Page 270 - Aircraft Stuctures for Engineering Student

8.6 Gust loads 251

From Eq. (8.19) we see that the load factor n in the turn is given by

n = sec4 (8.20)
Also, dividing Eq. (8.17) by Eq. (8.18)
V2
tan4=- (8.21)
gR
Examination of Eq. (8.21) reveals that the tighter the turn the greater the angle of
bank required to maintain horizontal flight. Furthermore, we see from Eq. (8.20)
that an increase in bank angle results in an increased load factor. Aerodynamic
theory shows that for a limiting value of n the minimum time taken to turn through
a given angle at a given value of engine thrust occurs when the lift coefficient CL is a
maximum; that is, with the aircraft on the point of stalling.

In Section 8.4 we considered aircraft loads resulting from prescribed manoeuvres in
the longitudinal plane of symmetry. Other types of in-flight load are caused by air
turbulence. The movements of the air in turbulence are generally known as gusts
and produce changes in wing incidence, thereby subjecting the aircraft to sudden or
gradual increases or decreases in lift from which normal accelerations result. These
may be critical for large, high speed aircraft and may possibly cause higher loads
than control initiated manoeuvres.
At the present time two approaches are employed in gust analysis. One method,
which has been in use for a considerable number of years, determines the aircraft
response and loads due to a single or ‘discrete’ gust of a given profile. This profile
is defined as a distribution of vertical gust velocity over a given finite length or
given period of time. Examples of these profiles are shown in Fig. 8.13.
Early airworthiness requirements specified an instantaneous application of gust
velocity u, resulting in the ‘sharp-edged’ gust of Fig. 8.13(a). Calculations of normal
acceleration and aircraft response were based on the assumptions that the aircraft’s
flight is undisturbed while the aircraft passes from still air into the moving air of
the gust and during the time taken for the gust loads to build up; that the aerodynamic
forces on the aircraft are determined by the instantaneous incidence of the particular
lifting surface and finally that the aircraft’s structure is rigid. The second assumption
here relating the aerodynamic force on a lifting surface to its instantaneous incidence
neglects the fact that in a disturbance such as a gust there is a gradual growth of
circulation and hence of lift to a steady state value (Wagner effect). This in general
leads to an overestimation of the upward acceleration of an aircraft and therefore
of gust loads.
The ‘sharp-edged’ gust was replaced when it was realized that the gust velocity built
up to a maximum over a period of time. Airworthiness requirements were modified on
the assumption that the gust velocity increased linearly to a maximum value over a
specified gust gradient distance H. Hence the ‘graded’ gust of Fig. 8.13(b). In the
UK, H is taken as 30.5 m. Since, as far as the aircraft is concerned, the gust velocity
builds up to a maximum over a period of time it is no longer allowable to ignore the