non certified wood use

Discussion in 'Wood Construction' started by expedition2166, Dec 2, 2007.

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  1. Dec 2, 2007 #1

    expedition2166

    expedition2166

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    im builidng a knock off of a fly baby bipe i been useing what i was told/read would be an acceptable wood but i recently found something i now have questions about the wood im useing is a non certified northern white pine , i strength tested it against a piece of stikas spruce and it actually did better against breaking. im useing the white pine through out the aircraft and have gotten 70 percent complete . but heres the problem as i just recently discovered the graining is 6 rings per inch as per regulation of certified wood since the 43-13 was not to explanitory of acceptable graining etc i followed the rules to the best of my and my a&p/ia mech boss ability ,long straight grain no knots very little deveation etc but it only has 6 grains in a 1x4 stick since we localy lost a pilot in a citabra due to a broken spar it kinda makes me want more thoughts on it besides my own the wingspars are a c channel with 1/4 90 deg ply (which is also non certified ) but they have 2 fly wires for front spar and 2 for the rear spar the lenth is only 8 ft per side anyone have any thoughts on the matter
     
  2. Dec 2, 2007 #2

    PTAirco

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    Does Northern White Pine have any other names? The figures i have for a type of white pine (Pinus Strobus) are substantially lower than Sitka spruce: 4,000 psi compression vs 4,700 fotr Sitka, modulus of rupture: 7,600 psi vs 9.400 psi for Sitka.

    And an E value of about 25% less.

    From that it does not sound like a good idea to replace Fly Baby Spars with pine, it may be less critical in other locations, but the spars would have to be re-sized.
     
  3. Dec 2, 2007 #3

    expedition2166

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    your absoluty correct on the numbers there certified white pine is 85-96 % the strength of stikas and you do have to over size the material for a direct replacement . we took all of that in consideration in fabrication it originally called for 3/4x3/4 spar caps so we opted for 7/8 sq and went one further instead of useing the 3mm ply spar web we used 1/4 in , one other thing the original mono wing design is over 15 ft these modified setup we're doin is only 8 ft with the same number of fly wires. the tesing i done was from the advice of an aussi they have a site somewhere on crude but effective method of testing strength .mounting a 1''x1''x1'stick inside a piece of 1x1 id square stock and with a fish scale add weight till it shears and the non certified pine i tested actually held 8 more lbs as far as the other tests i have no clue what kind of numbers i can get elsewhere .......... can you see why i'm slightly confused the numbers i come up with contradict ''certified wood '' but i can say the spars are from all of the local builders opinion are built like a tank im just looking for others voices on how i did it oh and its built useing t88 just incase anyone was wondering
     
  4. Dec 2, 2007 #4

    PTAirco

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    Well, if you looked at the numbers and adjusted things accordingly and tested the wood - then I think you'll be ok. A lot of other people just don't appreciate that "That looks about right" does not work for things like spars, and you have to some basic maths. If your bending tests gave you good results and the rest of the wood is of similar quality, then I don't think you need to worry about grain count as long as it's straight and defect free.

    I used "non-certified" Douglas Fir before and did the same thing - some bending tests and got equally good results and used it without any qualms.
     
  5. Dec 2, 2007 #5

    expedition2166

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    well what got me started on it was an older fellow bout 86 yrs young he's built about 17 ( and crashed a few too ) but he told me a story about a old popular tree in his yard he parked his new car underneath it and lo and behold a branch fell on it he was upset to say the least so he cut it down and sent it to a mill and a few months later he was flying that tree he advised me not to worry about the graining he told me to test it if it didnt break use it ( of course the part i left out was he crashed it and almost killed himself , due to an ignorant passenger , the tree got the last laugh ) but why does the ''man'' tell us not to use wood with a minimum number or rings is there a hidden reason for it no one ive talked to seems to give a good reason behind it , ive been told its because the slower the tree grows (higher number of rings) means the tree was grown with less water supply and has i higher fiberous count withing the grain structure and the less water means the tar/sap actually glues the graining tighter wich makes sense to an extent but doesnt explain why wood with less graining has roughly the same strength
     
  6. Dec 2, 2007 #6

    PTAirco

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    Yes, we need an expert in on this... I'm curious to know the answer too.
     
  7. Dec 2, 2007 #7

    wally

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    A short answer is: just the number of rings alone does not give you a definitive indication of how strong a piece of wood actually is. The kind (species) tells you a lot more. Even then there are all sorts of defects to consider beside the number of rings.

    Stika Spruce it is not the lightest wood or the strongest. However it was originally selected for building airplane spars and framework because of it's lightness vs. strength ratio, glue holding ability and screw/nail holding ability. And (years ago anyway) it was cheap and available in huge, long, absolutely straight, clear grained boards. This was from monster old growth trees that were being cut 60-75 years ago. Old pictures show trees 15-25 feet in diameter and the first limbs a hundred feet or more in the air. Sadly they are all gone. They only grow - and very slowly at that - along the northwest Pacific coast. Douglas Fir is a little stronger but it is a little heavier and a little more available and a little more affordable.

    I think most of the Stika spruce is now going to Japan and other foreign places to be made into music instruments. I have a Spinnet style piano with the frame made of big square posts of spruce. The grain is absolutely straight and beautiful - circa 1940.

    I don't know much about it either so I better quit.
    Wally
     
    Last edited: Dec 2, 2007
  8. Dec 2, 2007 #8

    George Sychrovsky

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    The minimum faa standard to which the wood can be certified for aircraft use doesn’t mean every designer designed to that low standard.
    A real designer will specify a specific gravity of the wood used for the critical parts like spars and or how many rings it should have.
    Each piece of the same wood species is different so pine with many rings can be easily stronger than Sitka spruce with few rings.
    The bottom line is you chose weaker wood then Sitka spruce and use the minimum rings count for Sitka spruce.
    Personally I wouldn’t use 6 rings per inch wood anywhere.
    George
     
  9. Dec 3, 2007 #9

    expedition2166

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    looks like i asked a million dollar question , well in the house and construction industry they use oriented strand board ,bunch of crap left over glued together , in I beams for floor and roof joysts and they have unbeliveable durability as far as load bearing and strength characteristics not too much grain count there most is just random but still reguardless i think i agree with the post on its up to the designer but as far as wouldnt use less than 6 i have to ask why what is it with more rings that make you feel safer or more comfortable ? im might be thinking waaaaay too far out of the box but i'm curious i just seems that its one of those things where people were told that thats the way it is and the way it will be and no one has asked why its that way
     
  10. Dec 3, 2007 #10

    George Sychrovsky

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    I wasn’t told anything , I built two wooden airplanes, one mostly white pine , one mostly Sitka spruce some Douglas fir was used also, I can tell the difference from one piece of wood to another and I don’t have any need to settle for the lowest grade when I can have better for the same price, its just a matter of looking at it and picking the right piece. The wood I use have at least 12 rings per inch in pine and maybe 10 in Sitka.

    George
     
  11. Dec 4, 2007 #11

    expedition2166

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    it seems though every one has there own preference and i'll agree if i see something i dont like i wont use it but the ring issue still has me buffleoedddd
     
  12. Dec 4, 2007 #12

    Topaz

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    Home construction is done to different criteria - vastly over-weight (or over-strength, however you like to phrase it) compared to aircraft construction, where we push materials very close to their limits.

    Me, I'd stick to what it says in the plans. The designer chose that material for a reason, and second-guessing that reason gets you into re-engineering the plane. If you're up to that, then have at it. Otherwise, remember it's your fragile body being hauled several thousand feet into thin air by this thing. Not a time to scrimp, IMHO.
     
  13. Dec 5, 2007 #13

    expedition2166

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    well the home construction comment was purly for comparison of people takeing things most would say wouldnt work and showing the world otherwise i agree its not a weight issue there but the plans i have leave tooo much for imagination there vague at best that being said its one of htose things where i can locally get good useable white pine and spruce would be a bugger to get and shipping via truck freight would murder the projects budget i agree i dont want something to fall to pieces at 7000 ft but i can say from experiance the industry uses wood i wouldnt use even for a door stop recently i had the joy of restoring a j-3 that had been forgotted for 30 some odd years and we replaced the spars and a few stringers recovered etc but the wood they used for the spars were not of todays standards one spar had a total of 12 grains in the entire almost six inch width, which made me wonder why does the grain make such a big difference if the industry can use sub par material but every other builder ''should and would look better '' if he/she used above par material
     
  14. Dec 5, 2007 #14

    orion

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    I haven't done any wood-based engineering in quite a few years but I did delve into the subject in doing my own research for several past associations. As such, I'll put in my own two cents worth in an attempt to clarify some of the issues at hand.

    The first thing I found in digging into the properties of trees (for aircraft application) is that with only a few exceptions, pretty much all functionally usable wood falls into a rather narrow range of specific strength values - in other words, the strength as a function of density does not tend to vary by a whole lot. Yes, there are exceptions that fall a bit outside the band but overall, the range is surprisingly narrow. The major reason for this is simply that all wood consists of the same types of building blocks at the molecular level (cellulose fiber). These building blocks organize and connect in pretty much the same manner regardless of whether you're looking at pine, spruce or oak. The only differences therefore are a function of the compactness of the assembly, which is a function of the type of tree: Simply said, higher density wood tends to have stronger physical properties.

    Looking closely at a typical cross section what you'll see is a series of darker rings separated by lighter areas. This "grain" consists of a slight variation of the basic material. The dark material is of a higher density and thus it provides the major component of the physical characteristics. The lighter material then is less dense and its physical properties are a bit lower. If anyone's ever built a piece of furniture and tried to sand it perfectly flat - the softer woods are much more difficult to work with since no matter how long you sand, the denser grain will tend to stand up a bit from the softer material surrounding it. How much each grain component contributes to the overal properties is then a function of the ring's width and of course, it own physical stiffness and strength.

    In a typical board the two variations of the same material work together so the overall characteristics are actually a mix of the pair. In typical proportion, the two components provide the wood with a certain amount of strength and a certain amount of toughness. Wood that is virtually all high density material may be stronger, but it will also be more notch sensitive and subject to abrupt grain failure at high loads (poor failure mode).

    In arriving at a proper design specification for this inconsistent material, the original designers had a challenge in front of them since they needed to come up with some criteria that would allow the material to be easily used for structures where predictability was a key to survival. But in order to do so, the material had to have more than a strength specification - it needed to undergo a quality assurance process that would allow material suppliers to provide the products to airframe manufacturers without having to undertake extensive scientific testing and chemical analysis for each board leaving the mill.

    The criteria selected therefore was ring density (and angle or run-out), material density and moisture content. The ring density is the most basic parameter since it identifies the most clearly measurable characteristic of the wood. The moisture content then standardizes the wood to a specific gravity so that the material density measurements then makes sense. The latter then is used to characterize the material, accounting for variations in the wood that occur as a function of environmental causes and growing conditions.

    The design values (which are statistically somewhat conservative) that were eventually published were then to be used only as a design minimum. Trying to design every piece of the structure to within a "gnat's ass" of the published values was not considered good practice since it took only one slightly flawed piece of wood to cause the whole thing to literally come crashing down.

    When it comes to substitutions, there is no reason why you can't go from one material to another, but only as long as you understand the choices and do the proper engineering and material selection. There are published standards for most materials available so provided you obtain a material that meets those standards, the proper analysis should result in a safe and durable structure, regardless of the wood you select.
     
  15. Dec 6, 2007 #15

    plncraze

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    The EAA has books that can help in understanding choices and selecting wood to meet the criteria that Orion mentions in the last post.
     
  16. Dec 6, 2007 #16

    expedition2166

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    so in retrospect all the stuff the ''old senile farts '' told me was correct just not technical well i can say this orion you definatly answered the question i had ( finally someone answered where it made sense ) and i'm glad im not the only one who thinks theres more then one way to skin a cat lol great info there and thanks again
     
  17. Dec 7, 2007 #17

    dgeronimos

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  18. Dec 7, 2007 #18

    expedition2166

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    thats the same link i sent chris m on about the same subjet lol but thanks for the link i do appreciate any feedback
     
  19. Jan 7, 2008 #19

    bob pearce

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    The reason for the growth ring specifications are as follows. Too many ring may indicate ver slow growth of the tree which will make the wood heavier than normal with a tendency toward brittleness. Too few rings indicates fater than normal growth and weak wood. If you have tested a piece from every plank you use and it test as good as or better than the table for Sitka Spruce I suggest the piece is satifactory. Re test though to make sure it does not bend ezcesively before breaking....bob pearce
     

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