drops. Divergent inlet designs change ram air
Navy/Marine aircraft. You will notice that
velocity into high static pressure at the compressor
included with the turboprop engines are two
inlet. Friction due to air passing through the duct
civilian engines, which have the manufacturer's
surfaces and bends in the duct cause pressure
designations.
drops and differences. Performance achieved
through proper duct design is only half the story.
Careful construction and maintenance is essential
to maintain designed performance. Small amounts
JET TURBINE ENGINE
of airflow distortion result in loss of engine
MAJOR ASSEMBLIES
efficiency and unexplainable compressor surges.
There are many different models of jet engines
This is caused by poor sheet metal work,
protruding rivet heads, or poor welds.
in the Navy today. Developments over the years
has produced a more efficient engine, both from
Engine inlet ducts take a variety of shapes,
a performance and maintenance point of view.
These modern engines are more complex, but they
depending on the position of the engine and
p u r p o s e of the aircraft. Two methods of
still operate according to the same basic principles.
This section discusses the major parts found in
classifying inlet ducts are as follows:
v a r i o u s gas turbine engines; their name
1. Single entrance and divided entrance
(nomenclature), construction, purpose and
operating characteristics. Major components of
2. Subsonic and supersonic ducts
all gas turbine engines are basically the same. The
nomenclature of the various engines in current
use, however, may vary slightly because of
Single Entrance/Divided Entrance
differences in manufacturers' terminology.
Through experience and reading the many and
The simplest type of air ducts are on engines
varied publications, the mechanic recognizes the
mounted in pods under the wing. The single
engine components, regardless of the terminology
entrance (fig. 1-14, view A) gets maximum ram
used.
pressure through the straight flow. It's used where
an unobstructed entrance lends itself readily to
A turbojet engine consists of the following
a single, short, straight duct.
sections and systems:
Some aircraft, because of fuselage design or
1. Air entrance section
internal parts, have to use a dual or divided
2. Compressor section
entrance, as shown in figure 1-14, view B. These
3. Combustion section
dual entrances are in the wing root, or in scoops
4. Turbine section
on the side of the fuselage. Either type presents
5. Exhaust section
more problems to the aircraft and engine designers
6. Accessory section
than the single entrance. Problems are caused by
7. Systems necessary for starting, lubrication,
boundary layer airflow and the difficulty in
fuel supply, and auxiliary purposes, such as anti-
obtaining enough entrance area without creating
icing, cooling, and afterburning
too much drag.
AIR ENTRANCE SECTION
Subsonic/Supersonic Ducts
The air entrance directs incoming air to the
Modern Navy aircraft capable of supersonic
compressor entrance with a minimum of energy
flight pose another problem to aircraft designers
loss. Additionally, it must deliver this air under
because the airframe can withstand supersonic
all flight conditions with as little turbulence and
velocity air, but the engine cannot. There are two
pressure variation as possible.
methods commonly used to diffuse the intake air
Normally, the engine inlet is part of the air-
and slow its flow to subsonic speeds during super-
frame. Because of its important part in engine
sonic flight. One is to create a shock wave in the
performance we include it here. Proper duct
intake airstream, which will disrupt the flow and
design contributes to aircraft performance by
cause a decrease in velocity. The other method
increasing ram recovery and limiting pressure
1-12
