Principles of Flight
Principles of Flight tests the physics behind control: lift, drag, stability, stall and load factor. The PPL level is qualitative plus a few reliable calculations.
Lift and stall
Lift depends on angle of attack, speed, air density and wing area. The critical angle of attack produces the stall regardless of speed, attitude or weight. A heavier aircraft stalls at a higher speed; a forward CG raises stall speed slightly; an aft CG lowers it but reduces stability.
Drag and power
Induced drag falls with speed; parasite drag rises with the square of speed. Total drag is lowest at the best glide/L-D speed. Excess power gives climb; excess thrust gives acceleration.
Stability and control
Static stability is the initial tendency to return to equilibrium; dynamic stability is the motion over time. Dihedral gives lateral stability; the fin gives directional stability. Load factor = 1/cos(bank angle) in a level turn.
Worked example
A clean stall speed is 50 kt at 1 g. In a 60° level bank, load factor = 1/cos60° = 2. Stall speed increases by √2, so new stall speed = 50 × 1.41 ≈ 71 kt. The aircraft stalls at a higher speed, not a lower one, because the wing must produce twice the lift.
Traps
Thinking stall is caused by low speed alone; ignoring load factor in turns; assuming aft CG is always good; confusing best glide speed with minimum sink; forgetting that downwind turns do not change the stall angle.