wood shrinkage

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

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  1. May 21, 2013 #21

    DangerZone

    DangerZone

    DangerZone

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    Someone on this forum mentioned a guy who was building an airplane with oak spars and longerons instead of spruce as written in the plans, oak seemed stronger to this novice builder. Apparently the empty weight exceeded the MTOW so the airplane was beautiful but never managed to take off and fly.

    Even boats which need to be light have some ribs made of ash for additional strength. Some wooden airplanes have reinforcements of ash but not oak which is considered too heavy. Whoever has the chance to see a comparison chart of wood properties can notice right away that oak in fact is NOT that much stronger than spruce for it's role in an airframe.

    But hey, what the hell, if the guy wants to build an airplane made of oak it's his choice. A time and money consuming one, without a guarantee for flying. In that case shrinking of the wood should be the least of all concerns.
     
  2. May 21, 2013 #22

    Brian Clayton

    Brian Clayton

    Brian Clayton

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    Everything else aside, I believe that furniture quality oak it air dried for at least a year, sometimes more, then kiln dried to a specific moisture content. You cant just kiln dry green wood without air drying it first and expect it not to shrink as it dries. You rough cut wood while it is green, because it is much softer and easier to cut in a mill. Plus large pieces of wood tend to check at the ends, ruining several feet of lumber. The only plane I know for sure that was built with a lot of oak was the Bleriot. While it did fly sucessfully, technology has come quite a ways in the 100 years since it was designed.
     
  3. May 23, 2013 #23

    Heroben

    Heroben

    Heroben

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    Well, oak is used for the center portion of parts like the stem, knees etc ( like a sandwich: ply, oak, ply). Yes it is a boat, think of a runabout with wings.
    I appreciate the opinions, kiln drying would not reduce the moisture content to around say 20 to 25% that fast, so, it doesn't solve the issue, air dry can take up to a year or so, also nonviable.
    Back to the ANC18 we go...LALALA
     
  4. May 25, 2013 #24

    DangerZone

    DangerZone

    DangerZone

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    The heavier the boat, the better it is. Airplanes on the other hand should be lighter and the lighter they are the better they fly.

    So if you need some stems, knees or whatever done in your airplane to be tough it is better to make the part of spruce and then cover it with epoxy or some woven material and epoxy. Done properly, this part would be quite stronger yet much lighter than oak. And it would not soak moisture when close to water, adding weight to your airplane. Bear in mind that if dry wood is not protected well it could gain too much weight by moisture penetration, specially if it is an amphibian on water. Wood is a great natural composite to build an airplane but you have to know it's limits and the way to treat it. Time needed to dry wood so you could use it to build an aircraft - one year at least for tiny parts. Time needed to get your dry wood airplane parts completely soaked with water - overnight. Think about it for a while before you start building, it is wiser to think three times and build only once than to think only once and build three times...

    Oh yeah, and once that wood gets completely soaked with water/moisture/humidity it does not shrink, it expands. That's even worse than shrinking, cause it breaks easily under weight.
     

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