Principles of Flight
What the exam tests
This domain tests lift and drag, angle of attack, stall, stability and control, load factor, turning, Mach effects, V speeds and wake turbulence.
Core rules
- Lift increases with angle of attack up to the critical angle, then decreases; a stall occurs at the critical angle of attack regardless of speed or attitude.
- Induced drag decreases as speed increases; parasite drag increases with the square of speed.
- Load factor increases in turns: at 60° bank, load factor is 2 g. Stall speed increases with the square root of load factor.
- Stability is the tendency to return to equilibrium; controllability is the response to control input.
- Swept wings and high Mach numbers bring Mach tuck, shock stall and buffet; respect the aircraft’s Mach limits.
Worked example
An aircraft stalls at 60 kt in level flight. In a 60° bank, load factor = 1/cos 60° = 2. Reasoning: stall speed increases by √2, so the new stall speed is about 60 × 1.41 = 85 kt. The aircraft will stall at a higher speed in the turn, even though the critical angle of attack is unchanged.
Traps
Believing a stall depends only on low speed; confusing induced and parasite drag; forgetting the load-factor effect in turns; ignoring wake turbulence behind heavy aircraft.