Page 273 - Aircraft Stuctures for Engineering Student

254 Airworthiness and airframe loads

of lift is
4 pvS(dCL/aa)u
An = (8.23)
W
where W is the aircraft weight. Expressing Eq. (8.23) in terms of the wing loading,
w = W/S, we have

(8.24)

This increment in gust load factor is additional to the steady level flight value n = 1.
Therefore, as a result of the gust, the total gust load factor is
; pV(dCL/aa)u
n=l+ (8.25)
W
Similarly, for a downgust

f pV(dCL/da)u
n=l- (8.26)
W
If flight conditions are expressed in terms of equivalent sea-level conditions then V
becomes the equivalent airspeed (EAS), VE, u becomes uE and the air density p is
replaced by the sea-level value po. Equations (8.25) and (8.26) are written

(8.27)

and

(8.28)

We observe from Eqs (8.25)-(8.28) that the gust load factor is directly proportional to
aircraft speed but inversely proportional to wing loading. It follows that high-speed
aircraft with low or moderate wing loadings are most likely to be affected by gust
loads.
The contribution to normal acceleration of the change in tail load produced by the
gust may be calculated using the same assumptions as before. However, the change in
tailplane incidence is not equal to the change in wing incidence due to downwash
effects at the tail. Thus if AP is the increase (or decrease) in tailplane load, then
AP = tPo V~S~AC,,~ (8.29)
where ST is the tailplane area and ACL,T the increment of tailplane lift coefficient
given by

(8.30)

is
in which dC~,~/da the rate of change of tailplane lift coefficient with wing
incidence. From aerodynamic theory
-- --(&E)
dCL,T
acL,T
&k! &T