A helicopter airframe, the helicopter having a longitudinal axis and the airframe comprising: a subfloor structure for supporting cabin floor panels and/or at least one fuel tank bladder, said subfloor structure having a bottom shell, ribs, main frames and floor panels, said ribs and main frames extending essentially in planes perpendicular to said longitudinal axis on one side of the bottom shell and the floor panels being in an essentially horizontal plane essentially parallel to the longitudinal axis on the same side of the bottom shell as the ribs and main frames and mounted on flat top edges of the ribs and of main frames opposite to the bottom shell,the bottom shell having an essentially flat shell portion and two curved lateral shell portions extending integrally from said flat shell portion along longitudinally oriented sides and being curved towards respective lateral borders of said ribs and main frames, in at least a central and an aft portion of the subfloor structure along the longitudinal axis, the airframe has no longerons but longitudinal support beams provided with attachments for the floor panels, the longitudinal support beams being attached at least partially to the ribs and/or the main frames and featuring along at least 2/3 of their longitudinal extension a height 2 to 20 times smaller than any distances of any of the floor panels to the bottom shell, characterized in that the helicopter airframe further comprises at least a pair of membrane elements of bulgy shape; each membrane element laterally protruding to the outside of the longitudinal support beams one each side of the subfloor structure; each the of membrane elements has a upper longitudinal hem and a lower longitudinal hem, the upper longitudinal hem being attached to the longitudinal support beams and the lower longitudinal hem being attached to the bottom shell; the membrane elements being attached to a longitudinal support beam for the membrane elements hence not contributing to the overall load bearing behavior with moments and shear loads, so that the membrane elements do not need to be stabilized. a subfloor structure for supporting cabin floor panels and/or at least one fuel tank bladder, said subfloor structure having a bottom shell, ribs, main frames and floor panels, said ribs and main frames extending essentially in planes perpendicular to said longitudinal axis on one side of the bottom shell and the floor panels being in an essentially horizontal plane essentially parallel to the longitudinal axis on the same side of the bottom shell as the ribs and main frames and mounted on flat top edges of the ribs and of main frames opposite to the bottom shell,the bottom shell having an essentially flat shell portion and two curved lateral shell portions extending integrally from said flat shell portion along longitudinally oriented sides and being curved towards respective lateral borders of said ribs and main frames, in at least a central and an aft portion of the subfloor structure along the longitudinal axis, the airframe has no longerons but longitudinal support beams provided with attachments for the floor panels, the longitudinal support beams being attached at least partially to the ribs and/or the main frames and featuring along at least 2/3 of their longitudinal extension a height 2 to 20 times smaller than any distances of any of the floor panels to the bottom shell, characterized in that the helicopter airframe further comprises at least a pair of membrane elements of bulgy shape; each membrane element laterally protruding to the outside of the longitudinal support beams one each side of the subfloor structure; each the of membrane elements has a upper longitudinal hem and a lower longitudinal hem, the upper longitudinal hem being attached to the longitudinal support beams and the lower longitudinal hem being attached to the bottom shell; the membrane elements being attached to a longitudinal support beam for the membrane elements hence not contributing to the overall load bearing behavior with moments and shear loads, so that the membrane elements do not need to be stabilized.