Editing Angle of attack

{{Short description|Angle between the chord of a wing and the undisturbed airflow}}
{{Redirect|Attack angle|rail technology|Attack angle (rail technology)}}

[[File:Airfoil angle of attack.jpg|thumb|upright=1.25|Angle of attack of an airfoil]]

In [[fluid dynamics]], '''angle of attack''' ('''AOA''', '''α''', or '''<math>\alpha</math>''') is the [[angle]] between a [[Airfoil#Airfoil terminology|reference line]] on a body (often the [[chord (aircraft)|chord line]] of an [[airfoil]]) and the [[vector (geometry)|vector]] representing the relative motion between the body and the fluid through which it is moving.<ref name="nasa">{{cite web|url=https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/foilinc|title=Inclination Effects on Lift|website=National Aeronautics and Space Administration|date=2018-04-05}}</ref> Angle of attack is the angle between the 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]], angle of attack specifies the angle between the chord line of the wing of a [[fixed-wing aircraft]] 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 [[Wing root|root of the wing]] is chosen as the reference line. Another choice is to use a horizontal line on the [[fuselage]] as the reference line (and also as the longitudinal axis).<ref>{{cite journal |title=Summary of Methods of Measuring Angle of Attack on Aircraft |url=https://ntrs.nasa.gov/api/citations/19930085167/downloads/19930085167.pdf |issue=NACA-TN-4351 |journal=NACA Technical Note |first=William |last=Gracey |year=1958 |pages=1–30 |publisher=NASA Technical Reports |access-date=2024-02-22}}</ref> Some authors<ref name="SHIF">John S. Denker, ''See How It Flies''. http://www.av8n.com/how/htm/aoa.html#sec-def-aoa</ref><ref name="SnH">Wolfgang Langewiesche, ''Stick and Rudder: An Explanation of the Art of Flying'', McGraw-Hill Professional, first edition (September 1, 1990), {{ISBN|0-07-036240-8}}</ref> do not use an arbitrary chord line but use the [[zero lift axis]] where, by definition, zero angle of attack corresponds to zero [[Lift coefficient|coefficient of lift]].

Some British authors have used the term [[angle of incidence (aerodynamics)|angle of incidence]] instead of angle of attack.<ref>Wolfgang Langewiesche, ''Stick and Rudder: An Explanation of the Art of Flying'', p. 7</ref>  However, this can lead to confusion with the term ''riggers' angle of incidence'' meaning the angle between the chord of an airfoil and some fixed datum in the airplane.<ref>Kermode, A.C. (1972), ''Mechanics of Flight'', Chapter 3 (8th edition), Pitman Publishing Limited, London {{ISBN|0-273-31623-0}}</ref>

==Relation between angle of attack and lift coefficient ==
[[File:MISB ST 0601.8 - Platform Angle of Attack.png|thumb|350px|Platform angle of attack]]
[[File:Coefficients of Drag and Lift vs AOA.jpg|thumb|300px|Coefficients of drag and lift versus angle of attack. Stall speed corresponds to the angle of attack at the maximum coefficient of lift (C<sub>L<small>MAX</small></sub>)]]
[[Image:Lift curve.svg|thumb|300px|right|A typical [[lift coefficient]] curve for an [[airfoil]] at a given [[airspeed]].]]
The [[lift coefficient]] of a [[fixed-wing aircraft]] varies 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.<ref name="nasa-lc">{{cite web|url=http://www.grc.nasa.gov/WWW/k-12/airplane/liftco.html|title=NASA Lift Coefficient}}</ref>

As the angle of attack of a fixed-wing aircraft increases, [[Flow separation|separation]] 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 [[Camber (aerodynamics)|cambered]] straight wing. Cambered airfoils are curved such that they generate some lift at small negative angles of attack.  A symmetrical wing has zero lift at 0 degrees angle of attack. The lift curve is also influenced by the wing shape, including its [[airfoil]] section and [[planform (aeronautics)|wing planform]].  A [[swept wing]] 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 the maximum lift coefficient. This is also called the "[[Stall (fluid dynamics)|stall]] angle of attack". Below the critical angle of attack, as the angle of attack decreases, the lift coefficient decreases. Conversely, above the critical angle of attack, as the angle of attack increases, the air begins to flow less smoothly over the upper surface of the [[airfoil]] 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 lift coefficient. As the angle of attack increases further, the upper surface flow becomes more fully separated and the lift coefficient reduces further.<ref name="nasa-lc" />

Above this critical angle of attack, the aircraft is said to be in a stall. A fixed-wing aircraft by definition is stalled at or above the critical angle of attack rather than at or below a particular [[airspeed]].  The airspeed at which the aircraft stalls varies with the weight of the aircraft, the [[load factor (aeronautics)|load factor]], the center of gravity of the aircraft and other factors. However, the aircraft normally stalls at the same critical angle of attack, unless [[icing conditions]] prevail. The critical or stalling angle of attack is typically around 15° - 18° 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, regardless 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.<ref>{{Cite web|title=Fly-by-Wire Systems Enable Safer, More Efficient Flight {{!}} NASA Spinoff|url=https://spinoff.nasa.gov/Spinoff2011/t_5.html|access-date=2022-01-04|website=spinoff.nasa.gov}}</ref>

In takeoff and landing operations from short runways ([[STOL]]), such as Naval Aircraft Carrier operations and [[STOL]] backcountry flying, aircraft may be equipped with the angle of attack or [[Airspeed indicator#Angle of attack and Lift Reserve Indicators|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 operations require the aircraft to be able to operate close to the critical angle of attack during landings and at the best angle of climb during takeoffs.  Angle of attack indicators are used by pilots for maximum performance during these maneuvers, since airspeed information is only indirectly related to stall behavior.

==Very high alpha==
[[File:Su-35 (12509727094).jpg|thumb|Su-27M / [[Su-35]] at high angle of attack]]

{{Further|High Alpha Research Vehicle}}

Some military aircraft are able to achieve controlled flight at very high angles of attack, but at the cost of massive [[induced drag]].  This provides the aircraft with great agility.  A famous example is [[Pugachev's Cobra]]. Although the aircraft experiences high angles of attack throughout the maneuver, the aircraft is not capable of either aerodynamic directional control or maintaining level flight until the maneuver ends. The Cobra is an example of [[Supermaneuverability|supermaneuvering]]<ref>[https://books.google.com/books?id=Ti3lNwAACAAJ Timothy Cowan]</ref><ref>{{Cite web |url=http://apps.dtic.mil/dtic/tr/fulltext/u2/a224126.pdf |title=DTIC |access-date=2022-06-02 |archive-date=2023-03-15 |archive-url=https://web.archive.org/web/20230315051852/http://apps.dtic.mil/dtic/tr/fulltext/u2/a224126.pdf |url-status=dead }}</ref> as the aircraft's wings are well beyond the critical angle of attack for most of the maneuver.

Additional aerodynamic surfaces known as "high-lift devices" including [[Leading edge extension#Leading edge root extensions|leading edge wing root extensions]] allow [[fighter aircraft]] much greater flyable 'true' alpha, up to over 45°, compared to about 20° for aircraft without these devices. This can be helpful at high altitudes where even slight maneuvering may require high angles of attack due to the low density of air in the upper atmosphere as well as at low speed at low altitude where the margin between level flight AoA and stall AoA is reduced. The high AoA capability of the aircraft provides a buffer for the pilot that makes stalling the airplane (which occurs when critical AoA is exceeded) more difficult. However, military aircraft usually do not obtain such high alpha in combat, as it robs the aircraft of speed very quickly due to induced drag, and, in extreme cases, increased frontal area and parasitic drag. Not only do such maneuvers slow the aircraft down, but they 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.{{citation needed|date=November 2012}}

==Sailing==
In [[sailing]], the physical principles involved are the same as for aircraft—a sail is an airfoil.<ref name="MIT - How Sails Work">{{cite web|last=Evans|first=Robin C.|title=HOW A SAIL BOAT SAILS INTO THE WIND|url=http://web.mit.edu/2.972/www/reports/sail_boat/sail_boat.html|work=Reports on How Things Work|publisher=Massachusetts Institute of Technology|access-date=14 January 2012}}</ref> A sail's '''angle of attack''' is the angle between the sail's chord line and the direction of the relative wind.

==See also==
*[[Air data boom]], measures angle of attack
*[[Advance ratio]]
*[[Aircraft principal axes]]
*[[Angle of sideslip]]
*[[Bernoulli's principle]]
*[[Drag equation]]
*[[Küssner effect]]
*[[Lift (force)]]

==References==
{{Reflist}}

* Lawford, J.A. and Nippress, K.R.; [http://spaceagecontrol.com/AD-CalibrationOfAirDataSystemsAndFlowDirectionSensors.pdf Calibration of Air-data Systems and Flow Direction Sensors] (NATO) Advisory Group for Aerospace Research and Development, AGARDograph No. 300 Vol. 1 (AGARD AG-300 Vol. 1); "Calibration of Air-data Systems and Flow Direction Sensors"; Aeroplane and Armament Experimental Establishment, Boscombe Down, Salisbury, Wilts SP4 OJF, United Kingdom
* USAF & NATO Report RTO-TR-015 AC/323/(HFM-015)/TP-1 (2001).

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{{DEFAULTSORT:Angle Of Attack}}
[[Category:Aircraft aerodynamics]]
[[Category:Aircraft wing design]]
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