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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gyroscope. A rapidly spinning wheel with its weight concentrated about its rim. Gyroscopes have two basic characteristics that make them useful: rigidity in space and precession. Because of its rigidity in space, the spinning gyroscope does not tilt its axis of rotation as the earth rotates. Precession is the characteristic that causes the gyroscope to react to an applied force at a point 90° away from the point of application, in the direction of its rotation. Gyroscopes are used in both attitude and rate indicating flight instruments. See rigidity in space and gyroscopic precession. gyroscopic precession. A characteristic of a gyroscope that causes it to react to an applied force as though the force were applied at a point 90° in the direction of rotation, from the actual point of application. Precession is the characteristic used in rate gyros. A rate gyro will precess, or lay over in its gimbal, an amount proportional to the rate at which the gimbal is rotated. Rate gyros are the heart of turn and slip indicators and turn coordinators. The rotor of a helicopter acts in much the same way as a gyroscope and is affected by gyroscopic precession. gyroscopic turn indicator. A rate-type flight instrument used to show the pilot the rate at which the aircraft is rotating about its vertical axis. Both turn and slip indicators and turn coordinators are gyroscopic turn indicators. H HAA (height above airport) (air traffic control). The height of the minimum descent altitude above the published airport elevation. HAA is published in conjunction with circling minimums. hacksaw. A hand-held saw using a narrow, fine-tooth, replaceable blade, held under tension in a metal frame. Hacksaw blades used for cutting metal and other hard materials are designed to cut only on the forward movement of the blade. The blade should be lifted from the work on its return stroke. hafnium. A brilliant, silvery, metallic chemical element. Hafnium’s symbol is Hf, its atomic number is 72, and its atomic weight is 178.49. Hafnium has an extremely high melting point (above 2,000°C) and is used in the manufacture of filaments for incandescent lamps. hail (meteorology). A form of precipitation which comes from cumulo nimbus clouds. Hail forms when drops of water are carried by up-currents of air inside the cloud to a level where the temperature is low enough that the water freezes into pellets of ice. Near the top of the cloud, the ice pellets are thrown from the up-current and fall into the warmer air below. They pick up more water and are again carried upward. Each trip up and down allows the ice pellets, called a hailstones, to gain another layer of ice. This process continues until the hail is too heavy to stay in the cloud. Some hailstones grow large enough to cause severe damage to people, property, or animals on the ground. hair hygrometer. An instrument used to measure the humidity of the air. The sensitive element in a hair hygrometer is a bundle of human hair held under a slight tension by a spring. The length of the hair expands or contracts as the humidity in the air changes. This change in length moves a pointer over a calibrated scale. |
