Here is the basic formula for air drag:
Fd = 0.5 * Cd * Rho * Velocity^2 * A
*Fd is the drag force in newtons, applied to the opposite direction of the velocity.
*Cd is the coefficient of drag - this is determined in wind tunnel test. It is a constant which you can tweak to your preference, but there are some examples of the Cd of different primitives to be found here: http://en.wikipedia.org/wiki/File:14ilf1l.svg
*Rho is the density of air. This can be tricky since it changes with temperature, and temperature drops at higher altitudes. Air humidity also affects density. I don't know the exact equation for this, but i suggest you use a constant air humidity, and then sample data from this calculator (http://www.denysschen.com/catalogue/density.aspx) and then make a function out of these points. You might have to find samples of air temerature at different altitudes aswell, but that should not be hard.
*Velocity relative to the airflow.
*A is the reference area in square meters.