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Thread: Cleaning the inside of the Jury Struts ??

  1. #1
    supercub's Avatar
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    Cleaning the inside of the Jury Struts ??

    Looking for suggestions on what to clean the inside of the jury struts with? I'm thinking the best way is to soak them in something, but not quite sure what to use, any suggestions?

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    www.SkupTech.com mike mcs repair's Avatar
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    If they are that bad they are cheap to replace. I think like $50 from airframes ak. Never have needed much more than sand blast ends. But then again that’s all taken care of at the powder coaters


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    I have never worried about them. Their only purpose is to keep the lift struts dead straight. They have very little strength beyond that. The horizontal piece will break now and then; a good guy with a very small torch can fix that. But it is really, really thin steel. Mine are 74 years old and still good.

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    Quote Originally Posted by supercub View Post
    Looking for suggestions on what to clean the inside of the jury struts with? I'm thinking the best way is to soak them in something, but not quite sure what to use, any suggestions?
    Try Marine CRC

  5. #5
    www.SkupTech.com mike mcs repair's Avatar
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    Cleaning the inside of the Jury Struts ??

    Quote Originally Posted by bob turner View Post
    I have never worried about them. Their only purpose is to keep the lift struts dead straight. They have very little strength beyond that. .
    I disagree. They are very important to the strength of the wing.


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    Really? They keep the spars straight, using the rigidity of those .032 steel lift struts?

    Having straightened (legally) a metal spar, and having seen how critical the spacing of the jury struts is to avoid bowed lift struts, I would say their only purpose is to keep the struts from buckling under negative g loads. The spar is so strong that a jury strut would collapse long before it did anything to keep the wing straight.

    Opinion.

  7. #7
    www.SkupTech.com mike mcs repair's Avatar
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    Quote Originally Posted by bob turner View Post
    Really? They keep the spars straight, using the rigidity of those .032 steel lift struts?
    ..
    exactly!....

    in team work with the rest of the MAJORITY of fuselage tubes (which are in fact made of .035" ) ........

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    Gordon Misch's Avatar
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    I agree with Bob on this one. FWIW
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    irishfield's Avatar
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    Kinda like saying the web of a roof truss isn't that important.. no...
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    Gordon Misch's Avatar
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    Respectfully, a couple points:
    1. The roof truss analogy does not hold. The roof truss is fully triangulated throughout its span. On the Cub, the inner bay, between fuselage and jury struts, is a trapezoidal four-bar linkage. It is not at all rigid like a triangle.

    2. Even if the spar/strut/jury strut assembly did form a true truss, in order for a truss member to carry or transmit any load it must be attached at each end to something that can apply or resist that load. Compare the bending resistance of a spar (in its vertical plane) to a strut. The spar is hugely stiffer than the strut, thus making the strut entirely ineffective in transmitting or resisting a meaningful load to or from the spar.

    3. Note that there is a member that attaches the two jury strut verticals, fore and aft. That member acts to increase the Euler buckling resistance of each strut (increases the moment of inertia in that axis). If the jury struts' function were to stabilize the spars, that member would not be needed.

    4. Consider the PA-12 jury strut arrangement. There is a 3/4? 5/8? round member from front strut to rear strut, and both the front and rear jury struts attach to the center of that member. That member isn't going to carry much of a bending load.

    5. On an assembled plane, absent the jury struts, just feel how floppy the lift struts are. They're crazy-floppy. They need stabilization for negative g's for sure, and possibly for oscillations.

    6. The outboard section of the wing is cantilevered at the strut attach point, which reduces bending moment on the spars in the inboard section. I could take some measurements on my wing tomorrow and do a rough stress analysis on the spars, but the bending moment on the spars at the jury strut attach point is low compared to other portions of the spar, maybe even close to zero. So the spars do not need any support from those flimsy struts, even if the struts could provide same.

    There I went again - - - But Bob is right.
    Gordon

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    Thanks, Gordon. Much better than I could have done, and one of my degrees is in this kind of analysis. It took a 50 ton hydraulic press to get the bent spar cold-straightened - you sneeze on an unsupported strut and it will bend.

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    wireweinie's Avatar
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    ...?...?...? . . . .

    Jury struts are cheap, in the grand scheme of things. If you're worried about them, just replace them. Once the aircraft is rebuilt and flying, pop open a cold one and start arguing about how strong they are and their 'true' purpose.

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    Good link. Mike makes a point that a wing on sawhorses without leading edges is loosey goosey. It surely is, but that rear spar is not bending. It is twisting a bit, but a jury strut does not inhibit twisting, since, among other things, it is pinned with a joint that can rotate.

    Spars simply do not deflect much in the vertical direction. Try it - you can stand on a rear spar, and not see more than maybe 1/8" deflection end to end. Without ribs, it could rotate, and then bend or fail, but if loads are vertical it will not move much.

    Interesting discussion. I am going flying. I have to admit - I really don't know what the J4 jury struts look like. Flies fine.

  15. #15
    Mauleguy's Avatar
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    Quote Originally Posted by bob turner View Post
    Good link. Mike makes a point that a wing on sawhorses without leading edges is loosey goosey. It surely is, but that rear spar is not bending. It is twisting a bit, but a jury strut does not inhibit twisting, since, among other things, it is pinned with a joint that can rotate.

    Spars simply do not deflect much in the vertical direction. Try it - you can stand on a rear spar, and not see more than maybe 1/8" deflection end to end. Without ribs, it could rotate, and then bend or fail, but if loads are vertical it will not move much.

    Interesting discussion. I am going flying. I have to admit - I really don't know what the J4 jury struts look like. Flies fine.
    I am no engineer but I am with Mike, jury struts are just as important as any other small component to the overall strength on the wing. My long wings flexed a lot in turbulence, but most of the flexing I could see was from the strut attach point on the wing out. Next time you go by a long wing cub ask the owner if you can shake the airplane at the wing tip and look at how much it moves up and down. It is way more then 1/8", if you give it a good shake I bet it is closer to an 1". When I fly an airplane in turbulence the last thing I want to be worried about are the components that keep everything working together. If you are worried about corrosion inside and can't determine how bad it is then buy some new parts.

    Greg

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    Gordon Misch's Avatar
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    I am no engineer but I am with Mike, jury struts are just as important as any other small component to the overall strength on the wing
    Translation: "I don't know what's really going on, but here's what I believe."

    Sometimes, myth and magic define the uncertain. The jury struts have their purpose, but the purpose is not constrained to what a "believer" would have it be. Engineering is about Sgt Friday's "just the facts, Ma'am". Dated myself there, eh?

    Think about my previous post (item 2 in post #10), about a member having to be pushed to or held firm to impose or resist a load. If it weren't, it would accelerate. Newton's Second Law. Simple concept.

    Nothing personal, neighbor Greg - just a point of view. But then, if I'm wrong someone should prove it, not merely assert it. And when someone proves me wrong, I will gladly learn.
    Last edited by Gordon Misch; 07-13-2019 at 02:33 AM.
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    skywagon8a's Avatar
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    Quote Originally Posted by Gordon Misch View Post
    5. On an assembled plane, absent the jury struts, just feel how floppy the lift struts are. They're crazy-floppy. They need stabilization for negative g's for sure, and possibly for oscillations.
    I agree with Gordon (I do not possess the ability to analyse this on paper) with the observation that it is likely the primary purpose is to control the oscillations or the natural resonant frequency of the entire wing assembly more than the negative G compression loads on the strut. The negative G compression load carrying capability of the struts will be doubled when the jury strut holds it stable at the center.
    As Greg noted when he shakes the wing it is very flexible from the strut attach point on the wing outboard. Take the jury struts off and shake it again. Notice how flexible the wing is then.
    At higher speeds the air flowing over the struts can create an oscillation, vibration or a "buzz". That is the natural frequency of that particular part with that amount of air flowing past it. That oscillation flows and is absorbed into the entire airplane, in turn effecting the natural frequency of the entire airplane. Most noticeably (but not exclusively) it would effect the flexing of the wing. When the wing flexes, the higher speed air loads react in a different direction than when the wing does not flex. This in turn alters that same natural frequency. When that happens the wing flexes to a different position or relationship to the air flow. This will happen with or without the jury struts. This wing flex can happen at a low frequency or at an extremely high frequency. There is no way to know in advance which, without a lot of advanced mathematics being involved and even then only actual flight testing will provide the proof. Without the jury struts there will be more flexibility. When there is more flexibility the flexing will take place at a different frequency and amplitude. This in turn determines the flutter speed of the airplane. You do not want to experience flutter first hand! Just take my word for it.
    For proof take your airplane up in smooth air and dive it to Vne. Does it feel solid with no adverse vibrations? It should. When it was certified, it was tested for flutter at Vd. Vne is 90% X Vd. Is yours an E-AB? You are the test pilot.

    Now, take off your jury struts and dive it to Vne or if you have a big set of round things in a sack go to Vd. Notice how flexible the wing is now. Apply a rapid input to the ailerons and release the stick. How flexible is it now? Does it flex on it's own or do you have to excite it? Is the wing still on the airplane?
    Do NOT attempt this without a parachute on your back!

    Bottom line: The jury strut is only there to stabilize the strut and control the natural resonant frequency of the wing assembly through the lift strut along with it's connection to the spars between the strut attach points and the fuselage attach points. The structural loads are minimal. The jury strut needs very little structural strength as it is only stabilizing the long lift strut.
    This in turn allows a higher flutter free Vne.
    Rust on the inside of the jury strut unless it is poking holes through the metal is only a personal issue.

    Here are some methods of removing rust:
    https://m.wikihow.com/Remove-Rust-from-Metal Try soaking the jury struts in a long container filled with white vinegar. After clean, flush them with linseed oil or thinned paralkeytone. https://www.aircraftspruce.com/catal...SABEgJ9MvD_BwE
    N1PA
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    Mauleguy's Avatar
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    I like having engineering behind the things I fly and have consulted with them on a few things. In my business over the years I have dealt with a lot of engineers on different projects, sometimes it mattered on whether they got it right and sometimes it did not. It is amazing to me how many times a little change made make things much different. For reasons that are confidential I can not point out a specific items that made a huge difference that were very very, very, small changes. On paper it worked in the real world it did not! They are only as good as the date as Gordon points out, then real testing comes into equation. Testing is good and it probably provides an ample margin for 99.9% of the stupid things people are going to do or try to do. I would never want to induce flutter so I will never be taking the jury struts off to see what happens. All I said was " they are just as important as any other small component to the overall strength on the wing". Maybe it is overkill to want my jury struts free of corrosion but that is where I am at...
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    The idea that jury struts only serve to keep primary struts straight doesn’t account for any wing flex or twist in flight. I’ve been told by a couple of Cub guys way smarter than me that Cub wings do flex and oscillate in flight, which is why the FAA is reluctant to allow wing mods like extended edges that might stiffen the wing, or combinations of big tanks and wing extensions. If jury struts only served to keep struts straight they’d be more effective if they were perpendicular to the strut, yet they’re perpendicular to the wing. Maybe somebody thought the point of attachment of the jury strut to spar mattered?

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    BC12D-4-85's Avatar
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    Have a look here at the + and - wing load tests. Note the struts and jury struts. I'd not want to remove anything while standing nearby.

    https://phantomaero.com/strut-braced-wings

    Edit: Observe the bending of the lift struts...down under simulated negative down loads, and up under positive up loads to maintain the triangle with the spars. The jury struts add support. Now we see how snow loads break struts.

    Gary
    Last edited by BC12D-4-85; 07-13-2019 at 07:45 PM.
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    Cub junkie's Avatar
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    Piper left jurys off the short wings cause they didn't need them. If they could have left them off anything else they would have because Piper was cheap.

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    Gordon Misch's Avatar
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    Pete wrote:
    The negative G compression load carrying capability of the struts will be doubled when the jury strut holds it stable at the center.
    Actually, it will be quadrupled because the critical buckling load varies inversely as the square of the unsupported length. (plagiarized from https://en.wikipedia.org/wiki/Euler%27s_critical_load)

    No way would I consider flying my 12 without the jury struts. Who knows where that next big downdraft might be lurking, and the possibility of strut flutter is a complete unknown (to me).
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    Gordon Misch's Avatar
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    Stewart wrote:
    If jury struts only served to keep struts straight they’d be more effective if they were perpendicular to the strut, yet they’re perpendicular to the wing.
    True IF the jury strut attachment to the lift strut were free to slide. But since it's clamped tight, it is rigidly triangulated from the spar attach points. As the wing flexes some axial load is imposed on the lift struts, but that load is small, especially in comparison to the axial load the lift strut is experiencing from doing its primary job. I'd suspect other reasons for the chosen geometry. If perpendicular to the lift struts, the jury struts would be longer, with more drag and weight. And their attach points at the spars would be a little more complex. And maybe simple aesthetics could have played into the decision. Thanks for the idea - interesting stuff to think about.
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