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There will be cases when the aircraft is required to land on runways that are shorter than those for which the aircraft can normally land. It is also beneficial to use less runway than that available in order to have a safety margin in the event of a failure. Although the brakes on the aircraft are designed to be adequate under all normal operating conditions, the engine can assist in shortening the landing distance by using the thrust reversal systems.In this system the engine thurst is vectored(direct (an aircraft in flight) to a desired point.) to the front to slow down the aircraft.
On a propeller engine this is very simply done by reversing the pitch of the propeller blades so that they now thrust air forward instead of backwards. Therefore the prop aircraft can change the direction of thrust by 180 degrees. But, This is impossible to achieve with a jet engine, The result of this is that the reversed thrust from a jet engine can never be as fully effective as that from a propeller. The thrust level obtained from the engine can be as high in reverse as it is in forward thrust but, because of the angle involved, the effect can never be the same as the forward thrust.
types of reversers
As far as jet engines are concerned there are three basic types of thrustreverser:
- Clamshell Door
- Retractable Ejector
- Cold Stream Cascade Reverser
1) Clamshell door
The clamshell door system is a pneumatically operated system. Normal engine operation is not affected by this system, because the ducts through which the exhaust gases are deflected remain shut until reverse thrust is activated by the pilot. When this happens, the clamshell doors rotate to uncover the ducts and close the normal exit. Then the thrust is directed in a forward direction by vanes to oppose the aircraft's motion.
2) Retractable ejector
The bucket target system is a hydraulically actuated system that uses bucket type doors to reverse the hot gas stream. The thrust reverse doors are actuated by a conventional hydraulic powered pushrod system. The actuator incorporates a mechanical lock in the extended position. In the forward thrust mode the bucket doors form the convergent-divergent final nozzle for the engine.
3) Cold stream cascade reverser
The cold stream reverse system is actuated by an air motor. The output is converted into mechanical movement by a series of flexible drives, gearboxes and screwjacks. During normal operation, the reverse thrust cascade vanes are covered by the blocker doors. On selection of reverse thrust, the actuation system folds the blosker doors to blank off the cold stream final nozzle, thus diverting the airflow through the cascade vanes.
source- http://www.aviationbirds.com/2014/05/thrust-reversers-in-jet-eingine.html