Basic Aerodynamics
Core content
Basic aerodynamics covers the atmosphere, standard atmosphere, airflow, lift, drag, thrust and weight, stability and control. You should understand the lift equation, the four forces in steady flight, and the difference between parasite drag and induced drag. Induced drag falls as speed increases, while parasite drag rises with the square of speed. Know how angle of attack, camber, wing area, air density and speed affect lift. Stability includes static and dynamic, longitudinal, lateral and directional stability, and control surfaces produce moments about the centre of gravity. High-speed flight introduces compressibility, critical Mach number, shock waves and swept wings.
Traps
Candidates often confuse indicated airspeed with true airspeed. IAS depends on dynamic pressure and density; TAS increases with altitude for the same IAS. Another trap is the stall: a stall depends on angle of attack, not directly on speed, although lowering speed raises the required angle of attack. Do not assume induced drag increases with speed; it decreases. Also remember that the centre of pressure moves as angle of attack changes, which affects pitch stability.
Worked example
Lift is proportional to the square of speed. If speed doubles and angle of attack and density stay constant, lift becomes four times greater. If weight doubles, the stall speed increases by the square root of 2, about 1.41 times. Rule of thumb: for a given configuration, stall speed varies with the square root of weight.
Key points
- Lift is proportional to air density and speed squared.
- Induced drag decreases as speed increases; parasite drag increases.
- A stall is caused by exceeding the critical angle of attack.
- IAS and TAS differ because density changes with altitude.
- Stall speed rises with the square root of weight.