时间：2011-03-11 23:13来源：蓝天飞行翻译 作者：admin 点击：次
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Heli-Coil inserts are also used to provide durable threads in soft metal castings. Some spark plug holes in cast-aluminum cylinder heads are fitted with Heli-Coil inserts to minimize the wear caused by repeated removal and installation of the spark plugs. helicopter. A heavier-than-air flying machine that is supported in the air by aerodynamic lift produced by an engine-driven rotor. Since the rotor is engine driven, a helicopter does not need forward motion through the air for the rotor to produce lift. 14 CFR Part 1: “A rotorcraft that, for its horizontal motion, depends principally on its engine-driven rotors.” helipad. A small designated area, usually with a prepared surface, on a heliport, airport landing/takeoff area, apron/ramp, or movement area used for takeoff, landing, or parking of helicopters. heliport. 14 CFR Part 1: “An area of land, water, or structure used, or intended to be used, for the landing and takeoff of helicopters.” helium. A colorless, odorless, tasteless, inert gaseous chemical element. Helium’s symbol is He, its atomic number is 2, and its atomic weight is 4.0026. Because helium will not combine with other chemical elements, it is used to force oxygen away from the molten metal in certain types of arc welding, and to fill some types of electron tubes to prevent oxygen from contacting the hot filaments. Because of its lightness and because it will not burn, helium is used to inflate lighter-than-air aircraft. helix. A screw-like, or spiral, curve. henry. The basic unit of inductance. One henry, H, is the amount of inductance needed to produce an electromotive force of one volt, when the current in the inductor changes at the rate of one ampere per second. heptane. A liquid organic hydrocarbon compound, CH3(CH2)5CH3. Heptane, which has a very low critical pressure and temperature, is used as the low reference fuel (octane zero) in determining the octane rating of a reciprocating engine fuel. heptode (electron tube). An electron tube, or vacuum tube, that has seven active electrodes. The electrodes in a heptode are the anode, the cathode, the control grid, and four special purpose grids, such as a screen grid, suppressor grid, and beam-forming grids. hermaphrodite calipers. Calipers having one sharp-pointed leg, similar to that used on dividers, and one leg of an outside caliper. Hermaphrodite calipers are used to scribe a mark along a piece of material a specific distance from the edge. hermetic seal. An airtight seal used to protect components or devices from the effect of moisture in the air. To achieve a hermetic seal, air is pumped out of the container or displaced by a dry inert gas, such as nitrogen, and the container is sealed so no moisture can get into it. |
