Ok, then what exactly is "yaw"? My understanding is that it is a change of direction of the longitudinal axis of the aircraft in the plane that is roughly coplanar with the wings.

Yaw, as we define the term in flight test as it relates to directional stability is defined as moments about the VERTICAL axis, not the longitudinal axis.

Yes, that is pretty much what I said. Change in direction =of= the longitudinal axis, =in= the plane (of the wings), [(therefore) =about= the vertical axis.] I am however using "vertical" and "horizontal" as referenced to the aircraft, not the earth, thus when the aircraft pitches up, the yaw axis (as I understand it defined) would change.

Jose
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