时间:2011-04-05 11:32来源:蓝天飞行翻译 作者:航空 点击:次
To view this page ensure that Adobe Flash Player version 9.0.124 or greater is installed. Advantages of this system include vibration isolation, simple maintenance, and the ability to start and warm up the engine without engaging the rotor. FREEWHEELING UNIT Since lift in a helicopter is provided by rotating airfoils, these airfoils must be free to rotate if the engine fails. The freewheeling unit automatically disengages the engine from the main rotor when engine r.p.m. is less than main rotor r.p.m. This allows the main rotor to continue turning at normal in-flight speeds. The most common freewheeling unit assembly consists of a one-way sprag clutch located between the engine and main rotor transmission. This is usually in the upper pulley in a piston helicopter or mounted on the engine gearbox in a turbine helicopter. When the engine is driving the rotor, inclined surfaces in the spray clutch force rollers against an outer drum. This prevents the engine from exceeding transmission r.p.m. If the engine fails, the rollers move inward, allowing the outer drum to exceed the speed of the inner portion. The transmission can then exceed the speed of the engine. In this condition, engine speed is less than that of the drive system, and the helicopter is in an autorotative state. MAIN ROTOR SYSTEM Main rotor systems are classified according to how the main rotor blades move relative to the main rotor hub. As was described in Chapter 1—Introduction to the Helicopter, there are three basic classifications: fully articulated, semirigid, or rigid. Some modern rotor systems use a combination of these types. FULLY ARTICULATED ROTOR SYSTEM In a fully articulated rotor system, each rotor blade is attached to the rotor hub through a series of hinges, which allow the blade to move independently of the others. These rotor systems usually have three or more blades. [Figure 5-5] The horizontal hinge, called the flapping hinge, allows the blade to move up and down. This movement is called flapping and is designed to compensate for dissymetry of lift. The flapping hinge may be located at varying distances from the rotor hub, and there may be more than one hinge. SEMIRIGID ROTOR SYSTEM |