As promised, I managed to get some video of a bolt with a shaft diameter sized head on it (i.e. no up-pitch created by a larger than shaft sized head on it), as it is being fired.  We are testing to see if that excessive gap between the string and the deck is causing the bolt to pitch downwards coming out of the machine.  This is important because I do not want to be restricted to only shooting bolts with oversize heads.

Here is the 4X slow motion video,  20111104142657

And breaking down the action into stills, we see the following:

The second photo in this sequence clearly shows the bolt with its tail raised up higher than its head relative to the flight deck.   This will never do if we are to test something already as  naturally down-pitched as a Dura style bolt.

It looks like Firefly will need to go into surgery for a few days as I correct the angle of the stock and flight deck relative to the vertical axis of the field frames.

4 Responses to “Fat heads not allowed.”

  1. Randi Richert says:

    One of the central points of the timeline and design theory I’m working on is that the string should actually ride/slide directly on the syrinx. The plane of rotation of the arms should be below the rail. That is how Marsden depicted the string on the gastraphetes. Wilkins says in his book that it is axiomatic that the aperture on wood-framers is in the dead center, but that’s not what the text and artifacts show. By keeping a slight downpressure on the string it is prevented from lifting the rear of the bolt (as your stills indicate) or over-riding it entirely.

  2. Nick Watts says:

    Groovy, man. I was hoping someone would write in and confirm that I’m on the right track here.
    I have a good plan to shift things around a bit to produce a bit of down pressure.

    If I build the MK II Phoenix, you can bet it will have some dedicated adjustment and control over these parameters.

  3. Nick Watts says:

    I have pondered this term “axiomatic” and find that while it may be appealing to think that a full power two armed torsion engine can be held in tune well enough to maintain a dead center deployment of the string, the practical challenge is to make sure that the string is biased downward to insure that it does not move into the highly unfavorable position of lifting the tail of the bolt. It is one of those practical vs. academic conundrums. The real axiom is, only results count.

  4. Randi Richert says:

    Wilkins cites this supposed axiom in the context of the wooden framers. He uses it to justify the invention of a “missing” panel that he re-invents for the Cremona find to make it conform to the rigid ideal formulaic dimensions. In fact, none of the frames yet found does. Admittedly there are very few data points to use, but the trend shows that full-sized wooden frames got shorter (undersprung) and were progressively lower in relation to the syrinx (underslung). This has other implications. The main one is that the arms, draw-length, and subsequently the case would get longer too. Conducting a wire-frame analysis of the Cupid Gem highlighted these changes and led me to believe that it depicts a very late-stage scorpion. With the advent of iron-framers there was no reason to lift the string.
    Applying the underslinging concept to Heron’s Cheiroballistra helps solve one of the lingering debate points in the dueling articles that Aitor and Alan published in JRMES. Wilkins uses the projection on the bottom of the case as a socket for the backstay for his imaginary base. Aitor put it up front, placed the frame behind it, and uses it as some sort of handgrip/forearam. It seems far more logical to me that its sole purpose was to ensure that the the frame was properly undersprung.
    In any case, making the string stay put on the rail should make it easier to focus on getting the pull of the arms balanced to remove yaw.

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