Also I think NASA have a little Java web page that works it out for you. A google search should find it. It's called the Markov effect, I think. Cheers
If the other answers dont help, then maybe the Drag force=Cd*1/2*density of air*velocity of air^2, and use a value between 0.6 and 1 for the Cd depending on what it looks like.
If you are designing a fan blowing air, I think you consider the drag torque and rate of air mass movement, if you are designing a wind generator, then probably lift is also a consideration.