Thrust reversal - CONSTRUCTION AND MATERIALS

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Thrust reversal - Turbo-propeller reverse pitch system

CONSTRUCTION AND MATERIALS

Fig. 15-6 Hot stream thrust reverser installations.

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.

Fig. 15-7 A cold stream thrust reverser installation.

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 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

Thrust reversal - Turbo-propeller reverse pitch system

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Thrust reversal - Cold stream reverser system

Turbo-propeller reverse pitch system

Fig. 15-5 A propeller pitch control 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 hydro-mechanical pitch control system (fig. 15-5). Movement of the throttle or power control lever 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 interconnect- ed 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.

Thrust reversal - Bucket target system

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Thrust reversal - Clamshell door system

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 incor- porates 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.

Thrust reversal - Clamshell door system

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Thrust reversal - PRINCIPLES OF OPERATION

Clamshell door system

Fig. 15-3 A typical thrust reverser system using clamshell doors.

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.
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.

Thrust reversal - INTRODUCTION

INTRODUCTION

Fig. 15-1 Comparative landing runs with and without thrust reversal.

1. Modern aircraft brakes are very efficient but on wet, icy or snow covered runways this efficiency may  be reduced by the loss of adhesion between the aircraft tyre and the runway thus creating a need for an additional method of bringing the aircraft to rest within the required distance.
2. A simple and effective way to reduce the aircraft landing run on both dry and slippery runways is to reverse the direction of the exhaust gas stream, thus using engine power as a deceleration force. Thrust reversal has been used to reduce airspeed in flight but it is not commonly used on modern aircraft. The difference in landing distances between an aircraft without reverse thrust and one using reverse thrust is illustrated in fig. 15-1.
3. On high by-pass ratio (fan) engines, reverse thrust action is achieved by reversing the fan (cold stream) airflow. It is not necessary to reverse the exhaust gas flow (hot stream) as the majority of the engine thrust is derived from the fan.