FAA Flight Instructor Airplane (FIA)Aerodynamics and Principles of FlightEasy
On a graph plotting coefficient of lift (CL) against angle of attack for a given airfoil, the maximum coefficient of lift (CLmax) occurs at which point?
- AThe zero-lift angle of attack, where CL equals zero
- BThe angle of attack corresponding to the airplane's best glide speed
- CThe critical angle of attack, just before airflow separation causes the stall
- DThe angle of attack that produces the airplane's minimum total drag
Show answer & explanationAnswer & explanation
Correct answer: C. The critical angle of attack, just before airflow separation causes the stall
CLmax occurs at the critical angle of attack, the highest angle at which the airfoil can maintain smooth, attached airflow before boundary layer separation causes a rapid loss of lift, i.e., the stall.
Why the other options are wrong
- A. Wrong — CL is zero at the zero-lift angle, the opposite extreme of the curve
- B. Wrong — best glide speed corresponds to L/D max, a different point on the curve
- D. Wrong — minimum drag occurs at a lower angle of attack than CLmax
CL vs Angle of Attack Curve
A graph showing how the coefficient of lift rises with angle of attack up to a maximum value (CLmax) at the critical angle of attack, beyond which lift drops sharply as the airfoil stalls.
- CL increases nearly linearly through most of the useful AOA range
- CLmax and critical AOA occur at the same point
- Beyond critical AOA, CL decreases rapidly as flow separates
Memory trick: The peak of the CL curve IS the stall point — climb the hill, then fall off the cliff