The angle of attack of an aircraft is the angle between the air coming at it from the front and a certain reference line along the plane (usually the chord line). It can simply be described as the angle between where the wing is going and where the wing is pointing.
The angle of attack of an aircraft or wing determines how much lift it produces. In general, the higher the angle of attack, the more lift is generated, up to a point. After that point, the lift generates starts dropping dramatically. this point is known as the critical angle of attack of an aircraft. (Right): A diagram describing angle of attack.
Critical Angle of Attack
The critical angle of attack of an aircraft is the angle of attack at which is produces the maximum lift. This angle is sometimes also known as the stall angle of attack.
At angles below the critical angle, life increases with increase in angle. Above it, lift decreases with increase in angle. Maximum lift is generate close to the critical angle. As the angle of a wing increases, air flowing over the top of the wing starts separating (i.e. not flowing smoothly over the top) from of the wing. At angles above the critical angle, this airflow separates completely, leading to drops in lift generation and stall, a condition where an aircraft cannot generate enough lift to keep itself in the air. (Above right): A diagram showing detached airflow at an angle above the critical angle of the wing.
Different planes have different critical angles. Most typical aerofoils stall at angles above 15-20 degrees. Commercial aircraft have built in software which prevents them from flying above their critical angle regardless of pilot input.
Fighter aircraft, on the other hand, can fly at much greater angles of attack before stalling. Modifications to the wings and fuselage can allow such planes to fly at angles of attack as high as 45 degrees stably. Such high angles can be useful at high altitudes for maneuvers in thin air, but they cause a massive loss in speed and also place significant stress on the plane itself. Thus, most fighter aircraft limit the angles of attack they can achieve to well below what they could. (Left): A F-35 flying at a high angle of attack.
Extreme angles of attack
There have been experiments into achieving stable flight at extremely high angles of attack. One such experiment was the High Alpha (angle of attack) Research Vehicle (HARV), a modified F-18 Hornet developed by NASA. This plane used modified lifting bodies and thrust vectoring, among other techniques, to achieve stable flight at a massive 70 degrees of angle of attack. The program started in 1987 and was terminated in 1996, and provided significant information on stable flight at high angles of attack. (Above right): The F-18 HARV performing a high angle of attack maneuver.