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Thread: Another Wing Spar Question

  1. #11

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    Default Re: Another Wing Spar Question

    Thanks guys.

    Back to square one, I've got enough doubts. Avidfoxflyers say stress riser is mentioned in the avid manual. I do not have an avid manual, and I did not see it mentioned in the 1989 KF manual (maybe its in there). 43.13 does have it so no excuse.

    Under load, I can see a stress riser that may originate from it.

    EAA around here mainly builds RV's, and they would have valuable input.
    Last edited by jeffcoaj; 10-23-2017 at 10:16 AM.

  2. #12
    Senior Member jiott's Avatar
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    Default Re: Another Wing Spar Question

    One other point that I feel needs to be addressed are the statements about sanding out the scratches starting with 220 grit. I believe the manual (which I don't have at hand right now) states in the build tips section, do not use sandpaper on aluminum if the grit has silica in it, which most sandpaper does. It can get into the surface (especially scratches) and cause corrosion. The manual recommends using only Scotchbrite (maroon) to scuff aluminum in prep for gluing or painting.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  3. #13

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    Default Re: Another Wing Spar Question

    Quote Originally Posted by jeffcoaj View Post
    Thanks guys.

    Back to square one, I've got enough doubts. Avidfoxflyers say stress riser is mentioned in the avid manual. I do not have an avid manual, and I did not see it mentioned in the 1989 KF manual (maybe its in there). 43.13 does have it so no excuse.

    Under load, I can see a stress riser that may originate from it.

    EAA around here mainly builds RV's, and they would have valuable input.
    My 2 cents.........I think you're making a good decision..........better to be safe then sorry.

  4. #14
    Super Moderator Av8r3400's Avatar
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    Default Re: Another Wing Spar Question

    The only downside to replacing this spar is finding the same material to replace it with. I don't believe anyone makes this extrusion any more.

    If you use the standard tube with reinforcing I-beam in the middle, it will have different flexing properties than the fully reinforced extrusion. This could lead to unintended handling problems.


    Food for thought.
    Av8r3400
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  5. #15

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    Default Re: Another Wing Spar Question

    Thanks, I did not realize the I beams did not go full length of the spar. In replacing the spar, 12’ lengths are standard for 6061. Possible to use this length? Thanks again.

  6. #16
    Senior Member jiott's Avatar
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    Default Re: Another Wing Spar Question

    When you insert the I-beams in your replacement spars be very careful not to scratch/gall the inside of the new spars. Follow the manual precisely and use baby oil in addition. If they hang up, NEVER pound them in. I learned this the hard way.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  7. #17

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    Default Re: Another Wing Spar Question

    Quote Originally Posted by efwd View Post
    Ummm, is it concerning at all that there are a dozen holes drilled into the spar under the mount. Their right in the bottom side of the spar where all the stress is. Sure there are rivets to be placed through them but it seems to me rivets don't hold a piece together. Sure, they hold two parts together but they wont strengthen the spar just by filling a hole in it. What if I drilled a hole two inches away from the bracket, oops, then just put a rivet in to repair. No way right? Scratches, all retained under the bracket and filled with Hysol? Dang, its a tough one. If the bracket and Hysol maintain the integrity of the spar after all those holes then I am not sure it would not retain the scratches. Just me thinking.
    Eddie
    Your point has been expressed by many I've talked to recently. An experienced guy said that the hysol is packed in tight underneath the sleeve, esp. with the rivets. West systems epoxy even recommends 40 grit on aluminum when making a structural bond for the epoxy to key (of course after al prep, and al oxide pads). Also, I don't see any evidence that the scratches could make the joint any weaker. Originally I thought stress risers could emerge, but they could emerge just as easily from the holes given constant tension and compression under the spar. Dang is right. Wish I had some pictures of other's spars to compare to mine. Back to still thinking.

  8. #18
    Senior Member Esser's Avatar
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    Default Re: Another Wing Spar Question

    What’s the wall thickness of the spar and what’s the depth of your scratch’s? I think you are going to be within tolerance.
    ------------------
    Josh Esser
    Flying SS7
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    Edmonton, AB, CWL3

  9. #19
    Senior Member jiott's Avatar
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    Default Re: Another Wing Spar Question

    FWIW, a smooth deburred hole is much less of a stress riser than a deep scratch with a sharp V bottom.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

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