Question

Which one of the following figures represents the drag polar of a general aviation aircraft? (Axes: vertical \(C_L\), horizontal \(C_D\)).

• A. A
• B. B
• C. C
• D. D

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 \(C_{D0}\).

Answer 1

The Correct Option is D

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). 

\[\boxed{\text{Option (D) — sideways-opening parabolic polar}}\]