wood shrinkage

Discussion in 'Wood Construction' started by Kristoffon, May 24, 2010.

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  1. May 24, 2010 #1

    Kristoffon

    Kristoffon

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    Does wood shrink/expand with varying humidity? Is it necessary to control humidity when cutting to avoing size fluctuation?

    Thanks.
     
  2. May 25, 2010 #2

    scuba72

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    As far as I know, wood will expand, contract with varied humidity and temperature, but how much depends on the size of the wood and probably the species. Working with 1/4" rib material its probably measured in the thousands of an inch. Working with large material may be in the hundredth of an inch. I built my fuselage sides in a jig last fall when temps were cool and low humidity. Today is 90 degrees with high humidity and they still slide in the jig just fine.
     
  3. May 26, 2010 #3

    FlightCenter

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    Wood does have some play in it, but the insulation/applications of wood can exceed that of metal. That's why they still create water towers out of wood, and if there was enough variability they would come crashing down, so it's a safe material to use.
     
  4. May 26, 2010 #4

    Kristoffon

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    Yes, that's the kind of information I was looking for. So it's probably not a significant fluctuation.

    My understanding is that once varnished the moisture is locked inside the wood and won't change anymore.
     
  5. May 26, 2010 #5

    mz-

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    Most people who have played wooden instruments know that the tone changes with moisture as the wood expands and shrinks, and you have to retune them. Also, furniture may loosen up and even the pieces that have been joined in ways that are sensitive to shrinkage may dislodge when used. Or you can make stuff break when it's just sitting there if you use materials the wrong way when you try to force the wood into not having space to expand or contract. An axe with a wooden handle can loosen in dry weather and the blade may separate dangerously - one way to solve that is to put it in a bucket of water for a while.
    I have never built wooden airplanes, but from these one could surmise it is an issue.
     
  6. May 26, 2010 #6

    PTAirco

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    It was common practice in the 1920-40s for the British aircraft industry to use cupped washers on nuts and bolts used in wooden members to take up shrinkage, the cupped washers being slightly compressed when tightening the nuts. Probably overkill, but they liked to things that way. Also a lot of their aircraft were used in widely varying climates from Canada to India, so perhaps it was a justified precaution.
     
  7. Mar 16, 2013 #7

    Workhorse

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    He got really clever woodworking techniques also.
     
    Last edited by a moderator: Mar 20, 2019
  8. Mar 19, 2013 #8

    DangerZone

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    It is a good video from Mathias but some of the conclusions in there might be misleading for a homebuilt wooden aircraft builder.

    Aircraft quality wood has to be dry, almost completly and the wood that he and his dad made furniture was never that dry. In fact, they were building with almost raw wood if it shrunk that much after some time. Aircraft building wood costs so much because it has to be cut the right way, dried the right way, then recut again to the exact profile and at the end left for some time to settle and dry completely. Once it has aged, dried and settled there is no concern for moisture shrinkage because it achieved it's maximum shrinking. Such dry wood can be curved then just like in the video by simply applying moisture to the part we want to bend. Let it stay that way for a day after drying it with a fan and you got a shape matching the contours of your jig. Dry it completely and it stays that way. Apply epoxy on that dry wood and it will be protected by any moisture for a hundred years.

    The wood does not only shrink, it has two other actions, it bends and it twists depending on the cut, part of the wood and the directions of the grains. That is why aircraft spruce has to be cut well, dried, settled, so it has the final form when built into the airframe. If moisture would somehow invade the system it should be let to get out of the system. This means that at altitude the thermal differences act upon the wood to let the humiditiy into the fuselage, but it also lets the same moisture out when the airplane has landed. Another solution is to completely seal the wooden parts with composites or waterproof insulation, like the De Havilland airplanes. Or the W.A.R. Replicas, they had wood in between composite layers on one side and styrofoam on the other and it did make a concern fo9r many engineers that the wood that is sealed so will eventually rot inside and threaten the airframe structure. Yet, it did not happen, there are airplanes that are built that way and still fly today.

    There is still a debate between people who say it compromises the airframe structure to seal the wood completely and then there are others who insist that some wooden boats have been laying in water for half a century (Goudeon brother - West System) without any moisture damage inside the wooden structure protected by good quality epoxy. The way I see it, both are correct and both designs fly safely, even after many years, it only depends on the quality of the work and design.

    Wood is an excellent natural composite, it can be used in a variety of ways. Is is in some cases stronger than glass fibers and matches carbon fiber structures in some cases. Wooden wings were used on some acrobatic airplanes and proved to be an excellent material. Yet the problem of producing it and the time needed to be ready for building turns most people to less demanding composite materials.
     
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  9. Apr 24, 2013 #9

    Southron

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    My only objection to wooden aircraft are termites. More than one aircraft has crashed due to a termite infestation.
     
  10. Apr 24, 2013 #10

    Pops

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    I have built a KR-2, Falconar F-12, and the little single seat Super Cub that I fly from wood and have never had a problem with the wood shrinking or expanding. I like flying a wood aircraft because the wood soaks up the noise and vibration unlike aluminum. My Falconar F-12 was fast,quiet, smooth and felt more like flying a much larger and heavier aircraft but handled almost as good as a RV-4. . I also like the insulating effect of wood. If the sun is shining, my little Cub is warm with no cabin heat down to 20 degrees.
     
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  11. Apr 25, 2013 #11

    Kristoffon

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    While that is of course a valid objection I have the feeling that if it comes down to termites that particular bird is in a pretty sorry state and might as well crash from any other number of factors.
     
  12. Apr 26, 2013 #12

    rheuschele

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    Isn't this also one of the reasons you would want to chose close grained wood, to keep the expansion and contraction to a minimum?
     
  13. May 18, 2013 #13

    DangerZone

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    Were they made and protected well during construction? The way we make wooden aircraft is to coat them with a preservative basecoat containing insecticidal and funghical protection. When it evaporates and dries, the chemical inside is completely poisonous to insects and funghi if they would somehow penetrate the epoxy coat finish. Are you sure these airplanes were made and protected the right way?

    The closed grained spruce in an excellent building material, it would shrink less in one of the three directions. Yet it has a down side, if it gets too dry it tends to delaminate easier in a composite sandwich along the grains. Thus a 45 degree grain bond would be the best solution, which is not always feasible. Spruce used in aircraft homebuilt construction is thin so the expansion and contraction would be minimal, within the elastic modulus of most other composite materials if you respect the grain orientation.
     
  14. May 19, 2013 #14
    Termites don't just eat the wood - they build their mud tunnels through the airframe adding weight as they go - they can completely colonise any enclosed space in a remarkably short time if there is a tiny hole for them to enter.
    Insecticide might stop some termites but I don't think it would even slow down the ones we used to get in West Africa.
     
  15. May 19, 2013 #15

    PTAirco

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    I recommend a 600 Nitro Express for keeping that kind under control.
     
  16. May 20, 2013 #16

    DangerZone

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    That is scary, so they eat through both the plywood/spruce treated with chemical insecticides and epoxy covering the airframe..? How do people protect wood efficiently against those in West Africa? I lived in the north of Africa and there was no such problem with termites, so I guess these might be related to some damp climate or something similar..?
     
  17. May 21, 2013 #17

    Heroben

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    I'm cutting down a white oak, and will run it thru the Alaskan mill right away. Its green wood, lets face it, now; is there any way or technique to " dry" these out? I really want to use it on the bulkheads and nose ( stem, knee, keel braces etc) and don't want to have to wait a year for the wood to dry out. Thanks guys!
     
  18. May 21, 2013 #18

    DangerZone

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    Is this a joke? An airframe made of oak? And you want to dry it now instantly just after you fell it down?
     
  19. May 21, 2013 #19

    PTAirco

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    Are you building a boat or an airplane?
     
  20. May 21, 2013 #20

    davidb

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    I believe he is building a flying boat. White oak is great for boats and whiskey barrels. Hopefully he is not planning to use too much of it since the specific gravity of dry white oak is .68. Heroben, I'd say even if you have access to a kiln you'll still have to wait several months for it to reach equilibrium of your environment. Did the flying boats of yore use white oak extensively?
     

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