A rotary wing aircraft (1) such as a helicopter comprises a force-transmission structure (5) for transmitting forces, in particular those coming from an undercarriage (2), and that, in longitudinal elevation view, presents an L-shape in which the arrangement (6, 19-21) forms a transversely centered box (6), such that the structure (5) transfers the forces, at least in part, to top and bottom working covering segments of the fuselage.