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
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