In aircraft

Aircraft

An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...

 construction, a longeron or stringer or stiffener is a thin strip of wood, metal or carbon fiber, to which the skin of the aircraft is fastened. In the fuselage, longerons are attached to former

Former

thumb|right|150px|Interior of an F-16B with the engine removed showing frames or formers.A former is a structural member of an aircraft fuselage, of which a typical fuselage has a series from the nose to the empennage, typically perpendicular to the longitudinal axis of the aircraft...

s (also called frames) and run the longitudinal direction of the aircraft. In the wing or horizontal stabilizer, longerons run spanwise and attach to ribs

Rib (aircraft)

In an aircraft, ribs are forming elements of the structure of a wing, especially in traditional construction.By analogy with the anatomical definition of "rib", the ribs attach to the main spar, and by being repeated at frequent intervals, form a skeletal shape for the wing...

.

Sometimes the terms "longeron" and "stringer" are used interchangeably. Historically, though, there is a subtle difference between the two terms. If the longitudinal members in a fuselage are few in number (usually 4 to 8) then they are called "longerons". The longeron system also requires that the fuselage frames be closely spaced (about every 4 to 6 in (10.2 to 15.2 cm)). If the longitudinal members are numerous (usually 50 to 100) then they are called "stringers". In the stringer system the longitudinal members are smaller and the frames are spaced farther apart (about 15 to 20 in (38.1 to 50.8 cm)). Generally, longerons are of larger cross-section when compared to stringers. On large modern aircraft the stringer system is more common because it is more weight efficient despite being more complex to construct and analyze. Some aircraft, however, use a combination of both stringers and longerons.

Longerons often carry larger loads than stringers and also help to transfer skin loads to internal structure. As stated above longerons nearly always attach to frames or ribs

Rib (aircraft)

In an aircraft, ribs are forming elements of the structure of a wing, especially in traditional construction.By analogy with the anatomical definition of "rib", the ribs attach to the main spar, and by being repeated at frequent intervals, form a skeletal shape for the wing...

. But stringers often are not attached to anything but the skin

Stressed skin

In mechanical engineering, stressed skin is a type of rigid construction, intermediate between monocoque and a rigid frame with a non-loaded covering:...

, where they carry a portion of the fuselage bending moment through axial loading. It is not uncommon to have a mixture of longerons and stringers in the same major structural component.