Just come across this thread...
Due to the profile drag (surface friction and normal pressure) any high speed rotating blade will also generate a centrifugal component of flow. Sweep forward causes a pressure trying to drive the flow inward. Since tail rotors operate at a fixed RPM it is relatively easy to choose the foward sweep so that the two effects cancel, so you get lower input torque for the same rotor thrust. It also reduces tip noise.
Fans used for cooling in particular are tending towards "sickle" shape fans. Again the forward sweep is to minimise tip outflow.
Mart
Edit: Thanks Vaqueroaero...
Last edited by Graviman; 15th April 2006 at 11:53.