Attack angle
In fluid dynamics, the attack angle (AOA, αor alfa{displaystyle alfa}) is the angle between a "reference line" of a body (often the string line of an elar profile) and the vector that represents the relative movement between the body and the fluid through which it moves.
Angle of attack is the angle between the reference line of the body and the approaching flow. This article focuses on the most common application, the angle of attack of a wing or airfoil moving through the air.
It is a parameter that decisively influences the ability to generate lift of a wing or the ability to generate traction on the blades of a propeller.
Normally, increasing the angle of attack increases the sustenance to a certain point where it decreases abruptly, a phenomenon known by the name of entry into loss. The dependency of the support with the angle of attack can be measured through a supporting coefficient CL{displaystyle C_{L}} whose variation with the angle of attack α is illustrated in Figure 2. The theoretical dependence on a flat plate is given by CL{displaystyle C_{L}}(α)=2πα.
Due to the direct interaction between angle of attack and lift, angle of attack control is the primary command of a fixed-wing aircraft or aircraft. In effect, the increase in lift generates an increase in aerodynamic drag, which opposes aerodynamic traction. In other words, there is a reduction in aerodynamic speed. This leads us to the conclusion that the primary regulation of speed in an airplane is effected by changing the angle of attack.
Since a wing may have torsion, a chord line of the entire wing may not be definable, so an alternate reference line is simply defined. The chord line of the wing root is often chosen as the reference line. Another option is to use a horizontal line on the fuselage as a reference line (and also as longitudinal axis). Some authors do not use an arbitrary chord line but use the zero line of lift where, by definition, the angle of Zero attack corresponds to a zero lift coefficient.
It should be noted that there are certain high lift devices that can increase the stall angle of attack, that is, reduce the stall speed.
Relation between angle of attack and lift coefficient
The coefficient of lift of a fixed-wing aircraft varies with angle of attack. Increasing angle of attack is associated with increasing lift coefficient up to maximum lift coefficient, after which the lift coefficient decreases.
As the angle of attack of a fixed-wing aircraft increases, the airflow separation from the upper surface of the wing becomes more pronounced, leading to a reduction in the rate of increase of the coefficient of lift. The figure shows a typical curve for a combined straight wing. Camber wings are cambered in such a way that they generate some lift at small negative angles of attack. A symmetrical wing has zero lift at an angle of attack of 0 degrees. The lift curve is also influenced by the shape of the wing, including its section or airfoil and wing configuration. A swept wing has a flatter lower curve with a higher critical angle.
Critical Attack Angle
The critical angle of attack is the angle of attack that produces the maximum coefficient of lift. Also called stall angle of attack (fluid dynamics). Below the critical angle of attack, as the angle of attack decreases, the coefficient of lift decreases. Conversely, above the critical angle of attack, as the angle of attack increases, air begins to flow less smoothly over the upper surface of the airfoil and begins to separate from the upper surface. In most airfoil shapes, as the angle of attack increases, the point of separation of the upper flow surface moves from the trailing edge toward the leading edge. At the critical angle of attack, the upper surface flow is further apart and the airfoil or wing produces its maximum coefficient of lift. As the angle of attack increases, the upper surface flow separates more completely and the lift coefficient is further reduced.
Above this critical angle of attack, the aircraft is said to be stalled. By definition, a fixed-wing aircraft stalls at or above the critical angle of attack, and not below a given airspeed. The airspeed at which the aircraft stalls varies with aircraft weight, load factor, aircraft center of gravity, and other factors. However, the aircraft always stalls at the same critical angle of attack. The critical or stall angle of attack is usually around 15° - 18° for many air profiles.
Some aircraft are equipped with a built-in flight computer that automatically prevents the aircraft from further increasing its angle of attack when a maximum angle of attack is reached, regardless of pilot input. This is called an "angle of attack limiter" or "alpha limiter". Modern aircraft using fly-by-wire technology avoid the critical angle of attack by using software in the computer systems that govern the flight control surfaces.
In short runway takeoff and landing (STOL) operations, such as naval aircraft carrier operations and backcountry STOL flights, aircraft may be equipped with the Angle of Attack Indicator or Lift Reserve Indicators. These indicators directly measure angle of attack (AOA) or potential wing lift (POWL) or lift reserve and help the pilot fly closer to the stall point with greater accuracy. STOL operations require the aircraft to be able to operate near the critical angle of attack during landings and at the best angle of climb during takeoffs. Angle of Attack Indicators are used by pilots to gain maximum performance during these manoeuvres, as airspeed information is only indirectly related to stall behaviour.
Very high angles of attack
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Some military aircraft are capable of achieving controlled flight with very high angles of attack, commonly known as alpha, but at the cost of enormous induced drag. This gives the aircraft great agility. A famous example is Pugachev's Cobra. Although the aircraft experiences high angles of attack throughout the maneuver, the aircraft is neither able to control aerodynamic direction nor maintain level flight until the maneuver is complete. The Cobra is an example of super-maneuverability as the aircraft's wings are well above the critical angle of attack for most of the maneuver.
Additional airfoils known as "high lift devices," including leading edge wing root extensions, allow fighter jets a 'true' much higher for flying, up to more than 45°, compared to about 20° for aircraft without these devices. This can be useful at high altitudes, where even a slight maneuver may require high angles of attack due to the low air density in the upper atmosphere, as well as at low speed at low altitude, where the margin between the AoA in level flight and the AoA in loss is reduced. The high AoA capability of the aircraft provides a buffer for the pilot that makes it more difficult for the aircraft to stall (which occurs when the critical AoA is exceeded). However, military aircraft do not typically get such a 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. These maneuvers not only slow down the aircraft, but also 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.
Candle
In sailing, the physical principles involved are the same as for airplanes. The angle of attack of a sail is the angle between the chord line of the sail and the relative wind direction.