The number of blades of axial flow fan is generally odd, because it is easier to correct the center of rotation when doing dynamic balance correction. If an even number of blades are used, the system will easily vibrate. If the material of the blade can't resist the fatigue caused by vibration, the blade or bearing is easy to break, so the asymmetric odd-numbered blade is more suitable. The same principle applies to helicopter propellers. The difference between three-leaf fan and five-leaf fan. The single leaf area of three-leaf fan is larger than that of five-leaf fan, and the interaction range with air is also enlarged, which will produce greater noise; However, compared with the five-leaf fan, the mold and manufacture of the three-leaf fan are much easier, the cost is lower, and the joint between the root of the fan blade and the central shaft is wider, so the blades of the three-leaf fan with the same material are much stronger than those of the five-leaf fan. So generally speaking, the fans with low noise are mostly five blades; The wind power of the five-leaf fan is softer. Originally, household fans were basically three-bladed, but now people pursue five-bladed fans such as mute and natural wind, and the price is relatively high.