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dielectric constant. A number used to indicate the ease with which a particular dielectric material
concentrates lines of electrostatic force.
A vacuum is used as the reference and is assigned a dielectric constant of one. The dielectric
constant of air is very near that of a vacuum; it is 1.0006. Mica allows the lines of electrostatic force to pass
much more easily, and it has a dielectric constant of 7.
The dielectric constant of the insulating material in a capacitor determines its capacity.
dielectric heating. A process used to heat certain types of nonconductive plastic materials. The material to
be heated forms the dielectric, or insulator, of a capacitor, and two metal electrodes act as the plates.
Alternating current with a frequency of between 20 and 80 megahertz is placed across the plates.
The material between the plates is heated by the dielectric losses caused by the friction of the
molecules moving under the influence of the rapidly changing electrostatic field. Dielectric heating is used
to seal bags made of vinyl film and to cure certain types of plastic resins.
dielectric loss. Electrical energy converted into heat in a dielectric material. Dielectric loss occurs when the
material is subjected to a rapidly changing electrostatic field. Power is lost, and heat is produced.
dielectric strength. The maximum amount of voltage that can be placed across a dielectric material
without it breaking down and conducting. Dielectric strength is normally measured in volts per millimeter
of thickness.
diesel engine. A form of compression-ignition reciprocating heat engine. In a diesel engine, the fuel-air
mixture inside the cylinder is ignited by the heat of compression rather than by an electric spark.
The air inside the cylinder is heated as it is compressed and, when the piston is near the top of its
stroke, the air is very hot. A fuel injector sprays a very fine mist of fuel into the hot air and the fuel ignites
and burns. Heat added by the burning fuel expands the air and pushes the piston down.
diesel fuel. A hydrocarbon fuel used in a compression-ignition engine. Diesel fuel, obtained by fractional
distillation of crude oil, is the fraction that distills off just after kerosine.
dieseling (reciprocating engine malfunction). A condition in which an aircraft reciprocating engine tries
to run after the ignition is turned off. Dieseling normally occurs when the engine is overheated. Carbon
particles inside the cylinders become incandescent and ignite the fuel-air mixture in the cylinder and keep
the engine running.
difference. The amount by which one quantity is greater or less than another. The answer in a subtraction
problem is called the difference.
differential aileron travel (airplane control system). The difference between the upward and downward
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 188
travel of an aileron. An aileron moves a greater number of degrees upward than downward to counteract
adverse yaw.
The downward-moving aileron produces both induced and parasite drag, but the upward-moving
aileron produces only parasite drag. To prevent the combined drag causing the nose of the airplane to start
to move toward the down aileron, the up aileron travels a greater distance, producing enough additional
parasite drag to overcome the induced drag caused by the down aileron. See adverse yaw.
differential amplifier (electronic amplifier). An amplifier with two inputs and one output. The signal on
the output is an amplification of the difference between the signals on the two inputs.
differential coil. A coil wound in such a way that its magnetic field opposes the field produced by another
coil.
differential compression check (reciprocating engine maintenance check). A maintenance check used
to determine the condition of the piston rings and the valves in the cylinders of a reciprocating engine. An
air pressure regulator holds a constant pressure of 80 psi on the input side of a calibrated orifice. The output
side of the orifice is connected to the cylinder through an adapter screwed into a spark plug hole.
Air flowing into the cylinder, to replace that which leaks past the piston rings and valves, causes a
pressure drop across the orifice. The amount of leakage is indicated by the difference in the readings of the
two pressure gages on the tester.
The sound of leaking air heard at the carburetor air inlet indicates a leaking intake valve. A
leaking exhaust valve is indicated by the sound being heard at the exhaust tail pipe. Worn piston rings are
indicated by the sound of leaking air being heard at the crankcase breather.
differential compression tester (reciprocating engine test equipment). Test equipment consisting of an
　
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