Angle of attack
![](http://upload.wikimedia.org/wikipedia/commons/thumb/0/02/Airfoil_angle_of_attack.jpg/280px-Airfoil_angle_of_attack.jpg)
In fluid dynamics, angle of attack (AOA, α, or ) is the
In
Some British authors have used the term angle of incidence instead of angle of attack.[5] 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.[6]
Relation between angle of attack and lift coefficient
![](http://upload.wikimedia.org/wikipedia/commons/thumb/f/f5/MISB_ST_0601.8_-_Platform_Angle_of_Attack.png/350px-MISB_ST_0601.8_-_Platform_Angle_of_Attack.png)
![](http://upload.wikimedia.org/wikipedia/commons/thumb/b/b1/Coefficients_of_Drag_and_Lift_vs_AOA.jpg/300px-Coefficients_of_Drag_and_Lift_vs_AOA.jpg)
![](http://upload.wikimedia.org/wikipedia/commons/thumb/d/d1/Lift_curve.svg/300px-Lift_curve.svg.png)
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.[7]
As the angle of attack of a fixed-wing aircraft increases,
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 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.[7]
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, 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° - 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.[8]
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 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
Some military aircraft are able to achieve controlled flight at very high angles of attack, but at the cost of massive
Additional aerodynamic surfaces known as "high-lift devices" including
Sailing
In sailing, the physical principles involved are the same as for aircraft—a sail is an airfoil.[11] 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
- ^ "Inclination Effects on Lift". National Aeronautics and Space Administration. 2018-04-05.
- ^ Gracey, William (1958). "Summary of Methods of Measuring Angle of Attack on Aircraft" (PDF). NACA Technical Note (NACA-TN-4351). NASA Technical Reports: 1–30. Retrieved 2024-02-22.
- ^ John S. Denker, See How It Flies. http://www.av8n.com/how/htm/aoa.html#sec-def-aoa
- ISBN 0-07-036240-8
- ^ Wolfgang Langewiesche, Stick and Rudder: An Explanation of the Art of Flying, p. 7
- ISBN 0-273-31623-0
- ^ a b "NASA Lift Coefficient".
- ^ "Fly-by-Wire Systems Enable Safer, More Efficient Flight | NASA Spinoff". spinoff.nasa.gov. Retrieved 2022-01-04.
- ^ Timothy Cowan
- ^ "DTIC" (PDF). Archived from the original (PDF) on 2023-03-15. Retrieved 2022-06-02.
- ^ Evans, Robin C. "HOW A SAIL BOAT SAILS INTO THE WIND". Reports on How Things Work. Massachusetts Institute of Technology. Retrieved 14 January 2012.
- Lawford, J.A. and Nippress, K.R.; 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).