Introduction to Aircraft Structural Analysis (Elsevier Aerospace Engineering)

14.4 Prediction of Aircraft Fatigue Life 411

Fig.14.3

Gust exceedance curve.

Integrating Eq. (14.19) over the whole range of gusts likely to be encountered, we obtain the total
damageDg/kmofflight.Thus,

Dg=−

∫∞

0

1

2 N(Su,e)

dE(ue)

due

due (14.20)

Further,iftheaverageblocklengthjourneyofanaircraftisRav,theaveragegustdamageperflightis
DgRav.Also,someaircraftinafleetexperiencesmoreguststhanothers,sincethedistributionofgusts
israndom.Thereforeif,forexample,itisfoundthatoneparticularaircraftencounters50percentmore
guststhantheaverage,itsgustfatiguedamageis1.5Dg/km.
ThegustdamagepredictedbyEq.(14.20)isobtainedbyintegratingoveracompletegustvelocity
rangefromzerotoinfinity.Clearly,therewillbeagustvelocitybelowwhichnofatiguedamageoccurs,
since the cyclic stress produced will be below the fatigue limit stress of the particular component.
Equation(14.20)isthereforerewrittenas

Dg=−

∫∞

uf

1

2 N(Su,e)

dE(ue)

due

due (14.21)

inwhichufisthegustvelocityrequiredtoproducethefatiguelimitstress.
We have noted previously that more gusts are encountered during climb and descent than during
cruise.Altitudethereforeaffectstheamountoffatiguedamagecausedbygusts,anditseffectsmaybe