The Ballista

The only hope for these limbs is if some kind of miracle of nature gets discovered when operating at the short draw lengths these longsters require.

Shot # 33. Velocity 301 fps. Bolt flight improved. Draw length is now down to 35 3/4″ from the 38″ used to date. I tighten the starboard top washer (STW) 7 1/2 degrees and prepare for a replay at this same draw length. However, I have my doubts about any velocity bonanza. The fizzle is sounding louder.

Shot # 34. ….. delayed due to weather, lumbars, and a general lack of any of that hopey/changey stuff. Not complaining here. Just recording.

Our metaphor is wearing pretty thin too. Complicated weapon systems of dubious utility, may well have presaged the fall of Rome, but that’s not going to raise the consciousness of anyone not already out here on vanity patrol. (Okay, now I’m grousing.) What exactly is the use of history anyway? Entertainment or enlightenment? Connecting the dots or making a comfy bower of smugly cloistered archania? As this blog grinds on, a miasma of mediocrity pervades our attempts to make sense of how the ancients bent nature to their will.

Snivel, snivel, snivel…. It’s enough to make me want to cut something.

A general feeling of dissatisfation with the performance of these long limbs promps me to locate a nice sharp saw and freshly sharpened chisel. But before pulling the plug, and cutting them down to the same length as the MK VIII’s, there are a few last things I’d like to explore with these lazy old bean-poles.

These last few shots have been pulling around 4300 lbs peak draw eight. Because these limbs are 3″ longer than the previous set, that means if we had the MK VIII’s installed at this level of strain, the reading on my old chum the dynanometer would be in squeakin’ distance of 5,500 lbs.
This is way too much effort for the ride we’re getting. I suspect it will soon be time to cut them down.

But first we must try reducing the draw length on these long limbs while adding more at-rest torsion into the washers. At this current limb length, so long as I dont let the PDW stray much above 4500 lbs, the field frames and kamarion should be cool with that.

There is nothing like a bit of good news to keep the ardent catapultist enchanted with his hobby. The release of all that gripping strain, at the moment of the shot, makes for amused brain chemistry. And amused brain chemistry is exactly what’s needed for the creative process to go forward. I have found that it is best to go forward with a sense of measured abandon. Measured abandon is very important when developing dangerous implements. The measured keeps things safe-ish. The abandon is needed to surf ahead of the usual. There is always something worth wincing for if you are a true enthusiast.

I wonder what pratfalls plagued the ancients. If the effort makes you laugh deep enough, it’s almost as good as a vaction. Maybe that’s why the sage advised, “everyone should have a hobby.”

Yes, it has come to this. A filming safari out in the wilds of my back-yard. We hope to document the ellusive Waggle-tail as it appears briefly in the heart of it’s natural habitat.

(wait for it….)

Shot # 32

Click for vid: 20120609124922(1)

No velocity reading, and this waggle-tail was not nearly as spectacular as the one from yesterday. Still, I believe that with these long limbs and the 38″ draw length, the string angle is too acute for stable shooting. I’ve seen this before. Will work to verify by decreasing draw length.

Knowing that there is a limit to stability, based on string angle, should tell us how long the draw length would have been for any given limb length. Shorter limbs allow much shallower string angles, and hence more stable bolt flight at increased draw lengths. The most effective average of this limb length/ string angle ratio should suggest to us the ideal draw length for the ideal limb length. From this we might infer how long the overall machine would have been.

Here’s that snapshot of the string angle at 38″. It may occur to the reader that these tight string angles could be tolerated better if those pesky fins were just trimmed a little or moved further up the shaft. Experience shows that those plans also run quickly afoul of stability issues. A longer bolt would improve stability, but if the flight of the bolt down the deck has already been marred by clearance issues at the start of the power stroke, well clearly there is no good way to recover from that and still have full power.

At 38″ of draw, these limbs have strayed into a zone of deleteriously skinny string angles.

Our next shot will be #33 and we’ll cut the draw length down to 35 3/4″.

Shot # 31 went off without a hitch. Velocity: 331 fps. 377 gram bolt. 1415 foot pounds. Flight was a huge waggle tail though. Must investigate. 4600 lb PDW, 3400 lb SDW. 38″ DL. Balance in question.

Still, it’s early days. And that is already more zip than the MK VIII’s could easily conjure up.

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And at the pity-party, the Lumbars issue cease and desist orders.

On a scale of one to stoopid, my aching back is telling me that the last few days exertions at the catapult winch rate as ……Oh!, let me see…… STOOPID.

Ah! the fantasy of youthful endevour — short lived when trumped by errant enthusiasm and arthritis. The bones do know. As soon as I pull this floundering carcass into some semblance of order, we will lay new torsions upon the long limbs. They at least show solid, at least next to this weeping stew of rotten cartilage and cheap analgesic. No creaks and groans with MK IX’s, thank the Gods.

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Latter that morning: Heating pads, ibuprophen, and a queasy 30 second exposure to Geraldo Rivera, have driven me off the couch. Firefly needs more torsion in her life. And those pretty limbs need to be stressed like the rest of us. Complacency is not an option.

Shot # 29. This is just a check of that 277 fps baseline set previously for the long limb testing. Set-up identical as before. Velocity, 273 fps. A little lower than before, but basically the same. Time to torsion.

Shot # 30. Added 15 degrees to topside, port washer. 7 1/2 degrees to topside starboard washer. This makes the tops 15 degrees advanced over the bottoms.

(Well! get on and shoot man. It’s like your’e afraid there’ll be no miracle.)

Velocity 294 fps with 377 gram bolt. 4300 PDW, 3500 SDW, 38″ DL. The string still feels less than “perfectly” taut.

More torque! Make ’em scream Scotty!

Shot # 31.

Added 7 1/2 degrees ST, for planing control. Added 7 1/2 degrees to SB & PB.

Velocity: (Who am kidding. It’s heating pad time. Back in a bit.)

The last three postings represent an avenue of work that I don’t think really tells us a lot. It would have been a fairer test if performed with limbs that were actually trimmed to length. Cutting back down to the medium length, and essentially duplicating the MK VIII’s, minus 6 pounds of baby fat, will only be an option after we’ve squeezed out every sliver of speed from the full length limbs.

The signs from the Nickmeter are that the string tautness is clearly too slack when compared to any memory of how the MK VIII’s felt at the peak of their performance. The plan is to crank up the torsion in the washers, while going back to the long limb length. If the best the MK IX’s can do does not exceed the best the MK VIII’s could do, that will be a sad indictment for long limbs. Especially given the lighter weight of these new limbs.

But, there’s not much chance of that, right? With what I’ve got planned that 277 fps average we had with the long limbs, should be fixing to jump up a mite. Either that, or we’ve entered the Twilight Zone, and ballista limbs have started behaving with unaccountably bad manners.

More tomorrow.

In this final nock station, the distance from the center axis of the spring bundle, out to the center line of the nock groove for the bowstring, is a mere 20″.

All draw weights for the following shots were 4100 lbs peak, 3500 lbs static. Draw length is 32″. Velocities as follows:

Shot # 26 6/7/12. Couldn’t sleep. Back at it 40 minutes after previous shot, in previous posting. Velocity with this 20″ limb is an eye-drooping, 228 fps.

Shot # 27 6/7/12. 15″ APS. Velocity 225 fps.

Shot # 28 6/7/12. 15″ APS. Velocity 230 fps.

That is enough of a pattern for me.

Bugger this. Old man concedes to the machine.

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Clearly these limb shortening alterations have only served to make the velocity plummet. Whether this is because of unfavorable leverage angles, and/or, the increased tare bourne in the shorter limb lengths, is not something we can know from these tests. In that sense, she’s gone a bit sideways here.

Time to regroup.

Best not to tarry too long in one place at this stage of development. The 26″ long limbs must now give way to testing the medium length, 23″ limbs. After that we will move on to the short 20″ length.

Step one. Form an analog, qualify what it is good for, start to implement it.

In this case we seek only a rough idea of how limb length affects velocity. This is largely because it is possible to apply nock whippings in several locations down the length of the limb, and thus achieve a form of limb shortening with relatively little effort. Because the limb will not be cut to length, we will, of course, be dragging all that extra weight through the sky. If any positive results are obtained, it will be enough to know that they will only get better after the amputations are finally performed. We’re still just cuttin’ fer sign here.

The more I feel that a direction has been established for the testing, the less I feel the need to pre-judge anything. Let’s just do the test and see what happens.

Step two.

Preparing to shoot. The following photo shows the new nock whippings. That string has been moved into the medium length position.

This medium 23″ length is actually the exact same length as the previous MK VIII limbs that saw so much use in 2011. Albeit they are 2 1/2 lbs lighter, and will become lighter still if finally cut to length.

Long time readers may recall my disinclination to perform any limb shortening tests, until all was set and ready. It is wise to be the master of your own calendar. It leaves less room for distraction to set in.

Shot # 19, medium length limbs. 6/6/12. Velocity 229 fps with the same 377 gram bolt. 4,000 lb peak draw weight, 3400 lb static draw weight. 35″ draw length. That is a fair chunk of velocity loss. Shoot more. Confirm if it is the new baseline….

Note: The difference between the peak and the static draw weight is that the former is the reading seen on the dynanometer when the cocking lever is depressed enough to take the load off the winch pawl. Static, of course, is when the load in the winch ropes is held in place by that pawl. (The locking system with the toothed sideplates on the stock is not a part of this scenario.) I have not made this distinction before, and it is unfortunate because all draw weights recorded before the MK IX testing showed only the static draw weight. Previously I would have called this a 3400 lb draw weight. Still pretty low compared to the 5,000 lb static load we had early on in this project, and the 4,000 lbs limit I imposed for safety later on. This means that after this limb length testing is finished, there is probably another 7 1/2 degrees all around that needs to be applied to all four washer locations. The lack of the supreme tautness in the bowstring tells the same story. For now we are only looking for a consistent trend line relative to limb length.

Sorry for any confusion. Right now I’m just catching up on dotting my i’s and turning over all the easy, stupid stones.

For the following shots, peak draw weight is 4,000 lbs on the nose. This is the measurement seen in those previous MK IX postings where I did not distinguish between peak and static. (Something I’ve just started recording for safety’s sake.) In each case, the static weight on the following shots was 3400 lbs.

Shot # 20. 6/7/12 This shot was made in the early morning of the day following shot #19. No changes in set-up were made from yesterday. Draw weight and length the same as previous.

Velocity, 253 fps. It sounds like the springs benefited from a nice nap too. They are oddly alive like that.

Shot # 21. 6/7/12 This shot made 10 minutes after the previous shot. Velocity 247 fps.

Shot # 22. 6/7/12 This shot made 2 hrs, 14 minutes after previous shot. Velocity 250 fps.

Shiot #23. 6/7/12 This shot made 45 minutes after previous shot. Velocity 248 fps.

Shot # 24. 6/7/12 This shot made 22 minutes, APS. Velocity 244 fps.

Shot # 25. 6/7/12 This shot made 1 hr, 33 minutes, APS. Velocity 242 fps.

Well that kinda sucked.

I’m beat now. Another nap ensues.