The 4700 lb. Warn electric winch showed up yesterday.  I had sworn I had dug my last ballista bolt out of the catalpa log, and would wait for a bale of compressed newspaper to use as a target.  As it was, it proved impossible not to give the new winch a try and so after today’s testing my supply of the heavy old bolts for the Gallwey are pretty much gone.   Just as well too.  I am eager to try the new Dura Europos style bolts as soon as I get some made up this weekend.  However, we did get off three 3 decent shots today.  They tabulate as below.

Bolt weight            Velocity            Energy           Draw length       Draw lbs.

First shot              5015 grains           249.1 fps         690.85 ft/lbs             47″                  2600 lb.

Second shot          5015 grains           244.3 fps         664.48 ft/lbs             47″                  2600 lb.

Third shot             5015 grains           244.0 fps         662.85 ft/lbs             53″                  3050 lb.

It became apparent after the third shot that the bowstring had stretched out a fair amount.  (Click for video,  stretch).  That would explain the drastic drop in relative power of that last shot, given that it was drawn back 6″ further than the first two.   Probably the expedient bowlines I had used to form the loops had crept a tad.  We will try it next time with timber hitches for the loops.  A locked splice, as was used on the Gallwey, might be best, but for now we are just getting a feel for how the machine performs under load.  There are many tuning parameters to be explored, not the least of which will be to tighten the bundles again.   If this machine is anything like the Gallwey, there  will be several cycles of shooting and tightening before the stretch in the bundles is stabilized.  With the Gallwey, the bundles finally achieved a state of equilbrium, and no further tightening was needed for the last 100 shots or so.  It is still a question as to whether  the extra rotation of an inswinger will cause problems with spring memory, and thus affect consistent velocities.

The following video is rendered in slow motion.  The sine wave that forms in the bowstring as it snaps out at the end of the shot is an interesting phenomenon.  Is this because the string is too loose?  Or is this simply a characteristic of an inswinger?  Just more stuff to investigate.

Thanks again to Scott Morrison for helping with today’s tests.  Click here for that slomo video,  mvi_2059

One Response to “Mid range testing.”

  1. Warhammer says:

    The sine wave is most likely formed either because your string is too slack or that you are shooting with too light an arrow.

    I dont work with rope bundles but do work with inswingers. A lot of the problem I believe you can eliminate by tightening the string a tad (tension). The other way is avoid using the lever arch much past the 90 degrees.

    The important angles with your particular machine is the string to lever relationship. At rest, it is at a 90 degree angle providing maximum leverage. As you draw and load the machine, the string angle to the limb again will be at 90 degrees where you again can excert maximum leverage.

    Generally this should happen before the levers have travelled a full 90 degrees. When the limbs are exactly horizontally opposed and at their closest together, for an arrow shooter, this is perhaps the farthest you want to go with such a light (2885 grain dura bolt). When released from this point, the arms travel in opposite directions (away from each other)so that no “jerk” is encountered.

    Once the limbs travel past 90 degrees, they take up “slack” which is then fed back into the stroke upon release.

    From the last 23 slot in your machine, probly 120 degrees? The leaves 40 degrees where the limbs when released accelerate towards each other, feed the slack into this movement.

    The machines energies are used in accelerating the limbs rather than the string and arrow by feeding slack or releasing string into the pull. The limbs are accelerating toward its other at the apogee. As the limbs come back to where they face each other again, and where the string is at its slackest position.

    NOW, here is where your strings trouble begins. In total the amount of distance it has covered is very small. At lower power you mind notice a sligh “lull” here. This is where the rapidly moving limbs accelerate the string instantly, JERKING it at the same time the limbs CHANGE DIRECTION and accerlerate OutWARDLY.

    The low power and slower shots allow you to observe this phenom. In later shots where everthing is tuned up, tightened up etc, would only serve to multiple the effect. ONe reason why you might now get more speed is that your arrow when jerked is the last contact with the string.

    In simplest terms lever arc past 90 degrees can be expressed as a negative, as it only serves to soak up draw.
    When the limbs pass a certain point it becomes positive and real movement is assigned to the arrow and string.

    If you ever get a slower camera watch it frame by frame. The string must wait for the limbs to once again provide pull and straighten it out.
    The last 45 degrees of movement is where the string speed (rate of change for acceleration doubles). Unfortunately while I cannot be sure as it happens too fast, your arrow has already departed by that time travelling just ahead of the string.

    Just me and my ramblings of course, but since you are going back to the thinner scalloped arms, first try a few shots using your new arms, but with a 10,000grain arrow.

    There also seems to be a second “jerking” action starting tghe last 40 degrees, and again a momentary lull in levers seem to occur as “turbo” kicks in. Again Im pretty sure your 2885 grain dura-bolt arrow has already departed.

    I really have to admire your build and the amount of punishment it takes. With 3600 lbs pull and just 2885 grains, that works out to 1.24 grains per lbs of pull.

    With your new(old) scalloped limb design, please consider reducing the length of each by a mere six inches. This way you can still get 3600lbs pull but have less stress and component strain as well. By decreasing the limb length you add to string length, which the main component you want to accelerate – not the limbs.

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