直升机教员手册 Helicopter Instructor’s Handbook
时间：2014-11-10 08:35来源：FAA 作者：直升机翻译 点击：次
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To view this page ensure that Adobe Flash Player version 9.0.124 or greater is installed. The three basic classifications of main rotor system are rigid, semirigid, and fully articulated. Main rotor systems are classified according to how the main rotor blades are attached and their movement relative to the main rotor hub. Show which rotor system is installed on the student’s particular helicopter and how it is identified. Discussions with the student regarding the different types of main rotor systems can be accomplished with additional manufacturer drawings. Pay particular attention to the type of main rotor system that the student will be flying. By now, the student probably has read about the different types of rotor systems but may not fully understand the differences. Explain to the student identifiable characteristics of each rotor system that make it different from the other system types. Also, be ready to answer the student’s questions about the advantages and disadvantages of each type of system, cost, and maintenance requirements versus ride quality, performance, reliability, and durability. Rigid Rotor System When introducing the rigid rotor system, instructors should explain that the system is mechanically simple, but structurally complex because operating loads must be absorbed in bending rather than through hinges. [Figures and 5-4] The rigid rotor was developed by Irven Culver (1911 to 1999) of Lockheed Aircraft Corporation to bring the simplicity of fixed-wing flight to helicopters. In a rigid rotor system, the blades, hub, and mast are rigid with respect to each other. The rigid rotor system is mechanically simpler than the fully articulated rotor system. There are no vertical or horizontal hinges so the blades cannot flap or drag, but they can be feathered. Operating loads from flapping and lead/lag forces must be absorbed by bending rather than through hinges. By flexing, the blades themselves compensate for the forces that previously required rugged hinges. The result is a rotor system that has less lag in the control response because the rotor has much less oscillation. The rigid rotor system also negates the danger of mast bumping inherent in semirigid rotors. The rigid rotor can also be called a hingeless rotor. Explain the other advantages of the rigid rotor system to the student (e.g., a reduction in the weight and drag of the rotor hub, higher control loads). Without the complex hinges, this rotor system is much more reliable and easier to maintain than the other rotor configurations. Rigid rotor systems require flexible rotor blades to produce a tolerable ride quality, but allow better maneuverability. Semirigid Rotor System Discuss with the student the main parts of the semirigid rotor system. Explain that it was named for its lack of the lead-lag hinge that a fully articulated rotor system has. The rotor system can be said to be rigid in plane because the blades are not free to lead and lag; however, they are not rigid in the flapping plane (through the use of a teeter hinge). Therefore, the rotor is not rigid, but not fully articulated either; it is semirigid. The parts of the semirigid rotor system that should be identified are teeter hinge, blade grip, blade pitch change horn, and pitch link. (NOTE: The swashplate assembly is described on page 5-12.) Also, discus the difference between teetering (flapping) versus feathering. On any rotor system, flapping occurs when the blade moves up and down. On a rigid rotor system, this occurs when the blade bends. On an articulated system, the blade flaps up and down around a teetering hinge. On a two-bladed, semirigid teetering system, the blades flap in unison around the flapping hinge, such as in a Bell 206.The semirigid main rotor system is designed such that as the blades cone and flap for different airspeeds, the rotor blade center of gravity centers around the teetering hinge such that the flap down is mostly cancelled out by flap of the other side. Examples of the semirigid rotor system are found on the Bell 230, the Bell 222 [Figures 5-5, 5-6, and 5-7], and the Bell 206. [Figure 5-8] Point out to the student that the Bell 206 head does not include coning hinges. Instead, the rotor head is designed with a pre-cone angle to the blade retention system, and other coning forces are simply dealt with by bending of the blades. Figure 5-6. Bell 230 semirigid rotor system. When discussing the semirigid rotor system, instructors should explain that some are designed with an underslung rotor system which mitigates the lead/lag forces by mounting the blades slightly lower than the usual plane of rotation so the lead and lag forces are minimized. As the blades cone upward, the center of pressure of the blades are almost in the same plane as the hub. Further explain that if the semirigid rotor system is an underslung rotor, the center of gravity (CG) is below the mast attachment point. This underslung mounting is designed to align the blade’s center of mass with a common flapping hinge so that both blades’ centers of mass vary equally in distance from the center of rotation during flapping. The rotational speed of the system tends to change, but this is restrained by the inertia of the engine and flexibility of the drive system. Only a moderate amount of stiffening at the blade root is necessary to handle this restriction. Simply put, underslinging effectively eliminates geometric imbalance. Fully Articulated Rotor System Fully articulated rotor systems can accommodate larger loads and faster airspeeds with good ride quality. Because there are more blades, the load can be spread among them resulting in lower initial angle of attacks which allows the retreating blade more margin above stall which allows increased forward airspeed before VNE. They have increased expenses due to the many parts that make up the rotor system, which also make preflight more complicated. The fully articulated rotor system is also susceptible of ground resonance if certain factors coincide. As with the other types of rotor systems, the student should have read about the fully articulated rotor system. [Figure 5-9] The student should be able to use the solidity ratio to explain how each blade carries only a portion of the total load. It is about the wing loading (in pounds) to the total area of the wing (in square feet). The instructor may need to review with the student basic aerodynamics of airfoils and airflows necessary to develop lift. Full articulation is also found on rotor systems with more than two blades. Using the rotor, show the student how the fully articulated system allows each blade to lead and lag, flap up and down, and feather. [Figure 5-10] |
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