航空術(shù)語詞典 Dictionary of Aeronautical Terms 上

時間：2011-03-11 23:11來源：藍(lán)天飛行翻譯 作者：admin 點擊：次

before bottom center (reciprocating engine piston position). The position of the piston in the cylinder of a reciprocating engine as the piston is moving downward, toward the bottom of its stroke, with the crankshaft turning in the direction of normal rotation.
The position is normally expressed in degrees BBC, which is the number of degrees the crankshaft must rotate before the piston reaches its bottom dead center position.
before top center (reciprocating engine piston position). The position of the piston in the cylinder of a reciprocating engine as the piston is moving upward, toward the top of its stroke, with the crankshaft turning in the direction of normal rotation.
The position is normally expressed in degrees BTC, which is the number of degrees the crankshaft must rotate before the piston reaches its top dead center position.
bel. A dimensionless number used to express the ratio of two powers or intensities. The number of bels is the logarithm of the ratio of the two values. A decibel (dB), one tenth of a bel, is more commonly used than the bel.
bell gear. The large gear in a planetary gear system. The bell gear may be either stationary or driven, and it surrounds, and meshes with, the planetary gears as they rotate around the sun gear.
bell jar. A heavy glass jar with a dome-shaped top and an open bottom. Bell jars are used as vacuum chambers in science laboratories.
The bottom of the bell jar is ground perfectly flat, so it forms an airtight seal with a flat surface plate, and a vacuum pump connected to the inside of the bell jar pumps all the air out of it.
bell mouth. The shape of a smooth convergent duct. The bell-mouth shape allows the maximum amount of air to be drawn into the duct with a minimum of loss.
bell-mouth inlet duct (gas turbine engine component). A form of convergent inlet air duct used to direct air into the inlet of a gas turbine engine. The area of a convergent duct gets smaller as the air flows into the engine.
A bell-mouth inlet duct is extremely efficient, and is used where there is little ram pressure available to force the air into the engine. Bell-mouth ducts are used in engine test cells and on engines installed in helicopters.
bell crank. A double lever in an aircraft control system used to change the direction of motion. Bell cranks are often used in the aileron system of an airplane to change spanwise movement of a push-pull rod into fore-and-aft movement of the control rod that raises or lowers the aileron.
bellows (pressure measuring device). Circular corrugated metal capsules that are either evacuated or filled with an inert gas under a specific pressure.
Bellows are used to measure pressure. As the pressure acting on the outside of the bellows changes, the bellows expand or contract. The change in the thickness of the bellows is measured, and this change relates to the change in pressure acting on the bellows.

Bellville washer. A cupped, spring steel washer used to produce a large amount of compressive force, but with a limited amount of movement.

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