Page 284 - Aircraft Stuctures for Engineering Student

8.7 Fatigue 265

Fig. 8.18 Gust exceedance curve.

Integrating Eq. (8.52) over the whole range of gusts likely to be encountered, we
obtain the total damage D, per km of flight. Thus
(8.53)

Further, if the average block length journey of an aircraft is R,, the average gust
damage per flight is D,RaV. Also, some aircraft in a fleet will experience more gusts
than others since the distnbution of gusts is random. Therefore if, for example, it is
found that one particular aircraft encounters 50% more gusts than the average its
gust fatigue damage is 1.5Dg/km.
The gust damage predicted by Eq. (8.53) is obtained by integrating over a complete
gust velocity range from zero to infinity. Clearly there will be a gust velocity below
which no fatigue damage will occur since the cyclic stress produced will be below
the fatigue limit stress of the particular component. Equation (8.53) is therefore
rewritten

(8.54)

in which uf is the gust velocity required to produce the fatigue limit stress.
We have noted previously that more gusts are encountered during climb and
descent than during cruise. Altitude therefore affects the amount of fatigue damage
caused by gusts and its effects may be determined as follows. Substituting for the
gust exceedance E(ue) in Eq. (8.54) from Eq. (8.50) we obtain

or
1
D, =--$ per km (8.55)
4 0