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Which one of the following statements is correct regarding the aerodynamic forces acting on an airfoil?

  • A
    Drag acts in the same direction as the relative wind and lift perpendicular to it.
  • B
    Lift acts perpendicular to the horizontal and drag parallel, and in a rearwards direction.
  • C
    Lift acts at right angles to the top surface of the aerofoil and drag acts at right angles to lift.
  • D
    Drag acts parallel to the chord and opposite to the direction of motion of the aerofoil and lift acts perpendicular to the chord.

Refer to figure.
The sum of all the reactive forces on an airfoil is known as the “Total Reaction”, which acts through the Centre of Pressure and at 90º to the chord line.

When an Angle of Attack of a symmetrical airfoil greater than zero exists, the Total Reaction tilts backwards and is split into two elements:

  • Lift, which is defined as the proportion of the Total Reaction perpendicular to the Relative AirFlow (RAF).
  • Drag, which is the proportion of the Total Reaction which is parallel to and in the same direction with the RAF.

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