has a "knee action" to provide a kind of passive locking function because when moved into its overcenter position, compression forces a can only try to overcenter it more, which is prevented by the contact lugs above the center pivot and it can only fold going the other way.

It works the same way as your knees when standing. You can keep your knees bent slightly so that your weight is trying to fold your legs up. If you bring your knees back while standing as far as they will go, they "overcenter" and your weight can only try to make them overcenter more, which is prevented by the bone structure of your knees. So when you stand up from a knee bend and your legs go into that overcenter lock mode that allows you to relax your thigh muscles, your legs are "down and locked".

While standing like that, some trickster might come along and bump your knee from behind and break the overcenter, making you start to fall before your thigh muscles can take over.

Or someone could lift you up just enough to let the compression load on your legs be released and then let you go, and might find that your knees moved forward of the overcenter axis as your weight comes back down on them, and down you go before you thigh/butt muscles take over.

enter image description here

The springs are there to prevent that in the case where temporary tension is applied when the gear comes under load, bringing the strut out of overcenter to on-center; when compression is re-applied, without the springs it could go either way, back into overcenter or out of overcenter, and collapsing, like your legs. The springs guarantee that the brace will always move back to the overcenter position unless forced out of it by the drag brace actuator.

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    $\begingroup$ The DownLock pin is used only on the ground by an engineer. It's used to prevent the undercarriage being inadvertently retracted while someone might be working on it. You can see the REMOVE BEFORE FLIGHT flag hanging down behind the Drag Brace. $\endgroup$ – CatchAsCatchCan Jul 6 '20 at 4:16
  • $\begingroup$ I think the picture is misslabeled. The arrow with the label down lock pin is actually pointing to the pivot axel of the drag brace jury strut. The actual down lock pin is on the other side of the jury strut and does not come through the whole assembly. $\endgroup$ – Jpe61 Jul 6 '20 at 19:56
  • $\begingroup$ @CatchAsCatchCan yes good catch. I was fooled by the label not knowing the details of that particular gear and assumed it was a plunger actuated from the other side. I deleted the last para. $\endgroup$ – John K Jul 6 '20 at 21:05
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If it is the actual steel springs we are wondering about, a thought about "convertible" sofa/beds came to mind today. Clever use of springs make these 300 lb items very easy to unfold.

Springs can be placed, with the proper leverage, to help lift or move a heavy object by opposing most of its weight, with enough weight left to keep it in place. The counter weights of a "dumb waiter" or elevator work on the same principle.

This would serve to make the actuators much smaller and lighter, a big plus on aircraft. Springs or "bungees" could be interchangeable, especially on smaller aircraft, but steel, especially for repeated cycles, may be favored.

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    $\begingroup$ I don't see how this is any more than speculation. It doesn't attempt to answer the question. $\endgroup$ – dalearn Jul 7 '20 at 16:18
  • $\begingroup$ @dalearn Think about the second spring there, and how you would lift a heavy set of gear. No, not speculation, and not in need of a computer, either. $\endgroup$ – Robert DiGiovanni Jul 7 '20 at 20:48
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    $\begingroup$ This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review $\endgroup$ – Vikki - formerly Sean Jul 7 '20 at 22:22
  • $\begingroup$ @Sean If you insist on piling one negative comment upon another, try reading the title of the question. Clearly being asked is "why are there springs when there is an actuator?". Go unfold a couch and let me know what you think. $\endgroup$ – Robert DiGiovanni Jul 8 '20 at 0:08
  • $\begingroup$ BTW if you think undamped abrupt movements are good for the actuators, remove the springs from the couch, unfold it, and let me know how your back feels. $\endgroup$ – Robert DiGiovanni Jul 8 '20 at 0:10

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