One of the very first maneuvers we learn is a stall. It’s one maneuver that tends to create the greatest unease in newer students. The best way to help that uneasy feeling in a student is to provide a thorough education on why we practice stalls. We practice them — Power-on or Power-off stalls — to provide recognition as to when it is likely to occur, the mistakes that could create the different types, identify the signs, and recover. I believe it is also important to understand some of the basic dynamics that go into a stall.
1) What is a stall?
Contrary to what some may think, the stalls we practice are not an effect of the engine shutting off. It is an aerodynamic condition that occurs once disruption of smooth airflow over the airplane’s wings, results in loss of lift. Simply, a stall occurs when the aircraft’s wings exceed its critical Angle of Attack (AOA). It is possible to exceed the critical AOA at any airspeed, at any attitude, and at any power setting. The wing does not totally stop producing lift during a stall. Rather, it is not generating adequate lift to sustain level flight.
2) The parts of a wing
To understand how an aircraft exceeds its critical AOA, we have to define some of the terms of a wing.
Photo credit: Researchgate.net
Leading edge: the portion of the wing that meets the airflow first
Trailing edge: the portion of the wing where the airflow over the upper surface (low pressure) rejoins the lower surface (high pressure) airflow
Flight path: the direction or path the airplane is traveling
Relative wind: the airflow which is parallel to and opposite the flight path
Cord line: an imaginary straight line drawn through the airfoil from the leading edge to the training edge
AOA (Angle of Attack): the angle between the chord line of the airfoil and the direction of the relative wind
3) Disruption of smooth airflow
Photo credit: DW.com—Why do airplanes stall and why is it so dangerous?
As illustrated above, as the AOA increases the wing is no longer streamlining with the relative wind and a disruption in the smooth airflow occurs. This creates turbulent air behind the wing which decreases lift significantly.
Due to the fact that the coefficient of lift (CL) increases with an increase in AOA, there is a point where the CL reaches its maximum tolerance and begins to drop off. This peak is called the CL-MAX. The wing produces a dramatically lesser amount of lift after exceeding the CL-MAX or critical AOA.
The wing is designed to stall at the wing root first. The wing root will usually reach its critical AOA first resulting in the stall progressing outward toward the wingtip. It is best to have the wing root stall first to maintain aileron effectiveness and controllability of the aircraft.
4) Recovery
If exceeding the critical angle of attack is what caused the stall, immediately decreased your AOA to regain the smooth airflow over the wings. Simultaneously, you will need to add power to avoid bleeding off any more airspeed. Lower the nose to the horizon by using slight and calm, yet responsive inputs on the elevator controls.
Different wing types and structures will produce different stall characteristics. It is always recommended to follow your standard operation procedures (SOP) to practice any stall specific to your aircraft type. Understanding stall training and prevention are critical skills for developing pilot mastery.
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