354 CHAPTER 11 Structural Components of Aircraft
Fig.11.4
Fuselage and wing bending caused by an unsymmetrical engine load.
In Chapter 12, we shall examine in detail the calculation of ground and air loads for a variety of
cases.
11.2 FunctionofStructuralComponents................................................................
Thebasicfunctionsofanaircraft’sstructurearetotransmitandresisttheappliedloads,toprovidean
aerodynamicshape,andtoprotectpassengers,payload,systems,andsoforthfromtheenvironmental
conditions encountered in flight. These requirements, in most aircraft, result in thin shell structures
where the outer surface or skin of the shell is usually supported by longitudinal stiffening members
andtransverseframestoenableittoresistbending,compressive,andtorsionalloadswithoutbuckling.
Suchstructuresareknownassemi-monocoque,whilethinshellswhichrelyentirelyontheirskinsfor
theircapacitytoresistloadsarereferredtoasmonocoque.
First,weshallconsiderwingsectionswhich,whileperformingthesamefunction,candifferwidely
intheirstructuralcomplexity,ascanbeseenbycomparingFigs.11.5and11.6.InFig.11.5,thewingof
thesmall,lightpassengeraircraft,theDeHavillandCanadaTwinOtter,comprisesarelativelysimple
arrangementoftwospars,ribs,stringers,andskin,whilethewingoftheHarrierinFig.11.6consistsof
numerousspars,ribs,andskin.However,nomatterhowcomplextheinternalstructuralarrangement,
thedifferentcomponentsperformthesamekindoffunction.Theshapeofthecrosssectionisgoverned
by aerodynamic considerations and clearly must be maintained for all combinations of load; this is
one of the functions of the ribs. They also act with the skin in resisting the distributed aerodynamic
pressureloads;theydistributeconcentratedloads(e.g.,undercarriageandadditionalwingstoreloads)
intothestructureandredistributestressarounddiscontinuities,suchasundercarriagewells,inspection
panels,andfueltanks,inthewingsurface.Ribsincreasethecolumnbucklingstressofthelongitudinal
stiffeners by providing end restraint and establishing their column length; in a similar manner, they
increasetheplatebucklingstressoftheskinpanels.
Thedimensionsofribsaregovernedbytheirspanwisepositioninthewingandbytheloadstheyare
requiredtosupport.Intheouterportionsofthewing,wherethecrosssectionmayberelativelysmallif