How to determine torque required to spin a fan?

[Spoolx]

So I want to design a propeller that will slow down a motor and I want to know what formulas or equations I would use to calculate that torque.
Also, if I decided I wanted to run it in a fluid, what it just be a matter of density change?

I have attached a picture of what I want to do.

Thanks

 

[DanPreLoaded]

It's mass moment of inertia will have a part to play in it