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Learning hub · Principles of Flight · Lesson 1

Lift, angle of attack and the stall

A stall is not the engine stopping, and it is not about being slow. It is the wing running out of lift because of the angle it meets the air. That one idea is the basis of safe flying.

About 12 minutes, plus the quiz. Part of Principles of Flight.

1. Four forces

In steady, level flight, lift balances weight and thrust balances drag. Lift comes from the wing pushing air downwards: the air is turned down and, in return, the wing is pushed up. The pressure over the top of the wing is lower than below it, which is another way of describing the same push. You do not need to choose between the explanations.

2. What lift depends on

Lift = ½ × air density × speed² × wing area × CL
  • Air density. Thinner air (hot, high, humid) gives less lift for the same speed.
  • Speed squared. Double the speed and lift is four times as large. Halve it and you have a quarter.
  • Wing area. Some types of flap increase the area, and all of them increase the curvature.
  • CL, the lift coefficient. It depends on the shape of the wing and, above all, on the angle of attack.

3. Angle of attack

The angle of attack is the angle between the wing’s chord line (an imaginary line from the leading edge to the trailing edge) and the airflow that meets it. It is not the pitch attitude of the aircraft: in a climb, a descent or a steep turn, the nose can be anywhere and the angle of attack is something else again.

As you raise the angle of attack, lift increases, up to a point. At the critical angle of attack (typically somewhere between about 15° and 20°, depending on the wing and flap setting) the smooth airflow over the top of the wing separates, lift drops sharply and drag climbs. That is the stall.

The key point: a wing stalls at the critical angle of attack, whatever the speed, attitude, weight or power. It is the angle that matters. Speed is only a convenient way of seeing it, because at a given weight and flap setting, a wing reaches the critical angle at the same speed in a level 1 g flight.

4. What changes the stall speed

  • Weight. A heavier aircraft needs more lift, so it stalls at a higher speed.
  • Flaps. They raise CL, so the stall speed is lower with flaps down.
  • Load factor. In a turn or a pull-up the wing must carry more than the aircraft’s weight. The stall speed rises by the square root of the load factor.
  • Altitude. The indicated stall speed barely changes with altitude.

In a steady level turn the load factor is 1 divided by the cosine of the bank angle: 1.41 g at 45° and 2 g at 60°. A stall speed of 60 kt in level flight therefore becomes about 71 kt at 45° (60 × √1.41) and about 85 kt at 60° (60 × √2). That is why you must not let the speed decay or pull hard in a steep turn, and why you keep a margin in the circuit.

5. Recognise it and get out

Warning signs: a high nose, a falling airspeed, soft and sloppy controls, buffeting, and the stall warning sound. The cure is always the same: reduce the angle of attack by easing the control column forward, add power as required, keep the aeroplane straight and coordinated, mainly with the rudder, then climb away. Follow your aircraft’s manual and your instructor: the exact technique differs between types.

Practise stalls only with an instructor, at a safe height and in the way the manual describes.

6. Try it yourself

Work each one out on paper first, then open the answer to compare. The numbers are invented.

  1. Your aeroplane stalls at 52 kt in level flight with the flaps up. What is the stall speed in a level turn at 45° of bank (1.41 g) and at 60° (2 g)? Is it safe to turn base at 65 kt in a 60° bank?
    Show the answer

    The stall speed rises with the square root of the load factor.

    At 45°: 52 × √1.41 = 52 × 1.19 ≈ 62 kt. At 60°: 52 × √2 = 52 × 1.41 ≈ 74 kt.

    At 65 kt in a 60° bank you are below the 74 kt stall speed, so the wing would stall. Keep the bank shallow near the ground and do not let the speed decay in a turn.

  2. In level flight at 80 kt the wing makes lift equal to weight. Everything else stays the same (density, wing area, angle of attack) and you speed up to 120 kt. How many times larger is the lift? What must you do with the angle of attack to stay level?
    Show the answer

    Lift is proportional to speed squared. The speed ratio is 120 ÷ 80 = 1.5, and 1.5 × 1.5 = 2.25 times the lift.

    Lift would now be much more than weight and the aeroplane would climb. To stay level you must lower the angle of attack, which lowers CL, until lift equals weight again. That is why the nose is lower in fast cruise than in slow flight.

  3. A pilot says: “I stalled at 80 kt in a steep pull-up, but the stall speed in my book is only 55 kt, so the book must be wrong.” What load factor does the story imply? Is the book wrong? What is the first recovery action?
    Show the answer

    Stall speed rises with the square root of the load factor, so 80 = 55 × √n. Then √n = 80 ÷ 55 ≈ 1.45 and n = 1.45 × 1.45 ≈ 2.1 g.

    The book is not wrong. A pull-up that loads the wing to about 2.1 g raises the stall speed from 55 kt to 80 kt. A wing stalls at the critical angle of attack at any speed or attitude.

    Recovery is the same as always: reduce the angle of attack by easing the control column forward, add power as required, keep the aeroplane coordinated, then climb away.

7. Watch

This video goes through the lift equation and angle of attack with animations, then stalls. It is American, so wording and some rules differ from European (ICAO/EASA) practice.

Open on YouTube if the player does not load. Not made by VectorEFB; check what you learn against your flying school’s material and your instructor.

More to watch

A short introduction to angle of attack and the stall from an experienced instructor. This video is American, so wording and some rules differ from European (ICAO/EASA) practice.

Open on YouTube if the player does not load. Not made by VectorEFB.

8. Check yourself

Five original practice questions. Answer them, press Check answers and read each explanation.

1. At which condition does a wing stall?

The stall depends only on the angle of attack. A wing can stall at a high speed in a steep pull-up, and at a nose-low attitude.

2. If the airspeed doubles and everything else stays the same, the lift becomes:

Lift is proportional to the speed squared, so double the speed gives 2 × 2 = 4 times the lift.

3. The wings-level stall speed is 60 kt. About what is it in a level 60° bank turn (2 g)?

The stall speed rises with the square root of the load factor. √2 ≈ 1.41, and 60 × 1.41 ≈ 85 kt. At 45° (1.41 g) it would be about 71 kt.

4. What is the first thing to do to recover from a stall?

You must get the wing back below the critical angle of attack. Easing the column forward does that. Pulling back keeps the wing stalled.

5. Extending flaps normally makes the stall speed:

Flaps increase the lift coefficient (and often the wing area), so the wing can fly slower before it reaches the stall. The stall speed with flaps down is lower.

Next: VFR radio phraseology. Back to the learning hub.

For training and situational awareness only. Not certified for navigation. Always use official charts, the AIP, NOTAMs and a proper weather briefing for real flights.