Why not plywood ribs

Discussion in 'Wood Construction' started by bcguide, Nov 30, 2013.

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  1. Nov 30, 2013 #1

    bcguide

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    Ok, this has been bugging me for a while. Why go through all the time gluing gusetts on rib, would it not be easier to make a jig and use a router to make them out of say 1/4 plywood
    Scott
     
  2. Nov 30, 2013 #2

    Autodidact

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    The Starduster uses plywood ribs, the Stewart headwind also uses them. I suspect they may be slightly heavier than built up ones.
     
  3. Nov 30, 2013 #3

    djschwartz

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    So did one of the makers of Pitts wing kits. They were heavier and they we also prone to cracking at the "joint" locations.
     
  4. Nov 30, 2013 #4

    TFF

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    Both have plusses and minuses. You may be somewhat quicker with ply, but a friend made 30 ribs in less than a month; closer to 20 days. Mainly tedious but it is considered better out of capstrip. SkyBolts and Starduster Toos originally were ply and both went to the option of capstrip; almost no one builds the ply option with those today. SparCraft pitts wings were ply to confuse the patent Pitts had on his wings. Flybabys are ply. WW1 Sopwiths are ply.
     
  5. Dec 1, 2013 #5

    Vipor_GG

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    I would think that due to the direction of the grain you would end up with a much heavier rib to have the same strength.
     
  6. Dec 1, 2013 #6

    TFF

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    Some people would rather gain weight than loose their sanity. Depends on the overall goal. I will say it is taking my friend more time to jig and glue the ribs and other parts into one wing panel than making all the ribs. Much more nerve racking than making the ribs in his words.
     
  7. Dec 1, 2013 #7

    dcstrng

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    As others said, stick and gusset is lighter, but early Avid-Flyer, Kitfox, the Kelly-D and others successfully employed this method... no reason it couldn't be used so long as the bird is engineered for it...
     
  8. Dec 1, 2013 #8

    Turd Ferguson

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    Key word there Scott. Planes like a Volksplane were designed to be simple and easy to build. You stack up plywood and saw out the ribs on a bandsaw. Compromise on other parameters perhaps but the objective was make them easier to build vs built up ribs.
     
  9. Dec 1, 2013 #9

    Dan Thomas

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    The Maranda had plywood ribs. I started one a long time ago and made the tailfeathers. Those ribs were heavy and not so strong. You need good-two-sides plywood, hard to find anymore, and much of the plywood I see now has really poor core plies so that the strength isn't there. If you use a really good ply like Baltic birch, they get far too heavy.

    A friend made an extra built-up rib when he built his biplane, made of 1/4" x 1/4" spruce capstrips. He glued it to a couple of short "spars," sat the spars on two sawhorses and hung a platform underneath it. He used a bit of string bracing to keep the rib from twisting under load, the same as it would be with the fabric on it. He put bags of sand on the platform, and got to 360 pounds before the rib broke.

    There's no way an equivalent plywood rib would take that.

    Dan
     
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  10. Dec 1, 2013 #10

    dcstrng

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    View attachment Mowhawk-RIB.pdf
    View attachment MOHAWK-RIB-TEMP.pdf

    To follow-up... above are a couple pdfs of a modified plywood rib from an Avid-clone (Raven/Mohawk). as you can see the rib is routed out, and then ply cap-strips are glued to finish... pretty light, although not as quick as a simply routed 1/4" might be, but about 60% the weight...
     
  11. Dec 1, 2013 #11

    Aerowerx

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    An alternative would be to cut the ribs out of extruded foam, and have thin plywood strips as rib caps. Can't think of any right now, but there have been some planes that use this method.
     
  12. Dec 1, 2013 #12

    Turd Ferguson

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    Sky Pup.
     
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  13. Dec 1, 2013 #13

    Brian Clayton

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    Other than weight, cost would be a big factor. Have you priced 1/4" aircraft ply lately? 25 or 30 ribs would gobble up several sheets. A lot of times, the ribs can be built out of scraps from the rest of the airframe, reducing the cost even further. If the ribs don't have super hard bend in them, you can make them in about 30 minutes each and pull them from the jig as soon as you nail/staple the gussets on. I made mine as I went along, building one or two a day while I was working on other parts. No reason why you couldn't build them the same time you build the spars and get everything else ready to assemble the wings. They are really not that bad, and for a lot of folks it is the first real time they get to work with these materials and start to practice precision woodwork. My hanger neighbor has done several stearman restos, and if I remember right, he said it takes him 15 minutes or so a rib....and there are a bunch of them on one of those. And unless you are doing them cnc, its a little harder than you think to get that many plywood pieces exactly the same. I could envision some other problems with end grain glue joints too.....
     
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  14. Dec 1, 2013 #14

    Vigilant1

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    Dana posted this very helpful look at alternative rib construction methods, including foam and ply. Even a bit of testing. Here's the discussion, see the PDF he posted, and go all the way to the end of the PDF. Foam ribs with wood caps were very light and strong, appeared to be approx twice as strong as built-up wood rib of the same weight (and probably a lot faster to build). It was a small sample, but I suppose this is better than a bunch of guessing.
     
  15. Dec 1, 2013 #15

    bcguide

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    Thank you guys, you covered lots of the other questions I had (using cap strips with the plywood). I am not sure if I will ever get around to building a plane but it kills time at work to plan and dream about it.
    The reason I asked is I think I could darn near get all the material out of the local dump. This brings 1 more question, could a 1/4 ply rib be coated in glue or epoxy to increase its strenght.
     
  16. Dec 1, 2013 #16

    akwrencher

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    Epoxy resin in this application will probably add little to the overall strength of the rib. usually this is done on ply to seal out moisture. If you used glass cloth with epoxy it may help, but would likely be heavy and way more work and expense than one of the other methods discussed. Were you able to view the link posted above from Dana?
     
  17. Dec 2, 2013 #17

    bcguide

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    Not I have not been able to have a look at Dana's post yet. It will have to wait till tomorow but I will get to it
    Scott
     
  18. Dec 2, 2013 #18

    mate88

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    I think the only way to be certain about ply vs stick is to make up a known design rib using both methods, compare weight, then bust 'em. It would be an interesting project at least. Also influential is the type of spar and their location re cord %. Keep the ends rigidly supported (x, y, and z axes).
    I would also add that if ply was better, it would rule already.
     
  19. Dec 2, 2013 #19

    deskpilot

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    Already been done. Follow the preceding links in this thread.
     
  20. Dec 2, 2013 #20

    Turd Ferguson

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    Only if you add some kind of fiber, i.e. spun glass, graphite, carbon, etc.

    A, B, OR C grade plywood you find in the dump isn't going to be suitable for aircraft ribs. Marine grade would work - maybe - but not regular building material grade.
     

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