In recent years, the greenhouse and global warming effects are becoming more and more severe, that partially explains why rainfall rates keep breaking record in all parts of the world. In general, helicopter or rotorcraft is an aircraft that is lifted and propelled by one or more horizontal main rotors, and it can take off and land vertically. Currently, helicopter has widespread utilization by air rescue unit in Taiwan during disaster situation under extreme weather situations. For this reason the understanding and improvement of helicopters under heavy rain condition is the main theme of this research. With our understanding of heavy rain physics, a CFD tool has first been developed and successfully tested on the standard helicopter blade. Once the heavy rain droplets are added, it is observed that rainfall indeed will decrease the total lift and might induce some detrimental effects on the main rotor blades performance. Therefore, it is highly recommended that every helicopter pilot should be aware of the information gained in this work and accepts extensive training in simulating heavy rain situations.