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Thread: Connecting wood box spar

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    Connecting wood box spar

    I'm planning to start building the Vampyr flying wing soon, and due to limited space (it's a garage project for now) I'd like to make the 24' wing in two sections, joined at the center. I can calculate the size of the connecting fittings/straps and bolts, but I'm wondering about drilling through the spar cap. The cap dimensions are 1 3/16" wide by 1 9/16" deep. Are there any guidelines for the size hole that can be put in a spar cap without compromising its strength? Also, would it make sense to add a solid piece inside the spar at the joint for extra support or is that just overkill if the fittings and bolts dimensions are calculated correctly?

    Thanks.
    Leland

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    Registered User fly2kads's Avatar
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    Re: Connecting wood box spar

    I would recommend checking out ANC-18, available here on HBA as a PDF file or as a photocopy from EAA's library. It has the info you need. A couple of options are to install blocks as you mentioned, or to splice in a higher-density material (plywood) to take the loads.

    Edit: ANC-18 is Design of Wood Aircraft Structures. Chapter 4 has information on detail design, plus there are additional examples at the end. Figure 4-37 shows an example of the general type of fitting you'd likely wind up with; this particular example shows both filler blocking and plywood faces.
    Last edited by fly2kads; January 4th, 2012 at 11:47 PM. Reason: Added title and section reference

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    Registered User Dan Thomas's Avatar
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    Re: Connecting wood box spar

    Be very, very careful about changing a design like that. Over the years we sometimes hear of inflight structural failures just because of such mods. Any interface between fittings and wood must be suitably reinforced and the fittings designed to take the loads.

    Dan

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    Registered User djschwartz's Avatar
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    Re: Connecting wood box spar

    Quote Originally Posted by LBarron View Post
    I'm planning to start building the Vampyr flying wing soon, and due to limited space (it's a garage project for now) I'd like to make the 24' wing in two sections, joined at the center. I can calculate the size of the connecting fittings/straps and bolts, but I'm wondering about drilling through the spar cap. The cap dimensions are 1 3/16" wide by 1 9/16" deep. Are there any guidelines for the size hole that can be put in a spar cap without compromising its strength? Also, would it make sense to add a solid piece inside the spar at the joint for extra support or is that just overkill if the fittings and bolts dimensions are calculated correctly? Thanks.Leland
    Any holes in the spar cap will compromise its strength. This is generally a bad idea; though, it may be possible with sufficient design work to maintain the structural integrity of the caps. Your plan would place the coupling joint at the point of maximum stress. This can be done, but it will require careful design and stress analysis. You will need to design metal fittings that bolt together at the center and then bolt to the front and back of each of the spars along a small portion of their length. The bolts through the spar should go horizontally through it and be relatively near the center top to bottom. They should not be in an exact line as this increases the likelihood of splitting along the grain. Needless to say the spars will now need to be solid for some distance out from the center to transfer the loads to the fittings. The advice to check AC43-13 is good; though, I'm not sure you'll find this exact situation covered there. AC43-13 is intended to address repairs, not major structural modifications.

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    Registered User Hot Wings's Avatar
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    Re: Connecting wood box spar

    The previous responders pretty much covered anything I might have said, except for one. Why break the wing at the center? To me it would be a far better idea to break the Vampyr wing at the outboard of the straight center section. The loads are less there, you then have the option to remove the outer panels for storage/transport, and the individual wing parts are even smaller in length. Shorter lengths makes individual part weight and building space requirements less.

    The only down sides would be having to make twice as many fittings and rigging the outer panels for identical dihedral.
    Conventional wisdom and practices yield conventional results. If that is good enough for you:
    Problem solved.

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    Re: Connecting wood box spar

    Thanks for your comments guys. I have ANC-18, have read parts, but forgot the bolt info was in there. Thanks for the reminder.

    The Vampyr is an ultralight, and the ultimate 4g bending load is less than 7,000 lbs at the root. One of my books has a series of examples for calculating the size of the fittings and bolts so I believe I can do that properly. Below are a few pictures similar to what I have in mind. It appears the bolts go through the spar caps, but there might also be solid piece in there - it's difficult to tell.

    I did consider putting the attachments at end of the center section - that certainly can be done, it just requires more pieces. The center section is 10 ft long, so making three sections vs two only saves 2ft. I thought reducing the attachment points would be a better option.

    I'll read through ANC-18 and come back with more questions. Or maybe just get a larger garage.


    Cheers.
    Attached Thumbnails Attached Thumbnails Connecting wood box spar-019.jpg   Connecting wood box spar-020.jpg  
    Last edited by LBarron; January 6th, 2012 at 08:06 AM.

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    Re: Connecting wood box spar

    i would look into corby starlet ,this plane has two versions of wing first is one solid piece and the second is two halfs connected in midle useing metal fittings ,you can download plans at ultraligero.net ,the think with vampire will be connect the fuselage to the wing not the wing together since the plane has fuselage build around spar or fuselage and the spar is glued in (as one piece airplane)

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    Re: Connecting wood box spar

    Thanks jany77 - I'll look at the Corby Starlet. One of the mysteries of the Vampyr is attaching the wing to the fuselage. One suggestion is four bolts at the corners of the reinforced fuselage frame but without any internal material in the spar those bolts would have to go through the spar caps. Two bolts would be used to attach the rear spar. Those details are not in the plans so I don't know what the designer originally did. Still not sure what to do....

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    Re: Connecting wood box spar

    i was also looking into build on of those vampyrs my self and make the wings foldable like mitchel wing but that was something i was unable to figure out strenghtwise ,i could build it and just guess the size of fittings etc ,but why bother there is several similar designs which will do just that ,i like vampyr because it look simple and flyies on low power i meet the designer when i was 19 just before i took of and came to us,i think he still lives in jihlava czech republic with his girlfriend even thou hes french,the vampyr was mostly ofered as kit where the fuselage was fiberglass body and one bulkhead was part of the main spar that was hold together by glue

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    Re: Connecting wood box spar

    I have been looking at the wing joining here...and gliders have clever ways to join the wings. Basically the only simple way to do it ?

    I have one Airplane Monthly copy where they say TA-183 wings were attached to fuselage with just one bolt. Wonder how that looks like ?

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