Stall

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For other meanings, see stall (enclosure)

A stall is the slowing or stopping of a process. It is commonly applied to the phenomenon whereby an engine abruptly ceases operating and stops turning (perhaps due to fuel starvation or a mechanical failure), or in response to a sudden increase in engine load. This increase in engine load is common in a manual car when the clutch is released too suddenly.

In aerodynamics, a stall occurs when the critical angle of attack is exceeded, causing loss of lift and a large increase in drag due to disruption of airflow.

Aircraft can stall at any indicated airspeed, however the stall speed varies with the angle of attack, the angle of bank, and the set up of the aircraft. Deployed flaps will lower the stall speed, but deployed landing gear will increase it. Increased angle of bank increases stall speed.

An aircraft in flight is usually not pointed directly into the oncoming airflow. The angle (when viewed from the side of the aircraft) between the airflow and the wing is called the angle of attack (not to be confused with the pitch angle). If a pilot allows the angle of attack to become too large, the airflow will be unable to flow smoothly over the wing and instead it will begin to separate from the wing, creating a dramatic increase in drag and a loss of lift. This condition is known as a stall.

Stall recovery usually involves reducing the angle of attack to "break" the stall, and, if the aircraft is powered, adding power to increase airspeed.

A wing doesn't completely stall at the same time. The design of the wing dictates where the wing will begin to stall first, which dictates where the stall warning device is positioned, as well as where the ailerons are placed. With plane wings, the stalls tend to develop from the body of the plane out to the tips, however in a V shaped wing, this is reversed.

The hazard inherent in a stall is that the aircraft may lose considerable height in the recovery process. If the stall recovery requires more altitude than the aircraft has available, a collision with the ground will be the result.

Rigorous definition

A stall is a condition in aerodynamics and aviation where the angle between the wing's chord line and the relative wind, defined as the angle of attack, exceeds the critical angle of attack. This angle is typically 12 to 15 degrees for many subsonic airfoils. The critical angle of attack is the angle of attack on the lift coefficient versus angle-of-attack curve at which the maximum lift coefficient occurs, and it defines the boundary between the wing's linear and nonlinear regimes. Flow separation begins to occur at this point, decreasing lift, increasing drag, and changing the wing's pitching moment. A fixed-wing aircraft during a stall will experience buffeting, a change in pitching moment (nose up or nose down depending on tailplane configuration), and changes in most stability derivatives. The critical angle of attack can at 1g only be attained at low airspeed. Attempts to increase the angle of attack at higher airspeeds merely cause the aircraft to climb. Consequently at 1g, stalling occurs only when the aircraft is flying slowly.

Because of the reduced airflow at low airspeeds aileron control of roll becomes less effective, whereas the tendency for the ailerons to generate adverse yaw increases. Any yaw will increase the lift from the advancing wing and will cause the aircraft roll. Pitch and roll damping decrease due to lower dynamic pressures above and below the wings and turbulence in the airflow. 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 boundary layer separation occurring on the upper surface of the airfoil, and therefore the critical angle of attack is dependent not only on the geometry of the configuration, but on the Reynolds number and surface roughness.

Depending on the aircraft's design, a stall can expose extremely adverse properties of balance and control. The ease with which a particular craft will recover from a stall depends on the dynamics of the aircraft itself and the skill of the pilot. If the stall persists a high rate of descent will occur and a spin may also develop.

Graph

The graph shows that the greatest amount of lift is produced just before the critical angle of attack is reached (which in early 20th century aviation was called the "burble point"). This angle is 17.5 degrees in this case but changes from airfoil to airfoil. The graph shows that as the angle of attack is exceeded beyond the critical angle, the lift produced by the wing decreases significantly. The airfoil is now stalled.

Note that this graph shows the stall angle, yet in practice most pilots discuss stalling in terms of airspeed. This is because in general terms one can relate the angle of attack to airspeed - a lower speed requires a greater angle of attack to produce the necessary lift and vice versa. Thus as speed falls, AoA can increase, until the critical angle is reached. The airspeed at which this angle is reached is the (1g, unaccelerated) stalling speed of the aircraft in that particular configuration. Deploying flaps/slats decreases the stall speed to allow the aircraft to land at a lower speed.

The stall speed will be higher if the aircraft is experiencing more than one-g of longitudinal acceleration. The stall speeds found in many aircraft manuals only apply to unaccelerated flight.

Aerodynamic description of a stall

Stalling an aeroplane

If attempting the stall for flight training purposes, be sure to carry out correct checks before hand such as the HASEL check. This ensures that the engine is in the right condition and the area around the aircraft is safe and acceptable.

An aeroplane can be made to stall in any pitch attitude or bank angle or at any airspeed but is commonly practised by reducing the speed to the unaccelerated stall speed, at a safe altitude. Unaccelerated (1g) stall speed varies on different aeroplanes and is represented by colour 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). 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 manoeuvre is normally quite safe and if correctly handled leads to only a small loss in altitude. It is taught and practised in order to help pilots recognize, avoid, and recover from stalling the aeroplane.

The most common stall-spin scenarios occur on takeoff (departure stall) and during landing (base to final turn) because of insufficient airspeed during these manoeuvres. Stalls also occur during a go-around manoeuvre 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. Stall speed is increased when the upper wing surfaces are contaminated with ice or frost creating a rougher surface.

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-centreline thrust aircraft), or from any number of possible sources of yaw.

Since most aircraft have an engine, some confusion exists between an aerodynamic versus engine stall. Many people seem to believe that an aircraft will drop out of the sky as soon as the engine stops in flight. In reality, the pilot can simply lower its nose to generate enough airspeed to maintain lift over the wings and so prevent a stall. The aircraft will then descend at a steady airspeed. The pilot then has time to find a suitable landing area or to restart the engine.

Put differently, all powered aircraft (even the biggest ones) become gliders when they lose all thrust. There have been cases of airliners running out of fuel at high altitude that landed successfully at airports a hundred kilometres away. However the distance which an aircraft can glide is directly related to the airspeed, but most of all the density altitude which the aircraft is at. The Gimli Glider is a celebrated example.

Stalls can occur at higher speeds if the wings already have a high angle of attack. Attempting to increase the angle of attack at 1g by moving the control column back simply causes the aircraft to rise. However the aircraft may experience higher g, for example when it is pulling out of a dive. In this case, the wings will already be generating more lift to provide the necessary upwards acceleration and so there will be higher angle of attack. Increasing the g still further, by pulling back on the control column, can cause the stalling angle to be exceeded even at a high speed. High speed stalls produce the same buffeting characteristics as 1g stalls and can also initiate a spin if there is also any yawing.

Symptoms of an approaching stall

One symptom of an approaching stall is slow and sloppy controls. As the speed of the aeroplane decreases approaching the stall, there is less air moving over the wing and therefore less will be deflected by the control surfaces (ailerons, rudder and elevator) at this slower speed. Some buffeting may also be felt from the turbulent flow above the wings as the stall is reached. However during a turn this buffeting will not be felt and immediate action must be taken to recover from the stall. The stall warning will sound, if fitted, in most aircraft 5 to 10 knots above the stall speed.

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. This can occur in some T-tailed aircraft where the turbulent airflow from the stalled wing can blanket the control surfaces at the tail.

Stall warning and safety 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 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 is called 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 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 movable metal tab that actuates a switch, and produces an audible warning in response.
An angle of attack limiter or an "alpha" limiter is a flight computer that automatically prevents pilot input from causing the plane to rise over the stall angle. Some alpha limiters can be disabled by the pilot.

If a forward canard is used for pitch control rather than an aft tail, the canard is designed to stall at a slightly higher speed than the wing (i.e. the canard stalls first). When the canard stalls, the nose drops, lowering the angle of attack thus preventing the wing from stalling. Thus the wing virtually never stalls.

If an aft tail is used, the wing is designed to stall before the tail. In this case, the wing can be flown at higher lift coefficient (closer to stall) to produce more overall lift.

Many aircraft have an angle of attack indicator among the pilot's instruments which lets the pilot know precisely how close to the stall point the aircraft is.

Spoilers

In most circumstances, a stall is an undesirable event. Spoilers, however, are devices that are intentionally deployed to create a carefully controlled stall over part of an aircraft's wing, in order to reduce the lift it generates, and allow it to descend without gaining speed.

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