Dear Marcos,
In BeamDyn, kp_xb, kp_yb, and kp_zb define the curvilinear local blade reference axis (natural geometry of the blade), which locates the origin and orientation of each local coordinate system where the cross-sectional 6x6 stiffness and mass matrices are defined. kp_xb, kp_yb, and kp_zb are defined relative to the blade root and pitch axis. The BeamDyn module does not include any aerodynamic effects, so, the location of the aerodynamic center is irrelevant to BeamDyn; instead the aerodynamic center is specified within AeroDyn v15. We’ve updated the BeamDyn documentation that we will release along with an update to FAST v8 soon (and where we have also renamed “kp_xb, kp_yb, and kp_zb” to “kp_xr, kp_yr, and kp_zr”). Included in this updated documentation is a figure that clarifies the blade reference and local blade coordinate systems, which I’ve also attached to my post dated Feb 18, 2016 in the following forum topic: Section coordinate system in BeamDyn.
When BeamDyn is enabled in FAST v8, the blade-related input specifications of ElastoDyn (including the DOFs, initial conditions, blade input files, etc.) are not used by FAST.
Best regards,