Today I assembled the cheiroballistra and shot quite a few bolts through the chronograph. I could easily get to 67m/s with 30 gram bolts (66 joules) with 225 degree washer rotation and a draw of 59 cm. I tried rotating the washers 270 degrees, but the stacking at around 10cm to full draw was pretty extreme. Drawing the thing may have been physically possible given enough courage, which I lacked. Having gotten scared, I reduced the washer rotation to about 250 degrees. At this rotation I only managed to pull of one successful shot, from which I can roughly estimate velocities for two different bolt sizes:
| Bolt weight (g) | Velocity (m/s) | Energy (J) | Real shot? |
|---|---|---|---|
| 29.50 | 75.65 | 84.41 | Yes |
| 20.58 | 83.44 | 71.65 | Estimated |
| 10.23 | 96.53 | 47.66 | Estimated |
The estimated velocities are based on my earlier results.
In any case, I learned several good lessons today:
- The shims and wedges keep the little ladder entirely rigid. There was absolutely no loosening of the wedges this time.
- The pins in the little arch work similarly well. There we zero issues with them.
- The new elliptically tillered arms worked well. There was no movement of the arms in the springs. The new arms bend visibly during pullback, but much smoother than the linearly tillered arms did. There seems to be small amount of permanent set in the arms, so the arms are clearly under considerable stress during pullback.
- Filing shallow grooves for the hoops in the cones seemed to have paid off: there was no hoop movement whatsoever, and the arms did not violently explode, either.
- Increasing draw beyond full minus 14 cm does not increase velocity. I've noticed this phenomenon earlier, and it seems odd. This has to be verified with lower power tests.
All this said, today was not without its complications. It seems that the little ladder has twisted slightly, so that the bowstring points upwards at rest:
Speaking of fingers, it's good that I still have them. The little ladder twist seemed to produce erratic bowstring movement, which apparently got worse as power levels were increased. This lead to my trusty 30 gram bolt to breaking, and the 20 gram bolt to completely disintegrating:
The complete disintegration was rather scary, because two pieces of the shrapnel actually rebounded from the target cushions and hit me. While I always take the precaution to wear a helmet and shooting glasses, increasing power levels starts to call for heavy duty protection. I'm seriously considering buying a chainmail hauberk, a gambeson and a chainmail hood. I will probably also attach a stand to the ballista so that I can:
- Winch the thing (from behind a shield) instead of pulling it from up close
- Fire it "remotely" using a cord
When all the parts are proven to work I can continue my stomach cocking adventures again.
The next step in quest for more joules are:
- Restretch the spring cords. Current springs were tuned to 250 hz, which is probably too little. I'll try 350 hz this time and see how it affects performance. At least the springs do not have to be twisted nearly as much, which should result in a lot smoother draw, less stack and hopefully more stored energy.
- Change the rather thin 4 mm little ladder steel beams to ones that are 6 mm thick. The basic structure no longer needs any modifications as far as I can see.
- Cut off the part of the slider that's under the locking mechanism, recreate it from elm and attach the two pieces together. Elm's resistance to splitting is better than that of birch, so it should be safe.