I don't see how it could be linear. The faster the blade is spinning to more resistance it will incounter from air. The slower it turns the less resistance it will incounter by air.
As an example (not real numbers):
Say it takes 12V to get 100CFM
It may only take 5V to get 50CFM being there is less resistance.
OR this shows how little I know about airodynamics.
Or airodynamics may not even play a part.
I will go back to shutting up now......