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Regarding a positively cambered aerofoil section, which statement is correct?
  • A
    It will always produce a nose-down pitching moment at every Angle Of Attack.
  • B
    A nose-down pitching moment exists when the Lift Coefficient equals zero.
  • C
    It will always produce a nose-up pitching moment at every Angle Of Attack.
  • D
    A nose-up pitching moment exists when the Lift Coefficient equals zero.

Refer to figures.
The curvature or camber of the upper surface of a cambered airfoil is greater than that of the lower surface. As a result, the negative pressure generated by the acceleration of the airstream over the upper surface is greater than that beneath the lower surface.

Because of the different curvatures of the upper and lower surfaces of the airfoil even when the angle of attack is zero the airfoil will still generate a small amount of total lift.

At small negative angles of attack the pressure distributions
over both surfaces of the airfoil are equal. Therefore, there is no reactive force and consequently no lift. To produce no lift at all a cambered airfoil must have a negative angle of attack.

However, as seen in figure 2, the total pressure vector for the upper surface is aft of the total pressure vector for the lower surface and the AC is exactly midway between them. Thus, a nose-down pitching moment is created about the AC.

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