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which is the vector sum of the total reactance (the difference between the inductive and the capacitive
reactances), and the resistance. See impedance.
LDA (localizer-type directional aid). A navaid used for nonprecision instrument approaches, with utility
and accuracy comparable to a localizer. An LDA is not a part of a complete ILS and is not aligned with the
runway.
L/D or lift/drag ratio (aerodynamic parameter). A measure of efficiency of an airfoil. L/D is the ratio of
the lift to the total drag, or the coefficient of lift to the coefficient of drag at a specified angle of attack. The
lift/drag ratio is commonly called the L-over-D ratio.
lead. A soft, malleable, ductile, bluish-white, dense, metallic chemical element. Lead’s symbol is Pb, its
atomic number is 82, and its atomic weight is 207.19. Lead is used in the manufacture of soft solder, type
metal, plates for lead-acid batteries, and radiation shielding.
lead-acid battery. A popular form of storage battery used in automobiles and airplanes. The positive plate
of a fully charged lead-acid battery is made of lead peroxide, and the negative plate is pure lead. The
electrolyte is a solution of sulfuric acid and water. When a lead-acid battery discharges, both plates change
into lead sulfate, and some of the acid in the electrolyte changes into water.
Passing electrical current through a lead-acid battery from its negative terminal to its positive
terminal reverses the chemical action which took place as the battery discharged, and the battery becomes
charged again. In the charging process, sulfate ions are driven from the plates back into the electrolyte.
leading current (electrical current). Current flowing in an AC circuit that has more capacitive reactance
than inductive reactance. Current must flow into a capacitive circuit before voltage can build up across it;
therefore, in a capacitive circuit, the current leads the voltage.
leading edge. The edge of a moving object that reaches a point in space or time ahead of the rest of the
object. In an airplane wing or a helicopter rotor, the leading edge is the part of the wing or rotor the moving
air touches first. In a pulse of electrical energy, the leading edge is the first part of the pulse that moves
away from the quiescent, or at-rest, state.
leading-edge flap (aerodynamic control). A type of aerodynamic control surface, used on some airplanes
to increase the lift produced by the wing. Part of the leading edge bends downward to increase the camber
(the curvature) of the airfoil. Increasing the camber increases the pressure difference across the wing, which
in turn increases the lift. Leading-edge flaps are used for both takeoff and landing, to increase the lift at low
airspeed.
lead-lag hinge (helicopter rotor system). The hinge in the root of a helicopter rotor blade that allows the
blade tip to move back and forth in its plane of rotation. Movement about the lead-lag hinge is called drag,
and this movement is opposed by drag dampers. A lead-lag hinge is also called an alpha hinge.
lead of a screw thread. The linear distance from the apex of one screw thread to the apex of the next
thread. Lead is also the distance the screw advances in one complete rotation.
leaf brake (sheet metal shop tool). A large bending tool used to make straight bends across a sheet of
metal. The metal to be bent is clamped between a flat lower jaw and an upper jaw fitted with a radius bar
having the proper bend radius.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
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A heavy leaf, mounted on a hinge, folds up to bend the metal over the radius bar. A leaf brake is
also called a cornice brake.
leaf spring. A flat spring hinged at one end and arched in the center. The load is applied to the center of the
arch and is absorbed as the spring alternately straightens out and returns to its arched shape.
leakage (electrical). Electrical current that passes through an insulator which is supposed to stop all flow.
The amount of leakage is determined by the dielectric strength of the insulator and the amount of voltage,
or electrical pressure, across the insulator.
leakage current (semiconductor characteristic). The flow of minority carriers in a semiconductor device
when the junction is reverse-biased. Both high voltage and high temperature cause electrons to break out of
their bonds and leave holes. These electrons and holes are attracted to the junction, where they recombine
and cause a small amount of current to flow. Leakage current increases as the temperature of the device
increases.
leakage flux (magnetism). Magnetic flux that does not pass directly between the poles of a magnet.
Leakage flux does no useful work.
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航空术语词典Dictionary of Aeronautical Terms 中(106)
