Definitions

angle of stall

Angle of attack

Angle of attack (AOA, alpha, 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. 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. 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.

The lift coefficient of a fixed-wing aircraft is directly related to the 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 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. 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.

Critical angle of attack

The critical angle of attack is the angle of attack at which the air no longer flows smoothly over the upper surface of the airfoil. At this point, the aircraft is said to be in a stall. A fixed-wing aircraft always stalls at the same critical angle of attack, rather than at the same airspeed. 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.

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°.

Sailing

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

References

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