I'm with you up to about
:
If lateral stability is stronger than directional stability then the aircraft will roll away from the sideslip more than it yaws into it. If it then rolls beyond wings level it will sideslip in the opposite direction. The process will then be repeated, with the aircraft rapidly banking from side to side in the phenomenon of Dutch Roll.
Most commercial jet aircraft possess dihedral, sweepback and large fins. Sweepback and large fins both increase directional stability, whilst all three factors increase lateral stability. Such aircraft are generally prone to Dutch Roll which suggests that they possess a fair degree of positive spiral stability.
It doesn't quite work like that. The combination of roll, yaw and sideslip get intertwined to produce three mixed modes.
In fact one is a heavily damped mode almost entirely in yaw called "roll damping". If the aircraft is rolling and you put the ailerons neutral, it stops rolling (gradually). And that's about all there is to that one.
Another is the "Dutch roll mode. That is typically stable but underdamped, so if you induce a Dutch roll the aircraft tends to wallow in the mode for a few oscillations which then disappear. I presume unstable Dutch roll modes are a Bad Thing, as it requires an interesting combination of feet and hands to get the aircraft back to neutral. Perhaps JF has played with them in that Bassett they had rigged up for stability demos at the ETPS?
The last is the "spiral" mode, which is usually unstable as the name implies (otherwise you'd call it the "wing rock mode" wouldn't you?
). The instability means that with no control input bank gradually increases into a spiral dive. I think that it's possible to make an aircraft stable in this mode in principle, but you pay a high price in terms of handling quality, and it generally causes more trouble than it's worth.
The tendency to Dutch roll does not necessarily imply a positive stability in the spiral mode, or vice versa.
[ 24 November 2001: Message edited by: bookworm ]