The fuselage of an aircraft is one of the most critical parts of the entirety of the plane, allowing for passengers, crew, cargo, and other equipment to be held safely during flight operations. The fuselage plays many roles, containing the cockpit, acting as a center of gravity, and may even house engines that provide power for aircraft systems and flight. Depending on the aircraft and its operations, the fuselage of the aircraft may differ. Throughout history, various designs and structures of the fuselage have come about, each using different materials and framing to achieve similar abilities. In this article, we will discuss the main types of fuselage designs that are present on aircraft.
The truss, or framework type, is an aircraft fuselage design that is denoted by box structures that are formed using beams. These beams may be created utilizing wood covered with hardwood, welded steel tubing, or light gauge steel tubes. Across aircraft types, the truss aircraft frame design is most prevalent in lightweight aircraft, typically using welded steel tubing. In its early uses, truss structures were created in rectangular shapes, and trusses were enclosed within cloth fabrics to remain lightweight. As this design continued to improve, cloth was replaced with aluminum, and soon the outer skin proved to provide much of the same support for flight loads. The development of such outer skin that could support loads is what led to monocoque and semi-monocoque construction.
Monocoque construction refers to the types of fuselage that feature stressed skin capable of supporting almost the entirety of the aircraft. In its early development, monocoque construction consisted of molded plywood, laminated into a three dimensional shape, or shell. This process was carried over from boat hull design, using a structure as a whole rather than for its individual components. During the early 1900’s, metal monocoque fuselages began to be introduced and aluminum fuselages became the standard during the mid 1930’s. The main benefit of monocoque construction for aircraft fuselages is the great reduction of weight, forgoing internal bracing for the stress carrying outer skin. While this greatly reduces the need for a heavy framework, it does cause a loss in structural integrity due to possible surface deformation. Much like a beverage can, the entirety of the structural stress is taken onto the outer skin. If there is a bent caused to the outer structure, the entirety may collapse. Because of this, the greatest problem faced with monocoque constructions is balancing weight with the amount of strength needed for structural integrity.
As a way to solve this problem, the semi-monocoque design implements reinforced skin that is achieved with the use of longerons and stringers. Longerons and stringers are components that are lightweight, attaching to the frame longitudinally and providing much more structural support to accommodate the bending stresses that occur in the fuselage. The longerons of the fuselage are typically constructed from aluminium alloy and will stretch across multiple frames. Stringers, on the other hand, are lighter than the longerons and greater in number, and they may be manufactured from aluminum alloy or formed aluminum. The fuselage itself also is typically aluminum, and still takes on much of the stress of operation as it would in a monocoque construction. The difference is, however, that the outer skin, longerons, stringers, and other fuselage structures & components all work together to provide ample support for the aircraft fuselage. By distributing stresses and loads across numerous structures and components, the fuselage becomes much stronger as a whole as no single part risks critical failure. Currently, the semi-monocoque design proves to be widely utilized for the fuselages of all aluminum aircraft. Many metal light aircraft also adopt this construction method for their fuselages.
Composite construction for fuselages may be considered a somewhat new practice, though its history began in the late 1950’s following World War II. During this time, sailplane construction started to use fiberglass for the main fuselage structures. In 1965, the Federal Aviation Administration made the first approval on an all fiberglass aircraft under the normal category. Over the years, leading into the early 2000’s, more aircraft began to adopt composite materials. Composite materials include Kevlar, fiberglass, carbon fiber cloth, and other similar materials that allow for smooth, lightweight skins that can take on complex designs much easier than aluminum. Composite construction proves to be cost efficient and provides a good strength to weight ratio. Their disadvantages, however, lay in their low conduction abilities for lightning protection, lack of visual damage for easy spotting, and other factors.
One of the major considerations that is taken into account when deciding which type of fuselage is best for a particular aircraft is pressure. Pressurization of aircraft has become a standard for the safety of all passengers and crew members. As pressurization begins during liftoff and the pressure difference between the inside and the outside of the aircraft increases alongside altitude, the structure of the aircraft needs to maintain structural integrity. The differences in pressure often exerts a great amount of stress on the fuselage, and a cycle of exertion and relaxing of pressure through each flight can begin to wear on the body. Because of this, virtually all aircraft that are pressurized choose to adopt the semi-monocoque fuselage due to its ability to bear such stress loads. As the aircraft continues to operate, the fuselage and correlating structures may be periodically inspected and maintained to ensure that there is no damage or weakness that is detected. If a weakness is found, the fuselage of the aircraft may be modified or re-engineered.
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