Drag as well as lift change with speed, so you can't possibly use one value for the speed in their formulas.
That formula looks like it's basically: s = 1/2 a t^2 with t = v/a
That only works for constant acceleration.
As i don't really have a clue myself i'll refer you to this page:
https://physics.stackexchange.com/qu...s-not-constant
To be specific your problem seems to be acceleration depending on velocity as described in case 3 here:
https://physics.stackexchange.com/a/15620
Now have fun with all those integrals.
(Because that's what you get when the acceleration isn't constant)