Micro Cooling Fan optimum blade of how many?

Regardless of various numbers of blades "Micro Cooling Fan" is more than a little, but the number of axial fan blades tend to be odd. This is due to the use of a symmetrically shaped blades even pieces, and did not adjust well balanced, it is easy to make the system resonance, if the blade material is unable to resist vibration fatigue, will make the blades or spindle breakage, so much design It is asymmetric with respect to the axis of the odd piece blade design. This principle is widely used in various parts of the helicopter propeller blade design includes including in. Fan blade number of options:
1, technical aspects: the number of leaves too much can make chord length shorter blade curvature becomes large, adverse pressure gradient increases, the air easily separated, resulting in a decline in efficiency. Therefore, increase "Micro Cooling Fan" Blades by minimum blade chord length restrictions.
2, the production cost considerations, 3 fan blades more economical.
3, from the appearance consider: 3 blade fan than 2, 4 a little more beautiful.
4, "Micro Cooling Fan" After long optimization iteration, taking the best leaf number three.