Thrust Reversal and its Types
Thrust reversal, also known as reverse thrust, is the momentary diversion of an aircraft engine’s thrust so that it acts counter to the forward travel of the aircraft, providing a means of deceleration. Thrust reversal systems are found on many jet aircraft, where they help the aircraft slow down upon touch-down, reduce wear on the brakes & landing gear, and enable shorter landing distances. These systems affect the aircraft significantly and are an integral component in its safe operation. In addition to jet aircraft, thrust reversal is used on many propeller-driven aircraft. In these aircraft, reverse thrust is achieved through the reversal of controllable-pitch propellers to a negative angle. There are multiple types of thrust reversal in both propeller-driven and jet aircraft.
As stated, propeller-driven aircraft generate reverse thrust by changing the angle of their controllable-pitch propellers, so the propellers direct their thrust forward. This feature was not available until the development of controllable-pitch propellers, which allow for changes in the propeller angle to make efficient use of engine power in a broad range of conditions. Reverse thrust is created when the propeller angle is reduced to a negative angle. This is known as the beta position.
Reverse thrust is particularly critical on propeller aircraft such as multi-engine seaplanes and flying boats. When landing on water, these aircraft have no conventional landing gear or braking method, and therefore must rely on reverse thrust and the drag of the water to stop. Additionally, thrust reversal is needed to maneuver on water, where it is used to make tight turns or propel the aircraft backwards such as when leaving a dock or beach.
On jet aircraft, thrust reversal is achieved by causing the jet blast to flow forward. The engine does not reverse, but rather thrust reversing devices block the air and redirect it forward. Aircraft engines with high bypass ratios typically reverse thrust by changing the direction of only the fan airflow, as the majority of the thrust is generated by this section, rather than the core.
Types of Thrust Reversal
There are three types of thrust reversal utilized by jet aircraft: target type, clam-shell type, and cold stream type.
1.Target type thrust reversal employs a pair of hydraulic ‘bucket’ doors to reverse the hot gas stream. These doors form a propelling nozzle, focusing the stream. In the most common version of this system, the buckets are hinged so that when deployed, they block the rearward flow of exhaust and redirect it forward. When deployed, target type thrust reversers are visible at the rear of the engine.
2.Clam-shell systems are pneumatically operated. When activated, the doors rotate to open the ducts and close the normal exit, thereby directing the thrust forward. This type is also known as a cascade system, and is only used on turbofan engines.
3.The third type of jet aircraft thrust reversal is the cold stream type. This is found primarily on high-bypass turbofan engines. In this configuration, doors in the bypass duct redirect air that is sped up by the engine’s fan but does not pass through the combustion chamber, resulting in reverse thrust. During normal engine operation, the reverse thrust vanes are blocked. When activated, doors within the system fold to redirect airflow through the vanes, allowing it to redirect exhaust flow of both the fan and core. Cold stream thrust reversal systems are known for their structural integrity, reliability, and versatility.
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