DC Cooling Fan Why is odd

We all know that "DC Cooling Fan" is the main impact of wind blades, the material and the number of blades of wind pressure will affect the size and the proper functioning of the fan, in everyday, we often see the fan blades are odd, Why is this? "DC Cooling Fan" of large cross-section blades were tested curve, angle and other basic comparable to the width of the blades depends on the height of the blade, in order to ensure that the exhaust fan blade pitch will not be too large, the impact of wind pressure, the smaller diameter high exhaust fan by increasing the blade amount to make up, but no matter how much the number of vanes is increased, the number of vanes "DC Cooling Fan" is often odd 3,5,7 etc. this is because even if the blade is used, the balance adjustment is not good, it is easy to cause the system to resonance, if the long blade material can not receive the impact of resonance, and the axis of the blade will fracture, and therefore, the odd blade asymmetrical design is widely used in a number of fans.