Wing mast wall-buckling

Discussion in 'Sailboats' started by idkfa, Feb 23, 2014.

  1. idkfa
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    idkfa Senior Member

    I'm considering building a thin-walled wing mast for a 15ft plywood beach cat 125lbs Disp, no trapeze. Mast diameter 90mm and wall thickness approx 1-1.5mm, t/D 1.1-1.6% but there should be enough 0deg carbon to take loading (estimate). The problem is then buckling,,, will placing shear webs/panels at A and C be more effective than at B, ie. area(s) of least curvature versus most loading? tks.

    Guess an alternative is to use a foam core to thicken skin?
     

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  2. TANSL
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    TANSL Senior Member

    To combat buckling you have to consider the solution that results in a higher polar moment of inertia. From this point of view, the panels in "A" and "C"appear more effective, as the panel on "B" adds virtually no inertia.
     
  3. idkfa
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    idkfa Senior Member

    Euler buckling, second moment of area and torsional resistance at Wiki, think I understand. An I beam (in column) versus a square column. Also dividing the skin with 2 panels is better than 1. Is it fair to say that the highest load is experienced along the skin at B?
     
  4. TANSL
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    TANSL Senior Member

    Yes, analyzing Euler's formula can be deduced that it would be better to increase the thickness of the skin. The material close to the axis does not provide buckling resistance. A tubular profile is always much more effective than an I or H profile. The skin provides most of the resistance.
     
  5. philSweet
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    philSweet Senior Member

    Euler buckling is only one of about a dozen different buckling type failure modes. Euler buckling occurs when a column moves out of plumb, then stabilizes with a bow. What I got from your post is that you are wondering about different sorts of thin wall buckling. These modes have been worked out theoretically to at least an 8th order differential equation, and it still doesn't agree with testing very well on things like an empty aluminum beer can, so this is entirely a matter of art at the present time.

    it gets ugly fast

    http://shellbuckling.com/papers/classicNASAReports/2009NASA-TP-2009-215778.pdf

    http://scholar.lib.vt.edu/theses/available/etd-07102002-020043/unrestricted/Chapter-2.pdf

    http://link.springer.com/chapter/10.1007/0-306-46954-5_20
     
  6. TANSL
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    TANSL Senior Member

    The unit stress at which a long column fails by elastic instability is given by the Euler formula.
    It is a very simple way, but correct, to analyze buckling problem, especially, in my opinion, to suggest that the critical stress for buckling is a function of the radius of gyration of the column. If this is correct, we can tell idkafa than solution with panels "A" and "C" is better than "B".
    Again, a very simple analysis of the problem, but valid. Am I wrong?
     
  7. Skyak
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    Skyak Senior Member

    Buckling is understood academically, but all that goes out the window with real world off-center loading. You have a mast, which is absolutely not perfect compression. Forget buckling, it is nowhere near being the critical load condition of your mast, the main luff is a large horizontal distributed load, the stays and the boom are big loads. And what about capsize at speed? Do you want to break the mast every time that happens? You need to diagram the loads, then use that to dictate the layup. When you have designed for real loads you can check buckling and see you have a safety factor 10+
     
  8. gggGuest
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    gggGuest ...

    I must admit I should be mildly surprised if your webs, which look like a nightmare to build, were significantly superior to simply putting the extra material on a tube. I'n no mast engineer, but I don't think I've heard of webs like that in as small as mast as that.
     
  9. idkfa
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    idkfa Senior Member

    Thanks for all the replies, I'll up the thickness to 2% and hold my thumbs. :)
     
  10. gggGuest
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    gggGuest ...

    Isn't a more important consideration what your mast bend should be?
     
  11. Gary Baigent
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    Gary Baigent Senior Member

    This is a small, basic and simple mast, not necessary to go into elaborate construction techniques and theory, just build it like hundreds of others have done - which is either one I beam at thickest chord area (your B) either in foam/glass or zut alors, 3mm plywood, or a thin walled alloy tube, then attach frames at 50cm intervals, leading and trailing edge stringers, the latter for main track, and skin it. Epoxy coat everything, lay in hounds reinforced (carbon) area ... and get sailing.
    Rocket science is not required.
     
  12. Petros
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    Petros Senior Member

    in real world designs for structural elements that have to actually get built, the equations keep evolving to consider all of the various design variables that can predict when a buckling failure will occur. They still are not very good at predicting buckling failure, Euler buckling is just one of many modes of possible failure. Hence the equations keep evolving to more conservative solutions.

    On a sailboat, as with a lot of applications, damage tolerance can be your most important design consideration. Even if performing fancy numerology yields a "safe" design, a slight manufacturing defect or a nick or dent in the wrong place, will cause it to fail long before reaching critical stress. It is not very often that mast handling when trailing, set-up or take down, can be done in such a way to keep it in perfect condition at all times, let alone when using it. So extra weight and strength has to be added to it just for practical reasons.

    For this reason I would also suggest that adding more thickness to the skin would be the most beneficial to prevent mast failure.
     
  13. Skyak
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    Skyak Senior Member

    Wingmasts are all too thick to to bend. They give up bending for mast rotation to add or take out sail power. I have wondered about having a bendable luff track but I have never seen it attempted.
     
  14. Baltic Bandit

    Baltic Bandit Previous Member

    in which case KISS applies. It would would be a lot simpler - albeit a touch heavier, to use a simple center spar for load carrying and to then shroud that with an airfoil fairing. Easier to build as well:
    identity the station points on the beam
    build horizontal ribs for those station points
    carve out a female mold from EPS
    seal the foam
    Lay in the skin
    glue ribs to spar
    glue skin to ribs.

    If you look at some AL masts, that's how they have been made as well: essentially a box sleeved inside of an aero fairing.
     

  15. gggGuest
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    gggGuest ...

    Not my (first hand) experience. The difference in bend characteristics between the lengthways and sideways axis is a big part of the sail control available.
     
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