Thrust Reversal Systems: Types and Mechanisms
Thrust reversal is one of the three techniques used to slow down the aircraft after landing (the other two being brakes and ground spoilers). During this process, the engine’s thrust temporarily diverges so that it can produce a thrust (a sudden force) in the direction opposite to the motion. Thrust reversers are the control systems that execute this process. This article will focus on the different types and science behind their operation.
Reversers are useful in scenarios like landing roll and rejected takeoffs A typical landing roll consists of aircraft touchdown followed by deacceleration to taxi speed or full stop. Rejected takeoff procedures are those in which the takeoff is aborted due to obstruction in the runway, emergency situations inside or outside the aircraft, unfavorable climatic conditions, or mismanagement due to heavy air traffic. They also help in normal landings when the aircraft weight is heavy or the runway is short. Their job is to assist the landing brakes after landing by producing an opposing force hence reducing the efficiency of engine thrust and wear on the brakes. Ideally, the gas stream should be directed in a completely forward direction but this is not possible for aerodynamic reasons. These systems direct the stream to an angle between 90 and 45 degrees. This produces reverse thrust up to 50% of forward thrust.
Types of Thrust Reversal Systems
Large aircraft with piston, turbine, or jet propulsion engines are usually designed to include reversal systems. In turboprop aircraft, the direction of thrust is controlled by changing the angle of the propeller blades. To reverse it, their pitch angle is reduced from fine to a negative value (also called beta position). A negative angle produces a forward thrust against the motion of the aircraft. This is only possible in aircraft like Cessna 208 Caravan with controllable-pitch propellers. These types of propellers are also known as reverse pitch propellers.
Based on their method to reverse the thrust, they can be classified into the following 2 types:
- Target Type (Bucket Thrust Reversal): This thrust reversal is used in aircraft with low bypass turbofan engines that produce more than 13 kilonewtons of thrust. They provide the reverse thrust by engaging a pair of hydraulic bucket-shaped doors to reverse the hot gas stream by closing the end of the engine. The bucket systems have an aerodynamic contour on both surfaces. Airflow which was supposed to exit from the ends passes through these contours and produces a reverse thrust. When undeployed, the doors seamlessly connect to the engine’s curved body and provide a streamlined outer surface.
Mechanism: The reversers are actuated by hydraulic pushrod systems. The position of any type of thrust reverser can be categorized into two types; forward thrust mode and reverse thrust mode. For target type reversers, the actuator deploys a mechanical lock in reverse thrust mode. In the forward thrust position, these reversers act as convergent-divergent nozzles for the engine and allow the usual airflow through the engine.
Target type reversers also include clamshell reversers. These are a modification of bucket type design. They are pneumatically controlled and perform the operation following the same principle. When deployed, clamshell doors rotate to uncover the ducts and close the normal exit. Unlike the bucket doors, this design has deflector doors in the front part of the engine. Doors are smaller, the deployment system is compressed, less complex, and has less weight.
2. Cascade Type: This type is used in both high bypass turbofan engines. They are less effective than target reversers. To incorporate a cascade reverser, the engine has radially arranged openings near the aft edge of the fan cowl. Within these openings, a cascade set of airflow turning vanes is mounted. This system also has blocker doors in the bypass duct with their actuating system placed in the inner wall of the fan cowl. These doors redirect the air entering the engine and prevents it from entering the combustion chamber. Hence, this system is also called cold stream type reversal. Outside the engine, the openings are covered by a sleeve-like sliding section of the cowl. Cascade type systems are very heavy, reliable, and versatile as compared to other systems.
Mechanism: Cascade systems are actuated by an air motor. The output is converted into mechanical motion by a series of flexible drives, gearboxes, and screw jacks. When deployed, this section slides back and uncovers the cascades. The linkage between the sliding cowl and blocking doors moves doors into the bypass airstream. The normal path of the stream is blocked and the airflow is redirected to cascade vanes. The air exits from these vanes and produces a reverse thrust.
Other Methods for Thrust Reversal
- Drogue parachute: These parachutes are deployed during landing to increase drag, deaccelerate the aircraft and provide more stability. They are used by aircraft and spacecraft recovery systems.
