Nice. D'Arcy- Weisbach surface roughness effect. The friction coefficient of PVC is .0015-.007 compared to galvanized steel at .15 . I did some numbers at one time but not sure what the effect of all the seams ,especially spiral wound are. If you have two ducts 6" in diameter, one PVC, one spiral gal steel at a velocity of lets say about 3500 FPM what would be the CFM difference between the two. Not stated properly but I think you can figure out what I'm trying to obtain.