Originally Posted by
Lascaille
The devices are 'self-erecting' i.e. designed to level with the horizon themselves without intervention.
What that means is they act as a gyroscopic artificial horizon in the short term and a earth-referenced 'plumb bob' horizon in the long term.
In short, if you hold a coordinated turn long enough the AH will turn towards 'wings level' as the coordinated turn produces a local 'cabin gravity' identical (in direction) to that experienced by the instrument when the aircraft is in a 'wings level' position.
Earlier I posted the same thing, that over time it should align itself with "cabin gravity" since that's the only up/down reference it knows. But after a quick bit of research I changed my mind and deleted it. Because the pendulous vanes are on the gyro case itself, i.e., the innermost part of the instrument that stays fixed in space, inside of all the airplane's free motions around it. So, let's say you're flying North and concerned with rigidity and errors while banking left or right... as far as the inner case, you shouldn't think of it as banking left or right, but rather banking West or East, because it's gonna (mostly) stay fixed in space.
If you start a right turn, the "cabin gravity" banks East (referencing the up direction of it, not down) and starts accumulating a slight bit of that error. Well, as you come through 180 degrees of turn headed South and stay in the right bank, the Cabin G is now banked West and the continued error accumulation cancels the East-banked error accumulated in the first 180 degrees. So every continued half-circle of a turn accumulates an error that cancels the previously accumulated error.
At least as far as I understand it... maybe I'll learn some more that causes me to reverse position once again!