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resistance (R) of the circuit with the combined values of the inductive reactance (XL) and capacitive
reactance (XC) in the circuit. If the capacitive reactance is larger than the inductive reactance, the formula
for impedance is:
If the inductive reactance is larger than the capacitive reactance, the formula for impedance is:
impedance coupling (electronic circuits). A method of using a transformer to join, or couple, stages of an
electronic device, such as an amplifier. The impedance of the primary winding of the coupling transformer
is the same as the output impedance of the first stage, and the impedance of the secondary winding is the
same as the input impedance of the following stage. Impedance coupling is primarily used in circuits
carrying audio-frequency AC because of the limited bandwidth of the coupling transformer.
impedance matching (electronic circuits). The process of matching the impedance of a source of
electrical power with the impedance of the load using the power. For maximum power transfer to occur, the
impedance of the source and the impedance of the load should be the same. Transformers can be used to
match impedance. The following formula is used to relate the impedance ratio to the turns ratio:
NP = number of turns in the primary winding
NS = number of turns in the secondary winding
ZP = impedance of the primary winding
ZS = impedance of the secondary winding
impedance matching transformer. A transformer used in an electronic circuit to match the impedance of
the load to the impedance of the source of electrical energy.
impedance triangle. A graphic method of showing the relationship existing between resistance, reactance,
and impedance in an AC circuit. An impedance triangle is a right triangle, with the horizontal side
representing the circuit resistance and the vertical side representing the combined capacitive and inductive
reactance in the circuit. The hypotenuse represents the circuit impedance.
The angle formed between the hypotenuse and the horizontal side is the phase angle (Ø) of the
circuit, the number of electrical degrees the current leads or lags the voltage.
impeller (centrifugal pump). A vaned wheel used to move and add energy to a fluid. The fluid enters the
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 316
center of the impeller and is thrown outward by centrifugal force. As the fluid leaves the impeller, it is
collected in a diffuser where it is slowed and its pressure is increased.
Impellers can move either liquids or gases. Some applications are: moving cooling fluid through a
reciprocating engine, compressing the air before it goes into the carburetor of a reciprocating engine, and
compressing the air in a gas turbine engine.
impingement starter (gas turbine engine starter). The simplest type of starter used to start a gas turbine
engine. A stream of high-pressure air, directed against the turbine blades or onto the centrifugal compressor,
spins the compressor fast enough to start the engine.
implode. To burst inwardly. Electron tubes, such as the cathode-ray tubes used in television sets and
computer displays, have the air evacuated from them so the pressure of the air outside the tube is much
greater than the pressure inside the tube.
If the glass envelope of the tube cracks, outside air will rush violently onto the tube and the tube
will implode.
improper fraction (mathematics). A common fraction in which the numerator, the number above the line,
is greater than the denominator, the number below the line.
impulse coupling (magneto component). A spring-loaded coupling between a magneto and an aircraft
reciprocating engine that causes the magneto to produce a hot and late spark for starting the engine. The
rotating magnet in a magneto must be turned at a fast rate to produce a hot spark, but when the engine is
being started, the magneto turns slowly.
Arms inside the impulse coupling contact stop pins in the magneto housing and hold the magnet
while the crankshaft turns. A heavy, flat spring winds up as the crankshaft continues to turn, with the
magnet held stationary. When the crankshaft turns about thirty degrees past the point at which ignition
normally occurs, a cam forces the arms off the stop pins, and the spring spins the magnet at a fast rate.
When the magnet spins, it produces a hot spark. And since the spark occurs late, the piston is
moving downward, preventing the engine kicking back. As soon as the engine is running normally,
counterweights on the arms sling outward and hold the arms in, away from the stop pins. This keeps the
impulse coupling disengaged so it has no effect on the ignition timing for normal engine operation.
impulse-reaction turbine (gas turbine engine component). A type of turbine in which hot, high-velocity
　
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