The principle that the cross section of a helicopter blade is similar to that of an airplane wing. When the rotor rotates at high speed, the speed of air flowing up and down the blades is different. So there is a pressure difference between the upper and lower blades. That is, there is lift. With this lift, the helicopter can take off and land vertically. The angle of helicopter blades can be changed, and this term is variable pitch. The aerodynamic performance of blades can be changed by changing the pitch, thus changing the lift.