Introduction to Aircraft Structural Analysis (Elsevier Aerospace Engineering)

398 CHAPTER 13 Airframe Loads

13.4.3 Gust Envelope

Airworthinessrequirementsusuallyspecifythatgustloadsshallbecalculatedatcertaincombinationsof
gustandflightspeed.Theequationsforgustloadfactorintheaboveanalysisshowthatnisproportional
toaircraftspeedforagivengustvelocity.Therefore,wemayplotagustenvelopesimilartotheflight
envelopeofFig.12.1,asshowninFig.13.13.Thegustspeeds±U 1 ,±U 2 ,and±U 3 arehigh,medium,
andlowvelocitygusts,respectively.Cutoffsoccuratpointswherethelinescorrespondingtoeachgust
velocitymeetspecificaircraftspeeds.Forexample,AandFdenotespeedsatwhichagustofvelocity
±U 1 wouldstallthewing.
Theliftcoefficient–incidencecurveis,aswenotedinconnectionwiththeflightenvelope,affected
bycompressibilityandthereforealtitudesothataseriesofgustenvelopesshouldbedrawnfordifferent
altitudes. An additional variable in the equations for gust load factor is the wing loadingw. Further,
gustenvelopesshouldthereforebedrawntorepresentdifferentconditionsofaircraftloading.
TypicalvaluesofU 1 ,U 2 ,andU 3 are20m/s,15.25m/s,and7.5m/s.Itcanbeseenfromthegust
envelopethatthemaximumgustloadfactoroccursatthecruisingspeedVC.Ifthisvalueofnexceeds
thatforthecorrespondingflightenvelopecase—thatis,n 1 —thenthegustcasewillbethemostcritical
in the cruise. Let us consider a civil, nonaerobatic aircraft for whichn 1 =2.5,w=2400N/m^2 ,and
∂CL/∂α=5.0/rad.TakingF=0.715wehave,fromEq.(13.33)

n= 1 +

1

2 ×1.223VC×5.0×0.715×15.25

2400

givingn= 1 +0.0139VC,wherethecruisingspeedVCisexpressedasanEAS.Forthegustcasetobe
critical

1 +0.0139VC>2.5

Fig.13.13

Typical gust envelope.