Principles of Flight
The four forces and the lift equation
A candidate must be fluent with lift, weight, thrust and drag as vectors in equilibrium and in accelerated flight. Lift comes from the equation L = ½ρV²SC_L, so lift varies with the square of speed and linearly with density and wing area. Understand angle of attack (AoA) as the angle between the chord line and the relative airflow, and the lift curve: lift rises linearly with AoA until the critical angle, then falls sharply. Recognise the lift coefficient curve, the drag polar, and that induced drag falls with speed while parasite drag rises with the square of speed, giving a minimum-drag speed.
Bernoulli, circulation and pressure distribution
Know that pressure is lower over the upper surface because the flow accelerates, and that the pressure distribution integrates to lift. Be aware of the centre of pressure movement: on a cambered aerofoil it moves forward as AoA increases toward the critical angle.
Traps
Common traps include confusing angle of attack with pitch attitude, assuming lift always equals weight (true only in unaccelerated level flight), and treating induced drag as constant with speed. Another trap is equating thrust with excess power, and confusing true airspeed with indicated airspeed when applying the lift equation.
Worked example
A wing of area 16 m² at sea level (ρ = 1.225 kg/m³) at 60 m/s and C_L = 0.5: L = ½ × 1.225 × 60² × 16 × 0.5 = 17,640 N. Rule of thumb: doubling speed quadruples dynamic pressure, so for the same C_L lift quadruples.