A study of aircraft wood

Discussion in 'Wood Construction' started by Stoyan, Apr 16, 2017.

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  1. Apr 16, 2017 #1

    Stoyan

    Stoyan

    Stoyan

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    Hello! My name is Stoyan, thats my first post here, so excuse me for not being professional. I am an aircraft mechanic and rc aircraft modeller, and in the recent years I am studying and preparing my self to build and ultralight all wood aircraft. I am living in Bulgaria, which is in eastern Europe, and getting aircraft grade wood here is almost as expensive as impossible. During the WW2 we have produced quite a few wooden aircrafts, so picking local wood is preferred for me.
    Any way, I will show a few pictures of 1/4x1/4 sitka spruce that I bought from aircraft spruce, and some wood i picked from local yards.

    18009760_10154917225780342_103681737_n.jpg
    The one on the right is the sitca spruce I bought, the one on the left is local wood. What I would like to ask you is about the grain on both samples. First of all aircraft wood should have like more than 8 rings per inch, so is it correct that the more rings per inch the better? The sitka spruce have few rings than the local sample and it is not exactly edge grained? So to sum up, which wood is better?
    I was keen on testing the real strength of both samples so I made the easiest test. I used bathroom tiles. One of them is 1.5 kg, both samples didnt break with 3 tiles load (4.5kg!) and when I applied the 4th they broke!
    On the last picture is the place where they broke. The local one is on top. Sitka spruce lower.
    Please share your experience and opinions with me! I have read a few books on aircraft wood and working with it but still have a lot of questions!
    cheers !

    17974655_10154917226410342_1489998983_n.jpg 17919074_10154917226930342_530288173_n.jpg
     
  2. Apr 16, 2017 #2

    plncraze

    plncraze

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    Welcome! Go to bowersflybaby.com for wood info. It looks like you have the right idea though. Buy local if you can, especially where you are.
     
  3. Apr 16, 2017 #3

    Aerowerx

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    Welcome, Stoyan!

    From the pictures, I would say that the local wood is better. If it is 1/4x1/4 inch then the grain is about 16 per inch, which is good, and it is almost vertical, which is also good.

    Your strength test is also interesting. The Sitka Spruce split along the grain lines. The failure on the local wood was at a single spot. I am not a failure mode expert, but I would say that the local wood is stronger.

    There is a copy of ANC-18 in the sticky folder at the top of this forum. You will find it beneficial to study. Grading your own wood is pretty easy once you know what to look for.
     
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  4. Apr 16, 2017 #4

    lr27

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    Since I've seen ANC-18 and an equivalent publication from Australia, my guess is that you can a publication like that from Europe that covers your local wood species better.

    (I don't remember where I saw the Australian publication, just that it exists.)
     
  5. Apr 17, 2017 #5

    bobm4360

    bobm4360

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    The German book, "Werkstattpraxis fur den Bau von Gleit- und Segelflugzeugen", by Hans Jacobs, has an excellent section on wood and wood testing, with emphasis on European woods. The wood won't care if its used in sailplanes or powered planes;)
     
  6. Apr 19, 2017 #6

    Stoyan

    Stoyan

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    Hello ! I am happy that there is still interest in wood building, thank you for the comments. I have red the ANC-18 and some more books covering the wood building method and wood its self. The german book about sailplanes is verry good, but hard for non german speaking like me. Still not shure about the actual strength of wood capstrips or planks. Thats why I think every one should perform some kind of testing. There are alot of methods for homebuilders that can provide enought accuracy. Some times i even bite :) the wood and determin the softness! Hand breaking small capstrips is also useful.
    The problem comes when trying to separate the actual type of wood. What I mean is, we all know what fir is...but is it a douglas fir or other from that tree familly. Thats the hardest part in finding local wood.. I also tried to import from Slovenia, still no success.
    I would like to ask you if I pick wood for spar caps, is there a max of grain rings per inch, or the more the better. Also I am reading different opinions of grain orientation in spar capstrips(minimax kind of spar)
    If thats the cross section of the spar capstrip?
    IMG-1492593092563-V.jpg
     
  7. Apr 19, 2017 #7

    skeeter_ca

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    I would consider it strange that they split so differently. I would suggest doing multiple test and see if the result are the same. What kind of wood is the local wood?

    Darrell
     
  8. Apr 21, 2017 #8

    Chris Young

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    Failure in one section instead of splitting on a longer area on the tension side is not good ! It shows poor resilience and should not be used for structural purposes.

    Very bad spruce :
    http://www.aero-constructeurs-amate...echGene/LesBois/boisCahiers/ba-4D.resized.jpg

    Very good spruce :
    http://www.aero-constructeurs-amate...echGene/LesBois/boisCahiers/ba-7D.resized.jpg

    I think you need something more precise than tiles for measuring the force and a precise point of loading so that you can calculate the bending moment and then the bending stress at failure.
     
  9. Apr 22, 2017 #9

    lr27

    lr27

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    If you've got a graduated cylinder or something else that measures water precisely, a jug of water hanging on a string can provide an adjustable weight.
     
  10. May 1, 2017 #10

    colinc

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    a spring balance is good, particularly one that has a indicator that shows the max force. We use one linked to a foot pedal to apply the load progressively.

    If your sections are small enough, a cheap suitcase weighing balance would work.

    Regards,

    Colin
     
  11. May 1, 2017 #11

    vtul

    vtul

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    If you'd like a really helpful method for testing wood samples and determining properties with simple equipment, the following link to a method from a bowyer's forum is something I've used in the past to help with unknown woods:

    http://www.ozbow.net/phpBB3/viewtopic.php?f=34&t=5450

    Scroll down about 9 posts for the method, and calculations for properties.

    Good luck with your project!
     

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