14.3 Fatigue Strength of Components 405fluctuations.Theundercarriageisretractedandlowered;flapsareraisedandlowered;thereisanimpact
onlanding;theaircrafthastocarryoutmaneuvers;finally,theaircraft,asweshallsee,experiencesa
greaternumberofguststhanduringthecruise.
Theloadscorrespondingtothesevariousphasesmustbecalculatedbeforetheassociatedstresses
canbeobtained.Forexample,duringtake-off,wingbendingstressesandshearstressesduetoshearand
torsionarebasedonthetotalweightoftheaircraftincludingfullfueltanks,andmaximumpayloadall
factoredby1.2toallowforabumpduringeachtake-offonahardrunwayorby1.5foratake-offfrom
grass.Theloadsproducedduringlevelflightandsymmetricmaneuversarecalculatedusingthemethods
describedinSection13.2.Fromthesevalues,distributionsofshearforce,bendingmoment,andtorque
may be found in, say, the wing by integrating the lift distribution. Loads due to gusts are calculated
usingthemethodsdescribedinSection13.4.Thus,becauseofasingleequivalentsharp-edgedgust,the
loadfactorisgiveneitherbyEq.(13.25)orbyEq.(13.26).
Although it is a relatively simple matter to determine the number of load fluctuations during a
ground–air–groundcyclecausedbystandardoperationssuchasraisingandloweringflaps,retracting
andloweringtheundercarriage,andsoon,itismoredifficulttoestimatethenumberandmagnitude
ofgustsanaircraftencounters.Forexample,thereisagreaternumberofgustsatlowaltitude(during
take-off,climb,anddescent)thanathighaltitude(duringcruise).Terrain(sea,flatland,mountains)also
affectsthenumberandmagnitudeofgusts,asdoesweather.Theuseofradarenablesaircrafttoavoid
cumuluswheregustsareprevalentbuthaslittleeffectatlowaltitudeintheclimbanddescentwhere
cloudscannoteasilybeavoided.TheEngineeringSciencesDataUnit(ESDU)hasproducedgustdata
basedoninformationcollectedbygustrecorderscarriedbyaircraft.Theseshow,ingraphicalform(l 10
versushcurves,hisaltitude),theaveragedistanceflownatvariousaltitudesforagusthavingavelocity
greaterthan±3.05m/stobeencountered.Inaddition,gustfrequencycurvesgivethenumberofgusts
ofagivenvelocityper1000gustsofvelocity3.05m/s.Combiningbothsetsofdataenablesthegust
exceedancetobecalculated—thatis,thenumberofgustcycleshavingavelocitygreaterthanorequal
toagivenvelocityencounteredperkilometerofflight.
Sinceanaircraftissubjectedtothegreatestnumberofloadfluctuationsduringtaxi–take-off–climb
anddescent–standoff–landing,whilelittledamageiscausedduringcruise,thefatiguelifeofanaircraft
doesnotdependonthenumberofflyinghoursbutonthenumberofflights.However,theoperational
requirementsofaircraftdifferfromclasstoclass.TheAirbusisrequiredtohavealifefreefromfatigue
cracksof24000flightsor30000hours,whileitseconomicrepairlifeis48000flightsor60000hours;its
landinggear,however,isdesignedforasafelifeof32000flights,afterwhichitmustbereplaced.Onthe
otherhand,theBAe146,withagreaternumberofshorterflightsperdaythantheAirbus,hasaspecified
crack-freelifeof40000flightsandaneconomicrepairlifeof80000flights.Althoughtheabovefigures
are operational requirements, the nature of fatigue is such that it is unlikely that all of a given type
of aircraft will satisfy them. Of the total number of Airbus aircraft, at least 90 percent will achieve
the above values and 50 percent will be better; clearly, frequent inspections are necessary during an
aircraft’slife.
14.3 FatigueStrengthofComponents...................................................................
InSection12.2.4,wediscussedtheeffectofstresslevelonthenumberofcyclestofailureofamate-
rial such as mild steel. As the stress level is decreased, the number of cycles to failure increases,