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Considering the polar curve of a subsonic aerofoil, the maximum lift-to-drag ratio is determined by...
• A
drawing a tangent to the curve from the origin.
• B
the maximum value of the lift coefficient.
• C
the point where total lift and total drag are equal.
• D
the minimum value of the drag coefficient.

Refer to figure.
The polar curve of an airfoil is the graphical relationship of the coefficients of lift and drag plotted against each other, the resulting graph is as shown in figure 1. It reveals that the total lift and total drag increase from point A to a maximum at point D, thus the angle of attack of the wings increases in the same manner. Therefore, the speed decreases from a maximum at point A to a minimum at point D.
Point B is the point at which a tangent from the origin touches the polar curve, which is at the point at which the total drag is minimum, the lift/drag ratio is maximum and the IAS in level flight is VMD. This is also the speed at which the longest glide distance will be achieved.

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