Re: spins from coordinated flight

> To define turn using axis reference is not the best way to explain turn
since once established in a stable turn there should be no movement on the
airplane's axis system.

I am not sure I'm getting this.

We'll ignore translation (straight line motion).

The axis system is (as I have been using it) fixed to the aircraft, and
the axis system moves whenever the aircraft rotates (around any point). 
It has nothing to do with the actual earth's horizon.  

Then, as I understand it, rotation of the aircraft involves a rotation
around one or more of the axes.  Yaw involves rotation (=of= the
longitudinal axis, and thus the airplane) =about= the (airplane) vertical
axis, =in= the plane (roughly described by the wing tips and tail) of the
(airplane) horizontal axes.

In a turn, slipping, skidding, or coordinated, the nose of the airplane
(dragging the longitudinal axis with it) is changing its direction.  The
airplane is changing heading.  If this happens at a constant rate (say, 3
degrees per second), I would say the aircraft is yawing at a steady
angular velocity, and undergoing no acceleration in yaw.  To =enter= or
=leave= this state would require an =acceleration= of yaw in one direction
or another.

Where do our understandings and vocabulary diverge?

Jose
-- 
You can choose whom to befriend, but you cannot choose whom to love.
for Email, make the obvious change in the address.