The shear force, bending moment and torque of the fuselage are all borne by the frame. Among them, the axial force caused by bending moment is borne by the four sides of the frame; The vertical shear force is borne by the braces and diagonal braces (or each pair of tension lines) on both sides of the frame; Horizontal shear force is borne by columns and inclined columns (or tension lines) on the upper and lower planes; The torque of the fuselage is borne by a three-dimensional structure composed of four plane frames. The torsional stiffness and aerodynamic performance of the frame fuselage are poor, and the internal volume is not easy to be fully utilized. Only some small low-speed aircraft fuselage adopts frame fuselage.
2. Single fuselage. The monocoque fuselage adopts frame and partition to form the fuselage shape, and the skin bears the main force. The monocoque fuselage structure has no longitudinal reinforcement, so the skin must be strong enough to maintain the rigidity of the fuselage. Its main problem is its heavy weight, which is rarely used in modern aircraft.
Three. Semi-monocoque fuselage
In order to make the stiffness of the fuselage structure meet the requirements of improving flight speed, it is necessary to make the skin participate in the stress of the whole structure. Therefore, at present, the fuselage structure generally adopts metal skin, and the skin is firmly riveted with the partition frame, main beam and stringer to form a stressed whole, which is usually called semi-hard shell fuselage.
In the semi-monocoque fuselage, the main beam and stringer are used to bear the axial force caused by bending moment; The skin not only bears different degrees of axial force, but also bears all shear and torque. The partition frame is used to keep the shape of the fuselage and bear local aerodynamic forces. In addition, it also bears concentrated loads from various components and transfers these loads to the skin in a decentralized way.