circular slide rule. A slide rule with its scales arranged in rings on the surface of a disk. Transparent cursors, attached at the center of the disk, may be moved over the scale to add or subtract portions of the scales to perform the various mathematical operations.
circumference of a circle. The total linear distance around the outside of a circle. The length of the circumference of a circle is always 3.1416, or pi (π), times the length of the diameter of the circle (the distance across the circle at its center).
circumferential coil spring. A coil spring that is formed into a ring. This type of spring, also called a garter spring, is used to hold ring-type carbon seals tightly against a rotating shaft.
circumferential frame (aircraft structural component). A frame that gives an aircraft fuselage or engine nacelle its shape. A circumferential frame is also called a belt frame, or a transverse frame.
circumscribed circle. A circle drawn around the outside of a regular polygon in such a way that all the points of the polygon touch the circumference of the circle.
cirriform (meteorology). All species and varieties of cirrus, cirrocumulus, and cirrostratus clouds. Cirriform clouds are composed mostly or entirely of small ice crystals, usually transparent and white. Cirriform clouds often produce a halo around the sun or moon that is not observed with other cloud forms.
The average height of cirriform clouds range upward from 20,000 feet in the middle latitudes.
cirrocumulus (meteorology). A cirriform cloud that appears as a thin sheet of small white puffs resembling flakes or patches of cotton without shadows. Cirrocumulus clouds are sometimes confused with altocumulus.
cirrostratus (meteorology). A cirriform cloud that appears as a whitish veil, usually fibrous, but sometimes smooth. Cirrostratus may totally cover the sky, and often forms a halo around the sun or moon.
cirrus (meteorology). A cirriform cloud in the form of thin, white featherlike patches or narrow bands. Cirrus clouds have a fibrous and/or silky sheen. Large ice crystals often trail downward for a considerable vertical distance in fibrous, slanted, or irregularly curved wisps called mare’s tails.
cistern. A reservoir used to store a liquid. The container for the mercury in a mercury barometer is called a cistern.
civil aircraft. 14 CFR Part 1: “Aircraft other than public aircraft.”
civil twilight. The period of time before sunrise and after sunset when the sun is not more than six degrees below the horizon.
clad aluminum. A sheet of aluminum alloy which has a coating of pure aluminum rolled on one or both of its sides.
Aluminum alloys corrode easily, but pure aluminum does not. By rolling a thin coating of pure aluminum onto the surface of an alloy sheet, the high strength of the alloy and the corrosion resistance of pure aluminum may be combined. Because the pure aluminum coating is weaker than the alloy sheet, cladding slightly reduces the strength of the material.
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