An aircraft stall results from a rapid decrease in lift caused by the separation of airflow from the wing’s surface, brought on by exceeding the critical AOA. A stall can occur at any pitch attitude or airspeed. Stalls are one of the most misunderstood areas of aerodynamics because pilots often believe an airfoil stops producing lift when it stalls. In a stall, the wing does not totally stop producing lift; rather, it cannot generate adequate lift to sustain level flight. Since the lift coefficient (CL) increases with AOA, at some point the CL peaks (CL-MAX) and then drops off. The amount of lift drops dramatically after exceeding CL-MAX or the critical AOA, but does not completely stop.
In most straight-wing aircraft, the wing is designed to stall at the root first, so the stall progresses outward toward the wingtip, maintaining aileron effectiveness and controllability. Design methods to achieve root-first stall include “twisting” the wing to a higher AOA at the root, or installing stall strips on the first 20–25 percent of the leading edge. Most training aircraft are designed for the nose to drop during a stall, reducing the AOA and “unstalling” the wing; this nose-down tendency is due to the CL being aft of the CG. The CG range is very important to stall-recovery characteristics: if operated aft of the rear limit, the pilot may not be able to generate sufficient elevator force to counteract the excess weight aft of the CG and recover.
The stalling speed of a particular aircraft is not a fixed value for all flight situations, but a given aircraft always stalls at the same AOA regardless of airspeed, weight, load factor, or density altitude. This critical AOA varies from about 16° to 20° depending on design, but each aircraft has only one specific AOA where the stall occurs.
There are three flight situations in which the critical AOA can be exceeded: low speed, high speed, and turning.
The aircraft can be stalled in straight-and-level flight by flying too slowly—as airspeed decreases, the AOA must increase to retain lift, until the wing no longer produces enough lift and the aircraft stalls.
Low speed is not necessary to produce a stall: the wing can be brought to an excessive AOA at any speed. For example, in a dive at 100 knots the pilot pulls back sharply on the elevator control. [Figure 4-32]
Gravity and centrifugal force prevent an immediate change of flightpath, but the aircraft’s AOA changes abruptly from quite low to very high, and the aircraft reaches the stalling angle at a speed much greater than the normal stall speed. The stalling speed is also higher in a level turn than in straight-and-level flight. [Figure 4-33] Centrifugal force is added to the aircraft’s weight, and the wing must produce additional lift to counterbalance the load; this lift is obtained by increasing the AOA, so the AOA must increase as the bank angle increases. If at any time during a turn the AOA becomes excessive, the aircraft stalls.
To balance the aircraft aerodynamically, the CL is normally located aft of the CG. Although this makes the aircraft inherently nose-heavy, downwash on the horizontal stabilizer counteracts this condition. At the point of stall, when the upward force of the wing’s lift and the downward tail force cease, an unbalanced condition allows the aircraft to pitch down abruptly, rotating about its CG; the AOA decreases, airspeed increases, smooth airflow over the wing resumes, lift returns, and the aircraft is again flying. Considerable altitude may be lost before this cycle is complete.
Airfoil shape and degradation of that shape must also be considered: if ice, snow, or frost accumulate on the surface, the smooth airflow is disrupted, the boundary layer separates at a lower AOA than the critical angle, and lift is greatly reduced. [Figure 4-34] As little as 0.8 millimeter of ice on the upper wing surface increases drag and reduces lift by 25 percent.
Pilots can encounter icing in any season, anywhere, at altitudes up to 18,000 feet and sometimes higher. Small aircraft, including commuter planes, are most vulnerable because they fly at lower altitudes where ice is more prevalent and they often lack the anti-ice mechanisms common on jet aircraft.
Icing can occur in clouds any time the temperature drops below freezing and super-cooled droplets build up on an aircraft and freeze. (Super-cooled droplets are still liquid even though the temperature is below 32 °F / 0 °C.)
