how does longitudinal stability change as Changes in Thrust are observed/encountered
- to amplify the 'mad one's ' post - with low slung engines, increasing thrust will raise the nose - ie it will affect the pitching
forces on the a/c and 'de-stabilse' the a/c, but not affect the actual 'longitudinal
stability' although this itself could be somehow affected by modified airflow over the tailplane. High mounted engines pitch you down with increased thrust
I think you are confusing pitching 'moments' with 'basic stability'? When the nose rises/drops with changed power, the speed will (normally) fall/increase as the a/c climbs or descends, and then the basic 'longitudinal stability' of the a/c will cause the nose to reverse its movement. Try it sometime in whatever to you are in (unless you have paying passengers, of course!
).
Edited to add: I think it would be best if 'npasque' clarified whether the second question was about pitching moments with power changes or actually about the basic 'longitudinal stability'.