According to the OpenFAST documentation (https://openfast.readthedocs.io/en/main/source/user/subdyn/theory.html#member-or-element-local-coordinate-system-x-e-y-e-z-e-element-cs), the local x_e beam axis is parallel to the global XY plane, and directed such that a positive, less than or equal to 180 deg rotation about it, would bring the local z_e axis parallel to the global Z axis.
In the above sketch, x_e is parallel to the global XY plane. However, I understand that a rotation of 270 deg around x_e is necessary to align z_e with the global coodinate system.
I have reproduced this in a easy SubDyn model with the longitudinal axis along the global X direction (in red I have added the coordinates of the global coodinate system):
I have also requested the direction cosine matrices for the 2 members in SubDyn (OutCOSM = True) and I get:
According to this output, the direction cosine is:
From here, we can see that x_e would be [0, -1, 0]. This is consistent with my expectation. Such x_e lies in the XY plane and a rotation of 90 deg around x_e would make z_e parallel to the global Z. It can also be observed that y_e is [0, 0, -1], following the right-hand rule.
Are we aligned? Am I misinterpreting something?
If we are aligned, we should modify the figure in the documentation since it would be misleading.
According to the OpenFAST documentation (https://openfast.readthedocs.io/en/main/source/user/subdyn/theory.html#member-or-element-local-coordinate-system-x-e-y-e-z-e-element-cs), the local x_e beam axis is parallel to the global XY plane, and directed such that a positive, less than or equal to 180 deg rotation about it, would bring the local z_e axis parallel to the global Z axis.
In the above sketch, x_e is parallel to the global XY plane. However, I understand that a rotation of 270 deg around x_e is necessary to align z_e with the global coodinate system.
I have reproduced this in a easy SubDyn model with the longitudinal axis along the global X direction (in red I have added the coordinates of the global coodinate system):
I have also requested the direction cosine matrices for the 2 members in SubDyn (
OutCOSM= True) and I get:According to this output, the direction cosine is:
From here, we can see that x_e would be [0, -1, 0]. This is consistent with my expectation. Such x_e lies in the XY plane and a rotation of 90 deg around x_e would make z_e parallel to the global Z. It can also be observed that y_e is [0, 0, -1], following the right-hand rule.
Are we aligned? Am I misinterpreting something?
If we are aligned, we should modify the figure in the documentation since it would be misleading.