How Do Thrust Reversers Work?

Since the earliest days of aviation, one of the most challenging technical aspects to overcome has been the landing phase. As aircraft's capacity and technology have continued to improve, so has the demand placed upon the landing systems due to increased airspeed and weight. One mechanism that has played a significant role in modern landing systems is the thrust reverser, which can immediately be implemented after touchdown to slow the plane in a short time. In this blog, we will discuss everything you need to know about this critical landing technology, including its design and application.

Thrust reversers work alongside the brakes to slow the aircraft's fast landing speed. When used correctly, thrust reversers take work off of the brakes, thereby increasing their lifespan and decreasing the amount of maintenance required. As such, every airline company operating passenger jet engine aircraft has made their installation a requirement. While working through a slightly different mechanism, many propeller-driven aircraft can also achieve reverse thrust.

Although modern aircraft brakes are advanced enough to completely bring the aircraft to a stop, the jet engine's natural state produces forward thrust, even while idling. They also lose a great deal of efficacy during rainstorms or other circumstances that may cause the runway to be wet. Thrust reversers help slow the aircraft by changing the direction of the exhaust stream to favor deceleration. To accomplish this, they may implement a target, cascade, clam-shell, or cold-stream system.

Target Type

Popularized by the Boeing 707, the target-type thrust reverser uses hydraulic actuated doors near the exhaust panel to immediately change the direction of thrust. Commonly found in larger jet aircraft, target-type reversers are compatible with engines producing upwards of 13kN of thrust. A standard target-type system can provide around a 55% reverse-thrust ratio, while specialized cowls can boost this to 84%. Though most variants call for the reversers to open immediately after landing, some aircraft initiate the opening sequence simultaneously with landing gear actuation. Meanwhile, other aircraft such as the Douglas DC-8 allow the doors to open mid-flight to make dramatic speed adjustments.

Cascade

Limited to high-bypass ratio turbofan engines, cascade reversers work by changing the direction of the fan's airflow. Since the majority of thrust created by this engine type comes from the fan, changing its direction is an effective means of achieving deceleration.

Clam-shell

Explicitly designed for turbojet engines, clam-shell reversers use pneumatic-actuated doors to block the exhaust exit. As such, the exhaust is only allowed to escape through a different vent at an angle suitable to provide reverse thrust.

Cold-Stream

Also implemented in turbofan engines, the cold stream reverser blocks the bypass regions while leaving the hot stream intact.

Propeller Driven Aircraft

Unlike jet engines, which rely upon actuated surfaces to block hot and cold exhaust, propeller-driven aircraft generate reverse thrust by changing the angle of their propellers. The prerequisite for this maneuver is a controllable-pitch propeller that may be altered in flight. While most piston-engine aircraft lack this ability, the majority of modern turboprop aircraft may perform this technique. Additionally, seaplanes rely extensively upon this landing method since they are not equipped with any type of braking system.

Conclusion

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