Cd = 0.1 for Reynolds number ~666k (10m/s)
http://simscience.org/fluid/red/reynolds.html
Turn CD into a Force at velocity x by using info from here:
http://en.wikipedia.org/wiki/Drag_coefficient
Extra credit
Integrate air drag loss to peak of trajectory and you get an idea of how many joules extra kick to give it vs doing this in a vacuum.
Regards
Frank