Regarding a symmetric aerofoil, which statement is correct?

• A. The angle of attack is negative when lift coefficient is zero
• B. A nose-up pitching moment exists when lift coefficient is zero
• C. Both statements are incorrect
• D. Both statements are correct

Answer: (C)

For a symmetric aerofoil, symmetry means no preferred direction in the pressure distribution when the angle of attack is zero. As a result, the lift coefficient is zero at zero angle of attack. This means the angle of attack at which lift is zero is 0 degrees, not a negative value.

About pitching moment when lift is zero: for a symmetric airfoil the pressure distribution at the zero-lift condition is symmetric about the chord, so there’s no net torque about the aerodynamic center. In other words, the zero-lift pitching moment is effectively zero, so there isn’t a nose-up moment when lift is zero.

Therefore, both statements are not correct.