Page 278 - Aircraft Stuctures for Engineering Student

8.7 Fatigue 259

During taxiing the aircraft may be manoeuvring over uneven ground with a full
payload so that wing stresses, for example, are greater than in the static case. Also,
during take-off and climb and descent and landing the aircraft is subjected to the
greatest load fluctuations. The undercarriage is retracted and lowered; flaps are
raised and lowered; there is the impact on landing; the aircraft has to carry out
manoeuvres; and, finally, the aircraft, as we shall see, experiences a greater number
of gusts than during the cruise.
The loads corresponding to these various phases must be calculated before the
associated stresses can be obtained. Thus, for example, during take-off, wing bending
stresses and shear stresses due to shear and torsion are based on the total weight of
the aircraft including full fuel tanks, and maximum payload all factored by 1.2 to
allow for a bump during each take-off on a hard runway or by 1.5 for a take-off
from grass. The loads produced during level flight and symmetric manoeuvres are
calculated using the methods described in Sections 8.4 and 8.5. From these values
distributions of shear force, bending moment and torque may be found in, say: the
wing by integrating the lift distribution. Loads due to gusts are calculated using the
methods described in Section 8.6. Thus, due to a single equivalent sharp-edged gust
the load factor is given either by Eq. (8.25) or Eq. (8.26).
Although it is a relatively simple matter to determine the number of load fluctua-
tions during a ground-air-ground cycle caused by standard operations such as
raising and lowering flaps, retracting and lowering the undercarriage etc., it is more
difficult to estimate the number and magnitude of gusts an aircraft will encounter.
For example, there is a greater number of gusts at low altitude (during take-off,
climb and descent) than at high altitude (during cruise). Terrain (sea, flat land,
mountains) also affects the number and magnitude of gusts as does weather. The
use of radar enables aircraft to avoid cumulus where gusts are prevalent, but has
little effect at low altitude in the climb and descent where clouds cannot easily be
avoided. The ESDU (Engineering Sciences Data Unit) has produced gust data
based on information collected by gust recorders carried by aircraft. These show,
in graphical form (Ilo versus h curves, h is altitude), the average distance flown at
various altitudes for a gust having a velocity greater than f3.05 m/s to be encoun-
tered. In addition, gustfrequency curves give the number of gusts of a given velocity
per 1000 gusts of velocity 3.05m/s. Combining both sets of data enables the gust
exceedmzce to be calculated, i.e. the number of gust cycles having a velocity greater
than or equal to a given velocity encountered per kilometre of flight.
Since an aircraft is subjected to the greatest number of load fluctuations during
taxi-take-off-climb and descent-standoff-landing while little damage is caused
during cruise, the fatigue life of an aircraft does not depend on the number of
flying hours but on the number of flights. However, the operational requirements
of aircraft differ from class to class. The Airbus is required to have a life free from
fatigue cracks of 24000 fights or 30000 hours, while its economic repair life is
48 000 flights or 60 000 hours; its landing gear, however, is designed for a safe life
of 32000 flights, after which it must be replaced. On the other hand the BAe 146,
with a greater number of shorter fights per day than the Airbus, has a specified
crack free life of 40 000 fights and an economic repair life of 80 000 flights. Although
the above figures are operational requirements, the nature of fatigue is such that it is
unlikely that all of a given type of aircraft will satisfy them. Thus, of the total number