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Thread: Calling All Shade Tree Physicists

  1. #1
    85gr Konrad's Avatar
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    Default Calling All Shade Tree Physicists

    Calling all Shade Tree Physicists


    I was lying in bed one morning not too long ago and thinking about arrows. OK, all you folks about to make snide remarks about poking things and other suggestive comments, here is the place to get it out of your systems.

    At any rate, I was thinking about arrows, the physics involved in impact drivers and energy transfer pulse/time intervals…OK, you can make a couple of more jokes here too. You won’t be the first (or last) to make fun of me…and I began to wonder about an experiment with the following conditions:

    1. With a “standard” arrow weight of 500 grains striking a target at a “standard” velocity of 250 fps obtain an energy transfer pulse length as a “base line”. In other words, obtain a measurement of the length of time the arrow actually delivers its energy to the target.
    2. Then, as a starting point, construct an arrow with a free floating, internal weight, occupying the rear one third of the inside of the arrow weighing 50 grains as a starting point.
    3. The Forward of Center (FOC) of the rear-weighted shaft would be 15% FOC.
    4. Then, the test arrow would be shot into the target at the same initial velocity of 250 fps and the energy transfer impulse time would be measured again.

    My belief is that the weighted arrow’s transfer time would occur over a more prolonged period than that of the un-weighted shaft. If so, the effect would be similar to an impact driver, assisting in penetration due to the act of the internal weight transferring its energy to the tip of the arrow. In effect hitting a “double tap” upon impact.

    My question is: Should I spend valuable sack time in more productive pursuits or might I be onto something?

    Thanks,
    K
    “The rifle...may be used by evil men for evil purposes, while the(y) can not be persuaded to...righteousness by propaganda, they can be corrected by good men with rifles.”

  2. #2
    145gr Big Boar's Avatar
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    Actually I had similar thoughts several years ago and was thinking more along the lines of a spring loaded center punch. As you probably know if you try to center punch a piece of metal by just pressing on a hardened tip you will not make much of a mark if any. However with a spring loaded punch you keep pushing against a spring until it unloads and the very short impact in time will put a large dent into the metal. So I think having a very short time impulse is better for bones and such than a very long time impulse. What kind on stopped me is the thought of putting this large weight on the back of the arrow and then having to counter balance it with at least the same amount of weight on the front in order to keep the FOC the same.

    So what have I actually done? I have created a much heavier arrow which will start out having more KE to start with. So have I actually improved anything since we all know that an arrow with more weight will penetrate more anyway?

  3. #3
    85gr GRIMWALD REAPER's Avatar
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    Recently Firenock released a notice concerning some of the faster, harder hitting crossbows and the effects they were having on his arrows.
    Long story short, the impact of these faster bows, creating more Kinetic energy, more momentum was to cause the Firenock batteries to blow through the extreme shock battery ends, down the shafts and impacting the broadhead inserts. The result at a minimum was to dislodge the insert and broadhead from the shaft and in some cases the end of the arrow shattered destroying the arrow.
    While this is not an apple to apples comparison it should lead you be careful in how much energy you wish to transmit down the shaft.

    GRIM

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    85gr Konrad's Avatar
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    Quote Originally Posted by GRIMWALD REAPER View Post
    Recently Firenock released a notice concerning some of the faster, harder hitting crossbows and the effects they were having on his arrows.
    Long story short, the impact of these faster bows, creating more Kinetic energy, more momentum was to cause the Firenock batteries to blow through the extreme shock battery ends, down the shafts and impacting the broadhead inserts. The result at a minimum was to dislodge the insert and broadhead from the shaft and in some cases the end of the arrow shattered destroying the arrow.
    While this is not an apple to apples comparison it should lead you be careful in how much energy you wish to transmit down the shaft.

    GRIM
    Interesting!
    This example shows the energy contained in just that small amount of mass. Were the insert better adhered to the shaft and the shaft able to withstand the generated impact, I have no doubt additional energy retained by the mass of the battery would have been converted to forward momentum.

    The weight I am going to use in testing is shot shell buffering media.
    It is non-hygroscopic, light spherical plastic, free-flowing and does not stick to anything.

    While it will easily be moved to the rear of the shaft, as soon as deceleration due to target contact is encountered, it will begin moving toward the front in one extended (hopefully) “pulse” without the crash associated with your battery example.

    Good feedback, thanks!
    “The rifle...may be used by evil men for evil purposes, while the(y) can not be persuaded to...righteousness by propaganda, they can be corrected by good men with rifles.”

  5. #5
    85gr GRIMWALD REAPER's Avatar
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    It should also be noted that the resistance of a target is much different than the body mass of a Deer.
    Obviously moving up to larger game will increase the resistance, but it is still less than a standard target.
    It should also be noted that he didn't really start having issues until the arrows Began to travel in excess of 350 fps.
    My point was simply a reminder to you to be careful.

    GRIM

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    85gr Konrad's Avatar
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    Yes, you are correct regarding the differences in target types and about keeping safety paramount in mind. I have no desire to perforate or otherwise deface my engaging countenance.

    My primary issue (as I have already decided to proceed with experimentation) is the manner in which quantitative data is to be collected. I have imagined a crude method would be the use of ballistic gelatin. First shooting a “solid” arrow into the medium and then shooting into a fresh block with the movable weight arrow, both tipped with the Saunders Combo-point. One sees the use of ballistic gelatin in firearms testing as a standard media for projectile/load experimentation. It would seem to me that the same media should give excellent results at least giving an “apples to apples” comparison.

    The mathematical equations could be backed out afterwards.
    “The rifle...may be used by evil men for evil purposes, while the(y) can not be persuaded to...righteousness by propaganda, they can be corrected by good men with rifles.”

  7. #7
    145gr Big Boar's Avatar
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    Quote Originally Posted by Konrad View Post
    Yes, you are correct regarding the differences in target types and about keeping safety paramount in mind. I have no desire to perforate or otherwise deface my engaging countenance.

    My primary issue (as I have already decided to proceed with experimentation) is the manner in which quantitative data is to be collected. I have imagined a crude method would be the use of ballistic gelatin. First shooting a “solid” arrow into the medium and then shooting into a fresh block with the movable weight arrow, both tipped with the Saunders Combo-point. One sees the use of ballistic gelatin in firearms testing as a standard media for projectile/load experimentation. It would seem to me that the same media should give excellent results at least giving an “apples to apples” comparison.

    The mathematical equations could be backed out afterwards.
    If you do this, I think you must still make the total weight of both arrows the same. The only difference should be movable versus stationary weight. If you just add more weight to an existing arrow and then compare it to the original arrow, the difference in KE will cloud the results.

    I think what you might find is that the results will be like trying to drive a nail with a dead blow hammer.

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  9. #9
    85gr Konrad's Avatar
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    Well, waddaya know!
    Thanks,
    K


    Quote Originally Posted by Archer View Post
    “The rifle...may be used by evil men for evil purposes, while the(y) can not be persuaded to...righteousness by propaganda, they can be corrected by good men with rifles.”

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