Which statement best describes the lift coefficient at zero-lift angle for a symmetric airfoil?

Which statement best describes the lift coefficient at zero-lift angle for a symmetric airfoil?

• A. For a symmetric airfoil, α_L0 = 0 and CL = 0 at α = 0.
• B. α_L0 = 5 degrees and CL = 0 at α = 5.
• C. α_L0 = -2 degrees and CL = 0 at α = -2.
• D. α_L0 = 0 degrees and CL = 0 at α = 5.

Answer: (A)

For a symmetric airfoil, there is no camber to generate lift when the angle of attack is zero. The angle of attack at which the lift coefficient becomes zero is therefore 0 degrees, and at that same angle the lift coefficient is zero as well. In aerodynamic terms, the lift curve for a symmetric airfoil crosses the CL = 0 axis at α = 0, so CL is zero when α = 0. The slope dCL/dα is positive, so for small positive angles the lift rises above zero, and for small negative angles it becomes negative. The other options propose a nonzero zero-lift angle or claim zero lift occurs at a different angle, which contradicts the symmetry.

For a symmetric airfoil, there is no camber to generate lift when the angle of attack is zero. The angle of attack at which the lift coefficient becomes zero is therefore 0 degrees, and at that same angle the lift coefficient is zero as well. In aerodynamic terms, the lift curve for a symmetric airfoil crosses the CL = 0 axis at α = 0, so CL is zero when α = 0. The slope dCL/dα is positive, so for small positive angles the lift rises above zero, and for small negative angles it becomes negative. The other options propose a nonzero zero-lift angle or claim zero lift occurs at a different angle, which contradicts the symmetry.