Flight testing

Stall (flight)

For other uses, see stall.

In aerodynamics, a stall is a sudden reduction in the lift forces generated by an airfoil. This occurs when the critical angle of attack of the airfoil is exceeded, typically about 14 to 16 degrees.

Because stalls are most commonly discussed in connection with aviation, this article discusses stalls mainly as they relate to aircraft. In simple terms, a stall in an aircraft is an event that causes the wing to lose lift suddenly. An aerodynamic stall does not necessarily mean that the engine(s) of an aircraft have stopped working, or that the aircraft has stopped moving.

Overview

Aircraft are supported in the air by an aerodynamic force called lift, which is generated by the wings of the aircraft as air flows past the wings as a result of the forward movement of the aircraft. The wings of the aircraft generate lift because of a pressure reduction above the wings. The lower pressure above the wing combined with the higher pressure under the wing constitutes lift. An immediate increase in lift can be obtained by increasing the angle between the wing chord and the relative airflow. (The angle is called the angle of attack, or AOA, and is often symbolized with the Greek letter alpha.)

But increasing the AOA also increases drag. Without a sufficient increase in engine power, the aircraft slows, and wing-lift decreases. Above a particular angle, the "critical angle of attack", the airflow behind the wings becomes turbulent, the wing-lift largely disappears, and the wing stalls—that is, it suddenly ceases to provide enough lift to support the aircraft. In addition, the turbulence dramatically increases drag, which further slows the aircraft as it moves through the air, further reducing wing-lift. Rapidly the aircraft begins to accelerate downward.

In many aircraft recovering from a stall is simple. Since the stall is caused by an excessive angle of attack, simply pointing the nose of the aircraft downward will arrest the stall by reducing the angle between the wings and the flow of air (this is for a fixed wing aircraft rather than a helicopter). Some aircraft have a natural tendency to pitch downward (sometimes dramatically) when the wings stall; others must be directed downward by the pilot. As soon as the angle of attack drops below the critical angle, the aerodynamic stall of the wings will cease: the wings will start to produce lift and far less drag. However, the aircraft may still be flying too slowly to generate enough lift to prevent the aircraft from continuing to descend: complete stall recovery includes regaining this necessary speed.

In some circumstances stalls can result in more complicated problems, such as a 'spin' or a 'deep stall'.

A stall is caused by the pilot attempting to fly the aircraft too slowly, or to pull up too quickly from a dive, or to turn too steeply. Each of these causes the nose to be lifted until the wing's critical angle of attack is exceeded. Increasing engine power counteracts the increased drag caused by the stall and also increases air speed, and this helps in recovery from a stall. The critical action in recovering from a stall is reduction in the angle of attack, i.e., lowering the nose.

Altitude (height above the ground) is lost by the aircraft during the stall itself but considerably more height can be lost during the recovery. If the aircraft is already at a high altitude this is not a problem. If the aircraft is very close to the ground, however, a stall may cause the aircraft to lose so much altitude that it hits the ground before recovery from the stall is possible. For this reason, pilots are especially careful to avoid stalls during take-off and landing procedures, when the aircraft is very close to the ground.

Stalls in aircraft usually do not occur without warning. In addition to sensors which alert the pilot when the aircraft is about to stall, experienced pilots can sense an incipient stall by noting changes in the behavior of the aircraft. Since the conditions that produce stalls are very well understood, pilots can easily avoid stalls, and many pilots never experience stalls outside of their pilot training. Standard pilot training includes training in the proper ways to avoid, recognize, and recover from stalls.

A few types of aircraft with a T-shaped tail or rear-mounted engines can enter a deep stall or superstall. This is a type of stall that produces turbulence behind the wings that can interfere with the operation of engines or the tail of the aircraft. Recovery from a deep stall can be impossible, resulting in a crash. Some aircraft with such characteristics are fitted with special control devices to prevent the aircraft from ever approaching a position that can cause a deep stall. An example of such a device is a stick pusher, which forces the nose of the aircraft down whenever it approaches a stall, regardless of any actions taken by the pilot.

The remainder of this article describes stalls in more technical terms.

Formal definition

A stall is a condition in aerodynamics and aviation where the angle between the wing's chord line and the relative incoming wind (the angle of attack) increases beyond a certain point such that the lift begins to decrease. The angle at which this occurs is called the critical angle of attack. This critical angle is dependent upon the profile of the wing, its planform, and its aspect ratio but is typically in the range of 8 to 20 degrees relative to the incoming wind for most 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 usually represents the boundary between the wing's linear and nonlinear airflow regimes. Flow separation begins to occur at this point, decreasing lift, increasing drag, and changing the wing's center of pressure. A fixed-wing aircraft during a stall may experience buffeting or a change in attitude (normally nose down in General aviation aircraft). Most aircraft are designed to have a gradual stall with characteristics that will warn the pilot and give the pilot time to react. For example an aircraft that does not buffet before the stall may have an audible alarm or a stick shaker installed to simulate the feel of a buffet by vibrating the stick fore and aft. The "buffet margin" is, for a given set of conditions, the amount of ‘g’, which can be imposed for a given level of buffet. The critical angle of attack in steady straight and level flight can only be attained at low airspeed. Attempts to increase the angle of attack at higher airspeeds can cause a high speed stall or may merely cause the aircraft to climb.

Any yaw of the aircraft as it enters the stall regime can result in autorotation, which is also sometimes referred to as a 'spin'. Because air no longer flows smoothly over the wings during a stall, aileron control of roll becomes less effective, whilst simultaneously the tendency for the ailerons to generate adverse yaw increases. This increases the lift from the advancing wing and accentuates the probability of the aircraft to enter into a spin.

Depending on the aircraft's design, a stall can expose extremely adverse properties of balance and control; particularly in a prototype.

Graph

The graph shows that the greatest amount of lift is produced as 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 exceeds the critical angle, the lift produced by the airfoil decreases progressively.

The information in a graph of this kind is gathered using a model of the airfoil in a wind tunnel. Steady operation of an aircraft at an angle of attack above the critical angle is not possible because, after exceeding the critical angle the aircraft behaves dynamically in a way that quickly causes the angle of attack to return to a value less than the critical angle. This dynamic maneuver indicates the stall of the aircraft.

This graph shows the stall angle, yet in practice most pilots discuss stalling in terms of airspeed. This is because all aircraft are equipped with an airspeed indicator, but very few aircraft have an angle of attack indicator. An aircraft's stalling speeds is published in the Flight Manual for a range of weights and flap positions, but the stalling angle of attack is not published.

As speed reduces, angle of attack increases 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 take off and land at a lower speed.

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 practiced 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, a result of the turbulent air separated from the wing hitting the tail of the airplane.

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.

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 air will be deflected by the control surfaces (ailerons, elevator and rudder) 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 speed”

Stalls depend more on angle of attack rather than airspeed. However, since, for every weight of every aircraft, there is an airspeed at which the wing's needed angle of attack will exceed the stall angle or critical angle of attack, airspeed in a given configuration is often used as an indirect indicator of approaching stall conditions.

There are multiple V speeds which are used to indicate when a stall will occur:

  • VS: the computed stalling speed with flaps retracted at design speed. Often has the same value as VS1.
  • VS0: the stalling speed or the minimum steady flight speed in landing configuration (full flaps, landing gear down, spoiler retracted).
  • VS1: the stalling speed or the minimum steady flight speed in a specific configuration (usually a "clean" configuration with flaps, landing gear and spoilers all retracted).
  • VSR: reference stall speed.
  • VSR0: reference stall speed in the landing configuration.
  • VSR1: reference stall speed in a specific configuration.
  • VSW: speed at which onset of natural or artificial stall warning occurs.

On an airspeed indicator, the bottom of the white arc indicates VS0 at maximum weight, while the bottom of the green arc indicates VS1 at maximum weight. While an aircraft's VS speed is computed by design, its VS0 and VS1 speeds must be demonstrated empirically by flight testing.

Turning flight stall

A turning flight stall is a stall that occurs while the aircraft is turning. In turning flight the stalling speed of an aircraft is faster than the stalling speed in straight, level flight.

A notable example of an air accident caused by a stall in turning flight is the 1994 Fairchild Air Force Base B-52 crash.

When an aircraft is turning, the lift required is equal to the weight of the aircraft plus extra lift to provide the centripetal force necessary to perform the turn. The lift required in a banked turn is:
L = nW
where L is the lift on the aircraft

n is the load factor during the turn
W is the weight of the aircraft

Regardless of the weight of an aircraft or the angle of bank, the stall always occurs at the same angle of attack on the wing, and the same aircraft lift coefficient. In turning flight, the wing of an aircraft must generate more lift than is required in straight, level flight, so the airflow over the wing separates from the upper surface of the wing when the aircraft is flying at a faster airspeed than in straight, level flight. If the stalling speed of an aircraft in straight, level flight is V_s, when that aircraft is turning with a load factor n its stalling speed V_{st} is:

V_{st} = V_s sqrt n

Deep stall

A deep stall (also called a superstall) is a dangerous type of stall that affects certain aircraft designs, notably those with a T-tail configuration. In these designs, the turbulent wake of a stalled main wing "blankets" the horizontal stabilizer, rendering the elevators ineffective and preventing the aircraft from recovering from the stall.

Although effects similar to deep stall had long been known to occur on many aircraft designs, the name first came into widespread use after a deep stall caused the prototype BAC 1-11 to crash, killing its crew. This led to changes to the aircraft, including the installation of a stick shaker (see below) in order to clearly warn the pilot of the problem before it occurred. Stick shakers are now a part of all commercial airliners. Nevertheless, the problem continues to haunt new designs; in the 1980s a prototype of the latest model of the Canadair Challenger business jet entered deep stall during testing, killing one of the test pilots who was unable to leave the plane in time. Also, paragliders are sometimes known to enter a deep stall condition.

Deep stall is possible with some sailplanes, as their most common designs are T-tail configurations. The IS-29 glider is one of the gliders that are vulnerable to deep stalls when the CG and the overall weight are between certain limits.

In the early 1980s, a Schweizer SGS 1-36 sailplane was modified for NASA's controlled deep-stall flight program.

A different type of stall affecting the F-16 fighter is also known as a deep stall because of its similar difficulty in recovery, but for a different reason. The aircraft is designed to be inherently unstable, which when kept under control by its "fly-by-wire" system allows for higher maneuverability. However, this design, coupled with the intent of the control computer to keep the fighter level, prevents the aircraft from pitching nose-down in a stall, which would allow the pilot to recover given sufficient altitude. This is known as a deep stall because the elevators are rendered useless by the flight computer even though, unlike a T-tail, air does contact the elevators, and even with the computer disabled it is difficult to recover from (the pilot must "rock" the aircraft with elevator input until it pitches nose-down, which can take several seconds).

Stall warning and safety devices

Aeroplanes can be equipped with devices to prevent or postpone a stall or to make it less (or in some cases more) severe, or to make recovery easier.

  • An aerodynamic 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.
  • 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 that protrude past the boundary layer into the free stream. As the name implies they energize the boundary layer by mixing free stream airflow with boundary layer flow thereby creating vortices, this increases the inertia of the boundary layer. By increasing the inertia of the boundary layer airflow separation and the resulting stall may be delayed.
  • 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. It pushes the elevator control forwards as the stall is approached, causing a reduction in the angle of attack. Generically, a stick pusher is known as a stall identification device or stall identification system.
  • A stick shaker is a mechanical 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 AOA Indicator or A.K.A Lift Reserve Indicator is a pressure differential instrument that integrates airspeed and angle of attack into one instantaneous, continuous readout. An AOA indicator provides a visual display of the amount of available lift throughout its slow speed envelope regardless of the many variables which act upon an aircraft. This indicator is immediately responsive to changes in speed, angle of attack and wind conditions and automatically compensates for aircraft weight, altitude, and temperature.
  • 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.

Stall warning system systems are often involve inputs from a broad range of sensors and systems to include a dedicated angle of attack sensor.

Blockage, damage, or inoperation of stall and angle of attack (AOA) probes can lead to the stall warning becoming unreliable and cause the stick pusher, overspeed warning, autopilot and yaw damper to malfunction.

If a forward canard is used for pitch control, rather than an aft tail, the canard is designed to meet the airflow at a slightly greater angle of attack than the wing. Therefore, when the aircraft pitch increases abnormally, the canard will usually stall first, causing the nose to drop and so preventing the wing from reaching its critical AOA. 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. Spoilers are also deployed asymmetrically (i.e. on one wing only) to enhance roll control. Spoilers can also be used on aborted take-offs and after main wheel contact on landing to increase the aircraft's weight on its wheels for better braking action.

To look at it in a more simple way spoilers are effectively lift dumpers. A roll to the left could be aided by the left wing spoiler erecting.

Popular misconceptions

Since most aircraft have one or more engines, some confusion exists within the general public and most news media between an aerodynamic stall and the so-called "stall" of an aircraft engine. The misunderstanding results from the common use of the word "stall" with respect to automotive engines. An aerodynamic stall is an abrupt loss of lift due to excessive angle of attack, as explained in this article. An engine "stall" is an abrupt loss of power from an engine and a resulting loss of thrust; the proper aviation term for this situation is engine failure. In all correctly described aviation contexts, stall means aerodynamic stall. Nevertheless, many reports in the media and eye-witness accounts by lay people (and even the law enforcement officers responding to the scene of an accident) quoted in these sources will still often incorrectly use the word "stall" when referring to what should properly be called an engine failure.

The above confusion is wide-spread in the general public because of the very common misuse of the term stall in the media. This misunderstanding also results in another less common popular misconception, namely, that aircraft that have suffered an engine failure will just fall from the sky. This is because an aerodynamic stall will usually result in a significant loss of altitude during the recovery from the stall. If one assumes that stalls have something to do with the engines then one can see how this misconception can arise. However, and to the contrary, all fixed-wing aircraft rely on their wings to generate lift, and can therefore safely glide, sometimes for great distances (depending on their altitude at the time of engine failure), without any thrust from the aircraft engine(s).

References

  • Chapter 4, "Slow Flight, Stalls, and Spins," in the Airplane Flying Handbook (FAA H-8083-3A)
  • Clancy, L.J. (1975), Aerodynamics, Pitman Publishing Limited, London. ISBN 0 273 01120 0
  • Alpha Systems AOA Website for information on AOA and Lift Reserve Indicators
  • How Does a Stick Shaker Sensor Work?
  • 4239-01 Angle of Attack (AoA) Sensor Specifications

Notes

See also

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