Angle of attack

Lore & Background

The angle of attack is defined using a reference line on the body, often the chord line of an airfoil, but for a wing with twist, an alternate reference line such as the root chord or a horizontal line on the fuselage may be used. Some authors use the zero lift axis, where zero angle of attack corresponds to zero coefficient of lift. British authors have sometimes used the term angle of incidence instead, though this can cause confusion with the riggers' angle of incidence.

The lift coefficient of a fixed-wing aircraft varies with angle of attack, increasing up to the maximum lift coefficient at the critical angle of attack, after which it decreases. As angle of attack increases, airflow separation from the upper surface becomes more pronounced, reducing the rate of lift increase. The critical angle of attack, also called the stall angle, is typically around 15°–18° for many airfoils. Above this angle, the aircraft is stalled, and the stall occurs at the same critical angle of attack unless icing conditions prevail.

Some aircraft are equipped with angle of attack limiters or fly-by-wire systems that prevent exceeding the critical angle. In STOL operations, angle of attack or lift reserve indicators help pilots fly close to the stalling point. Military aircraft can achieve controlled flight at very high angles of attack, such as in Pugachev's Cobra, but at the cost of massive induced drag. High-lift devices like leading edge wing root extensions allow fighter aircraft to achieve flyable angles of attack over 45°.

Reader's Guide

The angle of attack is a fundamental parameter in aerodynamics, directly governing lift generation and stall characteristics. Its significance lies in its role as the primary determinant of lift coefficient for a fixed-wing aircraft, with the critical angle of attack marking the boundary between normal flight and stall. The article emphasizes that stall is defined by exceeding the critical angle of attack, not by a specific airspeed, which varies with weight, load factor, and center of gravity. This understanding is crucial for safe flight operations, particularly in takeoff and landing from short runways, where pilots use angle of attack indicators to maximize performance near the stall point. The legacy of angle of attack extends to modern aircraft design, where fly-by-wire systems and angle of attack limiters automatically prevent pilots from exceeding the critical angle, enhancing safety. In military aviation, the ability to achieve very high angles of attack provides agility for maneuvers like Pugachev's Cobra, though at the cost of high induced drag and structural stress. The concept also applies to sailing, where a sail's angle of attack relative to the wind follows the same physical principles.

Did You Know?

• The critical angle of attack is typically around 15°–18° for many airfoils.
• A fixed-wing aircraft is stalled at or above the critical angle of attack, not at a particular airspeed.
• Some military aircraft can achieve controlled flight at angles of attack over 45° using high-lift devices like leading edge wing root extensions.
• In sailing, a sail's angle of attack is the angle between its chord line and the direction of the relative wind.