Longeron

In aircraft construction, a Longeron or Stringer is a thin strip of wood or metal, to which the skin of the aircraft is fastened. Longerons are attached to formers (also called frames), in the case of the fuselage, or ribs in the case of a wing, or empennage. In very early aircraft, a fabric covering was sewn to the longerons, and then stretched tight by painting it with dope, which would make the fabric shrink, and become stiff.

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 inches). 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 inches). 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. [Bruhn, E.F., page C11.29, "Analysis and Design of Flight Vehicle Structures", 1973]

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. But stringers often are not attached to anything but the skin, they perform the duty of preventing the skin deforming thus creating stress risers that would destroy the structural integrity of the monocoque. It is not uncommon to have a mixture of longerons and stringers in the same major structural component.

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