Discussion
Consider the following statements about a symmetric airfoil.
1. Its zero-lift angle of attack is zero.
2. Its moment coefficient about the quarter chord is zero.
3. It cannot produce lift at any angle of attack.
Which of the statements given above is or are correct?
Answer: D
Correct answer: 1 and 2 only
Statements 1 and 2 are both consequences of the symmetry: with no camber the section produces no lift at zero incidence, so its zero-lift angle is zero, and the moment about the quarter chord vanishes. Statement 3 is wrong - a symmetric section lifts perfectly well at any non-zero angle of attack, which is why tailplanes and control surfaces are usually symmetric.
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