The main shaft
 

If it is a sharp bend, just turn the main rotor head, you can see the main shaft center shake as circle shaped. But if it is slightly curved, you may need to remove the spindle and put them on the tablet or glass, use flow method to check. If the spindle bent, it will beat on the tablet;or flow route deviation in one direction. If put a straight precision spindle on flat plate flowing, you can find that its entire surface can always close to the flat. In the event of cast machine, you’d better remove the spindle to check. Please pay special attention, a main shaft bending of helicopter will directly affect the efficiency of the main rotor, and Joint of the flight performance, the body shaking violently set into a meaningless. Finally, it can not fly.