11.1 Loads on Structural Components 353Fig.11.3
Typical lift distribution for a wing/fuselage combination.
However,thereis,conveniently,apointintheaerofoilsectionaboutwhichthemomentduetothelift
and drag forces remains constant. We therefore replace the lift and drag forces acting at the CP by
lift and drag forces acting at the aerodynamic center (AC) plus a constant momentM 0 ,asshownin
Fig.11.2(b).(Actually,athighMachnumbersthepositionoftheACchangesduetocompressibility
effects.)
While the chordwise pressure distribution fixes the position of the resultant aerodynamic load in
thewingcrosssection,thespanwisedistributionlocatesitspositioninrelation,say,tothewingroot.
Atypicaldistributionforawing/fuselagecombinationisshowninFig.11.3.Similardistributionsoccur
onhorizontalandverticaltailsurfaces.
Therefore, we see that wings, tailplane, and the fuselage are each subjected to direct, bending,
shear,andtorsionalloadsandmustbedesignedtowithstandcriticalcombinationsofthese.Notethat
maneuvers and gusts do not introduce different loads but result only in changes of magnitude and
position of the type of existing loads shown in Fig. 11.1. Over and above these basic in-flight loads,
fuselagesmaybepressurizedandtherebysupporthoopstresses,wingsmaycarryweaponsand/orextra
fueltankswithresultingadditionalaerodynamicandbodyforcescontributingtotheexistingbending,
shear,andtorsion,whilethethrustandweightofenginesmayaffecteitherfuselageorwingsdepending
ontheirrelativepositions.
Ground loads encountered in landing and taxiing subject the aircraft to concentrated shock loads
throughtheundercarriagesystem.Themajorityofaircrafthavetheirmainundercarriagelocatedinthe
wings,withanosewheelortailwheelintheverticalplaneofsymmetry.Clearlythepositionofthemain
undercarriageshouldbesuchastoproduceminimumloadsonthewingstructurecompatiblewiththe
stabilityoftheaircraftduringgroundmaneuvers.Thismaybeachievedbylocatingtheundercarriage
justforwardoftheflexuralaxisofthewingandasclosetothewingrootaspossible.Inthiscase,the
shocklandingloadproducesagivenshear,minimumbendingplustorsion,withthelatterbeingreduced
asfaraspracticablebyoffsettingthetorquecausedbytheverticalloadintheundercarriagelegbya
torqueinanoppositesenseduetobraking.
Other loads include engine thrust on the wings or fuselage which acts in the plane of symmetry
butmay,inthecaseofenginefailure,causeseverefuselagebendingmoments,asshowninFig.11.4;
concentratedshockloadsduringacatapultlaunch;andhydrodynamicpressureonthefuselagesorfloats
ofseaplanes.