11.2 Function of Structural Components 355thewingistaperedandtheloadsarelight,ribsactprimarilyasformersfortheaerofoilshape.Alight
structureissufficientforthispurpose,whereasatsectionsclosertothewingroot,wheretheribsare
requiredtoabsorbandtransmitlarge,concentratedappliedloads,suchasthosefromtheundercarriage,
enginethrust,andfuselageattachmentpointreactions,amuchmoreruggedconstructionisnecessary.
Between these two extremes are ribs which support hinge reactions from ailerons, flaps, and other
controlsurfaces,plusthemanyinternalloadsfromfuel,armament,andsystemsinstallations.
Theprimaryfunctionofthewingskinistoformanimpermeablesurfaceforsupportingtheaerody-
namicpressuredistributionfromwhichtheliftingcapabilityofthewingisderived.Theseaerodynamic
forcesaretransmittedinturntotheribsandstringersbytheskinthroughplateandmembraneaction.
Resistancetoshearandtorsionalloadsissuppliedbyshearstressesdevelopedintheskinandsparwebs,
whileaxialandbendingloadsarereactedbythecombinedactionofskinandstringers.
Althoughthethinskinisefficientforresistingshearandtensileloads,itbucklesundercomparatively
lowcompressiveloads.Ratherthanincreasetheskinthicknessandsufferaconsequentweightpenalty,
stringersareattachedtotheskinandribs,therebydividingtheskinintosmallpanelsandincreasingthe
bucklingandfailingstresses.Thisstabilizingactionofthestringersontheskinis,infact,reciprocated
tosomeextent,althoughtheeffectnormaltothesurfaceoftheskinisminimal.Stringersrelychieflyon
ribattachmentsforpreventingcolumnactioninthisdirection.Wehavenotedinthepreviousparagraph
thecombinedactionofstringersandskininresistingaxialandbendingloads.
The role of spar webs in developing shear stresses to resist shear and torsional loads has been
mentionedpreviously;theyperformasecondarybutsignificantfunctioninstabilizing,withtheskin,
thesparflanges,orcaps,whicharethereforecapableofsupportinglargecompressiveloadsfromaxial
andbendingeffects.Inturn,sparwebsexertastabilizinginfluenceontheskininasimilarmannerto
thestringers.
Whilethemajorityoftheprecedingremarkshavebeendirectedtowardwingstructures,theyapply,
as can be seen by referring to Figs. 11.5 and 11.6, to all the aerodynamic surfaces, namely, wings,
horizontal and vertical tails, except in theobvious cases of undercarriageloading, enginethrust, and
soon.
Fuselages,whileofdifferentshapetotheaerodynamicsurfaces,comprisememberswhichperform
similarfunctionstotheircounterpartsinthewingsandtailplane.However,therearedifferencesinthe
generationofthevarioustypesofload.Aerodynamicforcesonthefuselageskinarerelativelylow;onthe
otherhand,thefuselagesupportslarge,concentratedloadssuchaswingreactions,tailplanereactions,
andundercarriagereactions,anditcarriespayloadsofvaryingsizeandweight,whichmaycauselarge
inertiaforces.Furthermore,aircraftdesignedforhigh-altitudeflightmustwithstandinternalpressure.
The shape of the fuselage cross section is determined by operational requirements. For example, the
mostefficientsectionalshapeforapressurizedfuselageiscircularoracombinationofcircularelements.
Irrespectiveofshape,thebasicfuselagestructureisessentiallyasinglecellthin-walledtubecomprising
skin,transverseframes,andstringers;transverseframeswhichextendcompletelyacrossthefuselageare
knownasbulkheads.ThreedifferenttypesoffuselageareshowninFigs.11.5to11.7.InFig.11.5,the
fuselageisunpressurizedsothat,inthepassenger-carryingarea,amorerectangularshapeisemployed
tomaximizethespace.TheHarrierfuselageinFig.11.6containstheengine,fueltanks,andsoon,sothat
itscross-sectionalshapeis,tosomeextent,predetermined,whileinFig.11.7,thepassenger-carrying
fuselageoftheBritishAerospace146ispressurizedandthereforecircularinitscrosssection.