Dear Rocio,
Yes, indeed this is confusing. Unfortunately, the two definitions of PitchAxis are not the same. The forum post you reference used the following definition (see NREL 5MW Rotor Geometry for details):
PitchAxis = 0.25 - AeroCent - AeroRef
But in ElastoDyn within FAST v8, PitchAxis is defined as (from the FAST v8 ReadMe file: wind.nrel.gov/nwtc/docs/README_FAST8.pdf):
PitchAxis = 0.5 – AeroCent
Regardless, the PitchAxis variable in the ElastoDyn module of FAST v8 is not used when AeroDyn v15 is enabled. This input in ElastoDyn is used to define the aerodynamic center offset from the pitch axis for AeroDyn v14. But when AeroDyn v15 is enabled, the aerodynamic center is defined within AeroDyn v15 instead of ElastoDyn. Once AeroDyn v14 is removed from OpenFAST (AeroDyn v15 is meant to replace AeroDyn v14), the PitchAxis input from ElastoDyn will be eliminated as well.
Best regards,