Stall

Theory and Concepts
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what happens when an airfoil stalls. Can we see the effect? does stall mean separation?

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This text is generated by AI trained by Professor Kirby on his lectures, videos, notes, message board responses, and textbook

PK Bot’s answer:
The definition of stall of an aircraft wing is when the lift coefficient vs angle-of-attack curve starts to have a negative slope. In practice, stall usually happens when a plane is flying too slowly, or perhaps during tight, steeply banked turns. These are the conditions that lead to aircraft wings being used at high angle of attack.

Stall is important because manual and automatic control of lift and pitch all assume that lift increases with angle of attack. When that is no longer true, simple control actions (e.g., pulling back on the control stick when you need more lift) will not work. Instead the pilot needs to point the nose down and, importantly, increase thrust. So stall defines an edge of the range of aircraft attitude in which normal control actions work.

Stall is closely tied to separation – sometimes stall happens precisely when separation occurs (this is most likely for a very thin airfoil, where separation occurs abruptly at the leading edge). More often, separation starts at the trailing edge, moves forward as the angle of attack increases, and the max lift point corresponds to some point in the middle of that transition. So usually separation happens before stall.

Stall, precisely, can only be observed with a force measurement on the airfoil. Separation can be observed visually, most easily by attaching strings to the airfoil and seeing when they show recirculation.

There are other contexts for stall. For example, the stall speed of a torque converter corresponds to the RPM difference at which the torque converter starts to have a sharp efficiency drop. That efficiency drop comes from flow separation as the fluid moves from impeller to turbine – the change in direction from impeller vanes to turbine vanes is too drastic. The physical mechanism is similar to stall over an aicraft wing.

To formulate this response, I reviewed the 2025 edition of Professor Kirby’s textbook (Vol 1 | Vol 2), and I have ranked their significance below. If they are marked as relevant, I encourage you to take a look at them:

Textbook sections:

  1. Chapter 10.5: Designing for Minimum Drag (HIGHLY RELEVANT)
  2. Chapter 9.5: Separation (HIGHLY RELEVANT)
  3. Chapter 9.3: Why We Use Irrotational Flow Analysis Part 1 (HIGHLY RELEVANT)
  4. Chapter 9.1: How Airfoils Work Part 1 (HIGHLY RELEVANT)
  5. Chapter 9.1: How Airfoils Work Part 2 (RELEVANT)

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