Page 269 - Aircraft Stuctures for Engineering Student

250 Airworthiness and airframe loads

We see from either Eq. (8.15) or Eq. (8.16) that the smaller the radius of the flight
path, that is the more severe the pull-out, the greater the value of n. It is quite possible
therefore for a severe pull-out to overstress the aircraft by subjecting it to loads which
lie outside the flight envelope and which may even exceed the proof or ultimate loads.
In practice, the control surface movement may be limited by stops incorporated in
the control circuit. These stops usually operate only above a certain speed giving
the aircraft adequate manoeuvrability at lower speeds. For hydraulically operated
controls 'artificial feel' is built in to the system whereby the stick force increases
progressively as the speed increases; a necessary precaution in this type of system
since the pilot is merely opening and closing valves in the control circuit and therefore
receives no direct physical indication of control surface forces.
Alternatively, at low speeds, a severe pull-out or pull-up may stall the aircraft.
Again safety precautions are usually incorporated in the form of stall warning devices
since, for modern high speed aircraft, a stall can be disastrous, particularly at low
altitude.

8.5.2 Correctly banked turn -
-__.)1_--- I
In this manoeuvre the aircraft flies in a horizontal turn with no sideslip at constant
speed. If the radius of the turn is R and the angle of bank 4, then the forces acting
on the aircraft are those shown in Fig. 8.12. The horizontal component of the lift
vector in this case provides the force necessary to produce the centripetal acceleration
of the aircraft towards the centre of the turn. Thus
wv2
Lsin4 =- (8.17)
gR
and for vertical equilibrium
Leos+= w (8.18)
or
L= Wsec4 (8.19)

Fig. 8.12 Correctly banked turn.