Calculating Force of Drag on Paper Helicopters

[DarkBlitz]

hello!
i'm doing a physics lab paper helicopters and changing the length of their wings.
i was wondering if i could find the coefficient of drag for each helicopter.
i have the mass of the helicopter, the time it takes to fall a certain distance and I'm assuming that the cross sectional area will be pi*length^2
however, I'm having a bit of trouble determining the force of drag, i am not sure at all how to go about it :S
how do you calculate the force of drag? thanks

 

[JaredJames]

Are you talking about the rotors or the helicopter itself?