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The best L/D ratio can be found on a polar curve at the...

  • A

    Point of highest CD.

  • B

    Point of lowest CD.

  • C

    Tangent from the origin to the curve.

  • D

    Point of highest CL.

Refer to figure.

Another way of illustriating an aerofoil's performance, in terms of lift and drag, is the polar diagram, which gives values for CL and CD, through a range of angles of attack:

  • POINT A: Zero CL, zero lift, which will be near minimum CD. On either side of this, CD increases as CL increases in the negative or positive sense, towards Point E or B.
  • POINT B: The tangent to the origin of the curve and is the best ratio of CL to CD. In other words, the best lift/drag ratio. This occurs at 4° angle of attack for a symmetrical aerofoil, but not at minimum CD.
  • POINT C: CL is maximum and the corresponding Alpha is called "Critical".
  • POINT D: CL falls, but CD continues to increase beyond the stall. It is a stalled condition, where the aerofoil produces lift, but less than the maximum and at a high drag penalty.
  • POINT E: Negative pitch angle is applied, resulting in a sucking force (downwards lift) and associated drag penalty.

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