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This paper provides, for the first time, a physical explanation of the dynamic fluid¿structure stability response of an unconventional aircraft configuration called PrandtlPlane, which has been recently acknowledged as a possible candidate to foster the ambition of a greener aviation. As observed in previous literature efforts on this configuration, flutter onset is significantly different when considering the aircraft being free in the air or fixed in space. Thanks to the formulation adopted in this effort, it is shown how the aerodynamic coupling of elastic and rigid modes has a beneficial effect on dynamic aeroelastic instability (flutter) onset. However, the different modal properties, consequence of the diverse boundary conditions, when switching from fixed-in-space to free-flying aircraft, also play a relevant role in determining the flutter occurrence. Whereas for the longitudinal case both effects are synergistic, contributing to increase flutter speed, for the lateral-directional case the variation in modal properties has a detrimental and dominating effect, leading to a flutter speed well within the flight envelope. Not only effects of rigid and elastic modes interaction have been addressed with respect to the aeroelastic side but the consequent effect on the flexible flight dynamics in terms of deterioration of the flying qualities has been quantified. Within the adopted formulation, unsteady aerodynamic forces are modeled by means of an enhanced Doublet Lattice Method able to take into account terms typically neglected by classic formulations. The work also discusses the relevance of such extra contributions to the dynamic response of the aircraft.