Angle of attack - AbsoluteAstronomy.com

Angle of attack is a term used in fluid dynamics

Fluid dynamics

In physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow—the natural science of fluids in motion. It has several subdisciplines itself, including aerodynamics and hydrodynamics...

to describe the angle

Angle

In geometry, an angle is the figure formed by two rays sharing a common endpoint, called the vertex of the angle.Angles are usually presumed to be in a Euclidean plane with the circle taken for standard with regard to direction. In fact, an angle is frequently viewed as a measure of an circular arc...

between a reference line on a lifting body (often the chord line

Chord (aircraft)

In aeronautics, chord refers to the imaginary straight line joining the trailing edge and the center of curvature of the leading edge of the cross-section of an airfoil...

of an airfoil

Airfoil

An airfoil or aerofoil is the shape of a wing or blade or sail as seen in cross-section....

) and the vector representing the relative motion between the lifting body and the fluid through which it is moving. Angle of attack is the angle between the lifting body's reference line and the oncoming flow. This article focuses on the most common application, the angle of attack of a wing or airfoil moving through air.

In Aerodynamics

Aerodynamics

Aerodynamics is a branch of dynamics concerned with studying the motion of air, particularly when it interacts with a moving object. Aerodynamics is a subfield of fluid dynamics and gas dynamics, with much theory shared between them. Aerodynamics is often used synonymously with gas dynamics, with...

, angle of attack is used to describe the angle between the chord line of the wing of a fixed-wing aircraft

Fixed-wing aircraft

A fixed-wing aircraft is an aircraft capable of flight using wings that generate lift due to the vehicle's forward airspeed. Fixed-wing aircraft are distinct from rotary-wing aircraft in which wings rotate about a fixed mast and ornithopters in which lift is generated by flapping wings.A powered...

and the vector representing the relative motion between the aircraft and the atmosphere. Since a wing can have twist, a chord line of the whole wing may not be definable, so an alternate reference line is simply defined. Often, the chord line of the root of the wing

Wing root

The wing root is the part of the wing on a fixed-wing aircraft that is closest to the fuselage. On a simple monoplane configuration, this is usually easy to identify...

is chosen as the reference line. Another alternative is to use a horizontal line on the fuselage as the reference line (and also as the longitudinal axis). Some books adopt the so called absolute angle of attack: zero angle of attack corresponds to zero coefficient of lift

Lift coefficient

The lift coefficient is a dimensionless coefficient that relates the lift generated by a lifting body, the dynamic pressure of the fluid flow around the body, and a reference area associated with the body...

.

Some British authors have used the term angle of incidence instead of angle of attack. However, this can lead to confusion with the term riggers' angle of incidence meaning the angle between the chord of an aerofoil and some fixed datum in the aeroplane.

Relation between angle of attack and lift

The lift coefficient

Lift coefficient

of a fixed-wing aircraft

Fixed-wing aircraft

varies uniquely with angle of attack. Increasing angle of attack is associated with increasing lift coefficient up to the maximum lift coefficient, after which lift coefficient decreases.

As the angle of attack of a fixed-wing aircraft increases, separation

Flow separation

All solid objects travelling through a fluid acquire a boundary layer of fluid around them where viscous forces occur in the layer of fluid close to the solid surface. Boundary layers can be either laminar or turbulent...

of the airflow from the upper surface of the wing becomes more pronounced, leading to a reduction in the rate of increase of the lift coefficient. The figure shows a typical curve for a cambered

Camber (aerodynamics)

Camber, in aeronautics and aeronautical engineering, is the asymmetry between the top and the bottom surfaces of an aerofoil. An aerofoil that is not cambered is called a symmetric aerofoil...

straight wing. A symmetrical wing has zero lift at 0 degrees angle of attack. The lift curve is also influenced by wing planform

Planform

In aviation, a planform is the shape and layout of a fixed-wing aircraft's fuselage and wing. Of all the myriad planforms used, they can typically be grouped into those used for low-speed flight, found on general aviation aircraft, and those used for high-speed flight, found on many military...

. A swept wing

Swept wing

A swept wing is a wing planform favored for high subsonic jet speeds first investigated by Germany during the Second World War. Since the introduction of the MiG-15 and North American F-86 which demonstrated a decisive superiority over the slower first generation of straight-wing jet fighters...

has a lower flatter curve with a higher critical angle.

Critical angle of attack

The critical angle of attack is the angle of attack which produces maximum lift coefficient. This is also called the "stall angle of attack". Below the critical angle of attack, as the angle of attack increases, the coefficient of lift (Cl) increases. At the same time, below the critical angle of attack, as angle of attack increases, the air begins to flow less smoothly over the upper surface of the airfoil

Airfoil

An airfoil or aerofoil is the shape of a wing or blade or sail as seen in cross-section....

and begins to separate from the upper surface. On most airfoil shapes, as the angle of attack increases, the upper surface separation point of the flow moves from the trailing edge towards the leading edge. At the critical angle of attack, upper surface flow is more separated and the airfoil or wing is producing its maximum coefficient of lift. As angle of attack increases further, the upper surface flow becomes more and more fully separated and the airfoil/wing produces less coefficient of lift.

Above this critical angle of attack, the aircraft is said to be in a stall

Stall (flight)

In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded...

. A fixed-wing aircraft by definition is stalled at or above the critical angle of attack rather than at or below a particular airspeed

Airspeed

Airspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are: indicated airspeed , calibrated airspeed , true airspeed , equivalent airspeed and density airspeed....

. The airspeed at which the aircraft stalls varies with the weight of the aircraft, the load factor

Load factor

Load factor may refer to:* Load factor , the ratio of the lift of an aircraft to its weight* Load factor , the ratio of the number of records to the number of addresses within a data structure...

, bank angle, the center of gravity of the aircraft and other factors. However the aircraft always stalls at the same critical angle of attack. The critical or stalling angle of attack is typically around 15° for many airfoils.

Some aircraft are equipped with a built-in flight computer that automatically prevents the aircraft from increasing the angle of attack any further when a maximum angle of attack is reached, irrespective of pilot input. This is called the 'angle of attack limiter' or 'alpha limiter'. Modern airliners that have fly-by-wire technology avoid the critical angle of attack by means of software in the computer systems that govern the flight control surfaces.

Takeoff and landing operations from short runways, such as Naval Aircraft Carrier operations and STOL

STOL

STOL is an acronym for short take-off and landing, a term used to describe aircraft with very short runway requirements.-Definitions:There is no one accepted definition of STOL and many different definitions have been used by different authorities and nations at various times and for a myriad of...

back country flying, aircraft may be equipped with angle of attack or Lift Reserve Indicators. These indicators measure the angle of attack (AOA) or the Potential of Wing Lift (POWL, or Lift Reserve) directly and help the pilot fly close to the stalling point with greater precision. STOL

STOL

operations require the aircraft to be able to operate at the critical angle of attack during landings and best angle of climb during takeoffs. Angle of attack indicators are used by pilots for maximum performance during these maneuvers since airspeed information is of less value.

Very high alpha

Some military aircraft are able to achieve very high angles of attack, but at the cost of massive induced drag. This provides the aircraft with great agility. A famous military example is Pugachev's Cobra

Pugachev's Cobra

In aerobatics, Pugachev's Cobra is a dramatic and demanding manoeuvre in which a plane flying at a moderate speed suddenly raises the nose momentarily to the vertical position and slightly beyond, before dropping it back to normal flight. It uses a potent engine thrust to maintain approximately...

.

Using a variety of additional aerodynamic surfaces — known as high-lift devices — like leading edge extension

Leading edge extension

A leading edge extension is a small extension to an aircraft wing surface, forward of the leading edge. Different kinds of extensions have been used for different reasons.-Leading edge slats:...

s (leading edge wing root extensions), fighter aircraft

Fighter aircraft

A fighter aircraft is a military aircraft designed primarily for air-to-air combat with other aircraft, as opposed to a bomber, which is designed primarily to attack ground targets...

have increased the potential flyable alpha from about 20° to over 45°. However, military aircraft usually will not obtain such high alpha in combat, as it robs the aircraft of speed very quickly. Not only do such maneuvers slow the aircraft down, but they can cause significant structural stress at high speed. Modern flight control systems tend to limit a fighter's angle of attack to well below its maximum aerodynamic limit.

Sailing

In sailing

Sailing

Sailing is the propulsion of a vehicle and the control of its movement with large foils called sails. By changing the rigging, rudder, and sometimes the keel or centre board, a sailor manages the force of the wind on the sails in order to move the boat relative to its surrounding medium and...

, the angle of attack is the angle between a mid-sail and the relative wind. The physical principles involved are the same as for aircraft. See points of sail

Points of sail

Points of sail describes a sailing boat's course in relation to the wind direction.There is a distinction between the port tack and the starboard tack. If the wind is coming from anywhere on the port side, the boat is on port tack. Likewise if the wind is coming from the starboard side, the boat...

A simple and non-technical explanation

The angle of attack can be simply described as the difference between where a wing is pointing and where it is going. While the angle of attack is less than a critical angle (which will vary for different wings depending on the shape of the aerofoil) the wing will generate lift. If the critical angle is exceeded then the wing will stall (i.e. stop producing lift) and this can happen at any speed above the so-called ‘slow stall speed.’
A wing does NOT stall because it is going at, or less, than what is called the ‘slow stall speed.’ What happens is that because the wing is going at that speed it does not generate sufficient lift and the critical angle of attack is exceeded because of gravity. The wing may be pointing parallel to the ground but gravity is pulling it down so the difference between where it is pointing and where it is going exceeds the critical angle and the wing stalls.

High Speed Stalls. A wing can stall at any speed above the ‘slow stall speed’ for the same reason. For example, an aircraft may go into a steep dive. At the bottom of the dive the pilot may pull back on the stick to make the aircraft flatten out and then climb. If done too quickly, or because of poor aircraft design, the wings can be pointing up but momentum i.e. gravity, continues to make the aircraft go down. If the wings are pointing up but the direction of travel is down the critical angle is exceeded and a high speed stall occurs.

To counter a stall the pilot pushes the stick forward, or applies power, or does both. Pushing the stick forward makes the direction of pointing and travel go below the critical angle and gravity accelerates the aircraft so the wing starts producing lift and therefore goes into an unstalled condition. That is, the direction of travel and pointing direction come in to harmony. Adding power can prevent the aircraft dropping very far because of gravity (which is very useful if near the ground) but the nose still needs to be lowered which again reduces the angle of attack below the critical level.
A cunning design for a wing is in existence which can help prevent a stall across an entire wing. See washout (aviation)

Washout (aviation)

Washout refers to a feature of wing design to deliberately reduce the lift distribution across the span of the wing of an aircraft. The wing is designed so that angle of incidence is higher at the wing roots and decreases across the span, becoming lowest at the wing tip...

for details.

Relation between angle of attack and lift

Critical angle of attack

Very high alpha

Sailing

A simple and non-technical explanation

See also