[mannykannot]

I don't think so. Firstly, in your scenario, and with an airplane that is unconditionally stable in roll, there is no tendency for the bank to increase. As the pilot pulls back, the airplane will slow down to the target speed, the pilot will adjust the elevator to maintain that speed, and the airplane will have entered a stable fixed-radius turn, albeit slowly descending because the power is set for straight-and-level flight at that speed. But there has been no aileron input, so the roll stability will bring the wings level. If the airplane was initially trimmed for straight and level flight, and the pilot gets it back to the target speed, it will resume straight and level flight, though not on its original heading.

It doesn't work out this way in practice precisely because the airplane is not unconditionally stable in roll, and exhibits spiral divergence.

[mpweiher]

Not the planes I've flown.

[mannykannot]

Well, were the planes you have flown immune to spiral divergence? - that's the point here.

[mpweiher]

No, that's not the point at all, because none of them were left alone long enough for that ever to matter.

Just as in the case of the Graveyard Spiral.

But that's not a point I seem to be able to get across, so we can just let it rest.

[mannykannot]

You have correctly described what happens to start a graveyard spiral, and when you say "the plane's behaviour is basic aerodynamics" you are correct - but it is the aerodynamics of an airplane undergoing the onset of spiral divergence, and it is that spiral divergence, together with the pilot's failure to notice what is happening and correct appropriately for it, that leads to the increasing bank and falling nose. The bank increases despite the fact that the airplane has some static roll stability and despite the fact that the pilot has taken no action to command it.

"Three types of airplane motion can result from the interaction of yaw and roll:

1. Spiral divergence results when the static directional stability is great in comparison to the static lateral stability (dihedral effect). If a wing is lowered, the directional stability is greater than the roll stability and the aircraft will not sideslip readily. Thus, the dihedral effect is weak and the wing will not rise to the level position. The airplane tends to enter an ever-tightening spiral dive commonly called a graveyard spiral.

Flight Theory and Aerodynamics: A Practical Guide for Operational Safety (Charles E. Dole & James E. Lewis), page 274.

https://books.google.com/books?id=8DkA9ZcrcisC&lpg=PA274&ots...