HFD
As bookworm said, yaw isn't the angle between the longitudinal axis and the direction of travel - in the yawing plane this angle reflects slip or skid (angle of attack in the pitching plane - well alright, that's chord line and direction of travel, but near enough!). It isn't really relevant to talk about yaw as being any angle ... it describes a motion about the normal axis, or a moment that may give rise to such a motion. Think about roll as an analogy - roll describes the motion about the longitudinal axis, bank angle describes the primary result (using an earth reference).
Your instructional 'brief' seems fine, as far as it goes. Unfortunately, it only covers the specific case where the angle of attack isn't increased sufficiently to enable a level turn to take place and which therefore results in a spiral dive. Quite a lot of this thread, however, has been concerned with what is happening in the level banked turn ... and that's where most of the misunderstanding has arisen. Furthermore, what about a climbing turn? In all cases, the aeroplane is yawing (and pitching) in the turn, and the ball may well be centred. How? Doesn't the student need to comprehend these other cases too?
As you said, your brief is the one that been used since time immemorial. Whilst not statistically significant, the number of incorrect descriptions that have been put forward here suggests that it's somewhat inadequate for a more-general understanding of the principles of flight as applied to turning!
Last edited by Islander2; 30th March 2007 at 15:03.