Basic Aerodynamics
Core concepts
The International Standard Atmosphere and its layers, and how temperature, pressure and density vary with altitude. Airflow: streamline and turbulent flow, Bernoulli's theorem, venturi effect, and the boundary layer. Aerofoil terminology - chord, camber, angle of attack, leading and trailing edge, mean camber line - and the generation of lift, with the lift coefficient. Drag: parasite drag (form and skin friction) and induced drag, and how total drag varies with speed. Thrust, weight and the four forces in steady flight; the stall, critical angle of attack, and the factors affecting it. Stability: longitudinal, lateral and directional, the role of the tailplane, dihedral, sweep and fin. Also high-speed flight: Mach number, shock waves, critical Mach number and mach buffet.
Traps in questions
- Stall is a function of angle of attack, not speed alone - an aircraft can stall at any speed and any attitude.
- Induced drag decreases with speed; parasite drag increases with the square of speed. Total drag is minimum where they cross.
- Lift is proportional to the square of speed - double the speed and lift quadruples at the same angle of attack.
- Angle of attack is measured relative to the relative airflow, not the horizon.
- Do not confuse dihedral (lateral stability) with sweep (high-speed and directional effects).
Worked example
Using L = ½ ρ V² S C_L with ρ = 1.225 kg/m³, S = 16 m², V = 50 m/s and C_L = 0.5: L = 0.5 × 1.225 × 2500 × 16 × 0.5 ≈ 12 250 N. To hold the same lift at 100 m/s, C_L must fall to 0.125 - the aerofoil must fly at a much smaller angle of attack.
Rule of thumb
At constant angle of attack, doubling speed quadruples lift. Increase angle of attack cautiously-the critical angle is fixed.