Principles of Flight
Principles of Flight
Principles of Flight tests lift, drag, thrust, weight, stability, control, stall, spin and high-speed effects. The CPL exam expects the candidate to explain why an aircraft behaves as it does and to predict the effect of changes in speed, mass, configuration and CG.
Core rules
- Lift depends on angle of attack, airspeed, air density and wing area. Induced drag decreases with speed; parasite drag increases with the square of speed. Total drag is lowest at the speed for best glide or maximum range.
- A stall occurs at the critical angle of attack, not at a fixed speed. Mass, load factor and configuration change the stall speed: stall speed increases with the square root of load factor.
- Stability: an aircraft with a forward CG is more stable but has higher stall speed and pitch-down tendency; an aft CG reduces static stability. Dihedral gives lateral stability; fin and rudder give directional stability.
- Controls: elevator controls pitch, ailerons roll, rudder yaw. Adverse yaw is countered with rudder; trim reduces control force. Ground effect reduces induced drag near the surface.
Worked example
An aircraft stalls at 60 kt in level flight at 1 g. In a 60° bank, load factor is 1/cos 60° = 2. Stall speed becomes 60 × √2 = 84.9 kt. The pilot must increase speed and power in the turn; pulling harder at 60 kt would stall the aircraft.
Traps
- Thinking a stall is caused by low speed alone.
- Ignoring load factor in turns.
- Assuming aft CG improves stall recovery.
- Confusing best glide speed with minimum sink speed.