The mystery of the power loss seems to be solved.  Several more shots quickly revealed that the bolts were not only dropping an extra foot, but they had also started  to strike to the right by maybe a foot or so.  A sudden lateral divergence in the strike of a two armed torsion catapult is nearly always the result of one of the bundles suddenly depowering.  In this case the starboard bundle  needed another 7 1/2 degrees of rotation induced into both its top and bottom washers to restore the balance.  It seems that things are going along crackingly fine,  and then, out of the blue, some fatigue point in the spring is suddenly reached, which causes some of the fibers to take a permanent set, and thus lose some of the preload.   I have seen this pattern before, and recognize that rather than heralding a catastrophic failure of the spring as a whole, that it is in fact, one of several cycles of laxation and tightening that push the spring towards a state of equilibrium, relative to its chore.  It is all just part of the tuning process.  What is tight must loosen,  and what is loose must tighten.

In the photo  below I have numbered the last four shots since our work from the other day.  The point of aim in all four shots is the middle bulls eye in the left hand column.

In shots one and two, the weakened starboard bundle is allowing the tighter port bundle to kick the bolt out of the machine in a rightward direction.  They are also grouping pretty poorly.  Shots numbers three and four were made after inducing 7 1/2 degrees of rotation into the top and bottom washers, starboard side.  This is the minimum amount the hole pattern in our washers and vernier plates will allow.  The fact that this adjustment has created the start of a group (3 & 4), and moved that group a good foot to the left of the previous impact points (1 & 2), is an indication of the effect that 7 1/2 degrees of washer rotation can have on the lateral dispersion of the shots.   I wouldn’t want any less than 7 1/2 degrees of rotational discrimination in attempting to balance one of these machines.   I suspect the ancients would have been similarly inclined.

The shot in this next video is perhaps 2″ directly to the right of its intended victim,  (the apple on top of the pumpkin).   As such it is really just a duplicate of the previous shot, number four on the cardboard. A slight sight adjustment should put our next attempt spot on.    The video shows a one pound bolt flying along at an indeterminate speed,  probably about 275 fps.    I don’t have my chronograph turned on because it imposes a time limit for making the shot.  This is not at all the thing for accuracy work.  Click for vid.   20111026155201(1)

While William and Son would have been underwhelmed by missing the apple like this, I am thrilled to see the machine responding with such sensitivity to these tuning adjustments.

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