- Angle of attack
Angle of attack (AOA, α, Greek letter alpha) is a term used in fluid dynamics to describe the angle between a reference line on a lifting body (often the chord line of an airfoil) 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, angle of attack is used to describe 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 root of the wing 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.
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 of a 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 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 straight wing. A symmetrical wing has zero lift at 0 degrees angle of attack. The lift curve is also influenced by 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 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 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. 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, 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 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 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.
Using a variety of additional aerodynamic surfaces — known as high-lift devices — like leading edge extensions (leading edge wing root extensions), fighter aircraft 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.
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) for details.
- Advance ratio
- Angle of incidence
- Angle of sideslip
- Bernoulli's principle
- Drag equation
- Lift coefficient
- Lift (force)
- Zero lift axis
- ^ Gracey, William (1958). "Summary of Methods of Measuring Angle of Attack on Aircraft". NACA Technical Note (NASA Technical Reports) (NACA-TN-4351): 1–30. http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930085167_1993085167.pdf.
- ^ John S. Denker, See How It Flies. http://www.av8n.com/how/htm/aoa.html#sec-def-aoa
- ^ Wolfgang Langewiesche, Stick and Rudder: An Explanation of the Art of Flying, McGraw-Hill Professional, first edition (September 1, 1990), ISBN 0-070-36240-8
- ^ Kermode, A.C. (1972), Mechanics of Flight, Chapter 3 (8th edition), Pitman Publishing Limited, London ISBN 0-273-31623-0
- ^ Wikipedia: High Alpha Research Vehicle
- 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).
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