Bob wrote:

Clearly there must have been some small difference
(ailerons not quite central, or some such) that made
an important difference. Whatever the cause, I couldn't
get it to repeat, all subsequent spins were unremarkable.

Rogue spins can happen, IN ANY TYPE.


Could it be that a spinning glider is governed by laws that are
not well described by more traditional linear equations of
aerodynamics (or JAR-22)? I am reminded of some articles related to the
loss of ships at sea to "Rogue Waves" or "Freak Waves".
http://members.shaw.ca/diesel-duck/l...ogue_waves.htm

Ships designed to the standard engineering models of expected maximum
wave heights for the worst predicted conditions were being lost and
those describing witnessing these waves at sea were dismissed as
crackpots until only very recently.

Now, application of a non-linear model i.e. chaos
theory, seems to be describing these freak ocean waves more accurately,
where there very existance was doubted until jut a few years ago. Your
description of a very small variation in the initial conditions,
resulting in a very much different situation describes an outcome that
might be predicted by chaos theory better than some of these other
explanations. The non-linear SchrÖdinger equation was originally
developed in the field of quantum mechanics but is now being applied to
modeling freack ocean waves. Could we be dealing with a "quantum"
phenomenon when dealing with a spinning glider where it behaves nicely
according to our traditional model most of the time, but every once in a
while it produces a "Freak Spin" do to the complex interaction of all
the forces involved?

Maybe you don't need to invoke such complex things like chaos. Things
which are not taken in account by JAR-22 and usual procedure for weight and
balance, as somebody pointed it, are the moments of inertia around the
3 axis of the glider. Of special importance is the moment of inertia around
the pitch axis, a higher inertia around this axis favors flattening the
spin. And 2 gliders may have exactly the same weight and same CG postion
with different such moments.