Principles of Flight
Lift, drag and control
Candidates must understand the four forces, the generation of lift, the lift coefficient and angle of attack, the stall and its recovery, and the drag curve. Learn the relationship between induced and parasite drag, the speed for minimum drag and the speed for maximum endurance and range. Understand stability (static and dynamic, longitudinal, lateral and directional), control effectiveness, and the effects of flaps, slats, spoilers and trim. Also learn the principles of the turn, load factor and the stall speed increase with bank.
Traps in questions
A frequent trap is assuming a stall occurs only at low speed - it is an angle-of-attack phenomenon, so it can occur at any speed and any attitude. Another is confusing the speed for minimum drag with the speed for minimum power. Questions also test the effect of CG on stability, and the change in stall speed with load factor.
Worked example
"Stall speed in level flight is 60 kt. What is the stall speed at 60° bank?" Answer: load factor = 1/cos 60° = 2, so stall speed = 60 × √2 ≈ 85 kt. Rule of thumb: stall speed rises with the square root of load factor.
Key points
- A stall is an angle-of-attack phenomenon, not a speed phenomenon.
- Distinguish minimum-drag speed from minimum-power speed.
- CG position affects stability and control.
- Stall speed rises with the square root of load factor.
- Understand induced versus parasite drag.