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Thread: Ultralight Spar

  1. #1
    Registered User mstull's Avatar
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    Ultralight Spar

    I've engineered a construction technique to make a weight efficient U/L spar. I want to bounce it off y'all for any problems or suggestions.

    For the spar caps I start with a very long piece of thin wall aluminum tubing, about 5/8" OD, and cut about a 1/8" wide slot down its full length.

    I lay it down flat with the slot facing up, tape off its ends, and fill it about 1/4 full with high strength epoxy glue.

    Then I drop full length carbon rods through the slot, until the tubing is 2/3 or 3/4 of the way full of carbon rods and glue.

    Then I push the edge of 1/8" carbon and/or plywood sheer web sheeting into the slot, until it touches the far side of the tubing. The sheer web can be made in segments. The sheer web sheeting would have to be supported with a long jig until the glue dries.

    Then I make the second spar cap the same way, and push the sheer web sheeting into it.

    Finally the aluminum tubing could be peeled off or not. The result is a sheer web with a bundle of carbon rod glued to its top and bottom. It should be quick and easy to make.

    Please forgive the crude drawing.
    Attached Thumbnails Attached Thumbnails Ultralight Spar-spar.jpg  
    Mark E. Stull
    mstull@wtxs.net

  2. #2
    Registered User xj35s's Avatar
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    Re: Ultralight Spar

    nice concept. I'm thinking the glue is not going to mix well with the rods. You'd have to ensure they are covered all around equaly. The glue is thick isn't it?

    Maybe after getting all the rods in you could spin it in a drill? That might rotate the rods and get them all wound up with the epoxy. You don't want to peel the aluminum then find some of the rods just fall off.

  3. #3
    Registered User Tom Nalevanko's Avatar
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    Re: Ultralight Spar

    Seems elegant in its simplicity. Have you built one yet?

    Blue skies,

    Tom

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    Registered User Tom Nalevanko's Avatar
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    Re: Ultralight Spar

    On 2nd thought, if it turns out that the adhesive is not coating the rods well enough. For example where they touch together. You could possibly use a larger diameter tube and make the slot wider by 2 rod diameters; put the shear web in first and then drop the rods in one by one. This might work out better but be more work.

  5. #5
    Registered User mstull's Avatar
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    Re: Ultralight Spar

    No, I haven't started experimenting with it yet. I wanted to get your input first. It does seem like it would work out best with very low viscosity glue, that would quickly wet the surface of each carbon rod as I drop them in.

    I wouldn't want to twist the rods for a couple reasons. Over that much length, with the resistance of the glue, the rods might twist apart (break). Also, it's important that the rods lay very parallel so when I push the shear web in, none of the rods block the shear web sheeting from reaching the far side of the aluminum tubing. It might be possible to work the rods from end to end a bit, to work the bubbles out.

    It may be necessary to fill the tubing with extra glue, so each rod gets covered with glue as it sinks. Then when the shear web is inserted, the excess glue will be forced out... along with most of any bubbles.

    I'm not sure the rods will tend to sink very fast, if at all. It may be necessary to push each rod to the bottom, which would help coat them and release any bubbles.

    For testing/experimenting, it would be helpful to use some kind of rigid, clear, plastic tubing, to get a feel for how bubbles might form, and how to get them to float up. All in all, very low viscosity glue would seem the obvious solution. I'm not sure how to find the lowest viscosity glue available.
    Mark E. Stull
    mstull@wtxs.net

  6. #6
    Registered User Tom Nalevanko's Avatar
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    Re: Ultralight Spar

    Using a clear rigid tube sounds like a good idea; even if you only use a short test piece. One of the best moves I ever made was to use a clear plastic line on the header vent for my Stallion. I was utterly amazed at the bubbles that worked their way up that line. It gave me insight as to why my engine had been stopping under certain circumstances. Bubbles sometime behave in strange ways.

    My understanding is that the carbon rods are actually kind of "furry" when examined under a microscope and the glue has lots of contact area. I think that it is important that each rod sees glue before it might contact another rod. The danger is getting dry spots on one rod touching dry spots on another. Something like a laminating resin sounds preferable to a structural adhesive like Hysol 9430.

    I was thinking of doing something similar with the rods using resin infusion. I would separate and also bundle the rods by wrapping around them with a pre-preg carbon fiber tow every few but TBD inches. In my case the shear element would be internal to the rods bundle. And then post cure them and the set the prepreg tow.

  7. #7
    Registered User mstull's Avatar
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    Re: Ultralight Spar

    Thanks Tom,

    I've tried laminating resin with carbon rods. It didn't work. Laminating resin isn't nearly strong enough. The rods will break free of the resin. It was explained to me in another thread about spar caps, that epoxy glue can be in the order of 10 times stronger than laminating resin.

    In addition to the glue needing to be low viscosity, it would also help if it was very slow curing. That way any bubbles from one rod would have a chance to float up before the next rod was lowered into the liquid glue.
    Mark E. Stull
    mstull@wtxs.net

  8. #8
    Registered User Tom Nalevanko's Avatar
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    Re: Ultralight Spar

    You might look at the various Hysol products for the characteristics that you desire. I have found that you can call Hysol technical support and talk to the people who write the specs; they are generally quite helpful and go into a lot of detail about how they work with the stuff.

  9. #9
    Registered User BBerson's Avatar
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    Re: Ultralight Spar

    My experience is that resin will flow into the cavity from gravity if filled from above. It takes a few minutes to fill. A bit of heat will make it flow nicely into small cracks if needed.
    BB

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    Registered User rtfm's Avatar
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    Re: Ultralight Spar

    Hi,
    What an elegant and charming idea. Well done, that man! At the end of the day, the simplest solution which satisfies all the requirements is the winner. And I think you may well have a winner here...

    Duncan

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    Registered User flyoz's Avatar
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    Re: Ultralight Spar

    Check out this French web site - best ultralight spar design i have seen - nice plane too -http://quaetaers.club.fr/index.htm
    Use a translator - lots of photos about construction
    Flyoz

  12. #12
    Moderator Dana's Avatar
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    Re: Ultralight Spar

    The only thing that worries me is that there will be a large amount of epoxy in the voids between the round rods, contributing little to the ovverall strength but probably quite a lot to the weight. Efficient composites try to minimize the resin/fiber ratio for maximum strength. Have you figured what the weight would be compared to, say, an all aluminum flange of equivalent strength?

    -Dana

    Of course, the U.S. Constitution isn't perfect, but it's a lot better than what we have now.
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  13. #13
    Registered User mstull's Avatar
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    Re: Ultralight Spar

    Thanks Dana,

    Using carbon rods for spar caps is supposed to be the most weight efficient spar caps you can make... way better than aluminum. The little corners around the rods are there, no matter how you lay the rods on the spar caps. So this should be just as good as any other use of them. The reason carbon rod is so efficient is their extreme unidirectional strength and stiffness. All the fibers are perfectly straight and parallel. And they are so compactly formed, that their internal fiber to resin ratio is outstanding.

    You should experiment with some carbon rod. I've been using it for years. It has unique structural characteristics. I like the .0158" diameter rod. It is so stiff, some places it only takes one rod.

    For example, for sharp trailing edges on control surfaces... You buy that preformed aluminum trailing edge, which is kind of fragile. After it is installed on the ribs, you stand the control surface up on it's trailing edge and inject a large bead of epoxy micro into the trailing edge between each rib. Then you drop a large carbon rod into the micro that just fits between the ribs. Push the rod down as far as it will go with a little stick. Add a little more micro if necessary to make sure the rod is fully encapsulated in the micro. The trailing edge is now so strong, you can pound on it with your fist between the ribs without deforming it. It comes out stronger than any other trailing edge, and is extremely light.

    Another good use for the large carbon rods is for non-structural stringers, like on a turtle deck. They are way stronger and lighter than using wood or aluminum.

    You can buy it from Aircraft Spruce, part number 03-50708. It's on page 72 of this year's catalog, but the length of that item is a misprint. It's really $153.95 for a 100' roll. The smaller diameter rods are good for making curved parts, like main gear legs. In some layups, you can use the small rods to fill the little corners between the large rods, to eliminate most of the resin weight you are concerned about.

    Generally, you have to be careful about stacking more than one layer of rods in a spar cap or other layup. The rods can delaminate internally, rather than transfer the load to the shear web from the second layer.
    Mark E. Stull
    mstull@wtxs.net

  14. #14
    Registered User Tom Nalevanko's Avatar
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    Re: Ultralight Spar

    It would be helpful if you could describe or supply a sketch of the design where you had your pullout problem. I have some info, graciously supplied to me by Charlie Rogers & Jim Barr, on best practices. Charlie was a Bell Helicopter engineer who fostered and pioneered the use of the Graphlite rods. Jim Barr built a 6 place aircraft using the rods structurally. Unfortunately the aircraft caught fire on approach and burned just after landing....Their designs always use laminating resin and a layer of eglass or carbon fiber between rows of the rods. See the example below... You can see that the rods transfer shear force to the web via the layup.
    Attached Thumbnails Attached Thumbnails Ultralight Spar-iphoto.jpg  

  15. #15
    Site Developer Jman's Avatar
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    Re: Ultralight Spar

    Just an idea from the peanut gallery. Once you place the rods into the epoxy mix could you use vibration to help settle the rods and to allow the bubbles to work their way out? Sort of like what they do to concrete to make better contact between the re-bar and wet cement and to reduce the voids. Vibration in combination with heat might help settle things in place better than dropping them in cold.

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