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than a propeller-type engine, since its propulsive
efficiency depends largely on its forward speed; the
pure turbo-jet engine is, therefore, most suitable for
high forward speeds. The propeller efficiency does,
however, decrease rapidly above 350 miles per hour
due to the disturbance of the airflow caused by the
high blade-tip speeds of the propeller. These characteristics
have led to some departure from the use of
pure turbo-jet propulsion where aircraft operate at
medium speeds by the introduction of a combination
of propeller and gas turbine engine.
17. The advantages of the propeller/turbine
combination have to some extent been offset by the
introduction of the by-pass, ducted fan and propfan
engines. These engines deal with larger comparative
airflows and lower jet velocities than the pure jet
engine, thus giving a propulsive efficiency (Part 21)
which is comparable to that of the turbo-prop and
exceeds that of the pure jet engine (fig. 1-10).
Basic mechanics
7
Fig. 1-10 Comparative propulsive efficiencies.
18. The turbo/ram jet engine (fig. 1-11) combines
the turbo-jet engine (which is used for speeds up to
Mach 3) with the ram jet engine, which has good
performance at high Mach numbers.
19. The engine is surrounded by a duct that has a
variable intake at the front and an afterburning jet
pipe with a variable nozzle at the rear. During takeoff
and acceleration, the engine functions as a conventional
turbo-jet with the afterburner lit; at other
flight conditions up to Mach 3, the afterburner is
inoperative. As the aircraft accelerates through Mach
3, the turbo-jet is shut down and the intake air is
diverted from the compressor, by guide vanes, and
ducted straight into the afterburning jet pipe, which
becomes a ram jet combustion chamber. This engine
is suitable for an aircraft requiring high speed and
Basic mechanics
8
Fig. 1-12 A turbo-rocket engine.
Fig. 1-11 A turbo/ram jet engine.
sustained high Mach number cruise conditions
where the engine operates in the ram jet mode.
20. The turbo-rocket engine (fig. 1-12) could be
considered as an alternative engine to the turbo/ram
jet; however, it has one major difference in that it
carries its own oxygen to provide combustion,
21. The engine has a low pressure compressor
driven by a multi-stage turbine; the power to drive the
turbine is derived from combustion of kerosine and
liquid oxygen in a rocket-type combustion chamber.
Since the gas temperature will be in the order of
3,500 deg. C, additional fuel is sprayed into the
combustion chamber for cooling purposes before the
gas enters the turbine. This fuel-rich mixture (gas) is
then diluted with air from the compressor and the
surplus fuel burnt in a conventional afterburning
system.
22. Although the engine is smaller and lighter than
the turbo/ram jet, it has a higher fuel consumption.
This tends to make it more suitable for an interceptor
or space-launcher type of aircraft that requires high
speed, high altitude performance and normally has a
flight plan that is entirely accelerative and of short
duration.
Basic mechanics
9
Rolls-Royce/Snecma Olympus
Rolls-Royce RB37 Derwent 1
A straight-through version of the reverse-flow
Power Jets W2B, known as the W2B/26, was
developed by the Rover Company from 1941
to 1943. Taken over by Rolls-Royce in April
1943 and renamed the Derwent, it passed a
100hr. test at 2000 lb thrust in November 1943
and was flown at that rating in April 1944. The
engine powered the Gloster Meteor III which
entered service in 1945.
2: Working cycle and airflow
Contents Page
Introduction 11
Working cycle 11
The relations between pressure,
volume and temperature 13
Changes in velocity
and pressure 14
Airflow 17
INTRODUCTION
1. The gas turbine engine is essentially a heat
engine using air as a working fluid to provide thrust.
To achieve this, the air passing through the engine
has to be accelerated; this means that the velocity or
kinetic energy of the air is increased. To obtain this
increase, the pressure energy is first of all increased,
followed by the addition of heat energy, before final
conversion back to kinetic Energy in the form of a
high velocity jet efflux.
WORKING CYCLE
2. The working cycle of the gas turbine engine is
similar to that of the four-stroke piston engine.
However, in the gas turbine engine, combustion
occurs at a constant pressure, whereas in the piston
engine it occurs at a constant volume. Both engine
　
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