Beyond making sure that obvious parameters like the draw weight and draw length are the same between our two model mangonels, there are some subtleties to this experiment that need to be considered.

If we utilize ball-bearing races between the washers and the counter-plates, that should remove enough of the erratic, frictional element to make putting a spring scale on the end of the washer tightening wrench a viable technique for getting torque readings on the pre-load tension of the torsion spring.  Thus we can garner more data on the sinew/nylon dynamic. (And then again, it may be just as valid to compare the pre-tension directly off of draw weight readings taken just as the arm starts to move off of it’s impact buffer.   This is only about comparison.   The KISS  principle is everything.)

It will be important to make suitable gauges to insure that the volume of  each of the two springs is as nearly identical to one other as possible.  Nylon and sinew have different densities,  and so weight is not a good comparative measure.   Besides, the limiting factor in fitting a spring into a field frame is clearly the volume, not the mass.

The larger we make these models the more accurate they will be as analogs for full sized machines.  The nylon powered version will be easy enough to prepare a spring for,  however, the sinew spring  leaves me feeling greedy for more elk backstraps to process. When it comes to scale: the more, the bigger, the better.

While we can make the models very accurate dimensionally to one another, and situate them securely and level for the testing, that does not necessarily prove that the initial launch angle of the projectile is the same between the two machines.   For this we will need the video camera and some kind of graduated backdrop to film against.

There is going to be a fair amount of work in making the models for this comparative analysis between sinew and nylon.  Because our approach is that of an actual bricks and mortar model shop, rather than just mathematical modelling based on a variety of assumptions about modulus of elasticity and permanent and temporary deformation etc, etc., it follows that we must be as scrupulous as possible in how these little wood and steel torsion engines are  designed and constructed. The same is true for how they are tuned and tested.

If we tune and perfect the shooting of the sinew based mangonel before finishing up the nylon spring,  this will allow us to know exactly what diameter that latter should be when it has had it’s pre-tension applied.  Proper sequencing, like this, will obviate any concerns about the  “tension/diameter shrinkage” ratio, and insure that the two different springs have equal volume.

We only aim to predict that if a competently rendered,  modern reconstruction that is powered by nylon, could shoot a given payload to a  range of X, then all other things being equal, it would likely shoot that same payload to a range of Y if powered by sinew.  Trying different values of arm rotation, and also different levels of pre-tension, should tell us a lot about the performance envelope of both materials.   That is all this about.  It is very important to define the focus at the outset.

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