Keith,
I think there is an explanation of why a flat card will sometimes tumble LE down. If the card initially stays at a low CL, and is not completely symmetrical, then the CP could be aft of the CG. The CP should continue to stay aft of the CG as long as the CL says low. Ergo, the card can pitch down.
But this should never happen it the card can stay completely symmertrical, and at a positive aoa, since the upper and lower surfaces CP's would be directly above/below each other, and at the AC, approx 25% chord. Thus any positive aoa would produce a nose up pitch moment about the CG.
Do I make any sense at all?
Octas8,
I could find nothing on longitudinal dihedral (or decalage). I'm hesitant to use the internet since often erroneous data present. It these are other terms for longitudinal stability, OK
Octas8 & Milt
Octas, I don't see how "in some circumstances" the CG can be aft of the CP but still having positive static stabilty. You included "the aircraft as a whole" which makes sense since all factors affecting CG and CP need to be included. If the CG was aft of the CP, then for even a short time, a slight upgust (increase in aoa) would rotate the aircraft about its CG, further increase aoa etc. So I'm with Milt.
Alatriste,
I'm not familiar with your terms, or any bus, however I suggest that "1.2 DN" trim does not mean that the tail is producing a nose down force, ie the tail is not producing an up force.
Octas8,
Your quote "An aircraft may still be longitudinally stable as long as the horizontal stabiliser is operating at a lower coefficient of lift than the wing. Thus, some aircraft may use nose down trim and still be stable - as long as the trim is only slightly nose down!"
I'm assuming you're suggesting that both tail and wing produce an up force. The CL producing an up force from the tail will have a moment that depends on the arm (distance to tail CP to aircraft DB). So that will factor in as well as the the arm from the wing CP to the aircraft CG.
Based on your scenario, I'd suggest just the opposite, the tail needs a higher lift/arm moment than the wing. Thay way, an upgust (increased aoa), will cause the main wing to increaes lift AND the tail to increase lift. However if the tail produces more moment, the aircraft re aligns into the airflow, hence stable.
HOWEVER, I don't think conventionally controlled/configured aircraft, including air busses, use this positive lift from the tail scenario
Comments/corrections?
Hawk