Principles of Flight
What the exam tests
This domain tests lift, drag, angle of attack, stall, load factor, stability and control, including how speed, mass and bank angle change aircraft behaviour.
Core rules
- Lift increases with angle of attack up to the critical angle, then decreases. A stall is an angle-of-attack exceedance, not simply a low-speed condition.
- Induced drag decreases with speed; parasite drag increases with speed. Total drag is lowest at the speed for best glide.
- Load factor is the ratio of lift to weight. In a level turn, load factor equals 1 divided by cosine of the bank angle. Stall speed increases with the square root of load factor.
- Forward CG increases stability but raises stick force and landing speed; aft CG reduces stability and can make stall recovery difficult.
- Dihedral gives lateral stability; sweepback and tail surfaces affect longitudinal and directional stability.
Worked example
In a 60° bank, load factor is 1 divided by cosine 60°, or 2. If the stall speed in level flight is 50 kt, the accelerated stall speed is 50 multiplied by the square root of 2, about 71 kt. The aircraft stalls at a higher speed because the wing must produce twice the lift.
Traps
Believing a stall can occur only at low speed; confusing induced and parasite drag; forgetting that aft CG reduces stall margin; and ignoring the load-factor increase in steep turns.