Thất tốc (Stall)

Là Gì a Stall?

An aerodynamic stall occurs when a wing exceeds its critical angle of attack — the maximum angle at which airflow can remain attached to the upper wing surface. Beyond this angle, smooth (laminar) airflow separates and becomes turbulent, destroying lift almost instantaneously. A stall is not related to engine failure; it is purely an aerodynamic phenomenon that can occur at any airspeed or attitude.

Cách Hoạt Động

Under normal conditions, air flows smoothly over the wing's upper surface, generating low pressure and thus lift. As the angle of attack increases, the airflow must bend more sharply around the leading edge. At the critical AoA (typically 15–20° for most airfoils), the airflow can no longer maintain contact with the upper surface:

• Flow separation begins near the trailing edge and progresses forward.
• The coefficient of lift (CL) drops sharply — sometimes by 50% or more.
• Drag increases dramatically at the same moment.
• The nose pitches down as the center of pressure shifts rearward (in most conventional aircraft designs).

Modern aircraft are equipped with stall warning systems: stick shakers physically vibrate the control column, and angle-of-attack indicators alert crews before reaching the critical angle. The stall speed (VS) varies with weight, bank angle, and configuration — it is embedded in the V-speeds framework.

Ý Nghĩa Trong Hàng Không

Stalls are among the most consequential events in aviation. A stall at low altitude — during takeoff, departure, or approach — leaves insufficient height for recovery. The FAA mandates stall training in all pilot certification programs. Loss of control in flight (LOC-I), often stall-related, has historically been the leading cause of fatal airline accidents.

Aircraft certification requires demonstrated stall characteristics that give pilots adequate warning and a clear recovery path: reduce back pressure, lower the nose, apply full power, and restore airspeed before attempting climb.

Tác Động Thực Tế

Air France Flight 447 (2009) crashed into the Atlantic after pilots failed to recognize and recover from a high-altitude stall. The Colgan Air Flight 3407 accident (2009) involved a low-altitude stall during approach near Buffalo, NY, killing 50 people. Both accidents prompted worldwide changes to stall training requirements, including the introduction of upset prevention and recovery training (UPRT). Modern fly-by-wire aircraft like the Airbus A320 family incorporate alpha protection systems that prevent the computer from commanding angles of attack beyond the critical limit.