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    Pippi Kortborst (proof of concept model)

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    hullutiedemies
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    Pippi Kortborst (proof of concept model)

    Post by hullutiedemies on Fri Jan 31, 2014 7:23 am

    In medieval art European crossbows are typically pictured as braced to middle the bolt track.




    (above images from Ethnographic Arms & Armour forum at
    http://www.vikingsword.com/vb/showthread.php?p=164322#post164322 )

    I made a test model.

    This one pulls 40kg/115mm (~90#/5") (*)
    Matchbox shows the scale.
    Zero elevation trajectory tests from half meter height throw
    7g bolt 22m and
    2g bolt 32m
    Aiming directly at eye level target from 23m, 2g bolt hits 20cm low.
    This suggest speeds over 65m/s for 7g
    and around 100m/s for 2g bolt.
    Given 20J energy capacity this puts the virtual mass of the prod around 2grams. Prod mass is ca. 100grams.

    So the virtual mass is 1:50 of bow mass, about twice as good as that of a typical longbow.

    And as can be seen, the bow is rather crude, so there is room for improvement. - Was not the medieval rule of thumb "bolt weighting 1/40 or 1/50th of prod", suggesting that the bows back then were notably better still.

    The model above is about 1:2 scale, so a full size crossbow of similar design should be 8 times as poverful.

    So a even a simple crossbow of true medieval design can throw bolt 300fps with 60% efficiency - or better.
    This explains why long drawing ballistas were phased out and why compound bows were never developed in Europe even though the concept was understood.

    This also explains why crossbows were bothered at all.
    When properly set up, the ballistics are truly superior to hand bows. So the extra cost , bulk and clumsiness was worth having.


    (*) also notice this is an economical way to build a heavy weight wooden bow.

    kenh
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    Re: Pippi Kortborst (proof of concept model)

    Post by kenh on Fri Jan 31, 2014 9:08 am

    Pretty darn impressive for a couple sticks and some string!

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    Re: Pippi Kortborst (proof of concept model)

    Post by Anatine Duo on Sun Feb 02, 2014 10:22 am

    Awesome!  Thanks for helping de-mystify the high brace/ deflex on traditional crossbows.  Some people put a lot of emphasis on energy but flat trajectories are quite valuable too.

    Do you suppose that scaling the prod up will be problematic?

    kenh
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    Re: Pippi Kortborst (proof of concept model)

    Post by kenh on Sun Feb 02, 2014 2:26 pm

    I would not think scaling up the prod would be a problem.  The ancient Chinese made 200# or more prods made from several slats of bamboo held together with twine or leather bands.

    hullutiedemies
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    Re: Pippi Kortborst (proof of concept model)

    Post by hullutiedemies on Mon Feb 10, 2014 12:04 pm

    This thing worked so well that I decided to fi... swedish it




    It is a Skåne bow after all. (though Mora style is more Dalar than Scanian, but whatever )

    Prod is lilac, two small parallel tillered bows steam bend to allmost full deflex.
    Longer stick is 45 cm long, 15mm thick from middle tapering to 10x10mm tips, shorter 40cm long 12mm wide, same thickness- a small 9mm stick is stiffening mid section.
    Since working limbs are parallel the bow could in fact just as well be solid wood.

    So scaled up to 2:1 that would be 90cm long 60x30mm center, 50x22mm mid limb selfbow with 70 cm string.
    Pulling somewhere about 350#9" , mounted on tiller with 45cm bolt track.

    As for the energy storage, this approach allows a thick limbed bow with very high draw weight survive reasonably long power stroke. So what it looses in "cast per pound" it wins in simply having much more pounds in given space.

    kenh
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    Re: Pippi Kortborst (proof of concept model)

    Post by kenh on Mon Feb 10, 2014 2:59 pm

    Love the 'Swedish' paint job!  Nice compact design.  Sooo much nicer than any of the "crossbow pistols" with approx the same width and draw weight! cheers  You are the king

    hullutiedemies
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    Re: Pippi Kortborst (proof of concept model)

    Post by hullutiedemies on Sun Sep 21, 2014 4:16 am

    Just an update.
    After last winter I left this strung and hanged it away.
    Yesterday I shot couple bolts and tested the weight:
    It is still pulling the original 42 kg.
    So one more advantage from high deflex. Wooden bow can be left permanently strung without penalty.

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    Re: Pippi Kortborst (proof of concept model)

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