As a helicopter rotor is rotating about its vertical axis, the blades are constantly changing, not only in location, but also in pitch, and more importantly in angle with respect to the vertical axis. It is this variation in angle that gives the helicopter a forward motion. Each of these changes to the blades happens in turn, so let's look at each one in more detail.
The blade's change in location is the most obvious. As the blade rotates around the vertical axis, its location is changing. At one point in time, the blade will be aligned with the nose of the helicopter, and a tenth of a second later, that same blade will be aligned with the tail.
As the blades rotate, their pitch, which is defined as the rotation of the blade along its length, is constantly changing. There are two different types of these changes in pitch, and they are both the result of pilot inputs. The two different types of blade pitch changes are "collective" and "cyclic," and they share their names with the pilot controls used to control them. When a pilot wants to go up or down, he or she uses the "collective," which is mounted to the left of the seat. This control collectively changes the pitch of all the blades the same amount, increasing or decreasing the total lift of the rotor, and causing the aircraft to climb or descend. When a pilot wants to move forward, backward, or to the side, he or she uses the "cyclic" which is usually mounted between the legs. This control cyclically changes the pitch of the blades as they rotate. Let's consider the pilot wants to fly forward. He or she would then push the cyclic forward, which would cause the rotor blades to increase in pitch as they approach the rear of the helicopter, increasing lift in that area. If the pilot uses enough cyclic control, this can cause the entire helicopter to tilt forward, allowing the rotor to pull the helicopter through the air. This is often seen in war movies, since it is the best way to get going quickly, since a large portion of the rotor's thrust is pulling the helicopter forward. Believe it or not though, a helicopter can remain perfectly level and still move forward, backward or sideways, with the help of "flapping hinges."
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