In a rotary wing aircraft a bar (1) of composite matrix material has a longitudinal axis (8) perpendicular to a cross section with upper and lower surfaces (2, 3). At least one torsion box (17) comprises an upper and a lower layers (10, 21, 11, 22) respectively offset of a longitudinal axis (8) and of the upper and the lower surfaces (2, 3), said upper and lower layers (10, 21, 11, 22) being laterally joint, while cross sections are filled with the first group of fibers extending essentially parallel to the longitudinal axis (8).