GENERAL
The turbopropeller-powered airplane flies and handles
just like any other airplane of comparable size and
weight. The aerodynamics are the same. The major
differences between flying a turboprop and other
non-turbine-powered airplanes are found in the powerplant
and systems. The powerplant is different and
requires operating procedures that are unique to gas
turbine engines. But so, too, are other systems such as
the electrical system, hydraulics, environmental, flight
control, rain and ice protection, and avionics. The
turbopropeller-powered airplane also has the advantage
of being equipped with a constant speed, full feathering
and reversing propeller—something normally not
found on piston-powered airplanes.
THE GAS TURBINE ENGINE
Both piston (reciprocating) engines and gas turbine
engines are internal combustion engines. They have a
similar cycle of operation that consists of induction,
compression, combustion, expansion, and exhaust. In a
piston engine, each of these events is a separate distinct
occurrence in each cylinder. Also, in a piston engine an
ignition event must occur during each cycle, in each
cylinder. Unlike reciprocating engines, in gas turbine
engines these phases of power occur simultaneously
and continuously instead of one cycle at a time.
Additionally, ignition occurs during the starting cycle
and is continuous thereafter.
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The basic gas turbine engine contains four sections:
intake, compression, combustion, and exhaust.
[Figure 14-1]
To start the engine, the compressor section is rotated by
an electrical starter on small engines or an air driven
starter on large engines. As compressor r.p.m.
accelerates, air is brought in through the inlet duct,
compressed to a high pressure, and delivered to the
combustion section (combustion chambers). Fuel is
then injected by a fuel controller through spray
nozzles and ignited by igniter plugs. (Not all of the
compressed air is used to support combustion. Some of
the compressed air bypasses the burner section and circulates
within the engine to provide internal cooling.) The
fuel/air mixture in the combustion chamber is then burned
in a continuous combustion process and produces a very
high temperature, typically around 4,000°F, which heats
the entire air mass to 1,600 – 2,400°F. The mixture of
hot air and gases expands and is directed to the turbine
blades forcing the turbine section to rotate, which in
turn drives the compressor by means of a direct shaft.
After powering the turbine section, the high velocity
excess exhaust exits the tail pipe or exhaust section.
Once the turbine section is powered by gases from the
burner section, the starter is disengaged, and the
igniters are turned off. Combustion continues until the
engine is shut down by turning off the fuel supply.
High-pressure exhaust gases can be used to provide
jet thrust as in a turbojet engine. Or, the gases
can be directed through an additional turbine to drive a
propeller through reduction gearing, as in a
turbopropeller (turboprop) engine. |
