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Thread: Scarfing and laminating wing spars

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    Registered User Jeff R's Avatar
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    Scarfing and laminating wing spars

    Another request for opinions....

    The center spars for my F12A are a bit under 17' long. I know that laminating thin sections of spurce together to build up to the required thickness is acceptable, even encouraged for reasons that have been talked about elsewhere on this board.

    I have not heard opinions about splicing shorter pieces together to make up the required length for the laminations. Specifically, since the longest lumber than can be shipped by UPS is 8' (and I wish to avoid expensive trucking fees and added costs to get 17' long spruce - yikes), is there really any problem in scarfing together three or four shorter pieces for each lamination layer? Of course, the scarf joints would have to be offset, such as the following 4-lamination spar diagram shows, where each - is a foot of spar (or so) and each / is a scaft joint.

    --/------/-------/--
    ----/-------/------
    ------/--------/---
    ---/-------/-------

    Is there any ideal placement of the splices? That is, is it better to center them with the fuselage, such as:

    --/-------/-------/--
    ------/------/------
    --/-------/-------/--
    ------/------/------

    Would it be better to use longer lumber, such as 12', as follows, to reduce the number of splices?

    -----/------------
    -------/----------
    ---------/--------
    ------------/-----

    The process to make the spar would be to cut strips of spruce maybe 3/8" thick. The wood would be inspected, scarfed at 1:12 or so, cleaned and glued straight to make the required length plus some spare. After curing, each lamination layer would be run through a planer to make the surfaces flat and parallel, then the laminations would be stacked and glued together. After the stack dries, the completed spar would be planed and edged to the required final dimensions.

    Either T88 or possibly resorcinol would be used. I am using T88 for the ribs, but wonder if resorcinol might be better for the spars.

    Thanks,
    Jeff

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    Registered User jgnunn's Avatar
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    Re: Scarfing and laminating wing spars

    call dave stone at Steen aero. They make their own laminated spars that they have test-destructed. He will be able to tell you if they use shorter staggered pieces for laminating or not [they also do this in a vacuum environment]. I think Hughes epoxy would be a better laminating glue, t-88 could starve the joint on clamp-up.

    I noticed another one of your posts regarding substituting ply etc. [so i see a pattern of trying to 'save'].While most of us want to economise, just be careful that you don't end up in trouble....some things simply cost money and has to be accepted, especially when dealing with flight critical componants.

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    Registered User Jeff R's Avatar
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    Re: Scarfing and laminating wing spars

    Quote Originally Posted by jgnunn View Post

    I noticed another one of your posts regarding substituting ply etc. [so i see a pattern of trying to 'save'].While most of us want to economise, just be careful that you don't end up in trouble....some things simply cost money and has to be accepted, especially when dealing with flight critical componants.
    Thanks for the contact.

    My original intent was to use certified, full length wood in the spars, and common sense would say do it that way, as I sure don't want to be falling out of the sky when my spar snaps wishing I had spent a few extra hundred dollars on wood. However, this is a learning experience and I am trying to go through the thought processes, and ANC-18 certainly allows for splices and laminations. As we know, laminations actually increase strength. Splices are a different manner, but I really don't see a problem in what I am proposing by offsetting the splices within the laminations. But, I am presenting it here to see if anyone DOES see a problem. Even if no one sees a problem, I might still break down and use a single piece from ACS just for the peace of mind it would bring.

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    Re: Scarfing and laminating wing spars

    Jeff,

    Using a built up spar with laminated caps, even with spliced joints in the laminates, can actually be stronger than a single solid spar of spruce.

    The reason is that every piece of wood has some minor deformities and the thicker the piece the more likely there will be one or two, and the less likely you are to be able to spot them.

    Worse still, if there is an internal grain deformation, split, etc. there is the distinct possibility of it propogating through a substantial portion of the spar.

    With a laminated construction, any such deformity in a single piece will not extend further than that piece.

    Also, a laminated spar is generally more flexible than a solid one and if it is pushed to failure it will let go progressively as the weaker sections fail first. (Assuming, of course, that the overload is not of catastrophic proportions)

    There are a couple of caveats involved however:

    1. The splice joints need to be well made using a 12: 1 scarf or better - 15:1 is preferred. The mating faces must be in close contact over the length of the scarf.
    2. The glue joints between the laminates also need to be of good quality and free of voids.

    The bottom line is not to stress about having to source a large and expensive chunk of a tree for your spar, properly made laminated caps are perfectly acceptable

    Rob

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    Registered User Jeff R's Avatar
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    Re: Scarfing and laminating wing spars

    Thanks, Rob. Your thinking aligns with mine.

    If I build the spar from 4 laminations, then the inner scarf joints will have glue not only in the scarf joint, but on either side as well, adding a lot of strength and stability to the joint. The outer laminations will have the extra glue on just the inside, but this will still add strength.

    So, I do think that it should be ok -- provided the wood is of the necessary quality. I would be ripping down several 2 x 8's to obtain the laminations, so I will have some control over grain orientation. By using wood from different boards, the the safety margin should be increased further.

    Thanks.
    Jeff

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    Registered User jgnunn's Avatar
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    Re: Scarfing and laminating wing spars

    Quote Originally Posted by Falco Rob View Post
    ... properly made laminated caps are perfectly acceptable.Rob
    'Properly' being the keyword. It sounds like you have done your homework. The contact I gave you is the contact I would have gone to if I was contemplating on doing the same as you. It doesnt hurt to reinforce what you may already know by talking to a commercial body that is already successfully doing this. 'wouldn't hurt to ask them what glue they use too....

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    Re: Scarfing and laminating wing spars

    Jeff,

    Referring to your earlier post where you ask about location of the laminate splices, keep them as far outboard as possible.

    The bending moment of the wing decreases as you move outwards away from the fuselage (or centre of the span for a one piece spar)

    To use the Falco as an example, it has an 8m wing span and a one piece spar.

    The maximum length spar cap laminates I received where about 5 m long so I positioned them slightly offcentre in relation to the fuselage centreline and spliced about 1.25 metres on one end and 1.75 on the other.

    The splice location was alternated from side to side on subsequent laminations to keep the splices away from each other.

    Hopefully this sketch will explain it, although for ease of drawing the spar cap is shown with no dihedral.

    Sorry about the orientation - I rotated and saved it but it didn't seem to want to stay that way.
    Attached Files Attached Files

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    Registered User Jeff R's Avatar
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    Re: Scarfing and laminating wing spars

    Thanks, Rob. That does make sense.

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    Registered User Jeff R's Avatar
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    Re: Scarfing and laminating wing spars

    I have decided to take a dual approach. I will use certified spruce from ACS for the main wing spars, and either self-certified spruce from Public Lumber or self-certified Douglass Fir from some local area Home Depots for the rest of the structure.

    As the only fir I can get is 1-by material, which is actually a bit thinner than .75", and since most of my remaining requirements for other than the spars is for 7/8" square material, I need to laminate two pieces to build up the required cross section and, in some cases, scarf shorter pieces to get the required length. For example, I need two longerons that are 7/8" by 7/8" by about 17 feet; which I can make from a pair of 1 by 2's scarfed with a 10' piece and an 8' piece, then laminated. From there, the lamination will be planed and trimmed to the required size.

    I did see direct evidence of the desirability to use thinner laminations (though one could also argue the evidence supports using only certified wood). The attached picture shows an internal defect in one piece of fir that looked fine on the outside. Unless I had planed down the wood, I never would have seen this defect. I think things would have been fine had it not been discovered as it was being laminated with another piece and the defect is only about 1/8" deep. However, this case certainly shows that there can be hidden defects, and the thinner the wood is that we use, the better chance we have of seeing and finding those defects.

    In this particular case, I can plane the wood down some more and eliminate the defect, but had I not already been planing it down to prepare if for lamination, I would not have ever seen it and it might now be in one of my longerons.

    I suppose one can argue that if you use certified wood you would not have to worry about hidden defects. I am not sure that I could agree to that. Maybe. Perhaps the safest thing is to use thin slices of certified wood and laminate it to the required thickness, but that would be very, very expensive. I admit to some apprehension about using non-laminated wood in my main spars, just as I have some apprehension using laminated, non-certified wood! Well, some apprehension in this business is good I guess.

    Jeff
    Attached Thumbnails Attached Thumbnails Scarfing and laminating wing spars-lamination-defect.jpg  

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    rug
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    Re: Scarfing and laminating wing spars

    Sorry to dig up an old thread, but it seems worth posting my question here, rather than starting an entirely new one.

    Assuming that laminated spar caps can use scarfed joints, is there a preferred orientation for the slope ? i.e. is it better to have the slope going up towards the wingtip, or down? Or indeed, does it make no difference, or should there be a mix of the two ?

    I am presuming that a scarf joint where the slope runs across from the front to the back would be too inflexible in the vertical plane, and hence a bad thing to do.

    I ask because I'm pretty sure I won't get 5 metre (16') length of spruce very easily in the UK ...

    Thank you for any light you can shed on this.

    Matt

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    Registered User Jeff R's Avatar
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    Re: Scarfing and laminating wing spars

    It shouldn't make any difference at all. Properly prepared scarf joints, especially when part of a laminated beam, should be stronger than the wood, so orientation of the joints is simply a non-issue.

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    Registered User rheuschele's Avatar
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    Re: Scarfing and laminating wing spars

    this thread was started before I joined this group so I never have seen this before, but I would be curious if anyone had done this with the small pieces. I wonder if the amount of glue used to make this would add to the weight.

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    Re: Scarfing and laminating wing spars

    I may be wrong, but I disagree. The scarf should angle up towards the wingtip. Well, if we are talking a solid cantilever here. But with caps, it does not matter. But back to off topic: if it were cantilever with no caps, you would want to angle up to avoid concentrated peeling.
    So I agree with the above poster about spar caps.

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    Re: Scarfing and laminating wing spars

    Quote Originally Posted by rheuschele View Post
    this thread was started before I joined this group so I never have seen this before, but I would be curious if anyone had done this with the small pieces. I wonder if the amount of glue used to make this would add to the weight.
    That would be a good reason to keep the thickness of the pieces big compared to the thickness of the glue, since the glue is weaker than the grain of the wood. But as long as the glue layer is as thin as the pith between fibers, it should not be an issue.

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    Re: Scarfing and laminating wing spars

    Quote Originally Posted by rug View Post
    Sorry to dig up an old thread, but it seems worth posting my question here, rather than starting an entirely new one.

    Assuming that laminated spar caps can use scarfed joints, is there a preferred orientation for the slope ? i.e. is it better to have the slope going up towards the wingtip, or down? Or indeed, does it make no difference, or should there be a mix of the two ?

    I am presuming that a scarf joint where the slope runs across from the front to the back would be too inflexible in the vertical plane, and hence a bad thing to do.

    I ask because I'm pretty sure I won't get 5 metre (16') length of spruce very easily in the UK ...

    Thank you for any light you can shed on this.

    Matt
    Check out section 4 of AC 43.13 ch. 1, par 1-40. http://rgl.faa.gov/Regulatory_and_Gu...apter%2001.pdf

    It has a diagram of proper and improper grain orientation for a scarf joint.

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