# Calculating induced drag approximation using XFoil generated parasitic drag

I am trying to do rough predictions of induced drag using the XFoil results for parasitic drag. To do so, I am using the lifting line equation for induced drag and the Oswald efficiency number: $$C_D=C_{D_0}+C_L²/(\pi AR e_0)$$. However I am trying to understand the difference between the constant zero-lift drag coefficient $$C_{D_0}$$ from the above formula and the parasitic drag coefficient (which is not constant wrt angle of attack, as one can expect that pressure drag will increase when frontal area increases) that I can get from a 2D airfoil analysis like XFoil. I thought that parasitic drag (pressure and friction drag) was supposed to be used to calculate induced drag with this formula, however it is not constant. So how do the two (zero-lift and parasitic drag) relate? And how should I use the XFoil "infinite wing"/2D drag coefficient to make rough approximation of "finite wing"/3D drag coefficient? Thanks a lot

UPDATE 1/11: Might have found a possible way to go: my confusion came from the fact that it is often said that parasitic drag is another way to say zero-lift drag, which it isn't. It seems like in the equation $$C_D=C_{D_0}+C_L²/(\pi AR e_0)$$, the term $$C_L²/(\pi AR e_0)$$ accounts for both the effects of the variations of induced drag with lift, and the variations of the pressure drag with AoA (and thus with lift). And it differs from $$C_L²/(\pi AR e)$$ that only accounts for induced drag, where $$e$$ is the span efficiency this time and not the Oswald efficiency number. The way to go using XFoil would therefore be to use the drag coefficient at zero lift in the polars only?