If you would like to read a nice derivation of the dynamic
equations of blade motion about the flapping and the
lead/lag hinges, check out Ch 4, Rotating Blade Motion
in "Principles of Helicopter Aerodynamics" by Leishman.
Section 4.7 ends with an expression for phase angle for
a blade with an offset flapping hinge (as they are on
articulated heads). In the text he states that the value
of hinge offset only varies from 4 to 6% for an articulated
blade, so the natural frequency is only slightly greater
than Omega (1/rev). The forcing frequency from the
swashplate is, of course, at Omega, so the phase lag
will be slightly less than 90 deg, as it would with any
resonant system.
If you really want to understand the physics behind the
phase lag I think you are going to have to refer to a text
similar to Leishman. I don't believe you will get the level
of detail necessary in a few posts.
Shaun