时间：2011-03-11 23:18来源：蓝天飞行翻译 作者：航空 点击：次
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Selcal system. Selective calling system. Each aircraft operated by an airline is assigned a particular four tone audio combination for identification purposes. A ground station keys the signal whenever contact with that particular aircraft is desired. The signal is decoded by the airborne selcal decoder and the crew alerted by the selcal warning system. selective plating. A method of electroplating only a portion of a metal part. The portion not to be plated is masked with a nonconductive material, and only the unmasked area accepts the electroplating. selectivity (radio reception characteristic). The ability of a radio receiver to separate the signals it receives. Selectivity is a very important characteristic of a communications receiver because of the large number of transmissions in a narrow band of frequencies. The better the selectivity of a receiver, the narrower the band of frequencies it amplifies. selector switch. A multi-pole switch used to connect a single conductor to any one of several other conductors. See rotary switch. selector valve (fluid power system component). A flow control valve used in hydraulic systems to direct pressurized fluid into one side of an actuator, and at the same time direct return fluid from the other side of the actuator back to the reservoir. There are two basic types of selector valves: open-center valves and closed-center valves. The four-port closed-center valve is the most frequently used type. See closed-center selector valve and open-center selector valve. selenium. A steel-gray, highly toxic, nonmetallic chemical element. Selenium’s symbol is Se, its atomic number is 34, and its atomic weight is 78.96. Selenium, which is in the sulfur family, has the useful characteristic of changing its resistance as light falls on it. In the dark, selenium’s resistance is high, but in the bright light, its resistance is low. selenium rectifier. A type of rectifier using a cadmium and selenium junction to control the flow of electrons. When a layer of selenium is deposited on an aluminum base, and a cadmium alloy plate is pressed against the selenium, a barrier is formed between the selenium and the cadmium. Electrons can easily pass through this barrier from the cadmium, through the selenium to the aluminum, but they cannot pass through the barrier in the opposite direction. self-aligning bearing. A type of bearing used on a control rod end that functions properly, even though the control rod does not move at right angles to the surface to which it is attached. One of the most commonly used self-aligning bearings is the spherical bearing. The bearing surface has the shape of a sphere with a hole drilled through it. The two sides of the sphere perpendicular to the hole are ground flat so it can be attached to the component to be moved by the control rod. The control rod end has a socket into which the sphere fits, and the sphere rotates on a film of grease. The rod end can be out of alignment by a few degrees without affecting the operation of the bearing. Self-aligning bearings are used in control rods installed in helicopter rotor systems. |
