REVERSE THRUST Thrust Reversal is the temporary diversion of an aircraft engines exhaust so that the exhaust produced is directed forward, rather than aft. This acts against the forward travel of the aircraft, providing deceleration. Thrust reverser systems are featured on many jet aircraft to help slow down just after touch-down, reducing wear on the brakes and enabling shorter landing distances. Such devices affect the aircraft significantly and are considered important for safe operation. Reverse thrust is also available on many propeller-driven aircraft through reversing the controllable pitch propellers to a negative angle. - Types of Reverse System As we have said, Reverse Thrust systems are in used in both Prop and Jet aircraft. Depending on this, we find different systems: Propeller-driven Aircrafts: Propeller-driven aircraft generate reverse thrust by changing the angle of their controllable pitch propellers so that the propellers direct their thrust forward. This reverse thrust feature became available with the development of controllable-pitch propellers, which change the angle of the propeller blades to make efficient use of engine power over a wide range of conditions. Jet aircrafts: On aircraft using jet engines, thrust reversal is accomplished by causing the jet blast to flow forward. The engine does not run or rotate in reverse; instead, thrust reversing devices are used to block the blast and redirect it forward. High bypass ratio engines reverse thrust by changing the direction of the fan airflow, since the majority of thrust is generated by this section, as opposed to the core. There are three jet engine thrust reversal systems in common use: - Target type The target thrust reverser uses a pair of hydraulically-operated bucket type doors to reverse the hot gas stream. For forward thrust, these doors form the propelling nozzle of the engine. In the original implementation of this system on the Boeing 707 and still common today, two reverser buckets were hinged so when they deployed they blocked the rearward flow of the exhaust and redirected it with a forward component. This type of reverser is visible at the rear of the engine during deployment. - Clam-shell type The clam-shell door, or cascade, system is pneumatically-operated. When activated, the doors rotate to open the ducts and close the normal exit, causing the thrust to be directed forward. The cascade thrust reverser is commonly used on turbofan engines. On turbojet engines, this system would be less effective than the target system, as the cascade system only makes use of the turbine airflow and does not affect the main engine core, which continues to produce thrust. - Cold stream type In addition to the two types used on turbojet and low-bypass turbofan engines, a third type of thrust reverser is found on some high-bypass turbofan engines. Doors in the bypass duct are used to redirect the air that is accelerated by the engines fan section but does not pass through the combustion chamber (called bypass air) such that it provides reverse thrust. The cold stream reverser system is activated by an air motor. During normal operation, the reverse thrust vanes are blocked. On selection, the system folds the doors to block off the cold stream final nozzle and redirect this airflow to the cascade vanes. This system can redirect both the exhaust flow of the fan and of the core. The cold stream system is known for structural integrity, reliability, and versatility. During thrust reverser activation, a sleeve mounted around the perimeter of the engine nacelle moves aft to expose cascade vanes which act to redirect the engine fan flow. - Effectiveness The amount of thrust and power generated are proportional to the speed of the plane, making reverse thrust more effective at high speeds.For maximum effectiveness, it should be applied quickly after touchdown. If activated at low speeds, foreign object damage is possible. If applied before the nose-wheel is in contact with the ground, there is a chance of asymmetric deployment causing an uncontrollable yaw towards the side of higher thrust, as steering the plane with the nose wheel is the only way to maintain control of the direction of travel in this situation. ~ AL
Posted on: Tue, 03 Jun 2014 07:21:18 +0000
Trending Topics
Recently Viewed Topics
© 2015