Principles of Flight
Principles of Flight explains lift, drag, thrust, weight, stability and control. The exam often uses a simple calculation or a scenario about stall, load factor or Mach effects.
Core principles
- Lift increases with angle of attack up to the critical angle, then decreases. A stall is an angle-of-attack phenomenon, not merely a low-speed phenomenon.
- Induced drag decreases with speed; parasite drag increases with speed. Total drag is lowest at the speed where they cross.
- Load factor is lift divided by weight. In a level turn, load factor = 1 ÷ cos(bank). At 60° bank, load factor is 2.
- Stall speed increases with the square root of load factor. At load factor 2, stall speed rises by about 41%.
- Swept wings and high-speed flight introduce Mach effects, shock waves and buffet; Mach trim and speed limitations apply.
- Stability: positive static stability tends to return to equilibrium; aft CG reduces longitudinal stability.
Worked example
A level turn at 60° bank gives load factor 2. If the 1 g stall speed is 100 kt, the accelerated stall speed is 100 × √2 = about 141 kt. The critical angle of attack does not change; the speed at which it is reached increases.
Traps
Saying stall speed is fixed; confusing stall angle with stall speed; using bank angle instead of load factor; and ignoring that aft CG reduces stability.