Stall
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A stall is a (usually undesired) condition in aerodynamics and aviation where there is a sudden loss of lift. In unaccelerated flight, a stall is usually associated with a certain airspeed below which the aircraft will not continue to fly. More rigorously defined, a stall occurs when the critical angle of attack is exceeded.
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 on the upper surface of the airfoil.
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Graph
This graph shows the typical behaviour of most airfoils:
Lift_Curve.jpg
Image:Lift_Curve.jpg
Aerodynamic description of a stall
Stalling an aeroplane
An aeroplane can be made to stall in any pitch attitude or bank angle or at any airspeed but is commonly practised by pilots 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. 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, 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 maneuver is normally quite safe and if correctly handled leads to only a small loss in altitude. It is taught and practiced in order to help pilots recognize, avoid, and recover from stalling the airplane.
The most common stall-spin scenarios occur on takeoff (departure stall) and during landing (base to final turn). Stalls also occur during a go-around maneuver if the pilot does not properly respond to the out-of-trim situation resulting from the transition from low power setting to high power setting at low speed.
A special form of asymmetric stall in which the aircraft also rotates about its yaw axis is called a spin. A spin will occur if an aircraft is stalled and there is an asymmetric yawing moment applied to it. This yawing moment can be aerodynamic (sideslip angle, rudder, adverse yaw from the ailerons), thrust related (p-factor, one engine inoperative on a multi-engine non-centerline thrust aircraft), or from any number of possible sources of yaw.
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 warning and safety devices
Airplanes 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 slight twist can be introduced to the wing with the leading edge near the wing tip twisted downward. This is called washout and causes the wing root to stall before the wing tip. This makes the stall gentle and progressive. Since the stall is delayed at the wing tips, where the ailerons are, roll control is maintained when the stall begins.
- The wing can be built with aerodynamic twist; the airfoil changes shape toward the wing tip in such a way that the wing tip has a lower stall speed than the wing root. This serves the same purpose as washout.
- 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.
- Vortex generators, tiny strips of metal or plastic placed on top of the wing near the leading edge, lower the stall speed by preventing flow separation over the top of the wing.
- 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 high-lift canard can be placed near the nose of the aircraft. This adds to the overall lift of the airframe and can make the aircraft nearly stall-proof. In this case the canard is designed to stall at a slightly higher speed than the wing. When the canard stalls the nose drops, lowering the angle of attack thus preventing the wing from stalling.
- 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. The majority of aircraft contain some form of this device that warns the pilot of an impending stall. The simplest such device is a 'stall warning horn', which consists of either a pressure sensor or a moveable metal tab that actuates a switch, and produces an audible warning in response.
Military aircraft often have an angle of attack indicator which lets the pilot know precisely how close to the stall point the aircraft is.
Accelerated stalls
A type of stall that is related to the ordinary stall is the so-called Accelerated 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, a high-G pullup, or other manoeuvre where directions is changed with a significant amount of acceleration. This additional acceleration results in a high force that must be borne by the wings. In recent years there have been a number of accidents arising from Accelerated stalls in high-performance aircraft (e.g. the Jet Provost) that have been sold into the civilian sector from the military. Turbulence can cause an accelerated stall if the aircraft is flying below Vno (maximum structural cruising speed. If flying above Vno, turbulence can cause structural failure.
Note that all manoeuvres increase g to some extent - a 60° bank level 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 also
Disambiguation
Alternately, 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.
An instruction pipeline of a modern CPU may stall when execution cannot continue because previous events that the current instruction is dependent on has not yet completed.
A stall may also refer to:
- any small compartment (such as a shower stall or restroom stall)
- the living quarters for an animal in a barn or stable
- a holding compartment in a shed for one animal without much room for movement, a bale
- a high-backed booth at a restaurant
- a semi-permanent place for conducting salesde:Strömungsabriss