In this diagram, the black lines represent the flow of the wind. The wing is shown end on. The angle α is the angle of attack.

Angle of attack (AOA, α, Greek letter alpha) is a term used in aerodynamics to describe the angle between the chord line of an airfoil and the vector representing the relative motion between the airfoil and the air. The Greek alphabet (Ελληνικό αλφάβητο is a set of twenty-four letters that has been used to write the Greek language since the late 9th or early Alpha (uppercase Α, lowercase α; Αλφα is the first letter of the Greek alphabet. In Geometry and Trigonometry, an angle (in full plane angle) is the figure formed by two rays sharing a common Endpoint, called In reference to Aircraft, chord refers to the distance between the leading edge and trailing edge of a Wing, Horizontal stabilizer or Vertical stabilizer An airfoil (in American English) or aerofoil (in British English) is the shape of a Wing or blade (of a Propeller, rotor It can be described as the angle between where the chord line of the airfoil is pointing and where the airfoil is going.

In aviation, 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. Overview Fixed-wing aircraft range from small training and recreational aircraft to Wide-body aircraft and military cargo aircraft. In traditional British usage, the term angle of incidence is used instead of angle of attack.

The angle of attack is often confused with the pitch angle or body angle of an aircraft. Pitch angle and body angle are measured with respect to the horizon; whereas the angle of attack is measured with respect to the relative motion between the wing and the atmosphere.

A typical lift coefficient curve.

The lift coefficient of a fixed-wing aircraft is directly related to the angle of attack. The lift coefficient ( CL or CZ) is a non-dimensional coefficient that relates the lift generated by an Airfoil, the Overview Fixed-wing aircraft range from small training and recreational aircraft to Wide-body aircraft and military cargo aircraft. 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 on the wing 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. All solid objects travelling through a Fluid (or alternatively a stationary object exposed to a moving fluid acquire a Boundary layer of fluid around them where viscous At the critical angle of attack the wing is unable to support the weight of the aircraft, causing the aircraft to descend which, in turn, causes the angle of attack to increase further. This leads to stall of the aircraft. For other uses see Stall. In Aerodynamics, a stall is a sudden reduction in the lift forces generated by an Airfoil

Critical angle of attack

The critical angle of attack is the angle of attack at which the air no longer flows sufficiently smooth over the upper surface of the airfoil. An airfoil (in American English) or aerofoil (in British English) is the shape of a Wing or blade (of a Propeller, rotor At this point, the aircraft is said to be in a stall. For other uses see Stall. In Aerodynamics, a stall is a sudden reduction in the lift forces generated by an Airfoil A fixed-wing aircraft always stalls at the same critical angle of attack, rather than at the same 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 The airspeed at which the aircraft stalls is variable, depending on the weight of the aircraft, the load factor at the time and the thrust from the engine. The critical 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 lifting its nose any further when the 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 controls.

When takeoff and landing operations are critical, 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 these operations close to the stall point with greater precision. All STOL operations require the aircraft to be able to operate at full-stall during landings and maximum climb angle during takeoffs. Since the airspeed of maximum performance during these maneuvers varies, airspeed is of less value to the pilot than AOA or Lift Reserve.

Very high alpha

In some military aircraft that do not display a conventional critical angle of attack, the aircraft is able to achieve a very high angle of attack. This provides the aircraft with great agility. A famous military example is Pugachev's Cobra. The Pugachev's Cobra (or Pugachev Cobra) is an Aircraft cobra maneuver Currently, the highest angle of attack recorded is 123 degrees for a duration of 2-3 seconds, performed in an Indian Su-30MKI aircraft during the Singapore airshow 2004.

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°, and in some designs, 90° or more. Leading edge extensions or LEX (also referred to as leading edge root extensions or LERX or strakes or chines) are Leading edge extensions or LEX (also referred to as leading edge root extensions or LERX or strakes or chines) are 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 That is, the aircraft remains flyable when the wing's chord is perpendicular to the direction of motion.

Sailing

In sailing, the angle of attack is the angle between a mid-sail and the direction of the wind. Sailing is the art of controlling a Sailing vessel. By changing the Rigging, Rudder and dagger or centre board a Sailor manages the force The physical principles involved are the same as for aircraft. See points of sail. Points of sail describes a sailing boat's course in relation to the wind direction

References

• Summary of Methods of Measuring Angle of Attack on Aircraft
• Calibration of Air Data Systems and Flow Direction Sensors

See also

• Airfoil
• Angle of incidence
• Bernoulli's principle
• Camber
• Lift coefficient
• Drag equation
• Lift (force)
• Zero lift axis