Mart,
The Vortex Panel Method software looks interesting.
Have you seen the the freeware program
NVFoil, and had a chance to compare the two?
You may be underestimating the rigidity of Sikorsky's ABC blades.
[Source ~ Rotor-Wing Aerodynamics ~ W.Z. Stepniewski / C.N. Keys ~ Book I]
Sikorsky S-69 (XH-59A) ABC ~ Flapping hinge offset: Flapping frequency ratio (v/n) ≈ 1.4 [p.39]. From the graph [ p.18] it appears that the equivalent (virtual) flapping hinge offset will be approximately at 50% of rotor radius.
The information that is available on the new X2-ABC indicates that Sikorsky is increasing the rigidity even further.
This page on phase lag may be informative
Dave