Robert Ehrlich wrote:
...
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.

Correcting myself: what favors flattening the spin is rather the difference
of the moments of inertia around the 2 other axis (roll and yaw). Practically
what has influence on both (moment around pitch axis or difference between
moments around the 2 other axis) is the same thing, i.e. weights in the
fuselage far from CG, e.g. tail repair balanced by nose ballast.