Dear @Ian.Ammerman,
The flapwise and edgewise mode shapes used in ElastoDyn are mode shapes that follow the principle axes of bending (oriented by the local structural twist). So, I agree that you must take into account the local structural twist to find the out-of-plane and in-plane mode shapes (which are called “twisted shape functions” in the ElastoDyn theory documentation). However, I don’t agree with your equation because the mode shapes must be found by double integrating the curvature, where the curvature is defined about the principle axes of bending. A similar question was discussed in the following forum post: Coupled blade modes in FAST and the full equations implemented in ElastoDyn are documented in the ElastoDyn Theory Manual: 4.2.7. ElastoDyn Users Guide and Theory Manual — OpenFAST v3.5.3 documentation.
Best regards,