Aerodynamic question for you engineers

Tina wrote in news:c1b2a8de-4715-4731-9148-
:

Let's talk about something a little more simple. Consider a horizontal
loop, let's call it a coordinated turn. Let's be even more specific,
and talk about the pilot's frame of reference, and make it a turn
around a pylon.

Where is the center of rotation of the airplane.


CG


What, if the pilot
is skilled, will appear to be unmoving when SHE is flying the turn?
That is the center of rotation in the pilot's frame of reference.


Don't like the coordinated turn? Fly a loop around a little cloud
(question -- what would the loop diameter have to be for it to be
legal?). The pilot would be looking 'up' and trying to keep that cloud
in a fixed position -- SHE is turning around it, it is the center of
rotation.

Let's not resort to that old idea, where if one's idea is wrong the
solution is to shout.

Shout? I don't do that.
I'm proven wrogn al the time.



Here's an even more simple minded, but harder to do in real life,
idea, but it makes the point. Consider an airplane with a sturdy 10
foot mast extending up from its center of gravity, and in straight and
level flight, deply a big airbrake from the top of the mast (think if
it as a big off center thruster). Make it big enough so that when it
opens the top of the mast is stopped, right there. Where is the
center of rotation? If you can accept that the center of rotation is
at the top of the mast, then you have to accept that smaller forces,
deployed not thru the center of the gravity will also cause rotation
about a point not at the center of gravity. Think smaller and smaller
airbrakes, closer to the center of gravity., There is no discontinuity
in the Newtonian physics governing the motions, so the center of
rotation will move closer to the cg, but never get there!


Nope, the center of rotation is at the CG still. The airplane may
describe an arc at the smae time, but it's rotating about it's CG.

I am very sure you are a much better pilot than I am, and if it helps
you to be better by thinking all rotations are about the center of
gravity of the airplane, by all means do so. You don't have to
understand the physics to be a good pilot (unless you're really
messing with the far edges of the envelope).

I never said that all rotations ar about the CG. I said the airplane
rotates around it's CG.
In a spin, the airplane is rotating around it's CG. That does not mean
that the CG is describing a vertical line. It isn't.

But the airplane is rotating around it's mass. It can't do anything
else.


Bertie