Page 285 - Aircraft Stuctures for Engineering Student

266 Airworthiness and airframe loads

in which llo is a function of height h and

Suppose that the aircraft is climbing at a speed V with a rate of climb ROC. The time
taken for the aircraft to climb from a height h to a height h + Sh is Sh/ROC during
which time it travels a distance VShIROC. Hence, from Eq. (8.55) the fatigue
damage experienced by the aircraft in climbing through a height Sh is

The total damage produced during a climb from sea level to an altitude H at a
constant speed V and rate of climb ROC is

(8.56)

Plotting l/llo against h from ESDU data sheets for aircraft having cloud warning
radar and integrating gives
6000 dh 9000 dh
- = 303; - = 14, - = 3.4
j3000 111) 16000 110
From the above dh/llo = 320.4, from which it can be seen that approximately 95
per cent of the total damage in the climb occurs in the first 3000 m.
An additional factor influencing the amount of gust damage is forward speed. For
example, the change in wing stress produced by a gust may be represented by

= klueV, (see Eq. (8.24)) (8.57)
in which the forward speed of the aircraft is in EAS. From Eq. (8.57) we see that the
gust velocity uf required to produce the fatigue limit stress S, is

uf = Sco/kl Ve (8.58)
The gust damage per km at different forward speeds V, is then found using Eq. (8.54)
with the appropriate value of uf as the lower limit of integration. The integral may be
evaluated by using the known approximate forms of N(S,,J and E(u,) from Eqs
(8.48) and (8.50). From Eq. (8.48)
m
sa = su,e = K, (1 + c/JG)

from which

where Su?, = kl Veu, and S;,, = kl VeuF Also Eq. (8.50) is