[mpweiher]

> airplane to undergo spiral divergence

Hmm...I don't see the need for anything of the sort. The graveyard spiral can be achieved purely due to erroneous pilot inputs, the plane's behaviour is basic aerodynamics:

- you lose lift because the wings are at an angle. Nothing you can do about that relationship.

- the tightening of the spiral is also due to basic aerodynamics/geometry: once you are banked, the lift from the aerodynamic surfaces has a horizontal component in addition to a vertical component. You increase the lift from the surfaces by increasing the angle of attack, you get additional force in the horizontal component. Of course you also get vertical component, so in a normal turn this is fine.

Since there is continuous pilot input, even if the plane were stable in such a fashion as to automatically try to revert to straight and level (which most planes don't, you have to explicitly command exit from a turn), that wouldn't help you in a graveyard spiral.

Now the plane doing this by itself due to instability is an additional problem, sure, but it's not a necessary condition.

[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...