A longeron is part of the structure of an aircraft, designed to add rigidity and strength to the frame. It also creates a point of attachment for other structural supports, as well as the skin of the aircraft. They provide lengthwise support and the number of longerons present in an aircraft varies, depending on the size and how it is designed. Like other structural members, they need to be checked periodically for signs of damage that might compromise their function.
Materials like wood, carbon fiber, and metal can be used in longeron construction. Older aircraft were made almost entirely with wood, while it is a more rare construction material today because it does not provide as much strength and flexibility as other materials. The materials are carefully tested before being installed to make sure there are no cracks or other flaws that might cause them to fail once in place or while the plane is in use.
Each longeron attaches directly to the frame of the aircraft using bolts. In some planes, shorter longitudinal supports called stiffeners or stringers are fastened to the longerons. Confusingly, these terms are also sometimes used as alternate names for the longeron. The skin, whether made from metal, leather, canvas, or other materials, can be attached to the aircraft once the longerons are in place. Insulating material and lining may be installed on the other side, depending on how the plane is going to be used.
Historically, planes were skinned by stretching fabric over the body of the plane and doping it, which is to paint it with a chemical solution designed to stiffen and waterproof it. In addition, the dope also forced the fabric to shrink, creating a tight, even shell across the body of the aircraft. Unfortunately for pilots, doping materials were also extremely flammable and a single spark could send an aircraft up in flames. Modern aircraft usually use metal skins and have fire suppression systems to add to their safety.
People building a plane from scratch may custom-fabricate each longeron for their needs. In other cases, mass-produced structural supports are used. Most kits designed to help people build planes have all the components of the frame, and commercial aircraft manufacturers use their own fabrication facilities to produce longerons and other structural components for their assembly lines. When working with new aircraft designs, engineers spend time determining the required number, size, and shape of structural components to meet or exceed the load requirements.