Spinning a Grob, right-side up, has been a subject of some concern.
Supposedly, most of the time they behave "normally" but "1 time out of
a 100" they will spin flat and be very difficult to impossible to
recover. The story, as I heard it from my aerobatic instructor, is
that Les Horvath was giving spin instruction for the umteenth time in
a Grob and was not wearing a parachute. Normal spin entry that
transitioned flatter and became refractive to all attempts at
recovery. He is quoted as saying he would have exited the glider (a
tandem seat configuration) if he had a chute on but instead they
unbuckled their straps and climbed onto the glareshields over the
panels in order to shift the CG farther forward. Although this
maneuver is not advocated in the owners manual, it apparently was
successful in aiding the spin recovery.
One of the explanations that has been given to me for caution in
spinning a Grob is the offset rudder hinge. It has more rudder travel
in one direction than the other. So (I don't remember which) it is
easier to enter a spin from one direction but less opposite rudder
travel available for recovery. Likewise, it is more difficult to
enter a spin from the other direction but easier to recover. For
reasons I don't understand this does not seem to be a concern during
inverted spins... I avoid spinning a Grob. Gene (SZD-59)