What is the lift coefficient corresponding to zero-lift angle for a symmetric airfoil?

What is the lift coefficient corresponding to 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, the lift is zero at zero angle of attack because there is no camber to create an asymmetric pressure distribution. The angle of attack at which the lift coefficient CL becomes zero is called the zero-lift angle α_L0. Since the airfoil is symmetric, α_L0 is zero degrees, so CL is zero at α = 0. Beyond very small angles, CL changes with α roughly linearly, so positive α produces positive lift and negative α produces negative lift. The other options would require a nonzero zero-lift angle or a nonzero lift at zero angle, which doesn’t happen for a symmetric airfoil.

For a symmetric airfoil, the lift is zero at zero angle of attack because there is no camber to create an asymmetric pressure distribution. The angle of attack at which the lift coefficient CL becomes zero is called the zero-lift angle α_L0. Since the airfoil is symmetric, α_L0 is zero degrees, so CL is zero at α = 0. Beyond very small angles, CL changes with α roughly linearly, so positive α produces positive lift and negative α produces negative lift. The other options would require a nonzero zero-lift angle or a nonzero lift at zero angle, which doesn’t happen for a symmetric airfoil.