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power output for turbojets or
turbofans (turboprops use torque,
with balanced oil piston or phase shift
sensing systems). The EPR gauge
shows you the ratio of the tailpipe
total pressure divided by compressor
inlet total pressure.
When using EPR tables, enter with
the temperature and pressure
altitude, remembering to make
adjustments according to bleed air
usage. If you end up with an EPR
each for temperature and PA, use
the lower.
Inlet Guide Vanes direct air onto the
first rotor of the compressor. They
are closed at idle and fully open at
about 70% engine RPM.
Some engines have a small valve that
opens when the engine starts, to
correct the airflow so that the
compressor blades do not stall (a
compressor bleed) - for maximum
efficiency, and because engines have
to react quickly, you need to operate
as close to the stall as possible. At
low RPM, the engine is naturally is
not able to pump as much air, so you
need to "unload" it during start and
low power operations. A bleed air
system makes it see less restrictions
by staying open until a certain
pressure ratio is obtained. Other
engines may use such a bleed to
212 Canadian Professional Pilot Studies
prevent stalling when the throttle is
opened too suddenly.
A compressor stall reduces efficiency,
meaning less power. A cold stall only
affects a few blades or a small area
of the compressor, whereas a hot stall
involves it all, and may mean severe
damage caused by hot gases from
the combustor when the airflow
becomes reversed inside the engine.
There may or may not be a loud
noise to accompany this.
In fact, the compressor is an ideal
place from which to tap small
amounts of air (compressor bleed air)
for other purposes, such as cooling,
pressurising of oil systems or
operating anti-ice systems. However,
when doing this, the exhaust
temperature will tend to rise slightly.
For anti-ice systems, the bleed is
taken from the back end of the
compressor (that is, compressor
discharge air) that has already been
heated due to compression. It will
typically flow through the
compressor shell and hollow struts,
and the inlet guide vanes.
The Combustor
The air is divided into two streams
here, one for burning, and the other
for cooling. Once the flame is lit by
the spark igniter during engine start, it
stays that way till the engine is shut
down (you could say that the engine
is on fire all the time, and it's only
when the fire becomes uncontained
that it becomes an emergency). The
engine is spun initially by air from a
high pressure bottle or APU (or
even a V8 for the SR-71, or a
starter/generator in helicopters) and
the sparks ignited when the airflow
is high enough to keep the
temperatures down at the back end.
An auto relight system is designed to
restart the engine should it flame
out, providing a continuous spark
from the igniter all the time it is
switched on.
The Turbine
This is where hot air flows through.
As it spins, it drives the compressor,
as it is directly connected.
In a free turbine engine, used in
helicopters and some turboprops,
the exhaust goes through two
turbine stages, e.g. a compressor turbine,
and a power turbine. That is, there is
no direct connection between the
exhaust from the engine and the
gearbox it drives, which is important
for helicopters that need to
autorotate when the engine stops.
There is always a reduction gear
system to reduce the high RPM
coming from the power turbine
(rotor blades, for example, only go
round at about 300-400 RPM,
whereas a jet engine will be more like
33,000). A helicopter powered by a
free turbine may have a braking
system to keep the blades and
transmission stationary.
In a direct drive system, the propeller
is driven directly by the compressor
shaft through a reduction gear. You
can tell the difference between the
two when the propellers are stopped
– those driven by a free turbine will
be in the feathered position, whereas
those driven directly will be in fine
pitch. Start locks will fix the blades at
0° to reduce drag, and therefore
strain, when the engine is started.
They are metal pins that are held
open with centrifugal force.
The PT6, found on many aircraft,
has the compressor at the back so
Airframes, Engines & Systems 213
that the power section is nearest the
　
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