Page 274 - Aircraft Stuctures for Engineering Student

8.6 Gust loads 255

where dCL,T/aaT is the rate of change of CL,T with tailplane incidence and &/aa the
rate of change of downwash angle with wing incidence. Substituting for ACL:T from
Eq. (8.30) into Eq. (8.29), we have

(8.31)

For positive increments of wing lift and tailplane load
AnW = AL -k AP
:;;fa)
or, from Eqs (8.27) and (8.31)
An = i PO YE (~CL/~Q)~E 2
(
W + (8.32)

8.6.2 The 'graded' gust

The 'graded' gust of Fig. 8.13(b) may be converted to an equivalent 'sharp-edged' gust
by multiplying the maximum velocity in the gust by a gust alleviation factor, F. Thus
Eq. (8.27) becomes
PO ~E(~CL/~~)FUE
n=l+ (8.33)
It' , I
Similar modifications are carried out on Eqs (8.25), (8.26), (8.28) and (8.32). The gust
alleviation factor allows for some of the dynamic properties of the aircraft, including
unsteady lift, and has been calculated taking into account the heaving motion (that is,
the up and down motion with zero rate of pitch) of the aircraft only5.
Horizontal gusts cause lateral loads on the vertical tail or fin. Their magnitudes
may be calculated in an identical manner to those above, except that areas and
values of lift curve slope are referred to the vertical tail. Also, the gust alleviation
factor in the 'graded' gust case becomes Fl and includes allowances for the aero-
dynamic yawing moment produced by the gust and the yawing inertia of the aircraft.

8.6.3 Gust envelope
-=_^I~_I_II_II1-"---.~----,------- -_-.ll..--=_"*-._"-_._.__
Airworthiness requirements usually specify that gust loads shall be calculated at
certain combinations of gust and flight speed. The equations for gust load factor in
the above analysis show that n is proportional to aircraft speed for a given gust
velocity. Therefore, we may plot a gust envelope similar to the flight envelope of
Fig. 8.1, as shown in Fig. 8.15. The gust speeds fU1, fU2 and &Us are high,
medium and low velocity gusts respectively. Cut-offs occur at points where the
lines corresponding to each gust velocity meet specific aircraft speeds. For example,
A and F denote speeds at which a gust of velocity &U, would stall the wing.
The lift coefficient-incidence curve is, as we noted in connection with the flight
envelope, affected by compressibility and therefore altitude so that a series of gust
envelopes should be drawn for different altitudes. An additional variable in the