Rotary wing aircraft airframe

An airframe for a rotary wing aircraft comprising:

• a bottom shell having a flat shell portion and two curved lateral shell portions, each lateral shell portion extending outwardly from the flat shell portion;
• a series of ribs and main frames extending in planes perpendicular to a longitudinal axis of the rotary wing aircraft and on one side of the shell, each rib and main frame having a flat top edge positioned on an end of the rib and main frame opposite to the bottom shell, wherein the two lateral curved shell portions are curved towards respective lateral borders of the series of ribs and main frames;
• floor panels in a horizontal plane parallel to the longitudinal axis and on the one side of the shell, the floor panels mounted to the flat top edges of the ribs and main frames, wherein the bottom shell, the series of ribs and main frames, and the floor panels cooperate to form a subfloor structure to support cabin floor panels;
• first and second longitudinal support beams, each longitudinal support beam extending along the longitudinal axis and along at least a longitudinal section of the subfloor structure, each longitudinal support beam having an upper region with attachments for the floor panels, each longitudinal support beam having a series of front flanges spaced apart from one another and extending from the floor panels to the bottom shell, each longitudinal support beam having a height between adjacent front flanges being 2 to 20 times smaller than a height of an adjacent front flange such that an opening is defined within the region, each longitudinal support beam being connected to at least one of the ribs and main frames;
• first and second membrane elements, each membrane element having an upper longitudinal hem connected to the upper region of a respective longitudinal support beam and a lower longitudinal hem connected to the bottom shell for purely two-dimensional loading of the membrane element, each membrane element having a C-shaped cross sectional shape with a concave hollow side, the concave hollow sides of the first and second membrane elements facing one another, each membrane element protruding towards an adjacent lateral curved shell portion.