Documentation about ElastoDyn i and t Axis

[feanored]

I tried and could not find a good explanation about the q_t reference system, and comparation with the q_i reference, which I found to "i" stand for Inertial reference system. And about the "t", that is present also in the TwrBsF_ force variables?

[jjonkman]

Dear @feanored,

In ElastoDyn, the orientation of the tower-base coordinate system (t) is related to the inertial frame coordinate system (i) through the rigid-body rotation of the platform roll, pitch, and yaw degrees of freedom (the tower is cantilevered to the platform at its base). As of OpenFAST v4.0.0 and newer, the ElastoDyn platform uses a yaw-pitch-roll Euler-angle sequence (previous versions of ElastoDyn used small rotation angle approximations, where rotation sequence did not matter).

Best regards,

[feanored]

Thank you very much!

Is there any text/figure in the documentation about the definition of the tower-base coordinate system? (sorry if it's obvious)

[jjonkman]

Dear @feanored,

The various coordinate systems used by ElastoDyn are illustrated in the old FAST v6 User's Guide: https://openfast.readthedocs.io/en/main/_downloads/d8bd014121d6505cb25cf49bee5eaa80/Old_FAST6_UsersGuide.pdf.

Best regards,