Gas Turbine Glossary

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Glossary
A
Abradable strip. A strip of material in the compressor
housing of some axial-flow gas turbine engines. The tip of
the compressor blade touches the abradable strip, and wears,
or abrades a groove in it. This groove ensures the minimum
tip clearance.
Abradable tip (compressor blade tip). The tip of some
axial-flow compressor blades constructed so that it will
abrade, or wear away, upon contact with the compressor
housing, which ensures the minimum tip clearance between
the blade and the housing.
Absolute pressure. Pressure referenced from zero pressure
or a vacuum.
AC. Alternating current. Electrical current in which
the electrons continually change their rate of flow and
periodically reverse their direction.
ACC. Active clearance control. A system for controlling the
clearance between tips of the compressor and turbine blades
and the case of high-performance turbofan engines. When
the engine is operating at maximum power, the blade tip
clearance should be minimum, and the ACC system sprays
cool fan discharge air over the outside of the engine case. This
causes the case to shrink enough to decrease the tip clearance.
For flight conditions that do not require such close clearance,
the cooling air is turned off, and the case expands to its normal
dimensions. The control of the ACC system is done by the
FADEC, or full-authority digital electronic control.
Acceleration. The amount the velocity of an object is
increased by a force during each second it is acted upon by
that force. Acceleration is usually measured and expressed
in terms of feet per second, per second (fps²).
Accessory end. The end of a reciprocating engine on which
many of the accessories are mounted. Also, called the anti-
propeller end.
Accumulator. A hydraulic component that stores a
non-compressible fluid, such as oil, under pressure. An
accumulator has two compartments separated by a flexible
or movable partition with one compartment containing
compressed air. When oil is pumped into the other
compartment, the partition moves over, further compressing
the air which holds pressure on the oil.
AD (ashless dispersant) oil. A mineral-based lubricating oil
used in reciprocating engines. This oil does not contain any
metallic ash-forming additives, but has additives that disperse
the contaminants and hold them in suspension until they can
be removed by filters.
ADC. Air data computer. An electronic computer in an
aircraft that senses pitot pressure, static pressure, and total air
temperature. It produces an indication of altitude, indicated
airspeed, true airspeed, and Mach number. The output of ADC
is usable by any of the engine or flight control computers.
ADI (antidetonation injection) system. A system used with
some large reciprocating engines in which a mixture of water
and alcohol is sprayed into the engine with the fuel when
operating at extremely high power. The air/fuel mixture is
leaned to allow the engine to develop its maximum power,
and the ADI fluid absorbs excessive heat when it vaporizes.
Adiabatic change. A physical change that takes place within
a material in which heat energy is neither added to the
material, nor taken away. If a container of gas is compressed,
with no heat energy added to or taken from it, the gas will
become hotter; its temperature will rise.
Aeromatic propeller. A patented variable-pitch propeller
that has flyweights around the blade shanks and the
blades angled back from the hub to increase the effects of
aerodynamic and centrifugal twisting forces. This propeller
automatically maintains a relatively constant rpm for any
throttle setting.
Aft-fan engine. A turbofan engine with the fan mounted
behind the compressor section. The blades of an aft-fan are
normally extensions of the free turbine blades.
Afterburner. A component in the exhaust system of a
turbojet or turbofan engine used to increase the thrust for
takeoff and for special flight conditions. Since much of the air
passing through a gas turbine engine is used only for cooling,
it still contains a great deal of oxygen. Fuel is sprayed into
the hot, oxygen-rich exhaust in the afterburner, where it is
ignited and burned to produce additional thrust.
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