The whirl flutter stability of the free-flying X-57 is examined as well as the interaction between the longitudinal flight dynamic modes with the elastic wing modes. The X-57 has positive damping ratios for all the whirl flutter modes examined well beyond the expected flight envelope. The use of a multibody dynamics analysis has shown to be able to capture the flight dynamics as well be able to provide the stability derivatives for a linearized state-space analysis. The phugoid mode has a much lower frequency than the wing elastic modes and no interference is encountered. However, the short period mode has a frequency that increases with velocity and crosses the wing symmetric out-of-plane bending mode, affecting the damping ratios of both modes. This interaction does not drive either mode to become unstable, and the damping ratio rebounds once the short period mode frequency increases enough.
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