A fuselage serves as the main body of an aircraft, housing the pilots, crew, passengers, and cargo. Depending on the aircraft design, the wings and tail section may be attached to the fuselage. As such, it must be designed in such a way that the wings and tail are positioned so that the aircraft remains stable through the designed center of gravity envelope. As a requirement for all civil and general aviation aircraft, a stable vessel is one that tends to return to straight and level flight if the controls are released.
It is important to note that the fuselage is one of the primary contributors to the total drag force generated by aircraft in flight. That being said, it must be specially designed to be as aerodynamic as possible in an effort to mitigate drag. Nevertheless, aircraft engineers must strike a balance between low aerodynamic drag, payload, and passenger comfort to ensure safe operation.
Most Common Aircraft Fuselage Structure
In general, the most common type of fuselage airframe structure consists of a semi-monocoque structural design, where the substructure and skins work in tandem to absorb and transfer varying loads produced during flight. More specifically, the semi-monocoque structure of the outer skin and the internal substructure are load bearing, and they both contribute to the overall stiffness of the aircraft airframe. This design was born out of the use of aluminum, as opposed to steel or wood, as the main material utilized in airframe construction.
Aluminum has a number of advantages over other metals, including the density being one-third that of steel, allowing for thick structural sections that can be built without adding any issues with weight. Usually, thicker skins are more beneficial because they are less likely to buckle under heavy loads, resulting in a more efficient structure. A typical, semi-monocoque fuselage is composed of a few key elements: stringers or longerons, frames, and skins.
Stringers or Longerons
Stringers or longerons make up the longitudinal components of an aircraft structure. Their main purpose is to transmit the axial loads that source from the fuselage’s tendency to bend under a particular load. Moreover, the stringers also support the skin, and when combined with the frames, they create bays over which the skin is affixed.
Frames are defined as transverse elements that form the cross-section of the fuselage, and they are normally spaced approximately 20 inches apart. The frames and stringers are positioned in a way that ensures the resulting bays are capable of preventing the skins from buckling. In addition, frames provide a way to introduce point loads into the fuselage. Keep in mind that large frames are necessitated at the wing-fuselage and tail-fuselage interfaces to transmit loads generated by these lifting surfaces into the fuselage.
Load-bearing skins are generally attached to the stringers and frames of an aluminum aircraft through rivets. Additionally, the skins carry a load through shear movement, transmitting it into the aircraft stiffeners. In pressurized aircraft, the skin works alongside the frames to oppose the internal pressure load.
During flight, the fuselage bears a combination of loads from various sources. For instance, large bending loads are introduced from the wing and tail sections, and torsional loads are introduced from the pitching movement of the wings. Furthermore, landing loads are particularly important to consider as poorly executed landings can cause severe damage to the airframe structure. Lastly, crew and passenger movements, as well as baggage requirements, must also be considered in the final design of aircraft.
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