Don’t get confused
between minimum drag, and minimum CD, the two are not the same!
Polar diagrams use
coefficients to show how increasing/decreasing one variable affects the other
variable. As g10 correctly states if you're not producing any lift, CL zero, then CD will be will be at a minimum. As you start to
produce lift, CL increasing, then CD increases. There will be some point, shown
as the tangent to the polar diagram, where the maximum CL is achieved for the
minimum increase in CD. Not the minimum CD. This then is the best lift to drag
ratio which occurs at Vmd, the bottom of the total
drag curve.
Figure 11-3-16 shows
the wing at its most efficient, best L/D ratio at 4 degrees alpha.
Going back to Luc's
original query from Q294....this type of question is quite common where the
answer you were looking for, Vmd, is not there and the examiner is using an alternative
description of Vmd to test your knowledge.
For example when we
look at the tangent to the power required curve we find that this is also Vmd, best range speed prop, but also described as the point
where CL/CD is a maximum or where CD/CL is at a minimum.