From: perkins@MSUPA.PA.MSU.EDU
Subject: Re: Combat Archery
Reply-To: perkins@MSUPA.PA.MSU.EDU
Organization: MSU Dept. of Physics & Astronomy
Date: Sat, 15 Aug 92 15:57:25 GMT

In article <1992Aug13.192355.27846@javelin.sim.es.com>, 
krogers@javelin.sim.es.com (K. Rogers) writes:
>DAVID@bmeucl.medcor.mcgill.ca writes:
>[...]
>>What we really want to get a handle on is the energy that is transferred to 
>>the arrow (or bolt).  If we could measure the velocity of the arrow when 
>>it leaves the bow, K.E. = 0.5 m v^2.  (Try equipping every archery marshall
>>in AnTir with a police radar unit. Ya Right)
>
>No radar gun needed.  The standard ballistic measuring devices utilize
>the conservation of momentum to determine velocity.  What that means
>is you fire the projectile into a block of favorite material and do
>the momentum calculations.  Simple such gizmos consist of a blob of
>putty on a pendulum; measure the swing height after firing, etc.

(note in passing: *I* wouldn't want to be the one to go out onto the
archery field to do these measurements... but on to the main point)

>Precisely why some simple yet effective measuring device is needed.
>It's not as if there aren't a bunch of techno weenies in the SCA,
>certainly on the Rialto, who couldn't rig up something cheap, easy to
>use yet sufficiently accurate.

My suggestion, which would not require anyone actually *on* the archery
range, would be to find someone with a good camera with an accurate shutter
speed (there are ways of testing this) and station him/her along or just in
front of the firing line a few dozen meters away from the archer. Set up
some markers a known distance apart along the path the arrow will take.
Snap a photo just after the arrow clears the bow (I know, I know, easier
said than done, but I imagine that with some practice it should be just
possible, and it would be easier if the archer were to release on some
external cue also available to the photographer).  Suppose the speed of the
arrow was 80 m/s (about 175 mph -- I think this is not more than a factor
of two away from realistic values).  With a 1/30 s shutter speed, the image
of the arrow on the photo should correspond to a distance (measured by
reference to the images of the known-spacing markers and then scaled up) 
2.67 m + the length of the arrow long.  A slower arrow would leave a
shorter trail; a faster arrow would leave a longer one. Depending on how
well you know the shutter speed and how well you can measure the positions
of the markers, I think that a measurement to within 10 per cent should be
easily performed. A fast strobe and a marked arrow might combine to yield
even more precision. (This should be done at a practice, as it's not really
appropriate for an event). I suspect that any ballistic measuring device
portable enough to set up on an archery range would have a hard time
beating this level of precision and accuracy unless someone wants to cough
up some big bucks for a really fancy model.

Techno-weenily yours,
 ---------------------------------------------------------------------
 Jeremy de Merstone       George J Perkins    perkins@msupa.pa.msu.edu
 North Woods, MidRealm    East Lansing, MI    perkins@msupa (Bitnet)
Newsgroups: rec.org.sca