A wing stalls when its angle of attack passes the critical value for its section, at which the airflow separates from the top surface and lift drops; it is an angle, not a speed, so the same wing can stall at any speed if pulled hard enough. Level-flight stall speed is V = √(2 × weight ÷ (air density × wing area × maximum lift coefficient)), and it rises with weight, with the square root of the load factor in a turn, and with frost or roughness. Recovery is to lower the nose to reduce the angle, then add power, and it costs height that a low aircraft does not have.
Part of Aerodynamics and Flight
The loss of lift at high angle of attack, what moves the stall speed, and why stalls near the ground kill.
Stall Is An Angle, And The Wing Does Not Know How Fast It Is Going
As the angle increases, lift rises until the flow separates (Boundary Layer). Past the critical angle lift falls and drag rises. The critical angle depends on the section and its surface and cannot be given here; the carrier’s 15-18 degrees is a rule of thumb for some sections, and the value for your section comes from published tunnel data.
Because level-flight lift equals weight, a lower speed demands a higher angle, so a stall in level flight happens at a particular speed. In a turn or a pull-up the load is higher, so the same wing stalls at a higher speed.
The Stall Speed Formula, And What The Carrier’s Figures Imply
Setting lift equal to weight at the maximum lift coefficient gives V_stall = √(2W ÷ (ρ × S × CLmax)). Turned around, the coefficient implied by a stated stall speed is CLmax = 2W ÷ (ρ × S × V²).
Worked example. The carrier states a 55-70 km/h stall for 300-500 kg and 9-11 m span with 1.2-1.5 m chord. Take S = 13 m² (10 m × 1.3 m) and ρ = 1.225 kg/m³. 55 km/h = 15.28 m/s; 70 km/h = 19.44 m/s.
| Mass | at 55 km/h | at 70 km/h |
|---|---|---|
| 300 kg (W = 2,943 N) | 2 × 2,943 ÷ (1.225 × 13 × 15.28²) = 1.58 | 2 × 2,943 ÷ (1.225 × 13 × 19.44²) = 0.98 |
| 400 kg (W = 3,924 N) | 2.11 | 1.30 |
| 500 kg (W = 4,905 N) | 2.64 | 1.63 |
So the low end of the carrier’s stall range for the heavier aircraft needs a coefficient above 2, which a plain, unflapped wing of this kind should not be assumed to supply; check the section’s published maximum before trusting a stall speed at all. The upper end needs a modest one. A target stall speed is a requirement on the wing, not a result of building it.
What Raises The Stall Speed
- Weight: stall speed scales as the square root of weight. Adding 10% to the mass raises the stall speed by √1.10 = 1.049, about 5%.
- Turns and pull-ups: in a level turn the load factor is 1 ÷ cos(bank). At 60° it is 2, and the stall speed rises by √2 = 1.41 times (Envelope Expansion).
- Contamination: frost, rain, mud or fabric ripples.
- Flap setting and centre of gravity position: see Flaps and Slats and Tail Assembly.
Recognising It Before It Happens
Stall warning in a small aircraft is physical: a falling-off in control firmness, buffeting from the separated air on the tail, the nose dropping, a high nose attitude with a falling airspeed. Wool tufts on the wing’s upper surface show the separation begin at the root or tip. The airspeed indicator is poorest exactly here, where the scale is compressed (Airspeed Indicator).
Recovery And Spin
Step 1: Lower the nose. The only way out of a stall is to reduce the angle of attack. Pulling back deepens it.
Step 2: Level the wings with rudder, not aileron, if one wing has dropped. Aileron at a stalled wing can worsen the roll and start a spin.
Step 3: Add power once the angle is reduced and speed is rising.
Step 4: Ease out of the dive gently. A hard pull-out stalls the wing again.
A stall with one wing and yaw can become a spin, a stalled descent turning about a vertical axis. Recovery from a spin is a trained procedure that varies by aircraft and is not given here.
Common Mistakes
- Pulling back when the nose drops — it deepens the stall.
- Stalling in a turn at low height — a turn from base leg to the runway, a skidding turn, or a turn back to the field after a failure.
- Treating the published stall speed as fixed — it applies at one weight and one load.
Safety
A stall near the ground cannot be recovered
A stall recovery costs height, an amount that is specific to the aircraft and that nobody can tell you for a design that has not been flown. Do not deliberately stall an unproven aircraft. Learn the approach to the stall (buffet, mushy controls, tufts) and stop short of it, and never practise stalls below a height from which a spin could be recovered by someone trained to do it.
What’s Next
- Learn next: Wing Twist, how a root-first stall is arranged
- Combine with: Flaps and Slats, which raise the available coefficient