How to calculate the induced drag coefficient? In an exercice of Flight mechanic, I have to compute the drag coefficient at zero lift: $C_{D0}$.We saw in class two ways to find it :

$$
C_{D0} = \frac{\pi AR e}{4E_{max}^2}
$$

With $AR$ the aspect ratio, $e$ the Oswald number and $E_{max}$ the maximal Lift-to-drag ratio. I don't have the value for the two last ones in my exercice so I can't use this formula.

We saw another way, by using the polar definiton: $C_D = C_{D0} + kC_L^2$ with $C_D$ the drag coefficient, $C_L$ the lift coefficient and $k$ the induced drag coefficient. In order to use it I just need to find the value of $k$ but the only formula that I have is:

$$
k = \frac{1}{\pi e AR}
$$

And I still need the value of $e$ the Oswald number. Is there a way to compute $e$ ? I didn't found any on Internet. If not, is there another way to compute $k$ without $e$ ?

Thanks for your help and sorry if I don't have the proper english vocabulary.
There's no way to calculate $e$ without using computational fluid dynamics (CFD) since it involves the calculation of complex three-dimensional flow interactions with the wing geometry (lift loss due to wingtip vortices).

For most aircraft, 0.8 is a pretty good estimate, so this is probably the value you're expected to use. If you need a more accurate number for your particular problem, you'll either have to know the specific aircraft model (in which case you would likely have to calculate $e$ given $AR$ and ${C_D}_0$, $C_D$, and $C_L$ for a certain state) or be given a number to use for the problem.