CHAPTER 23 Fuselage Frames and Wing Ribs.......................................................
Aircraftareconstructedprimarilyfromthinmetalskinswhicharecapableofresistingin-planetension
andshearloadsbutbuckleundercomparativelylowvaluesofin-planecompressiveloads.Theskins
are therefore stiffened by longitudinal stringers which resist the in-plane compressive loads and, at
the same time, resist small distributed loads normal to the plane of the skin. The effective length in
compressionofthestringersisreduced,inthecaseoffuselages,bytransverseframesorbulkheadsor,
in the case of wings, by ribs. In addition, the frames and ribs resist concentrated loads in transverse
planesandtransmitthemtothestringersandtheplaneoftheskin.Thus,cantileverwingsmaybebolted
tofuselageframesatthesparcaps,whileundercarriageloadsaretransmittedtothewingthroughspar
andribattachmentpoints.
23.1 PrinciplesofStiffener/WebConstruction.........................................................
Generally, frames and ribs are themselves fabricated from thin sheets of metal and therefore require
stiffening members to distribute the concentrated loads to the thin webs. If the load is applied in the
planeofaweb,thestiffenersmustbealignedwiththedirectionoftheload.Alternatively,ifthisisnot
possible,theloadshouldbeappliedattheintersectionoftwostiffenerssothateachstiffenerresiststhe
componentofloadinitsdirection.Thebasicprinciplesofstiffener/webconstructionareillustratedin
Example23.1.
Example 23.1
Acantileverbeam(Fig.23.1)carriesconcentratedloads,asshown.Calculatethedistributionofstiffener
loads and the shear flow distribution in the web panels assuming that the latter is effective only in
shear.
WenotethatstiffenersHKDandJKarerequiredatthepointofapplicationofthe4000Nloadto
resist its vertical and horizontal components. A further transverse stiffener GJC is positioned at the
unloaded end J of the stiffener JK, since stress concentrations are produced if a stiffener ends in the
centerofawebpanel.Wenotealsothatthewebpanelsareonlyeffectiveinshearsothattheshearflow
isconstantthroughoutaparticularwebpanel;theassumeddirectionsoftheshearflowsareshownin
Fig.23.1.
Itisinstructiveatthisstagetoexaminethephysicalroleofthedifferentstructuralcomponentsin
supportingtheappliedloads.Generally,stiffenersareassumedtowithstandaxialforcesonlysothat
Copyright©2010,T.H.G.Megson. PublishedbyElsevierLtd. Allrightsreserved.
DOI:10.1016/B978-1-85617-932-4.00023-3 619