Somebody on previous blog post's talk page was asking if the project is still alive. The answer is "yes", but life (work, family, renovating the apartment) has prevented me from working on anything in a long while. Finding time for writing a blog post has been challenging. Anyways, here's a brief update of what I probably have not blogged about yet.
During previous shooting sessions I experienced an unusually high rate of dry-fires. I believe the the problem was caused by one of the field frames being slightly tilted backwards. I corrected that by adjusting the wedges in the pi-brackets. This is actually a very good reason for having such a wedge system.
Another issue was (and still is) that the bowstring is slightly too long. Based on earli…Read more >
In the previous shooting session I used a forged quadrangular handle that was locked into a simple pin. What was missing then was the pittarion, a goal-post shaped component at the end of the slider, something that I had in my previous slider.
The lack of the pittarion did indeed make cocking the weapon significantly harder: the handle consistently jumped up to a 90 degree angle when it contacted the pin. This required me to clap down the handle at full draw, which made it more difficult to focus on feeding as much energy into the weapon as possible. So pittarion definitely did serve a useful purpose. So I decided to bring it back but with some improvements, all of which are described in the new Pittarion article. In short, the pittarion …Read more >
Yesterday I finally had shooting session with the almost completely reconstructed cheiroballistra. Here's some background information
- Springs were tuned to ~350hz (up from ~320hz)
- The distance between torsion springs was correct
- The (nylon) bowstring was significantly shorter than previously
- Anchor points for the handle were at same places as in the previous case
- Cones were now correctly 11 dactyls long
- Draw length was reduced by ~3 dactyls (~6cm)
- Stomach pressure alone was used for cocking
- A new slightly longer quadrangular handle was used
- Draw length further reduced by ~1 dactyl (~2cm)
In previous cheiroballistra incarnation cones were incorrectly 14 dactyls long. This was only possible because the little arch and ladder were too long (see above).…Read more >
In the words of the classic video game Stronghold I can finally proclaim "Ballista ready". Here it is from the top:
And detail from front at a slight angle:
I unfortunately did not have time to shoot it yet, but will in a couple of days. Even so, I've already learned a couple things.
First, current pretension level is definitely about as high as I can with nylon. Even at mere ~45 degree washer rotation rotating the arms is quite challenging. This is partly because cones are now of correct length, 11 dactyls, and partly because of higher pretension. This is both good and bad:
- The force-draw curve should be about as good as it can ever be. Torsion spring stack should minimal torsion spring stack.
- There's the risk that tuning the two t…
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Once the nylon cheiroballistra is powerful enough - hopefully in the current configuration - I will move on to comparing authentic spring materials like sinew and horsehair with nylon. My plan is to have a "mini-onager", probably in 1:2 scale, except that it probably would not have a sling like the real one. The nylon version will act as a reference point: it will tuned to store (and release) the same amount of energy as the cheiroballistra in proportion, that is, (1/2)^3 * 1/2 = 0,0625 times that of the cheiroballistra. This translates to about 6-9 joules, assuming the cheiroballistra's energy output is 100-150 joules
After calibration the other spring cord materials will be tested to see if they can produce comparable energy levels (6-9 …Read more >