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If you look at the museum piece in the above pic and blow it up, and if the left half is a faithful reproduction of the one on the right, I am left to no other conclusion than it is assembled and displayed incorrectly.

The kam is on backwards. The longer tangs should be facing limb tips. While it might just be a trick on the eyes caused by the off angle shot, it appears if assembled as I say,it results in offsetting the rest position of the arm considerably.

Without the offset, velocities will always be less than what is possible. I fail to see how they missed that small but very important detail.

Whats weird is that when looking at your machine above, it appears you have done exactly that, putting the longer tangs toward the arm tips, yet it does not appear to achieved an offset…

Could that possibly be because of your brass inserts and limb design? From studying the stanchions on the original I came to the conclusion the arms were a fairly small diameter metal arm, possibly wrapped in easily replacable rope/leather as not to cause metal to metal contact and help cushion the blow reducing shock to the machine and lessoning the load for less kamerion resonance.

As we have previously agreed that a slack rope encourages stanchion use as a simple fulcrum with the rope bundle absorbing the impact, I would have to engineer it so that the load was distributed between the string, stanchion AND bundle. In this way no single componant bears the brunt of bringing the arms to a stop while similtaneously acting to quit residual vibrations.

While your impromptu short arm testing did not conclusively prove a benefit in velocity, it would be hard to deny it did result in an increase of two things, total mass to be accelerated, and an increase in pull for the same amount of draw length (power). Simplified, it means the machine loads a lot quicker, again providing more “low end torque” in the “velocity zone”.

Not sure if you will read this or when you will read this, but looking forward to next design mods and full power testing results.

May the CG’s be kind.

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This is useful if your machine is already operating at maximum pull capacity (5000lbs)but you still want higher velocities. If you need the same (or less) drawlength, but want extra power (heavier projectiles and or velocity)over 5000 without structural damage, the shorter limbs are the way to go.

data sampling from prototype with axle to axle spacing of 20.5 inches, six inch limbs, and no pulley or cams. The machine is at its maximum structural tolerances where a longer arm (steel rod) will bend with repeated use.

Conversely a shorter arm will allow a heavier pull without structural damage from repeated use. Increases in velocities have to found within the tolerances of material strength by altering the variables. Rather a windy way of saying shorter limbs will basically accomplish the same thing for Firefly without altering the original stanchion design or the limbs being at the 12 oclock position at rest.

Limb arc…………..draw in inches………Limb arc/draw 20……………………6″…………………. ..3.3
45……………………10…………………. ..4.5
60……………………12…………………. .5.2
90……………………15…………………. .6.0
120………………….18………………….. 6.3
135………………….20………………….. .6.75

( this post is one of mine I swiped from my RAT postings, as I destroyed most of my other experiment data on my Warhammer blog)

A basic compounding inswinger design with simple pullies. Should be a RAT first – data from simple inswinger with pulley wheels on limbs – YES a compounding design…Yes, Duncan told me they did have pullies in days of old… if only to wind machine! Surely one of them had thought of it.

Notice the change in limb arc per inch of draw compared to “basic” in-swinger design? Of course this would have reduced the pull capacity (the trade off) which could be countered by a similtaneous shortening of the limbs, the extra draw length of the compound machine would have been nullified by the shorter draw available with shorter limbs.

Lever arc……draw in inches…..Limb arc per inch of draw in degrees
15………………8……………. ………1.87
23……………..10……………. ……..2.3
45……………..12……………. ……..3.75
68…………….16……………. ……..4.25
90……………..21……………. ……..4.28
115…………….25…………….. ……..4.60
135…………..28…………….. ………4.82

At 90 degrees of rotation we have already exceeded 20 inches of draw meaning (maximum draw length) where as the basic design limbs must travel a full 130-135 degrees to accomplish the same amount of draw (35% increase in velocity potential).

The following agains drives home the point that shorter limbs can induce higher velocities. The limb length remains the same while the axle to axle spacing changes in multiples to make changes obvious or exaggerated.

The first measurement of a 13″inch axle to axle spacing allows one inch between limb tips at a 90 degree rotation – approximating Firefly’s limb length to axle spacing that leaves only the breath of the slider between them.

How does axle spacing to limb length effect such a ballista,crossbow or bow design? 6 inch levers, no pulley (causes extra length). The arc to draw is simply measuring the amount of limb rotation to cause one inch of draw,particularly useful in measuring how the velocity potential changes through out 135 degrees of limb rotation.

At 135 degrees of limb rotation 13″ spacing, the limb must rotate over 8 degrees before one inch of draw produced. At 90 degrees it only has to move 7.2 degrees to produce an inch of draw.

Since the highest possible velocity is the goal, not making use of my reproducable data is leaving out an obvious element crucial to attaining that goal.

Other than that I have maintained that a closely spaced axle to axle/limb length relationship is more condusive to rock chuckin.

The data table also suggests that a limb 1/3 that of axle spacing is in balance so that total width of machine is also the maximum draw length. Given the width at 20.5 inches and the limbs six inches, the ratio is VERY close to that of 3.3 or Pie – a number that comes up often when using optimization routines.

Spacing..Draw length@90..@135..arc to draw@ 90…….@135

13″…….12.5………..16.5…………..7.2…….8.18
15″…….13.5………..17…. ………..6.6…….7.9
18′…….15.0………..18.0. …………6.0…….7.5
24……..16………….20…. ………..5.6…….6.75
30……..18………….21…. ………..5.0…….6.43
36″…….20………….23.5…………..4.5…….5.74

**Note limb arc in degree measurements were “eyeballed” only, so a small amount of disparity creeps exists.

One data table missing is one where the arms or limbs are set up to rotate past the 12 oclock position which has already proved to provide extra draw resulting in higher velocities without higher pull.

My research into bundle tech is still ongoing but findings are encouraging so far, breaking the twisting action down into its most basic form, performing experiments first to understand and interpret results correctly, and then performance tweaking to commence on reproducable and measureable results.

I cannot blindly follow accepted doctrine and treatises blindly and without question, and will most likely end up with contraversial findings again.

Ballista freaks and builders are such a secretive lot, and I am no exception (except where Firefly is concerned of course).

Watt a lot we are eh Nick? Never satisfied, always looking to the next round of build and test build and test…

Summary of possible changes to improve overall performance, wider crossbar (possibly concave/convex shaped), string pretensioning, lighter and possibly shorter limbs,
changing rope from nylon to polypropelene (slightly less stretch). In lieu of shorter limbs, brass stanchion inserts may be removed to provide extra draw for same poundage pull, but strays from original Orsova design UNLESS curved limbs are implemented. No limbs survived the centuries so that is open to debate/experimentation.

For the life of me I cannot figure out why the Orsova limb design eliminated the double stanchion or stops. Without a “heel” to arrest movement on the backend, the job falls to the string and bundle. Also, I believe it does not assure the string comes to rest perfectly balanced or equal distance from the (keel?)slider and main beam. Thus the arrow may not correctly align in the middle when drawn making accuracy an issue at any distance.

There, enough grist to bury yuh for awhile perhaps. The wife has a weeks holidays so computer use will be limited for awhile. Again you can delete my comments if and when you get around to reading them because of auto bot spam finding its insidious way onto your blog.

W.

Previous testing producing 350fps with 7500 grains, 45 – 47 inches of draw (approx. 75 degrees limb rotation?)with 5000lbs pull, by calculation should result in 410 – 415 fps.

Introduction a small amount of pretention in the string along with dropping bolt weight to 7000 grains MAY result in attaining 420+ velocities.

I know the CG’s (and minions)on their lofty perch will be watching with rapt attention.

Should this goal not be attained on first two shots, retighten bundles or let them stand till next morn and move string to second position making arms shorter, keeping the same lengh draw boosting pull to 5300 – 5400lbs.

If you still have the audacity software (laptop?)(secondary chrongraph) be sure to position recording device mid-way between Firefly and target to eliminate noise from machine and string vibrations.

If now there remains a 394fps ceiling, I believe it is now your choice in letting it remain where it is. 425 fps will certainly help me in setting a new 450fps goal for Firefly.

Introducing you to “impact acceleration” still seems years away…

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