Dave, the definition I was looking for needs more detail. I don't want to put words into anyone's mouths because I want to see if there is actually consensus. But for a hint, is the angle measured along the span of the blade, along the mean azimuthal position (when able to lead and lag), etc. Also, is this just a rigging angle decided by the engineers, or is it a complex aerodynamic result of the design of the entire system that allows a predictable and intuitive stick plot?
As far as the delta 3 reducing phase angle, I thought you weren't convinced of that point and that is why I started with this questioning. The natural frequency of the blade (speed at which it flaps) is only one consideration that decides phase angle. It is a fairly significant one as it considers many attributes of the blade and the rotor, but not the only one.
Changing delta 3 should change phase angle even if all other factors (including blade freq) stay the same. Understanding delta 3 requires understanding what phase angle precisely is, and understanding the movement the blades make whilst flapping (especially with some delta 3).
Matthew.