Principles of Flight
What the exam tests
Principles of Flight tests lift and drag, angle of attack, stall, load factor, stability, control, and propeller effects in normal and accelerated flight.
Core rules
- Lift = ½ ρ V² S CL. Lift rises with airspeed squared and with angle of attack up to the critical angle.
- Induced drag falls with increasing speed; parasite drag rises with speed squared. Total drag is lowest at the speed for best lift/drag ratio.
- A wing stalls at the critical angle of attack regardless of weight, speed, or attitude. Weight affects the speed at which the critical angle is reached.
- Load factor n = 1 ÷ cos(bank angle) in a level turn. Stall speed increases by √n. At 60° bank, n = 2 and stall speed increases by about 41%.
- Stability: positive static stability means a disturbance produces a restoring force; positive dynamic stability means the oscillation decays.
Worked example
An aircraft stalls at 50 KIAS in level flight. In a 60° banked level turn, load factor = 1 ÷ cos 60° = 2. Accelerated stall speed = 50 × √2 ≈ 70.7 KIAS. If the pilot pulls to the buffet at 70 KIAS in that turn, the wing reaches the critical angle at a higher speed and a higher load factor.
Traps
Do not say a wing stalls because of low airspeed alone; angle of attack is the cause. Do not ignore the increase in stall speed with load factor. Do not confuse best glide speed with minimum-sink speed.