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mounted on the center of the cup, and the tachometer pointer is pressed onto the end of the shaft. A flat,
coiled hairspring attached to the shaft holds the cup against a stop so the pointer rests on zero when the
magnet is not spinning.
When the engine is running, the magnet spins, and its lines of flux cut across the aluminum drag
cup and generate a voltage in it. This voltage causes eddy currents to flow in the aluminum cup where they
produce a magnetic field. The magnetic field caused by the eddy currents opposes the field of the spinning
magnet. The faster the magnet spins, the more the cup is pushed against the restraint of the hairspring.
Most magnetic-drag tachometers incorporate an hourmeter, similar to the odometer in an
automobile speedometer. The hourmeter is a gear-driven counter which counts the revolutions of the drive
shaft and displays the number of hours the engine has run on the hourmeter wheels. The hours shown are
correct only at a specific RPM, which is normally the cruising speed of the engine. This speed is stamped
on the tachometer case.
magnetic drain plug (engine lubrication system component). A drain plug in the sump of either a
reciprocating engine or a gas turbine engine. Two small permanent magnets built into the drain plug attract
and hold ferrous metal chips or shavings that may be in the lubricating oil. Metal on the drain plug is an
indication of internal engine failure.
Some magnetic drain plugs in gas turbine engines have the two magnets connected into an
electrical warning system. When bits of metal short across them, a circuit is completed, and a warning light
is turned on to show that metal particles are loose in the engine.
magnetic field. The invisible, but measurable, force surrounding a permanent magnet or current-carrying
conductor. This field is produced when the orbital axes of the electrons of the atoms in the material are all
in alignment.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 368
When a piece of iron or steel is held in the magnetic field, it is pulled toward the magnet, and
when a conductor passes through a magnetic field, current is induced into it.
magnetic flux. Invisible lines of force passing between the poles of a magnet. These invisible lines are
considered to leave the north pole of a magnet and enter its south pole, and they always follow the path of
least resistance. Lines of flux form complete loops through the magnet and across the gap between the
poles, and the loops never cross one another. When an electrical conductor cuts across the lines of magnetic
flux, a voltage is produced that causes current to flow in it.
magnetic flux density. The number of lines of magnetic flux per unit of area. Magnetic flux density is
measured in gausses. One gauss is equal to one line of flux (one maxwell) per square centimeter.
magnetic heading (navigation). The direction an aircraft is pointed with respect to magnetic north.
Magnetic north is the direction to the magnetic north pole, rather than the geographic north pole.
magnetic hysteresis. The characteristic of a magnetic material that causes it to retain some magnetism
after the magnetizing force is removed. In order to remove all the magnetism from a part, it must be
demagnetized with a force opposite to the one that magnetized it.
Soft iron has very little hysteresis. This means that as soon as a magnetizing force is removed, iron
becomes almost demagnetized. Hard steel, on the other hand, has a large amount of hysteresis, and a strong
force is needed to demagnetize it.
magnetic north pole. The location on the earth near the north geographic pole to which compass magnets
point. The earth is a huge magnet spinning about its axis in space. Lines of magnetic flux leave the earth at
its magnetic north pole and enter at its magnetic south pole, a location near its south geographic pole.
Magnetic compasses align with this magnetic flux, with one end pointing to the magnetic north pole.
magnetic particle inspection (nondestructive inspection method). A method of inspecting ferrous metal
components for cracks and other types of flaws. The part being inspected is magnetized and then flooded
with iron oxide suspended in a light oil much like kerosine. Any flaw, either on the surface or slightly
below the surface, forms a north and a south pole, and the iron oxide attracted to these poles helps locate
the flaw.
The iron oxide is often treated with a fluorescent dye, and the inspection is conducted in a
darkened booth. When an ultraviolet light (black light) is shone on the part, the treated iron oxide shows up
as a brilliant line.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 369
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航空术语词典Dictionary of Aeronautical Terms 中(120)
