Dear Felix,
Since the FAST User’s Guide was last published, we now recommend that BModes be used in place of Modes. BModes employs a higher-fidelity model than Modes and also includes the option for tower-top (nacelle and rotor) inertia (in addition to a point mass) and tower-base (platform) DOFs. As described in the forum topic found here: Tower fore-aft modes shapes, the tower-top inertia can have a large impact on the mode shapes and natural frequencies of a tower. While Modes may be predicting a higher frequency, the correct frequency of the 1st fore-aft and side-to-side tower-bending modes when the tower-top mass/inertia are included is around 0.32 Hz for the land-based NREL 5-MW turbine. We’ve obtained very similar frequencies with BModes, FAST, and MSC.ADAMS.
I hope that helps.
Best regards,