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On Jan 26, 9:52 am, Phil J wrote:
After thinking about this question some more, it strikes me that this
situation is equivalent to a lever and fulcrum. The lever doesn't
rotate around it's CG, it rotates around the fulcrum point. In an
airplane, this point is the center of lift.

Regarding the CL moving around, I think even given that complication
the airplane would still rotate around the CL.


This might hold if the CL (or CG as some would argue) is
rigidly fixed. But in flight, we're in a rather elastic medium, and
things move around, even leaving out forward motion.
Imagine, for example, two kids on a seesaw or teeter-totter or
whatever name by which you know that playground thing. Two kids, same
distance from the pivot, same weight. The board rotates around the
pivot. The CG is at the pivot. No argument there. But suppose we had a
different mounting for that pivot, one where the pivot was suspended
by a couple of springs. Same kids, same weight, board level and kids
motionless. (Yeah, right: motionless kids.) Now I walk up to one kid
and shove down on him; where will the board *really* pivot? As i push
down, the pivot point will move down some, too, because of the mass
and inertia of the kid at the other end. Now the real point of
rotation is somewhere along the board between the pivot and the far
kid, and it'll move back toward the pivot as that kid starts to move
upward. At any instant in this process it's somewhere besides the
original CG.
We could complicate things: A heavier kid near the pivot, a light
kid at the other end, but this light kid is a little too light, so we
have a small spring pulling down under his seat, just enough to keep
the seesaw level. Just like the engine in our airplane (big kid near
the pivot), the mass of the airplane behind the CG (light kid) and the
elevator's downforce (little spring). The main pivot, still on big
springs (wing in the air) will still move downward at the instant I
shove down on the light kid and the real rotational point will be
somewhere on the big kid's side of the pivot.
Rotation about the CG works if we ignore all the other
variables. Trouble is, those variables are with us every time we fly.
We can watch an aerobatic airplane twisting around in the air,
appearing to rotate around its CG, but is it really? Can we see the
small displacement of that point (do we even know exactly where it is
just by looking at the airplane?) at the instant of any change in tail
forces or flight path?
Like I said earlier, CG is probably good enough for our puddle-
jumper purposes, but I think the guys who study advanced aerodynamics
would have something to add to it. I don't think it's really all that
simple.

Dan



In a sense it is that simple. The CG does move due to accelerations of the aircraft in flight (your spring analogy is
close), but the aircraft still rotates around the center of mass at any given moment (no pun intended!).

Dan also...d²