Stall speed: bank, altitude and wind
Bank: wings hold weight and turn the aircraft, so lift exceeds level flight (more than 1 g). This is the load factor (see load-factors-va). Stall speed rises with its square root (TP 1102):
- Formula:
Vs(φ) = Vs1g · √n- Vs1g: wings-level (1 g) stall speed
- Vs(φ): stall speed at bank angle φ
- n: load factor at that bank
- Why √n: wing stalls at same angle of attack; lift ∝ V² (V = airspeed), so n times the lift needs √n times the speed.
- Any load factor, not just turns (TP 1102, load factor):
- 4 g: stall speed doubles, 50 kt aeroplane stalls at 100 kt
- 9 g: stalls at 150 kt
- Vs1g valid only (published wings-level stall speed) in unaccelerated (1 g), coordinated flight, at one weight, usually max gross (PHAK).
| Bank φ | Stall speed × |
|---|---|
| 30° | 1.07 |
| 45° | 1.19 |
| 60° | 1.41 |
Indicated stall speeds (V-speeds are IAS).
Altitude and wind:
| Factor | Stall speed | Effect | Why | Ref |
|---|---|---|---|---|
| Altitude | IAS | Same at any altitude | Same AOA at stall | TP 1102, Stalls; TP 13014 Q24; PHAK |
| Altitude | TAS | Only this rises with altitude | Thinner air needs more true speed for same dynamic pressure | PHAK |
| Headwind or tailwind | IAS | Unchanged | Same dynamic pressure as still air at that airspeed | PHAK |
- Stall speed quoted in mph (older ASIs): convert to knots first (1 kt ≈ 1.15 mph), then scale by √n.
Trap: scale by the square root of n; multiplying by n itself overstates the rise.
References
- TP 1102 (Flight Training Manual)
- TP 1102 (Flight Training Manual), Stalls, effect of altitude (line 2768)
- TP 13014 Q24 (sample exam), key 24-(4)
- PHAK ch.5 Aerodynamics of Flight