Reverse Thrust | Thrust Reverser used in Aircraft

Thrust Reverser

Thrust Reversers

Jet aircraft's weight and speed cause them to have a high kinetic energy during the landing roll. With the nose wheel on the ground, a jet aircraft has little drag, and the engines continue to produce forward thrust even while the power levers are idle, making it difficult to release this energy. Wheel brakes can typically handle the situation, but another speed-reduction technique is clearly necessary. The drag created by reverse thrust satisfies this necessity.

A thrust reverser is a device installed in the exhaust system of an engine that successfully reverses the flow of exhaust gases. Although the flow does not change direction by 180°, the final path taken by the exhaust gases is roughly 45° away from the direction of travel. A net efficiency of roughly 50% is the result of this plus the losses in the reverse flow pathways. If the engine is running in reverse at less than maximum rpm, it produces much less.

Requirements of thrust reversal in aircraft

The braking action provided by slowly rotating propellers, which on larger aircraft are capable of going into reverse pitch and hence producing reverse thrust, is lacking in a jet-powered aircraft during its landing run. The issue is made worse by the faster landing speeds brought on by modern jet aeroplanes' highly streamlined, low drag fuselages and their bigger gross weights. Under these challenging circumstances, standard wheel brakes are no longer sufficient, and larger brakes would result in a significant weight and space penalty and reduce the aircraft's useable load. Brakes can also be particularly useless on snowy or wet runways.

The difficulty of halting an aircraft after landing has significantly risen as aircraft have grown heavier and can land at faster speeds. In many cases, it is no longer possible to rely on the aircraft brakes to immediately slow the plane down to a safe distance after touchdown.

Reversing the direction of engine thrust by reversing the exhaust gas stream is a straightforward but efficient technique for swiftly slowing down an aircraft after landing. With today's technology, the pilot may regulate the amount of reverse thrust. The modern thrust reverser can also be utilised for emergency descents, to slow the aircraft during approaches, to change the rate of sink, and to maintain a high rpm to reduce the amount of time required to accelerate the engine in the event of a "go-around." High-idle rpm will also supply enough bleed air from the compressor for the correct operation of air-driven accessories. Additionally, the employment of the thrust reverser is being looked at as a way to enhance air combat manoeuvres. The majority of thrust reverser devices fall into one of two categories: aerodynamic blockage or mechanical blockage.

Types of Thrust Reversers

The following are the two primary categories of thrust reversers:

1. Post-exit or target type

2. Pre-exit with cascades or doors that obstruct or deflect traffic

Placing an impediment in the jet exhaust stream roughly one nozzle diameter behind the engine is all that is necessary to perform post-exit reversing. Depending on where the engine is mounted on the aircraft, the gas stream might be diverted in either a horizontal or vertical orientation.

The gases are turned forward in the pre-exit type thrust reverser using usually stowed doors or airfoils that are blocked during forward thrust operation. Doors are relocated during reverse thrust so that they now obstruct the exhaust gas stream. Now that the gas is leaving, it is being directed forward through spinning vanes or by deflector doors.

Thrust Reverser Designs and Systems

A best designed thrust reverser should do the following:

  • Be mechanically strong and constructed of high-temperature metals to take the full force of the high-velocity jet and, at the same time, tum this jet stream through a large angle
  • Not affect the basic operation and performance of the engine, whether the reverser is in operation or not
  • Provide approximately 50% of the full forward thrust
  • Operate with a high standard of fail-safe operation
  • Not increase air drag by increasing engine and nacelle frontal area
  • Cause few increased maintenance problems
  • Not add an excessive weight penalty
  • Not cause the reinjection of the exhaust gas stream into the compressor nor cause the exhaust gas stream to impinge upon the airframe. That is, the discharge pattern must be correctly established by the placement and shape of the target or vane cascade.
  • Allow the flying crew to complete control of the amount of reverse thrust
  • Not affect the aerodynamic characteristics of the airplane adversely

Mechanical blockage type Thrust Reverser

Bucket Type Thrust Reverser
Bucket Type Thrust Reverser

By placing a removable obstruction into the exhaust gas stream, often a little to the back of the nozzle, mechanical blockage is achieved. An inverted cone, half-sphere, or clam shell is used to mechanically obstruct the engine exhaust fumes and guide them at an appropriate angle in the opposite direction. This is positioned to turn around the flow of exhaust gases. This design is frequently used with ducted turbofan engines, where the fan and core flow are combined in a single nozzle before leaving the engine. The clamshell-type or mechanical-blockage reverser functions by erecting a wall in the path of outgoing exhaust gases, cancelling out and reversing the engine's forward impulse.

The reverser system needs to be reliable, "fail-safe," mechanically robust, able to tolerate high temperatures, and relatively light in weight. It must be streamlined into the engine nacelle's design while not in use. The clamshell doors retract and neatly fit around the engine exhaust duct when the reverser is not in operation, typically forming the back portion of the engine nacelle.

Aerodynamic Blockage Type Thrust Reverser

Cascade vanes thrust reverser
Cascade Vanes Thrust Reverser

Only fan air is used in the aerodynamic blockage type of thrust reverser, which is typically employed with unducted turbofan engines, to slow the aircraft. The fan airflow is redirected by a translating cowl, blocker doors, and cascade vanes in a modern aerodynamic thrust reverser system in order to slow the aircraft. Moving the thrust levers aft triggers the translating cowl to open and close the blocker doors if the thrust levers are in the idle position and the aircraft is under weight. By doing this, the fan's airflow is prevented from moving aft and is instead redirected through the cascade vanes, which direct the airflow forward to slow the aircraft. The fan is the best source for reverse thrust because it can provide about 80% of the engine's thrust. The blocker doors open and the translating cowl shuts when the thrust levers (power levers) are brought back to their neutral position.

System Dependency on air/ground status

On an airworthy aircraft, the choice of thrust reverser deployment depends on whether the system has been signalled with "air" status or "ground" status, with the latter being a must. When an aircraft is "in flight," multiple defences against reverser deployment must be in place. However, during the brief transition between "air" and "ground" status and between "ground" and "air," there may be system use or malfunction problems that are directly related to the status being signalled. One illustration of the latter is the intentional connection between reverser unlocking and slat retracting.

When deployed or stowed, a thrust reverser cannot have any negative effects on engine performance. The flight deck often provides information about the reverser system's status. The thrust reverser system is made up of a number of parts that can move either the blocker door and translating cowl or the clam shell doors. Gearboxes, flexdrives, screwjacks, control valves, and air or hydraulic motors are used in conjunction with pneumatic or hydraulic actuators to deploy or stow the thrust reverser systems. Until the flight deck gives the order to deploy the systems, they are locked in the stowed state. Requirements for maintenance and inspection are crucial because there are many moving parts. The reverser system must be mechanically locked out from deploying while people are in the region of the reverser system while performing any kind of maintenance.

Rejected Landings

The ability to reject the landing will almost always be lost if thrust reversers are activated after touchdown because it will take a long time to get back to full thrust. The attempt to halt will nearly always be pursued more effectively if such runway distance is available. If so, there may be various ways to avoid a risky runway detour that would certainly be preferable than trying to take off again. In any case, once thrust reversers have been deployed, several aircraft types are operated with a strict ban on going around.

Rejected Take Offs

Although the choice to reject a takeoff at high speed will often result in the selection of reverse thrust, it should not be assumed that reverser deployment will take place if the cause for the reject decision is connected to a potential or actual loss of airworthiness. Deploying a thrust reverser might not be wise in some circumstances.

Thrust Reverser Unserviceability

It is typically acceptable to dispatch under MEL relief when one or both thrust reversers have been identified by maintenance as being unserviceable. Any usage of any remaining functional thrust reverser(s) shall be based on the existence of flight crew instructions given by or approved by the aircraft operator, absent MEL conditions that completely exclude doing so. Prior to every landing with such status, this advice should be specifically given and then followed.

Use of Reverse Thrust

Reverse thrust should only be used on low wing aircraft with mounted engines while the aircraft is on an active runway. Ingestion of FOD or contamination of the air conditioning system with too much surface de-icing chemicals occasionally encountered on taxiways might cause engine damage if the vehicle is used in even idle reverse during runway exit and first taxi in.

Thrust Reverser Safety Features

Thrust reversers accidentally being deployed while in the air is a very hazardous emergency. Therefore, this possibility is considered when designing thrust reverser systems. The systems typically have multiple lock mechanisms: one to prevent reverser operation while in the air, another to stop reverser operation when the thrust levers are not in the idle detent, and/or a "auto-stow" circuit to order reverser stowage whenever reverser deployment would be inappropriate, such as during takeoff and while in the air. It is crucial that pilots are aware of both the standard operating procedures and restrictions associated with the usage of thrust reversers, as well as how to handle uncommanded reverse. These situations call for an immediate and precise response.

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