Shouldn't it scale linearly with length?
It's basically a cylinder, who's volume is heightPiradius^2.
Unless you're also making it wider and taller, it's not a cubed growth
The way I read it, is that drag increases ^2 for length, but because volume increases lift for airships (as opposed to other aircraft) you get this ^3 increase in lift by increasing length. So airship economy increases with size, as opposed to an airplane.
For the square-cube law, a cursory wiki search yields:
> The square–cube law can be stated as follows:
> When an object undergoes a proportional increase in size, its new surface area is proportional to the square of the multiplier and its new volume is proportional to the cube of the multiplier.
> Cargo airships would need to be big—bigger than the Hindenburg. Airships are blessed and cursed with a square-cube law: the drag of the airship, which is proportional to its cross-sectional area, scales with length squared, but the volume of lifting gas, and thus the gross lift, scales with length to the third power. Therefore, the lift-to-drag ratio, a critical parameter in aircraft performance, gets better as the airship gets bigger.