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system, which changes the blade angle
to give the braking action under the response of the
power or throttle lever in the aircraft.
5. Ideally, the gas should be directed in a
completely forward direction. It is not possible,
however, to achieve this, mainly for aerodynamic
reasons, and a discharge angle of approximately 45
degrees is chosen. Therefore, the effective power in
reverse thrust is proportionately less than the power
in forward thrust for the same throttle angle.
PRINCIPLES OF OPERATION
6. There are several methods of obtaining reverse
thrust on turbo-jet engines; three of these are shown
in fig. 15-2 and explained in the following
paragraphs.
7. One method uses clamshell-type deflector doors
to reverse the exhaust gas stream and a second
uses a target system with external type doors to do
the same thing. The third method used on fan
engines utilizes blocker doors to reverse the cold
stream airflow.
8. Methods of reverse thrust selection and the
safety features incorporated in each system
described are basically the same. A reverse thrust
lever in the crew compartment is used to select
reverse thrust; the lever cannot be moved to the
reverse thrust position unless the engine is running
at a low power setting, and the engine cannot be
opened up to a high power setting if the reverser fails
to move into the full reverse thrust position. Should
the operating pressure fall or fail, a mechanical lock
holds the reverser in the forward thrust position; this
lock cannot be removed until the pressure is
restored. Operation of the thrust reverser system is
indicated in the crew compartment by a series of
lights.
Clamshell door system
9. The clamshell door system is a pneumatically
operated system, as shown in detail in fig. 15-3.
Normal engine operation is not affected by the
system, because the ducts through which the
exhaust gases are deflected remain closed by the
doors until reverse thrust is selected by the pilot.
10. On the selection of reverse thrust, the doors
rotate to uncover the ducts and close the normal gas
stream exit. Cascade vanes then direct the gas
stream in a forward direction so that the jet thrust
opposes the aircraft motion.
Thrust reversal
160
Fig. 15-1 Comparative landing runs with and without thrust reversal.
Thrust reversal
161
Fig. 15-2 Methods of thrust reversal.
Thrust reversal
162
Fig. 15-3 A typical thrust reverser system using clamshell doors.
Thrust reversal
163
Fig. 15-4 A typical fan cold stream thrust reversal system.
11. The clamshell doors are operated by pneumatic
rams through levers that give the maximum load to
the doors in the forward thrust position; this ensures
effective sealing at the door edges, so preventing
gas leakage. The door bearings and operating
linkage operate without lubrication at temperatures of
up to 600 deg. C.
Bucket target system
12. The bucket target system is hydraulically
actuated and uses bucket-type doors to reverse the
hot gas stream. The thrust reverser doors are
actuated by means of a conventional pushrod
system. A single hydraulic powered actuator is
connected to a drive idler, actuating the doors
through a pair of pushrods (one for each door).
13. The reverser doors are kept in synchronization
through the drive idler. The hydraulic actuator incorporates
a mechanical lock in the stowed (actuator
extended) position.
14. In the forward thrust mode (stowed) the thrust
reverser doors form the convergent-divergent final
nozzle for the engine.
Cold stream reverser system
15. The cold stream reverser system (fig. 15-4) can
be actuated by an air motor, the output of which is
converted to mechanical movement by a series of
flexible drives, gearboxes and screwjacks, or by a
system incorporating hydraulic rams.
16. When the engine is operating in forward thrust,
the cold stream final nozzle is ’open’ because the
cascade vanes are internally covered by the blocker
doors (flaps) and externally by the movable
(translating) cowl; the latter item also serves to
reduce drag.
17. On selection of reverse thrust, the actuation
system moves the translating cowl rearwards and at
the same time folds the blocker doors to blank off the
cold stream final nozzle, thus diverting the airflow
through the cascade vanes.
Turbo-propeller reverse pitch system
18. As mentioned in para. A, reverse thrust action is
affected on turbo-propeller powered aircraft by
changing the pitch of the propeller blades through a
　
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