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ratios, i.e. in the order of 1:1, the two streams are
usually mixed before being exhausted from the
engine. The fan engine may be regarded as an
extension of the by-pass principle, and the
requirement for high by-pass ratios of up to 5:1 is
largely met by using the front fan in a twin or triplespool
configuration (on which the fan is, in fact, the
low pressure compressor) both with and without
mixing of the airflows. Very high by-pass ratios, in the
order of 15:1, are achieved using propfans. These
are a variation on the turbo-propeller theme but with
advanced technology propellers capable of operating
with high efficiency at high aircraft speeds.
19. On some front fan engines, the by-pass
airstream is ducted overboard either directly behind
the fan through short ducts or at the rear of the
engine through longer ducts; hence the term ’ducted
fan’. Another, though seldom used, variation is that of
the aft fan.
Working cycle and airflow
17
De Havilland H1 Goblin
Development of the de Havilland Goblin
began in 1941 with the Halford H1 with a
design thrust of 3000 lb. The engine passed a
25 hr special category test in September 1942
and was cleared for flight at 2000 lb thrust.
This took place in a Gloster Meteor on 5
March 1943 and was also the first flight of that
aircraft type. In September 1943 the first flight
of a de Havilland DH100 Vampire was made
with a Goblin of 2300 lb thrust.
Rolls-Royce RB211-22B
3: Compressors
INTRODUCTION
1. In the gas turbine engine, compression of the air
before expansion through the turbine is effected by
one of two basic types of compressor, one giving
centrifugal flow and the other axial flow. Both types
are driven by the engine turbine and are usually
coupled direct to the turbine shaft.
2. The centrifugal flow compressor (fig. 3-1) is a
single or two stage unit employing an impeller to
accelerate the air and a diffuser to produce the
required pressure rise. The axial flow compressor
(fig. 3-7 and fig. 3-8) is a multi-stage unit employing
alternate .rows of rotating (rotor) blades and
stationary (stator) vanes, to accelerate and diffuse
the air until the required pressure rise is obtained. In
some cases, particularly on small engines, an axial
compressor is used to boost the inlet pressure to the
centrifugal.
3. With regard to the advantages and disadvantages
of the two types, the centrifugal compressor is
usually more robust than the axial compressor and is
also easier to develop and manufacture. The axial
compressor however consumes far more air than a
Contents Page
Introduction 19
The centrifugal flow
compressor 21
Principles of operation
Construction
Impellers
Diffusers
The axial flow compressor 22
Principles of operation
Construction
Rotors
Rotor blades
Stator vanes
Operating conditions 28
Airflow control 29
Materials 29
Balancing 33
19
centrifugal compressor of the same frontal area and
can be designed to attain much higher pressure
ratios. Since the air flow is an important factor in
determining the amount of thrust, this means the
axial compressor engine will also give more thrust for
the same frontal area. This, plus the ability to
increase the pressure ratio by addition of extra
stages, has led to the adoption of axial compressors
in most engine designs. However, the centrifugal
compressor is still favoured for smaller engines
where its simplicity and ruggedness outweigh any
other disadvantages.
4. The trend to high pressure ratios which has
favoured the adoption of axial compressors is
because of the improved efficiency that results,
which in turn leads to improved specific fuel
consumption for a given thrust, ref. fig. 3-2.
Compressors
20
Fig. 3-1 A typical centrifugal flow compressor.
Fig. 3-2 Specific fuel consumption and
pressure ratio.
THE CENTRIFUGAL FLOW COMPRESSOR
5. Centrifugal flow compressors have a single or
double-sided impeller and occasionally a two-stage,
single sided impeller is used, as on the Rolls-Royce
Dart. The impeller is supported in a casing that also
contains a ring of diffuser vanes. If a double-entry
impeller is used, the airflow to the _rear side is
reversed in direction and a plenum chamber is
required.
Principles of operation
6. The impeller is rotated at high speed by the
turbine and air is continuously induced into the
centre of the impeller. Centrifugal action causes it to
flow radially outwards along the vanes to the impeller
　
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