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Stall

Normally, increasing the angle between a wing and the airflow causes the lift produced to increase. This can continue until a point is reached where maximum lift is generated and this is known as the stall or stall angle. Any further increase in angle does not produce a corresponding increase in lift, and may in fact lead to a sudden reduction in lift, a change in pitching moment or a wing drop.

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Aerodynamic description of a stall

Stalling an aeroplane

An aeroplane can be made to stall by reducing the speed to the stall speed (which corresponds to the stall angle described above). When an aeroplane approaches the stall speed it has already adopted an extremely nose-high attitude, and the pilot will notice the controls have become less responsive. The pilot may also notice some buffeting, an aerodynamic vibration caused by the airflow starting to detach from the wing surface. In most cases, as the stall is reached the aircraft will start to descend (because the wing is no longer producing enough lift to support the aeroplane) and the nose will pitch down. Recovery from this stalled state usually involves the pilot lowering the nose and increasing the speed, until normal flight can be resumed. The manoeuvre is normally quite safe and if correctly handled leads to only a small loss of height. It is normally taught and practiced purely in order to help pilots recognise and avoid it.

Stalling characteristics

Different aircraft types have different stalling characteristics. A benign stall is one where the nose drops gently and the wings remain level throughout. Slightly more demanding is a stall where one wing stalls slightly before the other, causing that wing to drop sharply, with the possibility of entering a spin. A dangerous stall is one where the nose rises, pushing the wing deeper into the stalled state and potentially leading to an unrecoverable deep stall.

Stall devices

Aeroplanes can be equipped with a variety of devices to prevent or postpone a stall or to make it less (or in some cases more) severe, or to make recovery easier. A stall strip is a small sharp-edged device which, when attached to the leading edge of a wing, encourages the stall to start there in preference to any other location on the wing. If attached close to the wing root it makes the stall gentle and progressive; if attached near the wing tip it encourages the aircraft to drop a wing when stalling. An anti-stall strake is a wing extension at the root leading edge which generates a vortex on the wing upper surface to postpone the stall. A stick-pusher is a mechanical device which prevents the pilot from stalling an aeroplane by pushing the controls forwards as the stall is approached. A stick-shaker is a similar device which shakes the pilot's controls to warn of the onset of stall. A stall warning is an electronic or mechanical device which sounds an audible warning as the stall speed is approached.



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