Lift supports weight in level flight. Drag resists motion and must be overcome by thrust — or accepted as sink rate on a glider.
Lift
Force perpendicular to the freestream. On airfoils we usually talk about coefficient Cl = L / (q S).
Drag
Force parallel to the freestream. Coefficient Cd = D / (q S). Includes pressure and skin-friction parts.
Why coefficients?
Forces depend on speed, density, and size. Coefficients remove those scales so you can compare shapes when Reynolds number and Mach are stated. Dynamic pressure q = ½ρV².
Lift-to-drag ratio
L/D (or Cl/Cd) is efficiency. High peak L/D helps endurance and glide. High Cl max helps takeoff and landing. You rarely maximize both with the same geometry.
Image placeholder — Cl–α and Cl–Cd polar pair
Common beginner mistakes
- Comparing Cl without matching Reynolds number
- Choosing “lowest Cd at one alpha” instead of Cd at the mission Cl
- Ignoring pitching moment until the tail is too small
Next: How to Read Cl–α and Cl–Cd Polars · practise in HFOIL