Direct lift control

The traditional control surfaces only directly produce turning moments. Thus, the conventional elevator control produces a pitching moment, which alters the pitch attitude, and hence angle of attack. This action only indirectly produces the desired effect of increasing the lift. However, by deflecting some form of flap or flaperon, while simultaneously deflecting the elevators downwards, it is possible to increase the overall lift, directly, without changing the aircraft’s pitch attitude. If the centre of gravity lies behind the wing centre of lift, as illus­trated in Fig. 10.16, then both wing and tail can contribute positive increases in lift whilst still keeping the moments in balance. Placing the centre of gravity aft, however, produces low natural stability unless a canard layout is used. To maintain stability and synchronise the controls it is almost essential to use an automatic system.

Such direct lift control is useful in combat manoeuvres. Very effective use of direct lift control has been demonstrated by the British Aerospace Harrier (Fig. 7.12). The Harrier however, uses the thrust vectoring capability of the engines rather than control surfaces. Direct lift control causes the aircraft to jump suddenly upwards, or downwards, an unorthodox manoeuvre that was found to be particularly useful for dodging missiles, and in aerial dog-fights. The Harrier was also the first production aircraft, other than a helicopter, to be able to fly both sideways and backwards.

Active control of individually adjustable surfaces can also be used to reduce structural loading, as described in Chapter 14.