Irlandés
It is a fairly complex subject. There have been numerous threads on it, in this forum, over the past year. A search and reading of these threads may help clarify the subject.
"You refer in your post to 'phase angle' and on your web-page it states at the end that "The phase angle of a specific helicopter can vary depending on the forward velocity and other aerodynamic events at a specific time." So I can only assume that phase angle and advance angle are totally different concepts...."
Fortunately or unfortunately, they are the same concept.
The note on the web page will be clarified. What should have been said is;
A helicopter's
optimal phase lag angle will change as its flight conditions change. Because it is difficult to change the physical phase lag angle on the helicopter, the designer selects an angle that is approximately the mid-point of the desired range. On the Robinson, this angle is 72-degrees.
This is the same situation as the undersling dimension on hinge of a teetering rotor. The optimal dimension will vary with flight conditions (greater payload = greater coning angle = desire for greater undersling) but the designer must obviously select a single fixed value.
Hope this makes sense.
Dave J.