Mainplane is inherently unstable
As glider pitches up, C of P moves forward
and the upsetting
moment is
increased
As glider pitches down, C of P moves
backwards and the
upsetting moment is also increased
Therefore use tailplane
But note canards (elevators at the front
-Wright Bros, Saab
Viggen, Eurofighter, LongEZ), which essentially give longitudinal instability (but are stall proof)
With a tail plane more deviation in pitch means more restoring moment
= STABLE
Pitch stability is not an absolute
The degree of stability
depends on the moment/position
of the CofG