Which one of the following figures represents the drag polar of a general aviation aircraft? (Axes: vertical \(C_L\), horizontal \(C_D\)).
Which one of the following figures represents the drag polar of a general aviation aircraft? (Axes: vertical \(C_L\), horizontal \(C_D\)).
Show Hint
Remember \(C_D=C_{D0}+kC_L^2\). If the graph's axes swap roles (plotting \(C_L\) vs \(C_D\)), the usual "upward" parabola becomes a "right-opening" one with vertex at \(C_{D0}\).
- A
- B
- C
- D
The Correct Option is D
Solution and Explanation
Step 1: Drag polar for aircraft.
For typical subsonic aircraft, \[ C_D=C_{D0}+k\,C_L^{\,2}\quad (k > 0). \] This is a parabola in the \(C_D\)-\(C_L\) plane with minimum drag \(C_{D0}\) at \(C_L=0\).
Step 2: Plotting with given axes (\(y=C_L,\ x=C_D\)).
Solving for \(C_L\) gives \[ C_L=\pm \sqrt{\frac{C_D-C_{D0}}{k}}, \] which is a sideways-opening} parabola to the right, vertex at \((C_{D0},0)\), symmetric about \(C_L=0\). This matches the sketch in option (D).
\[\boxed{\text{Option (D) — sideways-opening parabolic polar}}\]
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