In fluid dynamics, angle of attack) is the angle between a reference line on a body and
the vector representing the relative motion between the body and the fluid through which
it is moving.
Angle of attack is the angle between the 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 specifies 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 choice is to use a horizontal line on the fuselage as the reference line.
Some authors do not use an arbitrary chord line, but use the zero lift axis instead —
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 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 increases.
At the same time, above 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, 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° - 20° 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.
In 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 or the Potential of Wing Lift 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 behaviour.
Very high alpha
Some military aircraft are able to achieve controlled flight at 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 sometimes thought to be Pugachev's Cobra.
Although the aircraft experiences high angles of attack throughout the maneuver, the aircraft
is not capable of either aerodynamic directional control or maintaining level flight until
the maneuver ends.
The Cobra is an example of supermaneuvering as the aircraft's wings are well beyond the
critical angle of attack for most of the maneuver.
Additional aerodynamic surfaces known as "high-lift devices" including leading edge wing root
extensions allow fighter aircraft much greater flyable 'true' alpha, up to over 45°, compared
to about 20° for aircraft without these devices.
This can be helpful at high altitudes where even slight maneuvering may require high angles
of attack due to the low density of air in the upper atmosphere as well as at low speed
at low altitude where the margin between level flight AoA and stall AoA is reduced.
The high AoA capability of the aircraft provides a buffer for the pilot that makes stalling
the airplane more difficult.
However, military aircraft usually do not obtain such high alpha in combat, as it robs
the aircraft of speed very quickly due to induced drag, and in extreme cases, increased
frontal area and parasitic drag.
Not only do such maneuvers slow the aircraft down, but they 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.
Sailing In sailing, the physical principles involved
are the same as for aircraft.
A sail's angle of attack is the angle between the sail's chord line and the direction of
the wind.
A boat's angle of attack is the angle between the boat's course and the wind direction.
See points of sail.
See also Advance ratio
Airfoil Angle of incidence
Angle of sideslip Bernoulli's principle
Camber Drag equation
Lift coefficient Lift
Pitch Stall
Zero lift axis References
Lawford, J.A. and Nippress, K.R.; Calibration of Air-data Systems and Flow Direction Sensors
Advisory Group for Aerospace Research and Development, AGARDograph No. 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(HFM-015)/TP-1.