The limbs seen here are what might be termed “proof test” specimens.  They are built extra stout so we can explore the maximum draw weight possible with these springs (around 5500 pounds).  Our best efforts so far have yielded a consistent 330 feet per second and 1800 foot pounds of projectile energy when firing steel tipped bolts.  Further increases in performance will be inevitable if the limbs can be lightened to a significant degree.  Our old Gallwey ballista shot the very lightweight 1/2 pound bolts 700  plus yards.  Firefly should top 1000 yards when we get the new limbs installed.   It will be interesting to see what she can do with heavier projectiles.

One Response to “And then there were two.”

  1. Warhammer says:

    Lightening the arms will only do so much for velocity. You will need to shorten them to offset the drawlength gain unless you have added extra travel to trigger.

    5000lbs and 7500 grains is exactly 1.5 grains per lbs of pull. Through testing you know that bolt weight greatly affects the velocity with this set up…

    Now consider the experiment with the shorter arms (moved attachment position. Its not hard to see that in addition to a 6500 grain glandes, there was a couple of lbs of uneccessary weight per arm in the six inches or so of wood and metal in front of the string.

    Despite this fact this probly tripled the total mass the arms had to accelerate, the total loss was how much? two or three fps. Aside from the bolt, there was probly four lbs of limbtip they were accelerating.

    When you shortened the arms a couple of inches, you assigned more speed (rate of retraction per degree of rotation).

    I have no doubt that without four lbs of uneccessary weight the shorter arms would have proved out with the higher velocities, adding to the 22% gain of the new limb positioning.

    My prediction is that the new positioning will allow your 400fps to occur at roughly 75 total degrees of rotation or less if freshly tightened and tuned.

    I’d sure be interested to see what a full 120 – 125 degrees of rotation would do for velocity on your machine. I suspect you would need to back off on the bundle tension at rest to accomplish this however, to still keep it at 5000 – 5500 lbs draw weight.

    With the new arm position, ideal release point IMHO is another 15-20 degrees past the maximum leverage point (where stacking begins). As you winch the string in, there will come a point where the arms and the string form a 90 degree angle to each other. That would be the max. leverage point where the tension I would be set at 5000lbs the extra 15 degrees allowing the arm to accelerate before reaching its peak leverage point. At 15 degrees past the max leverage point is where the max draw weight of 5500 would be achieved.

    This simple adjustment allows the machine a huge boost in available torque throughout the last 55 degrees where approx 65% of the total draw occurs.

    With the old arm positioning (12 oclock position)The first 35 degrees of rotation provided 50% of total draw (string has to double speed). That measly 15 extra degrees of rotation then translates in a 22% in velocity by extending that acceleration for an extra millisecond.

    Ask Duncan to send you a photo copy of VG Harts paper as the rate of change in acceleration and angular velocity is plotted out. There are some interesting conclusions one can make with his pure research. I wish he would have plotted the spring friction which accounts for 72% of availble energies.

    Initial missile kinetic energy at launch amounted to only 38%, friction gobbling up the rest. Still, it was 6% higher than the outswinger used in comparison.

    As my work involves maximizing the effeciency of the design, it appears I have managed to have increased that a fair bit!

    Given that the experiment with the shorter limbs experienced no real negligible loss in velocity despite tripling the mass, its fair to say the shorter limb is the more effecient in transfering energy to the projectile than the longer one. Considerably.

    Lastly,the above picture shows the positioning of the string when adjusted to make it “shorter”. Just me perhaps but that six or seven inches of wood in front of it (string position)seems to have considerable mass. Especially if it contains a layer or two of steel for added strength?

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