 |
 |
|
 |
 |
|
|
|
|
|
|
Encyclopedia :
S :
ST :
STA :
Stall |
|
|
StallA stall is a (usually undesired) condition in aerodynamics and aviation where there is a sudden loss of lift. A stall is usually associated with a certain airspeed at which a stall occurs and below which the aircraft will not continue to fly. Normally, increasing the angle of attack between an airfoil and the airflow causes the lift and drag 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 but will in fact lead to a sudden reduction in lift, a change in pitching moment or a wing drop. This is due to flow separation occurring near the leading edge of the airfoil. GraphThis graph shows the typical behaviour of most airfoils:
Aerodynamic description of a stallStalling an aeroplaneAn aeroplane can be made to stall in any pitch or roll angle or at any airspeed but is commonly practised by pilotss reducing the speed to the stall speed, at a safe altitude. Stall speed varies on different airplanes and is represented by color codes on the air speed indicator. As the plane flies at this speed the angle of attack must be increased to prevent any loss of altitude or gain in airspeed (which corresponds to the stall angle described above). Any attempt to prevent the plane from descending by applying increasing up elevator control input or increasing the airspeed by use of the throttle will cause the airplane to stall. When an aeroplane approaches the stall speed smooth air over the top and bottom of the wings no longer remains smooth causing the relationship of high pressure below the wing and low pressure above the wing to lose its effect. The pilot will notice the flight controls have become less responsive and may also notice some buffeting, an aerodynamic vibration caused by the airflow starting to detach from the wing surface. In most light aircraft, depending on the stability charactistics of the airplane, as the stall is reached the aircraft will start to descend (because the wing is no longer producing enough lift to support the aeroplane's weight) and the nose will pitch down. Recovery from this stalled state usually involves the pilot decreasing the angle of attack and increasing the air speed, until smooth air flow over the wing is resumed. Normal flight can be resumed once recovery from the stall is complete. The manuoevre is normally quite safe and if correctly handled leads to only a small amount of loss in altitude. It is normally taught and practiced purely in order to help pilots recognize and avoid stalling the airplane in critical phases of flight such as landing and takeoff where there is insufficient altitude to recover. A special form of asymmetric stall in which the aircraft also rotates about its yaw axis is called a spin. Stalling characteristicsDifferent 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 warning and safety devicesAeroplanes 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. Military aircraft often have an angle of attack indicator which lets the pilot know precisely how close to the stall point the aircraft is. G-stallsA type of stall that is related to the ordinary stall is the so-called G-stall. This is a condition where the wing cannot produce enough lift to support the aircraft's weight and centrifugal force, in spite of otherwise flying at a reasonable airspeed and angle of attack. This can occur when an aircraft is in a tight turn or other manoeuvre where a significant amount of Gee is being pulled. This additional force must be borne by the wings. In recent years there have been a number of accidents arising from G-stalls in high-performance aircraft (e.g. the Jet Provost) that have been sold into the civilian sector from the military. In most types of GA aircraft, high-speed G-stalls are unknown because structural failure will usually occur before the G-stall condition could be met, but in military aircraft this is not necessarily true. Note that all manoeuvres increase g to some extent - a 60° bank turn will produce 2g, which will raise the stalling speed by 1.4142 times, or 41%. Quoted aircraft stalling speeds must be taken as the 1g stalling speed (i.e. straight and level flight). See alsoDisambiguationAlternately, a stall is a phenomenon whereby an internal combustion engine abruptly ceases operating and stops turning. This can happen spontaneously (perhaps due to fuel starvation or a mechanical failure), or in response to a sudden increase in engine load (perhaps due to incorrect manual transmission driving technique). Since most aircraft have an engine, too, a lot of language confusion exists between the two completely different types of stall that can be experienced. Many people seem to believe that an aircraft will drop out of the sky as soon as the engine stops in flight. This is entirely untrue. When the propulsion from the engine is no longer there, most aircraft will first slow up and then simply drop their nose. This gains them additional speed, and the aircraft will now be descending (in a controlled fashion) at more or less the same airspeed as it had before. The pilot has enough time to find a suitable landing surface on the ground. Put differently, all powered aircraft (even the biggest ones) are also gliders. There have been cases of airliners running out of fuel at altitude that landed successfully at airports a hundred kilometers away. The Gimli Glider is a celebrated example. A stall may also refer to:
|
|
|
This article is from Wikipedia. All text is available under the terms of the GNU Free Documentation License. |
|
| © 2008 Chamas Enterprises Inc. |
