For other uses, see stall.

In aerodynamics, a stall is a sudden reduction in the lift forces generated by an airfoil. In the context of a Fluid flow relative to a body the lift force is the component of the Aerodynamic force that is Perpendicular to the flow An airfoil (in American English) or aerofoil (in British English) is the shape of a Wing or blade (of a Propeller, rotor This occurs when the critical angle of attack of the airfoil is exceeded, typically about 14 to 16 degrees. Angle of attack ( AOA, \alpha Greek letter alpha) is a term used in Aerodynamics to describe the Angle between the

Because stalls are most commonly discussed in connection with aviation, this article discusses stalls mainly as they relate to aircraft. Aviation refers to activities involving man-made flying devices ( Aircraft) including the people organizations and regulatory bodies involved with them In simple terms, a stall in an aircraft is an event that causes the wing to lose lift suddenly.

Note that an aerodynamic stall does not mean that an aircraft's engines have stopped 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. In the context of a Fluid flow relative to a body the lift force is the component of the Aerodynamic force that is Perpendicular to the flow 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. ^[1] An immediate increase in lift can be obtained by increasing the angle between the wing chord and the relative airflow. In reference to Aircraft, chord refers to the distance between the leading edge and trailing edge of a Wing, Horizontal stabilizer or Vertical stabilizer (The angle is called the angle of attack, or AOA, and is often symbolized with the Greek letter alpha. Angle of attack ( AOA, \alpha Greek letter alpha) is a term used in Aerodynamics to describe the Angle between the )

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 Fluid dynamics, turbulence or turbulent flow is a fluid regime characterized by chaotic Stochastic property changes ^[2] In addition, the turbulence dramatically increases drag, which further slows the aircraft as it moves through the air, further reducing wing-lift. In Fluid dynamics, drag (sometimes called fluid resistance) is the force that resists the movement of a Solid object through a Fluid (a 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. A stick pusher is a device installed in some Fixed-wing aircraft to prevent the aircraft from entering an aerodynamic stall.

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. Aviation refers to activities involving man-made flying devices ( Aircraft) including the people organizations and regulatory bodies involved with them In reference to Aircraft, chord refers to the distance between the leading edge and trailing edge of a Wing, Horizontal stabilizer or Vertical stabilizer The angle at which this occurs is called the critical angle of attack. This critical angle is dependant 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. A planform or Plan view is a vertical Orthographic projection of an object on a horizontal plane like a Map. The aspect ratio of a Shape is the ratio of its longer Dimension to its shorter dimension 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. The lift coefficient ( CL or CZ) is a non-dimensional coefficient that relates the lift generated by an Airfoil, the The word linear comes from the Latin word linearis, which means created by lines. This article describes the use of the term nonlinearity in mathematics Flow separation begins to occur at this point, decreasing lift, increasing drag, and changing the wing's center of pressure. In Fluid dynamics, drag (sometimes called fluid resistance) is the force that resists the movement of a Solid object through a Fluid (a The center of pressure is the point on a body where the sum total of the aerodynamic Pressure field acts causing a Force and no moment about that point A fixed-wing aircraft during a stall may experience buffeting or a change in attitude (normally nose down in General aviation aircraft). Aeroelasticity is the science which studies the interaction among inertial, elastic, and aerodynamic forces General aviation (abbr GA) is one of two categories of Civil aviation. 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. A stick shaker is a mechanical device to rapidly and noisily vibrate the control yoke (the "stick" of an aircraft to warn the pilot of an imminent stall 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'. In Aviation, the word autorotation is applied to operation of Fixed-wing aircraft and Rotary-wing aircraft. 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. For the band with a similar name see The Ailerons Ailerons are hinged control surfaces attached to the Trailing edge of the Wing of a Fixed-wing Adverse yaw is a secondary effect of the application of the Ailerons in Aircraft. 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.

An example of the relationship between angle of attack and lift of a wing. The exact relationship depends on the wing section profile/planform & its aspect ratio.

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 is exceeded beyond the critical angle, the lift produced by the wing decreases significantly. The airfoil is now stalled.

This graph shows the stall angle, yet in practice most pilots discuss stalling in terms of airspeed. Airspeed is the speed of an Aircraft relative to the air There are several different measures of airspeed indicated airspeed calibrated airspeed equivalent airspeed and true This is because in general terms the angle of attack can be related to airspeed - a lower speed requires a greater angle of attack to produce the necessary lift and vice versa. In the context of a Fluid flow relative to a body the lift force is the component of the Aerodynamic force that is Perpendicular to the flow 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. Flaps are hinged surfaces on the trailing edge of the wings of a Fixed-wing aircraft.

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

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. Overview Fixed-wing aircraft range from small training and recreational aircraft to Wide-body aircraft and military cargo aircraft. Flight dynamics is the science of air and space vehicle orientation and control in three dimensions Unaccelerated (1g) stall speed varies on different aeroplanes and is represented by colour codes on the air speed indicator. The airspeed indicator or airspeed gauge is an instrument used in an Aircraft to display the craft's Airspeed, typically in knots, to the 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. Light aircraft is a generic category of aircraft that comprises many official designations in many countries 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. In aviation a spin is an aggravated stall resulting in rotation about the center of gravity wherein the aircraft follows a downward corkscrew path 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. In aviation a spin is an aggravated stall resulting in rotation about the center of gravity wherein the aircraft follows a downward corkscrew path 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. For other uses see Stall. In Aerodynamics, a stall is a sudden reduction in the lift forces generated by an Airfoil 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”

Airspeed Indicator

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 angle of attack will exceed the critical angle of attack, airspeed in a given configuration is often used as an indirect indicator of approaching stall conditions. Airspeed is the speed of an Aircraft relative to the air There are several different measures of airspeed indicated airspeed calibrated airspeed equivalent airspeed and true Angle of attack ( AOA, \alpha Greek letter alpha) is a term used in Aerodynamics to describe the Angle between the Angle of attack ( AOA, \alpha Greek letter alpha) is a term used in Aerodynamics to describe the Angle between the

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

• V_S: the computed stalling speed with flaps retracted at design speed. In Aviation, V-speeds or Velocity-speeds are standard terms used to define Airspeeds important or useful to the operation of Aircraft, such Flaps are hinged surfaces on the trailing edge of the wings of a Fixed-wing aircraft. Often has the same value as V_S1.
• V_S0: the stalling speed or the minimum steady flight speed in landing configuration (full flaps, landing gear down, spoiler retracted). In Aviation, the undercarriage or landing gear is the structure (usually wheels that supports an Aircraft on the ground and allows it to taxi In Aeronautics a spoiler (sometimes called a lift dumper) is a device intended to reduce lift in an aircraft
• V_S1: 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).
• V_SR: reference stall speed.
• V_SR0: reference stall speed in the landing configuration.
• V_SR1: reference stall speed in a specific configuration.
• V_SW: speed at which onset of natural or artificial stall warning occurs.

On an airspeed indicator, the bottom of the white arc indicates V_S0 at maximum weight, while the bottom of the green arc indicates V_S1 at maximum weight. The airspeed indicator or airspeed gauge is an instrument used in an Aircraft to display the craft's Airspeed, typically in knots, to the While an aircraft's V_S speed is computed by design, its V_S0 and V_S1 speeds must be demonstrated empirically by flight testing.

Accelerated stall

Illustration of an accelerated stall, occurring during a co-ordinated turn.

• 1994 Fairchild Air Force Base B-52 crash

Deep stall

Illustration of a 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. The B-52 crash at Fairchild Air Force Base was a fatal air crash that occurred on June 24 1994 killing the four crew members of a United States Air Force (USAF B-52 A T-tail is an aircraft tail stabilizer arrangement in which the horizontal surfaces ( Tailplane and elevators are mounted to the top of the 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. WikipediaWikiProject Aircraft. Please see WikipediaWikiProject Aircraft/page content for recommended layout 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. The 1980s was the decade spanning from January 1 1980 to December 31 1989. Business jet, private jet or colloquially bizjet is a term describing a Jet aircraft, usually of smaller size designed for transporting groups of Also, paragliders are sometimes known to enter a deep stall condition. Paragliding is a recreational and competitive flying sport A paraglider is a free-flying foot-launched Aircraft.

Deep stall is possible with some sailplanes, as their most common designs are T-tail configurations. A T-tail is an aircraft tail stabilizer arrangement in which the horizontal surfaces ( Tailplane and elevators are mounted to the top of the 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. WikipediaWikiProject Aircraft. Please see WikipediaWikiProject Aircraft/page content for recommended layout Terminology A "glider" is an unpowered Aircraft. The most common types of glider are today used for sporting purposes The center-of-gravity (CG is the point at which an aircraft would balance if it were possible to suspend it at that point

In the early 1980s, a Schweizer SGS 1-36 sailplane was modified for NASA's controlled deep-stall flight program. The 1980s was the decade spanning from January 1 1980 to December 31 1989. The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program ^[3]

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. The wing root is that part of the wing on a fixed-wing aircraft that is closest to the Fuselage. 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. For the band with a similar name see The Ailerons Ailerons are hinged control surfaces attached to the Trailing edge of the Wing of a Fixed-wing
• 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. A stall strip is a fixed aerodynamic device employed on fixed-wing Aircraft to modify the Airfoil used 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. A vortex generator is an Aerodynamic surface consisting of a small Vane that creates a Vortex. In Physics and Fluid mechanics, a boundary layer is that layer of Fluid in the immediate vicinity of a bounding surface 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. The vis insita or innate force of matter is a power of resisting by which every body as much as in it lies endeavors to preserve in its present state whether it be of rest or of moving 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. V erification of the O rigins of R otation in T ornadoes Ex periment or VORTEX, is a field project that seeks to understand how a
• 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 pusher is a device installed in some Fixed-wing aircraft to prevent the aircraft from entering an aerodynamic stall.
• A stick shaker is a mechanical device which shakes the pilot's controls to warn of the onset of stall. A stick shaker is a mechanical device to rapidly and noisily vibrate the control yoke (the "stick" of an aircraft to warn the pilot of an imminent stall
• A stall warning is an electronic or mechanical device which sounds an audible warning as the stall speed is approached. A buzzer or beeper is a signalling device usually electronic typically used in Automobiles household appliances such as a Microwave oven, or 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. Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument A switch is a mechanical device used to connect and disconnect an electric Circuit at will
• 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.

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. In Aeronautics, canard ( French for Duck) is an airframe configuration of Fixed-wing aircraft in which the Tailplane is ahead of the 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. The lift coefficient ( CL or CZ) is a non-dimensional coefficient that relates the lift generated by an Airfoil, the

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. In Aeronautics a spoiler (sometimes called a lift dumper) is a device intended to reduce lift in an aircraft 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.

Popular misconception

Since most aircraft have one or more engines, some confusion exists within the lay public and news media between an aerodynamic stall and a stall of an aircraft engine. 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 halt of an engine and resulting loss of propulsion. In most aviation contexts, stall means aerodynamic stall.

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 [1]

Notes

See also

• Airfoil
• Wing twist
• Angle of attack
• Air safety
• Coefficient of lift
• Compressor stall
• Spin (flight)
• Spoiler (aeronautics)
• Dynamic stall