Principles of Flight
Principles of Flight explains why the aircraft flies and how control inputs change its behaviour: lift, drag, angle of attack, stall, stability, load factor, wake turbulence and propeller effects.
Lift, drag and stall
- Lift increases with angle of attack up to the critical angle, then decreases sharply at the stall.
- Induced drag decreases with speed; parasite drag increases with speed squared. Total drag is lowest at the speed for best glide or endurance, depending on the configuration.
- A stall is an angle-of-attack phenomenon, not a speed phenomenon; weight, load factor and configuration change the stall speed.
- Load factor increases in turns: in a 45° bank it is 1 ÷ cos 45° = 1.41; stall speed increases with the square root of load factor.
Stability and control
- Longitudinal stability is provided by the tailplane and CG position; an aft CG reduces stability and can make stall recovery harder.
- Dihedral gives lateral stability; rudder and aileron adverse yaw require coordination.
- Ground effect reduces induced drag and can cause float during landing.
- Wake turbulence sinks and drifts; avoid the preceding aircraft’s flight path and land beyond its touchdown point.
Worked example
Stall speed in level flight is 50 kt. In a 45° bank, load factor = 1.41, so the new stall speed is 50 × √1.41 ≈ 59 kt. If the bank steepens to 60°, load factor = 2 and stall speed rises to about 71 kt.
Traps that cost marks
- Thinking a stall can be avoided by speed alone at high load factor.
- Confusing best glide speed with best angle of climb speed.
- Forgetting that an aft CG reduces pitch stability.
- Ignoring wake turbulence because the preceding aircraft is “light”.