Lift Generation and the Lift Equation
How lift arises
Lift is the net pressure and shear force perpendicular to the relative airflow. Both the pressure difference between upper and lower surfaces and the downward momentum imparted to the air are valid descriptions; exam questions may use either. What matters is that lift requires circulation, which requires a sharp trailing edge, viscosity and a starting vortex. Candidates should be able to explain why a flat plate at an angle generates lift and why a stalled wing loses much of it.
The lift equation
L = ½ ρ V² S C_L. Know what each term means and how it scales. Lift is proportional to density, to the square of true airspeed, to wing area and to lift coefficient. In steady flight lift equals weight, so any change in one term forces a compensating change in another. This is the basis of nearly every performance and handling question in the syllabus.
Traps
Common traps: using indicated rather than true airspeed in the equation; forgetting that S is the reference (gross) wing area, not the exposed area; assuming C_L is fixed when load factor changes; and confusing the lift coefficient with lift itself. Another trap is the claim that lift is produced only by the upper surface; the lower surface contributes, particularly at high angle of attack.
Worked example
An aircraft of mass 1200 kg flies at sea level where ρ = 1.225 kg/m³, wing area 16 m², C_L 0.5. Weight is 1200 × 9.81 = 11 772 N. Then V² = 2W / (ρ S C_L) = 23 544 / (1.225 × 16 × 0.5) = 23 544 / 9.8 = 2402, so V ≈ 49 m/s, about 95 kt.
Rule of thumb: for level flight, speed for a given C_L varies as the square root of weight; a 21% increase in weight needs about 10% more speed.