Principles of Flight
Principles of Flight tests lift, drag, stability, control, stall and high-speed effects. The CPL exam favours applied reasoning over formula derivation.
Lift and drag
- Lift increases with angle of attack up to the stall. Induced drag decreases with speed; parasite drag increases with speed. Total drag is lowest at the speed for minimum drag.
- Load factor increases in turns. At 60° bank, load factor is 2 g; stall speed increases by √2, about 41%.
Stability and control
- Static stability means an initial tendency to return to equilibrium; dynamic stability describes the motion over time.
- Dihedral gives lateral stability; sweepback and fin area affect directional and longitudinal stability.
- Flaps increase lift and drag; slats delay stall; spoilers destroy lift and increase drag.
Worked example
An aircraft stalls at 50 kt in level flight. In a 60° bank, load factor is 2 g. New stall speed = 50 × √2 ≈ 71 kt. If the aircraft is also heavier, stall speed increases with the square root of the mass ratio.
Traps
Thinking induced drag rises with speed; using 1 g stall speed in a steep turn; confusing static and dynamic stability; assuming flaps improve climb performance; forgetting that a forward centre of gravity increases stall speed and reduces pitch stability.