The structures of a helicopter are designed to give it its unique flight characteristics, such as vertical takeoff and landing, and hovering in place. A helicopter flies because the rotors serve as rotating airfoils that provide lift, similar to how the wings of an airplane create lift. Air flows faster over the curved upper surface of the rotors, creating negative pressure, and thus lifting the aircraft. Changing the angle that the blades meet the wind increases or decreases lift, raising or lowering the helicopter. Tilting the rotor plane of rotation causes the helicopter to move horizontally.