Angle of attack is the angle between a wing’s chord line and the oncoming air. It is one of the most important ideas in aviation because it helps determine how much lift a wing produces. As angle of attack increases, the wing usually makes more lift, but only up to a limit.
If that limit is passed, the wing can stall and lose much of its lift.
A stall happens when airflow can no longer stay smoothly attached to the top surface of the wing. The flow separates, becomes turbulent, and the low-pressure region above the wing weakens. The critical angle of attack is the angle where this separation becomes severe enough to cause a major lift drop.
Pilots prevent stalls by controlling pitch, speed, weight, and configuration so the wing stays below the critical angle.
Key Facts
- Angle of attack, AoA, is the angle between the chord line of a wing and the relative wind.
- Lift generally increases as angle of attack increases, until the critical angle is reached.
- At the critical angle of attack, airflow begins to separate strongly from the wing and lift drops.
- A stall is caused by exceeding the critical angle of attack, not directly by flying too slowly.
- Lift can be modeled by L = 1/2 rho v^2 S CL, where CL depends strongly on angle of attack.
- Stall speed increases when weight increases or when the aircraft is in a bank because the wing must produce more lift.
Vocabulary
- Angle of Attack
- The angle between a wing’s chord line and the direction of the oncoming relative wind.
- Relative Wind
- The airflow that moves opposite to the aircraft’s motion through the air.
- Critical Angle of Attack
- The angle of attack at which airflow separation becomes large enough to cause a significant loss of lift.
- Stall
- A condition in which a wing exceeds its critical angle of attack and loses much of its lift.
- Airflow Separation
- The point where airflow no longer follows the wing surface smoothly and instead breaks away into turbulent flow.
Common Mistakes to Avoid
- Thinking a stall only happens at low speed is wrong because a stall occurs when the critical angle of attack is exceeded, even at high speed.
- Confusing pitch angle with angle of attack is wrong because pitch is the airplane’s attitude relative to the horizon, while angle of attack is measured relative to the oncoming air.
- Assuming more angle of attack always means more lift is wrong because lift drops after the critical angle when airflow separates.
- Ignoring bank angle during turns is wrong because a banked aircraft needs more lift, which can require a higher angle of attack and raise stall risk.
Practice Questions
- 1 A wing has a critical angle of attack of 16 degrees. If it is flying at an angle of attack of 10 degrees, is it stalled? How many degrees below the critical angle is it?
- 2 Using L = 1/2 rho v^2 S CL, suppose rho = 1.2 kg/m^3, v = 30 m/s, S = 12 m^2, and CL = 1.0. Calculate the lift force.
- 3 An airplane is flying level, then the pilot pulls the nose up sharply while the airspeed is still high. Explain why the airplane could still stall.