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What statement is correct regarding a symmetrical airfoil?

  • A

    The distances between the upper and lower surfaces and the chord line are equal.

  • B

    Opposite points of the airfoil have the same distance all over the airfoil.

  • C

    The distance between the upper surface and the chord line is greatest than the distance between the lower surface and the chord line.

  • D

    The distances between the upper and lower surfaces and the mean camber line are not equal.

Refer to figure.
Chord line is an imaginary line drawn between the middle of the leading edge and the trailing edge and is used to measure the angle at which the aerofoil is set as it moves through the air.

The Mean Camber Line of an airfoil is the line between the leading and trailing edges and equidistant from the lower and upper surfaces along its length.

When an airfoil has identical upper and lower surfaces, it is said to be “symmetric”. In this case, the mean camber line coincides with the chord line, thus the distances between the upper and lower surfaces and the chord line is equal.

When it has different curves on the upper and lower surfaces, it is said to be “asymmetric” or “cambered”:

  • On a positively cambered airfoil, the mean camber line sits slightly above the chord line.
  • On a negatively cambered airfoil, the mean camber line is below the chord line.

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