Originally Posted by Lemain
suninmyeyes: That is without doubt the most interesting post I've read. Why the oscillation?
The aircraft trades potential energy (altitude) for speed as it noses down and accelerates. This eventually creates additional lift, reversing the descent. The process repeats. But a glider can't do this forever and eventually comes down.
This clear illustrated example from Aerospaceweb can help explain it more fully:
Aerospaceweb.org | Ask Us - Lift, Wind & Porpoising