Dear Paul,
Good question.
In reality, both methods have some problems. The blade mode shapes would be more accurately predicted by BModes using method #1 than method #2, but as you point out, the structural model of FAST v7 or the ElastoDyn module of FAST v8 expects uncoupled blade modes, which FAST then couples through the structural pretwist, so, the use of method #1 is not completely consistent with what FAST expects. Using method #2 is more consistent with what FAST expects, but the mode shapes predicted by BModes will not be as accurately predicted because the lack of the coupling will have an influence on the effect of centrifugal stiffening.
The ADAMS-derived mode shapes for FAST for the NREL 5-MW blade were based on method #1. Which method are you using in BModes?
From my experience, both methods have yielded adequate results as the structural response predicted by FAST is not too sensitive to the specified blade mode shapes, as long as the natural frequencies are predicted accurately. I’d be curious if you arrive at a different conclusion.
Best regards,