Dear Ngoc Ha Tran,
See my post dated Jan 25, 2013 in the following forum topic for my definitions of the various centers: Coordinate System in FAST - #6 by Neelabh.Gupta. By referring to the elastic axis as the “locus of elastic centers”, this means that a line (or curvilinear line) is passed from the root to the tip through the elastic center of each cross section. Note that some authors refer to the “elastic center” as the “tension center” and other authors refer to the “elastic center” as the “shear center”.
The principal axes of bending are defined as the orthogonal axes in a cross section whereby the cross bending stiffness terms are zero. The origin of the principal axes of bending is the tension center (the neutral axis).
A tip brake is one example where there is a large lumped mass near the tip of the blade. In most blades, there is no large lumped mass near the tip, so, the tip-mass inputs in BModes or FAST can usually be set to zero.
I know nothing about the blade you analyzing, but I would be surprised if a tip mass was so large that it reduced a blade natural frequency from 400 to 20 Hz.
I hope that helps.
Best regards,