Difference between forward and backward-swept axial fan blades ?

[petrojelly]

Hi,

I have been researching why most axial fan blades are forward swept like this picture:

http://www.turbomoni.com/fan1.gif

while there are very few with backward swept blades:

http://www.turbomoni.com/fan4.gif

One explanation I have found is that forward swept fans have less induced drag from tip vortex, because flow tends to travel span-wise towards root of blade. Is this true ?

 

[Compositepro]

My guess is that forward swept-blades (and wings) will increase in angle of attack under aerodynamic loading. The opposite is true for back-swept blades. Forward sweep allows light and flexible plastic blades to be used to push the maximum amount of air up to the limit of the motor. For this kind of fan application, high-slip motors are commonly used. This means that they will just slow down and not burn-up when overloaded.

 

[rb1957]

i too would guess they're like wings ... backward sweep is common, forward sweep very uncommon ('cause it needs advanced composites to tailor the structural axes and the twist of the wing to avoid divergence). there might also be something in having the center of mass behind the attachment point, with centripedal accelerations to be considered.