Principles of Flight
Principles of Flight tests lift, drag, stability, control, stalls, load factor and high-speed effects. The examiner wants you to explain why an aircraft behaves as it does, especially near the stall and in turns.
Lift and drag
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. The lift-to-drag ratio is best at the speed for minimum total drag. Ground effect reduces induced drag and can make the aircraft float.
Stability and control
Longitudinal stability is provided by the tailplane and CG position. Dihedral gives lateral stability; sweepback and keel area give directional stability. Static stability is the initial tendency to return to equilibrium; dynamic stability describes the subsequent motion. Controls are elevator, aileron, rudder and trim.
Stalls and load factor
A stall occurs at the critical angle of attack, independent of speed, weight and bank angle. However, the speed at which the critical angle is reached increases with weight and load factor. In a level turn, load factor n = 1 / cos(bank angle). Stall speed increases by the square root of the load factor.
Worked example
An aircraft stalls at 60 kt in level flight. In a 45-degree banked level turn, n = 1 / cos 45 = 1.41. The stall speed factor is the square root of 1.41 = 1.19. New stall speed = 60 x 1.19 = about 71 kt. The aircraft also needs more power because induced drag rises with load factor.
Traps
Saying stall speed is fixed; forgetting that the critical angle of attack is the defining factor; ignoring the downwind turn illusion; confusing static and dynamic stability.