Control at high angles of attack

At low speeds, where the angle of the wing is high, and the aircraft is flying in a nose-up attitude, the tail surfaces may be partially immersed in the wake of the wing. To prevent this, the horizontal tail surface may either be mounted low so that it is out of the wake, as on the Tornado (Fig. 3.14), or high, as on the C-17 (Fig. 10.20). When the high option is chosen, the tail may need to be

WING TIP ROLL REACTION CONTROL VALVE

Fig. 10.21 Reaction controls are necessary on the VTOL Harrier (Diagrams courtesy of Rolls-Royce pic)

mounted very high indeed, as on the C-17, otherwise it may come under the influence of wing wash just before the stall. This creates a very dangerous con­dition known as superstall, where not only is the aircraft stalled, but the lack of control means that the pilot can do nothing about it. The stall is likely to deepen, and recovery at low altitude is impossible; a situation that has caused a number of fatal accidents.

Because of the dangers of stalling, particularly at low altitude, a number of automatic devices may be fitted to warn, or otherwise help the pilot to avoid a stall. These include a flashing light, and an audible warning that is triggered off if the angle of attack is sensed to be too high.

On small aircraft, the onset of a stall can often be sensed by the shaking of the control stick, caused by buffeting of the control surfaces by the turbulent separated flow. On aircraft with powered controls, this buffeting may not be transmitted, and a stick shaker mechanism can be fitted to shake the stick artificially, and thus provoke the required conditioned response from the pilot.

In some cases, particularly on large airliners, a stick pusher may be incor­porated. This is a device that automatically pushes the stick forwards to reduce

the angle of attack when it is sensed to be too high. A good description of such devices is again given by Davies (1971).

The use of reliable fly-by-wire systems enables aircraft to be flown much closer to the stall than was previously considered either possible or advisable. Some control systems may even allow the aircraft to be flown in a partially stalled condition.