From Technic Wiki
A Rotor Blade is an important piece of a Multiblock Turbine's interior. Rotor blades use energy from the turbine's intake fluids to spin the turbine's Rotor Shaft. They must be attached in straight lines, beginning from a Rotor Shaft block and extending directly away from it.
Each rotor blade is capable of fully converting the energy from 25 milli-buckets of intake fluid flow. If the turbine's fluid flow exceeds the conversion capability of the blades, then some of the excess energy will be lost.