That is some advanced aerodynamics you have there.
You'll need to know at least the dimensions of the whole ship, and the coefficient of friction between air and whatever material the bag is made of. I don't think the total mass is going to be too important.
To calculate air friction on a cylindrical balloon ship, use the drag equation F d r a g = 2 1 C d A ρ v 2 . You need to determine the frontal area, drag coefficient, air density, and speed of the ship. Plug these values into the equation to find the drag force.
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