Bending Plywood for Leading Edge

Discussion in 'Wood Construction' started by bradyaero, Apr 18, 2011.

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  1. Apr 18, 2011 #1

    bradyaero

    bradyaero

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    If use 3mm baltic birch for my leading edges will it bend properly with a hot water bath or do I need to go to 2mm 'KOSKIPLY' type plywood?

    Thanks for any suggestions!
     
  2. Apr 19, 2011 #2

    rheuschele

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    3mm ply for leading edge? What on earth are you building?
     
  3. Apr 19, 2011 #3

    bradyaero

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    I was wondering if 3mm had ever been used for a leading edge and if it was bendable.
     
  4. Apr 19, 2011 #4

    fly2kads

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    Plywood of that thickness has been used as wing skins of homebuilts before. Gene Turner designed his T-40 and T-40A with 1/8" (3mm) mahogany skins, for example. (See TURNER T-40 Airplanes) However, he did not wrap them around the leading edge. He used a two-piece solid leading edge, a common model airplane practice. (See image from Sport Aviation, July 1965.)
    Turner_Wing_Const.jpg
    I am not aware of other examples that actually wrapped that thickness all the way around from top to bottom. You could certainly laminate that thickness from 2x1.5mm or 3x1mm and achieve a reasonable radius. It sounds like some of the current Pitts builders are using laminated leading edges (but not 3mm) and are happy with how stiff they are turning out.
     
  5. Apr 19, 2011 #5

    fly2kads

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    Ok, sometimes it helps to take a peek into "the bible," AC 43.13-1B. Table 1.2 has minimum bend radii for various combinations of plywood thickness, grain orientation, and preparation. If you run the grain parallel to the span or at a 45 degree angle, and soak/form over a mandrel, it says you can get the radius down to 1.2 inches. You'd have to see if that is tight enough for your application. I would buy some small pieces and test it first before investing a lot of money in a large panel.
     
  6. Apr 19, 2011 #6

    Dan Thomas

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    You can bend birch ply easier if you orient it with the outer ply grain chordwise, spray water on it, and iron that water in using an old clothes iron on high heat. The outer plies get soft enough to bead easily and the inner ply, which runs spanwise, offers little resistance. If you orient the outer plies spanwise you run into lots of bending reistnace beaue it's hard to get the inner ply steamed enough to bend across its grain.

    I've wrapped a one-inch wide strip of 1.5mm ply in this manner around a pencil in spiral fashion. It would never do that if I tried it with the outer grain parallel to the pencil.

    Dan
     
  7. Apr 19, 2011 #7

    conestogaman

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    The Mitchellwing aircraft (U-2 & B-10) use a 1mm ply for wingskins and the leading edges (to make a 'D' tube attached to the spar. Also used for the leading edges of the stabilators.

    It is first wetted and formed, allowed to dry, and coated with epoxy on the entire inner surface just before bonding to nose ribs and spars.
     
  8. Apr 20, 2011 #8

    bradyaero

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    The Turner T-40 look great, thanks for the link. In my case I can't have a solid leading edge without alot of work, so the plywood is going to have to take the full bend. I've settled on 2mm as my choice and have actually found a supplier for the stuff! Thanks for the advice on bending Dan, I would not have thought to orient the outer grain chordwise. I'm also using a D tube structure like the Mitchell but a little more beefed up.
     
  9. Apr 20, 2011 #9

    NorthwestJack

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    this is how I bend my 1.6 MM Skins.
    I used a sponge to wet the outside and a torch heating the interior of the steel tube.
    1.6 MM Finnish birch ( Available Locally here in the USA) is incredibly strong, I could tap it with a hammer and it would just bounce off. Dont know why you would need anything heavier?
    Jacq
     

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  10. Apr 20, 2011 #10

    topspeed100

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    In a original Spurce Goose video they make plywood themselves at the site..bending it to form. 0,4 mm ply could come out strong when laminated in between a sheet of carbon....maybe ?
     
  11. Apr 24, 2011 #11

    autoreply

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    The Concordia project (a nec plus ultra sailplane) uses the same reasoning with balsa-wood, where the balsa carries a significant part of the load.
     
  12. Apr 25, 2011 #12

    aerosteve

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    I've just completed the re-skinning of some vintage Dragon Rapide wings. We upped the ply from 1 to 1.2mm birch, applied with outer grain chordwise. Also have just helped a builder do his Turbulent wings with 2mm birch applied spanwise. Never heard of 3mm for this job before......
    The way we do it is to use a steam-type wall-paper remover (which is just a watertight box with a kettle element inside), take the steam via a flex hose to a metal tube epoxied to a plank. Fit another hose to the output end of the metal tube. Clamp the ply under the plank, get steam up and play it on both sides of the ply. We found that we could form 2mm ply around a 3/4" tube without any drama. Leave it all to cool and dry thoroughly before removing from the rig.
     
  13. Apr 25, 2011 #13

    rheuschele

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    I never thought to use my wall paper box for this. Thanks for the great idea
    Ron
     
  14. Apr 25, 2011 #14

    harrisonaero

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    Have any of you tried ammonia or lye to soften the ply for bending?
     
  15. Apr 25, 2011 #15

    Norman

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    If you can do it with steam alone it's best to avoid chemicals. I've heard that ammonia weakens the wood probably by altering the lignin.
     
  16. Apr 25, 2011 #16

    highspeed

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    Ammonia or lye would be a no go because they will likely weaken the wood.
     
  17. Apr 25, 2011 #17

    Lucrum

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    I bought one recently for that very purpose after seeing it somewhere on the internet.
     
  18. Jul 31, 2011 #18

    hogheadv2

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    NorthwestJack- Great photo for sharing information. Shows me what I wanted to know.... Thanks.
     
  19. Jul 31, 2011 #19

    Sacha

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    Guitar builders bend the sides often by only using heat. They have a heated tube and move the wood against it until it is hot enough, it weakens and the wood gets surprisingly bendable! The wood sets immediately after cooling down and keeps the shape it was bent in. Of course they use only single ply wood. I experimented with it and was amazed how easy wood can be bend in this way. Radius can be very tight with 2 to 3 mm wood without breaking the grain.

    Sacha
     
  20. Jul 11, 2012 #20

    Vector

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    So I bought a 1/8" sheet of bendable ply today; saturated it water and tried to bend it for a leading edge but instead of bending around the radius, it cracked and broke. I am not sure what else I should be doing. I was being along the grain so it was not as if I was fighting it from the on set. It just would not hold the radius I need which is not small either. Any suggestion would be appreciated.
     
    Last edited: Jul 11, 2012

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