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Thread: Inter-rib sag reduction

  1. #1

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    Question Inter-rib sag reduction

    Hello!

    I am looking at covering an aluminum tube rib wing, and I am trying to minimize the inter-rib sag as much as possible.
    I am currently very uniformed about the fabric covering process.

    Is there any way to reduce the sag between the ribs after the fabric has been heat shrunk?
    Does the ammount of sag depend on the type or weight of the fabric?
    Or would the only way to reduce the sag be to cover the leading edge with a rigid covering such as sheet aluminum?

    Thank you

  2. #2
    Dana's Avatar
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    If you don't have a rigid sheet around the leading edge, there will be some sag. More spanwise tension can reduce the sag to some extent. But is it really an issue? Many aircraft designs have unsheeted leading edges, with resulting sag between ribs, and fly just fine.

  3. #3
    cwilliamrose's Avatar
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    One well known builder/restorer says shrinking first with span-wise passes of the iron helps to minimize the sag a lot. I have not had a chance to try this myself but I have seen his work.

  4. #4

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    Hi thanks for the replies.
    I was concerned with sagging because the wing we are trying to build will be one with significant sweep, (28 deg)
    and I was also concerned the sag would effect the characteristics in a way that the CFD did not predict.

    I think I may end up attaching a rigid aluminum sheet to the wing's leading edge, would you all recommend
    blind riveting the sheet aluminum to the aluminum tube ribs?
    Thanks again.

  5. #5
    rwanttaja's Avatar
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    Quote Originally Posted by timi View Post
    Hi thanks for the replies.
    I was concerned with sagging because the wing we are trying to build will be one with significant sweep, (28 deg)
    and I was also concerned the sag would effect the characteristics in a way that the CFD did not predict.

    I think I may end up attaching a rigid aluminum sheet to the wing's leading edge, would you all recommend
    blind riveting the sheet aluminum to the aluminum tube ribs?
    Thanks again.
    Shouldn't need many rivets. I'll leave it to guys like Frank and Joe to give a more-knowledgeable assessment, but it sounds good to me.

    Standard practice on the Fly Baby is to install ~0.025" gutter flashing across the front of all the wing, horizontal stabilizer, and fin. For more of an antique look, Kurt Gruber left it off the vertical tail:

    You can see this plane with the stock metal under the fabric Note that the fin ribs are almost invisible in comparison:


    Ron Wanttaja

  6. #6
    Sam Buchanan's Avatar
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    Quote Originally Posted by timi View Post
    Hi thanks for the replies.
    I was concerned with sagging because the wing we are trying to build will be one with significant sweep, (28 deg)
    and I was also concerned the sag would effect the characteristics in a way that the CFD did not predict.

    I think I may end up attaching a rigid aluminum sheet to the wing's leading edge, would you all recommend
    blind riveting the sheet aluminum to the aluminum tube ribs?
    Thanks again.
    Your wing sounds quite similar in construction with the tubular wings on the Airdrome WWI replica aircraft. Here is the leading edge skin on one of the wings for my Fokker D.VII kinda replica:

    Name:  wing-65.jpg
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Size:  46.3 KB

    The leading edge is attached to the spar and ribs with pulled rivets. I also added aluminum angle to the trailing edge of the sheeting to prevent the fabric from deforming the sheeting:

    Name:  wing-53.jpg
Views: 721
Size:  32.4 KB

    Hope this helps.
    Last edited by Sam Buchanan; 11-25-2018 at 05:24 PM.
    Sam Buchanan
    The RV Journal RV-6 build log
    Fokker D.VII semi-replica build log

  7. #7

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    Extra nose ribs to reduce the fabric span between main ribs are often used. The majority of lifting force and pull on the fabric is near the leading edge.

  8. #8
    George Sychrovsky's Avatar
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    You can reduce the sagging significantly by making the fabric a lot tighter in span-wise direction and loser in chord direction
    The way to do it you first attach the fabric at the leading and trailing edges but you create about 1.5 inch slack of extra fabric in the chord dimension.
    Then shrink the fabric at the root and the tip enough to remove this slack and attach the fabric at the root rib and tip keeping it as tight as possible span-wise.
    Now shrink the rest.
    At least that's what I did it on my two airplanes where the sagging even tends to contact the internal wing parts and It works.
    Disclaimer ; opinions of others will vary depending on what they’re selling.

    http://the-grand-design.com/

  9. #9

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    Quote Originally Posted by rwanttaja View Post
    You can see this plane
    Ron have you ever considered putting together a CD of Fly Baby pictures? No words, just pictures. That's the kind of books I like, just pictures.

  10. #10
    rwanttaja's Avatar
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    Quote Originally Posted by martymayes View Post
    Ron have you ever considered putting together a CD of Fly Baby pictures? No words, just pictures. That's the kind of books I like, just pictures.
    Nahhhh, too much work. But most of the photos are online.

    http://www.bowersflybaby.com/pix/index1.htm (and click the "next" buttons)

    http://www.bowersflybaby.com/stories/altered.HTM

    http://www.bowersflybaby.com/goofy_pix.html

    http://www.bowersflybaby.com/pix/panels.html

    http://www.bowersflybaby.com/pix/portraits.html



    Ron Wanttaja

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