Principles of Flight
Principles of flight questions test why the aircraft behaves as it does, from the four forces to stability and control. Build the subject in layers.
Key concepts: the four forces (lift, weight, thrust, drag); angle of attack and the critical angle; the lift formula and the effect of speed, density and area; induced drag and its relationship to speed (high at low speed) versus parasite drag (high at high speed); the drag curve and the speed for minimum drag; stability (static and dynamic, longitudinal, lateral and directional); control about each axis (aileron, elevator, rudder); stall and its recognition; load factor and the stall speed increase with load factor (stall speed rises with the square root of load factor); and mach effects (critical Mach number, Mach tuck, shock stall).
Traps: assuming lift is produced only by the pressure difference on the upper surface; confusing induced and parasite drag; forgetting that stall speed increases with the square root of load factor; and mixing static and dynamic stability.
Worked example: an aircraft stalls at 60 kt at 1 g. At 4 g, stall speed = 60 × √4 = 120 kt. Rule of thumb: stall speed scales with the square root of load factor - double the load factor, multiply stall speed by about 1.41.