Slow down too much

Raise the angle of attack and the lift grows, for a while. Then something changes. At the critical angle of attack, about 15° in the toy model, the smooth airflow over the top of the wing separates from the surface and breaks up into swirls. The lift drops and the drag rises. This is called a stall.

Two things to fix in your head straight away:

• A stall has nothing to do with the engine stopping. A plane can stall with the engine at full power, and a glider has no engine at all.
• A stall is caused by angle of attack, not by speed. A wing stalls when α goes past the critical angle, and that can happen at any speed.

The stall speed. In level flight the lift equals the weight. At the largest possible lift coefficient, C_Lmax, the speed is the lowest at which the wing can still carry the weight:

V_s = √( 2W / (ρ S C_Lmax) )

For the toy plane: W = 9,810 N, ρ = 1.225, S = 16 m², C_Lmax = 1.6. Then V_s = √(19,620 / 31.36) ≈ 25 m/s, which is about 90 km/h or 49 knots. This matches the last line of the table in lesson 2.

What moves the stall speed?

• Weight. It enters under a square root. 20 percent more weight (1,200 kg) gives V_s × √1.2 ≈ 27.4 m/s.
• Density. Higher up, the air is thinner. At 3,000 m in the standard atmosphere (ρ ≈ 0.91) the stall happens at a true airspeed of about 29 m/s.
• Flaps. They raise C_Lmax. With C_Lmax = 2.0, V_s ≈ 22.4 m/s.
• Load factor. In a turn or a pull-up the wing must carry more than the weight: the stall speed grows by √n. At n = 2 the toy plane stalls at about 35 m/s. See World 2 for how this goes with the bank angle.

Why does the flow separate? Along the top of the wing the air first speeds up and its pressure falls, then must slow down and recover pressure towards the trailing edge. The air very close to the skin (the boundary layer) is slow and tired. At a steep angle the pressure recovery is so abrupt that this slow layer cannot push against it, stops, reverses and lifts off the surface. Everything downstream of that point is a turbulent wake, and the lift collapses. The Flow Lab shows why potential flow never separates and why real wings do.

What does it feel like? Before the stall a wing may shake and buffet as the flow starts to break up. After it, the aircraft often drops a wing or the nose. The physics-level principle for recovery follows from the cause: the problem is a too-large angle of attack, so the cure is to reduce the angle of attack (by easing the pull and lowering the nose relative to the airflow). How a particular aircraft is flown out of a stall is taught by instructors, in training, for that type.