GATE 2024 AE – Question 20

Flight Mechanics & Space Dynamics · Airplane performance: drag polar, take-off and landing, climb and descent, ceilings, range and endurance, load factor, turning flight, V-n diagram, winds · 1 mark · Multiple choice

Which one of the following figures represents the drag polar of a general aviation aircraft?

Four plots of $C_L$ (vertical) against $C_D$ (horizontal). (A) A parabola opening upwards with its minimum on the $C_D$ axis, drawn with $C_L$ as the horizontal-looking axis. (B) A closed bulb shape from the origin. (C) An arch starting and ending on the $C_D$ axis. (D) A curve that starts on the $C_D$ axis at a small positive drag, bulges slightly to the left and then rises to the right.

Option A in the figure
Option B in the figure
Option C in the figure
Option D in the figure

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Correct answer: (D) Option D in the figure

Explanation

The drag polar is $C_D = C_{D0} + K C_L^2$ (with a small shift for camber), which with $C_L$ on the vertical axis is a parabola opening to the right, with a positive drag $C_{D0}$ even at zero lift. Plot D has this shape. The others are not single-valued or start at the origin, which would mean zero drag at zero lift.