Longeron

File:Truss-type fuselage structure.png type fuselage structure{{cite web |title=Pilot’s Handbook of Aeronautical Knowledge |url=https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak |publisher=Federal Aviation Administration |access-date=16 January 2023 |date=August 24, 2016}}{{rp|3–4}}]]

In an aircraft fuselage, stringers are attached to formers (also called frames){{Cite journal|last=V|first=Vignesh|title=Reconnaisance [sic] aircraft design project|url=https://www.academia.edu/35242413|language=en}} and run in the longitudinal direction of the aircraft. They are primarily responsible for transferring the aerodynamic loads acting on the skin onto the frames and formers. In the wings or horizontal stabilizer, longerons run spanwise (from wing root to wing tip) and attach between the ribs. The primary function here also is to transfer the bending loads acting on the wings onto the ribs and spar.

The terms "longeron" and "stringer" are sometimes 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) and run all along the fuselage length, then they are called "longerons". The longeron system also requires that the fuselage frames be closely spaced (about every {{convert|4|to|6|in|cm|disp=or|abbr=on}}). If the longitudinal members are numerous (usually 50 to 100) and are placed just between two formers/frames, then they are called "stringers". In the stringer system the longitudinal members are smaller and the frames are spaced further apart (about {{convert|15|to|20|in|cm|disp=or|abbr=on}}). 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 use a combination of both stringers and longerons.{{cite book |author=Bruhn, E. F |title=Analysis and Design of Flight Vehicle Structures |page=C11.29 |year=1973 |url=http://www.ihpa.ie/carbon-dragon/images/carbon-dragon/Manuals%20and%20Publications/73%20Bruhn%20analysis%20and%20design%20of%20flight%20vehicles.pdf |publisher=Tri-State Offset Company}}

Longerons often carry larger loads than stringers and also help to transfer skin loads to internal structure. Longerons nearly always attach to frames or ribs. Stringers are usually not attached to anything but the skin, where they carry a portion of the fuselage bending moment through axial loading.{{cite book |author=Michael C. Y. Niu |year=1988 |title=Airframe Structural Design |url=https://air.flyingway.com/books/Airframe-Stuctural-Design.pdf |publisher=Conmilit Press LTD. |page=376}} It is not uncommon to have a mixture of longerons and stringers in the same major structural component.

Stringers are also used in the construction of some launch vehicle propellant tanks. For example, the Falcon 9 launch vehicle uses stringers in the kerosene (RP-1) tanks, but not in the liquid oxygen tanks, on both the first and second stages.{{cite web |title=Falcon 9 Launch Vehicle Payload User's Guide, Rev 2 |page=11 |url=http://www.spacex.com/sites/spacex/files/falcon_9_users_guide_rev_2.0.pdf |access-date=29 January 2016 |date=21 October 2015 |archive-url=https://web.archive.org/web/20170314002928/http://www.spacex.com/sites/spacex/files/falcon_9_users_guide_rev_2.0.pdf |archive-date=14 March 2017 |url-status=dead }}

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Category:Aircraft components

Category:Mechanical engineering