CHAPTER 21 Fuselages.................................................................................
Aircraftfuselagesconsist,aswesawinChapter11,ofthinsheetsofmaterialstiffenedbylargenumbers
oflongitudinalstringerstogetherwithtransverseframes.Generally,theycarrybendingmoments,shear
forces, and torsional loads, which induce axial stresses in the stringers and skin together with shear
stressesintheskin;theresistanceofthestringerstoshearforcesisgenerallyignored.Also,thedistance
betweenadjacentstringersisusuallysmallsothatthevariationinshearflowintheconnectingpanelwill
besmall.Itisthereforereasonabletoassumethattheshearflowisconstantbetweenadjacentstringers
sothattheanalysissimplifiestotheanalysisofanidealizedsectioninwhichthestringers/boomscarry
all the direct stresses, while the skin is effective only in shear. The direct stress-carrying capacity of
theskin may beallowed for by increasing thestringer/boomareas as described in Section 19.3. The
analysisoffuselagesthereforeinvolvesthecalculationofdirectstressesinthestringersandtheshear
stressdistributionsintheskin;thelatterarealsorequiredintheanalysisoftransverseframes,aswe
shallseeinChapter23.
21.1 Bending...............................................................................................
The skin/stringer arrangement is idealized into one comprising booms and skin as described in
Section19.3.ThedirectstressineachboomisthencalculatedusingeitherEq.(15.18)orEq.(15.19),in
whichthereferenceaxesandthesectionpropertiesrefertothedirectstress-carryingareasofthecross
section.
Example 21.1
Thefuselageofalightpassenger-carryingaircrafthasthecircularcrosssectionshowninFig.21.1(a).
Thecross-sectionalareaofeachstringeris100mm^2 ,andtheverticaldistancesgiveninFig.21.1(a)are
tothemidlineofthesectionwallatthecorrespondingstringerposition.Ifthefuselageissubjectedto
abendingmomentof200kNmappliedintheverticalplaneofsymmetry,atthissection,calculatethe
directstressdistribution.
ThesectionisfirstidealizedusingthemethoddescribedinSection19.3.Asanapproximation,we
shall assume that the skin between adjacent stringers is flat so that we may use either Eq. (19.1) or
Eq. (19.2) to determine the boom areas. From symmetry,B 1 =B 9 ,B 2 =B 8 =B 10 =B 16 ,B 3 =B 7 =
Copyright©2010,T.H.G.Megson. PublishedbyElsevierLtd. Allrightsreserved.
DOI:10.1016/B978-1-85617-932-4.00021-X 577