Introduction to Aircraft Structural Analysis (Elsevier Aerospace Engineering)

12.2 Load Factor Determination 375

Table 12.1

Category

Load Factorn Normal Semiaerobatic Aerobatic

n 1 2.1+24000/(W+10000) 4.5 6.0

n 2 0.75n 1 butn 2 <\2.0 3.1 4.5

n 3 1.0 1.8 3.0

12.2 LoadFactorDetermination.........................................................................

Several problems require solutions before values for the various load factors in the flight envelope
can be determined. The limit load, for example, may be produced by a specified maneuver or by an
encounterwithaparticularlyseveregust(gustcasesandtheassociatedgustenvelopearediscussedin
Section13.4.Clearly,someknowledgeofpossiblegustconditionsisrequiredtodeterminethelimiting
case.Furthermore,thefixingoftheproofandultimatefactorsalsodependonthedegreeofuncertaintyof
design,variationsinstructuralstrength,structuraldeterioration,andsoforth.Weshallnowinvestigate
someoftheseproblemstoseetheircomparativeinfluenceonloadfactorvalues.

12.2.1 Limit Load

Anaircraftissubjectedtoavarietyofloadsduringitsoperationallife,themainclassesofwhichareas
follows:maneuverloads,gustloads,undercarriageloads,cabinpressureloads,buffeting,andinduced
vibrations.Ofthese,maneuver,undercarriage,andcabinpressureloadsaredeterminedwithreasonable
simplicity since maneuver loads are controlled design cases, undercarriages are designed for given
maximumdescentrates,andcabinpressuresarespecified.Theremainingloadsdependtoalargeextent
ontheatmosphericconditionsencounteredduringflight.Therefore,estimatesofthemagnitudesofsuch
loadsareonlypossibleifin-flightdataontheseloadsareavailable.
Obviously, it requires a great number of hours of flying if the experimental data are to include
possibleextremesofatmosphericconditions.Inpractice,theamountofdatarequiredtoestablishthe
probableperiodofflighttimebeforeanaircraftencounters,say,agustloadofagivenseverityisagreat
dealmorethanthatavailable.Therefore,it becomesaprobleminstatisticstoextrapolatetheavailable
dataandcalculatetheprobabilityofanaircraftbeingsubjectedtoitsprooforultimateloadduringits
operationallife.Theaimwouldbeforazeroornegligiblerateofoccurrenceofitsultimateloadand
an extremely low rate of occurrence of its proof load. Having decided on an ultimate load, then the
limitloadmaybefixedasdefinedinSection12.1,althoughthevalueoftheultimatefactorincludes,
aswehavealreadynoted,allowancesforuncertaintiesindesign,variationinstructuralstrength,and
structuraldeterioration.

12.2.2 Uncertainties in Design and Structural Deterioration

Neitherofthesepresentsseriousproblemsinmodernaircraftconstructionandthereforedonotrequire
large factors of safety to minimize their effects. Modern methods of aircraft structural analysis are
refined,and,inanycase,teststodetermineactualfailureloadsarecarriedoutonrepresentativefull-scale