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hydro-mechanical pitch control system (fig. 15-5).
Movement of the throttle or power control lever
Thrust reversal
164
Fig. 15-5 A propeller pitch control system.
Thrust reversal
165
Fig. 15-6 Hot stream thrust reverser installations.
directs oil from the control system to the propeller
mechanism to reduce the blade angle to zero, and
then through to negative (reverse) pitch. During
throttle lever movement, the fuel to the engine is
trimmed by the throttle valve, which is interconnected
to the pitch control unit, so that engine power and
blade angle are co-ordinated to obtain the desired
amount of reverse thrust. Reverse thrust action may
also be used to manoeuvre a turbo-propeller aircraft
backwards after it has been brought to rest.
19. Several safety factors are incorporated in the
propeller control system for use in the event of
propeller malfunction, and these devices are usually
hydro-mechanical pitch locking devices or stops.
CONSTRUCTION AND MATERIALS
20. The clamshell and bucket target doors (fig. 15-
6) described in paras. 9 and 12 form part of the jet
pipe. The reverser casing is connected to the aircraft
structure or directly to the engine. The casing
supports the two reverser doors, the operating
mechanism and, in the case of the clamshell door
system, the outlet ducts that contain the cascade
vanes. The angle and area of the gas stream are
controlled by the number of vanes in each outlet
duct.
21. The clamshell and bucket target doors lie flush
with the casing during forward thrust operation and
are hinged along the centre line of the jet pipe. They
are, therefore, in line with the main gas load and this
ensures that the minimum force is required to move
the doors.
22. Both the clamshell door system and the bucket
target system are subjected to high temperatures
and to high gas loads. The components of both
systems, especially the doors, are therefore
Thrust reversal
166
Fig. 15-7 A cold stream thrust reverser installation.
constructed from heat-resisting materials and are of
particularly robust construction.
23. The cold stream thrust reverser casing (fig. 15-
7) is fitted between the low pressure compressor
casing and the cold stream final nozzle. Cascade
vane assemblies are arranged in segments around
the circumference of the thrust reverser casing.
Blocker doors are internally mounted and are
connected by linkages to the external movable
(translating) cowl, which is mounted on rollers and
tracks. Because the thrust reverser is not subjected
to high temperatures, the casing, blocker doors and
cowl are constructed mainly of aluminium alloys or
composite materials. The cowl is double-skinned,
with the space between the skins containing noise
absorbent material (Part 19).
Thrust reversal
167
Turbo-Union RB199
Metrovick F2/4 Beryl
Development of the F2, the first British axial
flow turbo-jet, began in f 940. After initial flight
trials in the tail of an Avro Lancaster, two F2s
were installed in a Gloster Meteor and first
flew on 13 November 1943. After early
problems the F2/4 Beryl was developed which
gave up to 4000 lb thrust and was used to
power the Saunders Roe SR/A1 flying boat
fighter.
16: Afterburning
Contents Page
Introduction 169
Operation of afterburning 170
Construction 173
Burners
Jet pipe
Propelling nozzle
Control system 173
Thrust increase 175
Fuel consumption 178
INTRODUCTION
1. Afterburning (or reheat) is a method of
augmenting the basic thrust of an engine to improve
the aircraft take-off, climb and (for military aircraft)
combat performance. The increased power could be
obtained by the use of a larger engine, but as this
would increase the weight, frontal area and overall
fuel consumption, afterburning provides the best
method of thrust augmentation for short periods.
2. Afterburning consists of the introduction and
burning of fuel between the engine turbine and the jet
pipe propelling nozzle, utilizing the unburned oxygen
in the exhaust gas to support combustion (fig. 16-1).
The resultant increase in the temperature of the
exhaust gas gives an increased velocity of the jet
leaving the propelling nozzle and therefore increases
the engine thrust.
3. As the temperature of the afterburner flame can
be in excess of 1,700 deg. C., the burners are usually
arranged so that the flame is concentrated around
the axis of the jet pipe. This allows a proportion of the
turbine discharge gas to flow along the wall of the jet
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