Angle of attack is the angle between an airfoil's chord line and the oncoming airflow. It is one of the most important controls of lift because it changes how strongly the wing turns air downward. As angle of attack increases, lift usually increases, but only up to a limit.
Understanding this limit is essential in aircraft design, piloting, wind turbines, propellers, and any machine that uses airfoils.
Key Facts
- Angle of attack α is measured between the chord line and the relative wind.
- Lift can be estimated by L = 0.5ρv^2SCL.
- For small angles, lift coefficient often increases approximately linearly: CL = CL0 + aα.
- Stall begins when flow separates strongly from the upper surface and lift drops.
- The critical angle of attack for many aircraft wings is roughly 12 degrees to 18 degrees, but it depends on airfoil shape and conditions.
- Basic stall recovery: reduce angle of attack, keep wings level, add power as needed, then return to the desired flight path.
Vocabulary
- Angle of attack
- The angle between an airfoil's chord line and the direction of the oncoming airflow.
- Chord line
- A straight reference line drawn from the leading edge to the trailing edge of an airfoil.
- Relative wind
- The airflow direction seen by the moving wing, opposite the wing's motion through the air.
- Flow separation
- The condition in which air no longer follows the wing surface smoothly and breaks away into turbulent motion.
- Critical angle of attack
- The angle of attack at which the wing reaches maximum lift before stall begins.
Common Mistakes to Avoid
- Confusing pitch angle with angle of attack is wrong because the aircraft nose angle is measured relative to the horizon, while angle of attack is measured relative to the oncoming airflow.
- Thinking higher angle of attack always means more lift is wrong because lift decreases after the critical angle due to flow separation.
- Using speed alone to define stall is wrong because a wing stalls when the critical angle of attack is exceeded, not at one universal speed.
- Pulling back harder during a stall is wrong because it increases angle of attack and can deepen the stall instead of restoring smooth airflow.
Practice Questions
- 1 A wing has a chord line tilted 8 degrees above the relative wind. What is its angle of attack?
- 2 An aircraft wing has ρ = 1.2 kg/m^3, v = 40 m/s, S = 16 m^2, and CL = 0.9. Use L = 0.5ρv^2SCL to find the lift force.
- 3 A pilot is flying slowly and notices buffet as the nose is raised. Explain why lowering the nose can help the wing recover from stall.