tailieunhanh - Principles of Helicopter Aerodynamics phần 2

hơn so với một chiếc máy bay cánh cố định của tổng trọng lượng như nhau. Tất cả những yếu tố này ảnh hưởng đến thiết kế, mua lại, và chi phí hoạt động của máy bay trực thăng. Bên cạnh việc tạo ra tất cả thang máy thẳng đứng, rotor cũng là nguồn gốc chính của kiểm soát và đẩy cho máy bay trực thăng, trong khi các chức năng này được tách ra trên một máy bay cánh cố định. | 2 1 Introduction A History of Helicopter Flight than for a fixed-wing aircraft of the same gross weight. All of these factors influence the design acquisition and operational costs of the helicopter. Besides generating all of the vertical lift the rotor is also the primary source of control and propulsion for the helicopter whereas these functions are separated on a fixed-wing aircraft. For forward flight the rotor disk plane must be tilted so that the rotor thrust vector is inclined forward to provide a propulsive component to overcome rotor and airframe drag. The orientation of the rotor disk to the flow also provides the forces and moments to control the attitude and position of the aircraft. The pilot controls the magnitude and direction of the rotor thrust vector by changing the blade pitch angles using collective and cyclic pitch inputs which changes the blade lift and the distribution of thrust over the rotor disk. By incorporating articulation into the rotor design through the use of mechanical flapping and lead lag hinges that are situated near the root of each blade the rotor disk can be tilted in any direction in response to these blade pitch inputs. As the helicopter begins to move into forward flight the blades on the side of the rotor disk that advance into the relative wind will experience a higher dynamic pressure and lift than the blades on the retreating side of the disk and so asymmetric aerodynamic forces and moments will be produced on the rotor. Articulation helps allow the blades to naturally flap and lag so as to help balance out these asymmetric aerodynamic effects. However the mechanical complexity of the rotor hub required to allow for articulation and pitch control leads to high design and maintenance costs. With the inherently asymmetric flow environment and the flapping and pitching blades the aerodynamics of the rotor become relatively complicated and lead to unsteady forces. These forces are transmitted from the rotor to the airframe