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Thread: Rear spar reinforcement

  1. #1

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    Rear spar reinforcement

    Hello everyone - I am building a Carbon Cub - it has the rear spar reinforcement very similar to PA-18 Wips 2000 GW Kit.
    It is essentially a piece of 1/8 inch 6061 strip that gets cherry-riveted to the top of rear spar.
    I did not clamp the cap strip to the spar tight enough (needed to use more clamps) and I now have a gap about 0.015 inch between the reinforcement and the spar.
    The gap is not full length of the cap strip, - it is about 1 ft long.

    My question to the member familiar with the subject:

    Does it represent a significant problem from structural integrity point of view?
    Should I go ahead and rerivet it with oversized cherry rivets? Normal size ones will not work as holes in spar and cap strip will not match after I clamp it to the spar...

    I am attaching images illustrating the issue.
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  2. #2
    www.SkupTech.com mike mcs repair's Avatar
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    Rear spar reinforcement

    It should be fine. Just for side to side load..

    On other thought, why do you think hole won’t line up close enough??? I bet they would...


    Sent from my iPhone using SuperCub.Org

  3. #3

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    Probably not going to make you fall out of the sky, but why the gap? Cherries usually suck down tight if they’re sized correctly; looks like maybe some burrs/debris between the pieces... Also agree with Mike- if you do a careful job of removing them, you wouldn’t need oversized...

  4. #4
    Grant's Avatar
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    Fix it. Like others , I agree that it probably will not be a problem...Probably..... However, it wil always be on your mind.

    It should line up any any slight misalignment can be chased and would be much better than the gap it has now.
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  5. #5
    Steve Pierce's Avatar
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    I'd do it right. Won't take long and see no reason why the oles wouldn't line back up. The gap between the two will allow movement when flexed and defeat to purpose of adding the doubler.
    Steve Pierce

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  6. #6
    skywagon8a's Avatar
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    Looks like the holes were not deburred after drilling. Carefully remove the doubler, deburr and reinstall. I thought the Wip reinforcement was a U shape?




    N1PA

  7. #7
    Steve Pierce's Avatar
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    The Wipair doubler is U shaped. If normal clecoes will not hold it flat use draw type clecoes.
    Steve Pierce

    Everybody is ignorant, only on different subjects.
    Will Rogers
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  8. #8
    skywagon8a's Avatar
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    Alternatively a clamp, top to bottom of the spar.
    N1PA
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  9. #9

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    Like Steve said, use draw clecoes. Having a gap defeats the purpose of the doubler. Be careful drilling out the old rivets. Debut both the spar and the doubler. Assemble and check hole size with go/no go gage and then decide if you need oversized cherries.


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  10. #10

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    I am also building an EX-3. It seems like a small thing, and it is probably not a problem, but I always fix something like this. I hope it makes up for the screw-ups that I'm sure I did but I didn't notice.

    John

  11. #11

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    when you drill all the holes in these spars dont at some point you start to go backwards? just askin.
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  12. #12

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    Guys, thanks for input. I think what happened - I match drilled the strip and spar but did not remove it after drilling all holes and did not deburr.. Building a first plane is a massive learning process of course...Second one will be easier.
    I will fix it as suggested.
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  13. #13

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    what you most certainly dont want is the nagging doubt about the re-inforcement that will follow you if you dont fix it..... you are asking the question because it bothers you (read between the lines me accusing you good naturedly of being a perfectionist...) ...... so you need to fix the bother.... then you will be a much happier pilot when you fly it.

  14. #14

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    read between the lines me accusing you good naturedly of being a perfectionist.


    haha, kind of. I was going to stick strain gauges to this strip (connected to arduino box) to see if it does take any load off the spar... this is how far I can go to find the truth
    in reality - it is just lack of knowledge. you truly never know what IS important and what is not important. It comes with experience which i dont have.

  15. #15
    RVBottomly's Avatar
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    Quote Originally Posted by Cooley View Post
    I was going to stick strain gauges to this strip (connected to arduino box) to see if it does take any load off the spar... this is how far I can go to find the truth

    You have strain gauges?! Heh, I'd love to have some of those instead of the fish scales and torque wrenches I've been using. LOL.

  16. #16
    skywagon8a's Avatar
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    Quote Originally Posted by Cooley View Post
    I was going to stick strain gauges to this strip (connected to arduino box) to see if it does take any load off the spar... this is how far I can go to find the truth
    in reality - it is just lack of knowledge. you truly never know what IS important and what is not important. It comes with experience which i dont have.
    I'd be inclined to believe the strip only stiffens that portion of the spar preventing buckling under load. It is the buckling which weakens the spar. Once buckled, it can collapse.
    The strip itself doesn't really carry any load to speak of.
    N1PA

  17. #17
    Gordon Misch's Avatar
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    The strip itself doesn't really carry any load to speak of.
    Yes, it shares that capstrip load with the spar, via shear in the rivets. It also increases the effective moment of inertia of the spar section, thereby increasing the load capacity. It's not "just" a stiffener, though it's that too.
    Gordon

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