Angle of attack

In this diagram, the black lines represent the flow of a fluid around a two-dimensional airfoil shape. The angle α is the angle of attack.

Angle of attack (AOA, \alpha, 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 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. 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).[1]

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.[2]

Contents

Relation between angle of attack and lift

A typical lift coefficient curve.

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. 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°.[3].

Sailing

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

See also

References

  1. ↑ 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/search.jsp?R=415182&id=6&qs=Ne%3D25%26Ns%3DHarvestDate%257c1%26N%3D4294756036%2B287. 
  2. ↑ Kermode, A.C. (1972), Mechanics of Flight, Chapter 3 (8th edition), Pitman Publishing Limited, London ISBN 0 273 31623 0
  3. ↑ Wikipedia: High Alpha Research Vehicle