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 operations by airlines.
While most modern aircraft brakes are adequate in normal conditions, an additional method of bringing the aircraft to a stop is required when runways become icy or snow covered. Reversing the path of the exhaust gas stream is an easy and efficient way of reducing the landing distance of an aircraft. Thrust reversal has been used on modern vehicles to decrease airspeed in flight, but it is not popular.
By shifting the direction of the ventilator airflow, many high by-pass ratio engines reverse thrust. As the bulk of the thrust is derived from the fan, reversing the flow of exhaust gas is unnecessary. By altering the pitch of the propeller blades, the propeller-powered aircraft reverses thrust operation. Usually, to adjust the blade angle, a hydro-mechanical device is used, providing a braking response when triggered.
Ideally, the gas should be guided in an entirely forward direction; this is not feasible, however, primarily for aerodynamic reasons. Typically, a discharge angle around 45 degrees is chosen, resulting in a reverse thrust that is proportionally less efficient than the thrust of the same engine in its usual direction.
On turbo-jet engines, there are many methods of obtaining reverse thrust: (1) camshell-type deflector doors to reverse the exhaust gas stream, (2) aim mechanism with external type doors to reverse the exhaust, (3) fan engines to reverse the cold stream airflow using blocker doors.
The camshell door system is a system that is pneumatically controlled. This mechanism is not affected by regular engine operation, since the ducts from which the exhaust gases are deflected remain shut until the pilot triggers the reverse thrust. When this occurs, to expose the ducts and close the usual exit, the clamshell doors rotate. Then the thrust is directed by vanes in a forward direction to oppose the motion of the aircraft.
The bucket goal system is a hydraulically actuated system which reverses the hot gas stream using bucket type doors. A traditional hydraulic driven pushrod system actuates the thrust reverse doors. In the extended position, the actuator integrates a mechanical lock. The convergent-divergent final nozzle for the engine forms the bucket doors in the forward thrust mode.
An air motor is actuated by the cold stream reverse system. A series of flexible drives, gearboxes and screwjacks turn the output into mechanical movement. The reverse thrust cascade vanes are protected by the blocker doors during normal operation. The actuation mechanism folds the blosker doors to blank off the final nozzle of the cold stream when reverse thrust is chosen, thus diverting the airflow through the cascade vanes.
Author – Sam Thomas
