Where should most of the mass be for an industrial fly wheel? Why? can't find any thing about this in my book. need help answering this.

The object of a flywheel is to resist changes in angular momentum.

That means you want the largest possible moment of inertia for a given mass.
That means you put as much mass out at the maximum radius as you can.
therefore you have a thin web, just enough for needed strength, and a big outside rim.