BASIC PRINCIPLE
To ensure a good braking effect for the aircraft also on contaminated runways (with water or slush) and for the reduction of brake wear, transport aircraft are equipped with thrust reversers. A thrust reverser allows the generation of a rearward-directed thrust force when deployed. To achieve this it redirects the exhaust gas flow at an angle of approximately 120 degrees. Figure 1 shows the direction of the airflow during reverse thrust operation. On turbofan engines with high bypass ratios, only the secondary gas flow is redirected by the thrust reverser because this gas flow generates the larger portion of the engine thrust. This results in a reverse thrust force high enough for braking purposes. To redirect the secondary gas flow only makes mechanical deflector components in the hot gas flow unnecessary. This results in a simpler reverser kinematics with less weight and costs.
Fig . 1 Direction of the secondary airflow for the generation of the reverse thrust force
Because the effect of the reverse thrust is independent from the tire friction, the thrust reverser ensures a good deceleration of the aircraft on a contaminated runway with reduced tire friction. During normal runway conditions the use of the thrust reverser requires less use of the wheel brakes for the same aircraft deceleration. The results are less wear of the wheel brakes and a longer operating life of the brake disks.
To have these advantages the higher weight of the engine nacelle must be accepted. To minimize the weight penalty caused by a thrust reverser the designers use a high percentage of composite material for the reverser
and nacelle structure. Usually all engines of an aircraft are equipped with a thrust reverser. But this is not a general rule in any case. An exception is the A380. For weight reduction purposes this aircraft has only two thrust reversers. They are installed on the two inboard engines.
REVERSER OPERATION
A thrust reverser system is designed for the use on ground only. The system is equipped with safety features preventing the deploying of the reverser during flight. During landing the thrust reverser is deployed shortly after touchdown by selection of the pilot. The best braking effect is achieved at the higher speeds during the landing run because the propulsive efficiency for the reverse thrust has its highest values at the high forward
speeds. With the decreasing aircraft speed it also decreases. During typical flight operation the thrust reverser is used down to a speed of 80 knots. The pilot selects as much thrust as needed. This procedure
ensures an operation with the highest propulsive efficiency and prevents the ingestion of dirt at slow taxi speeds. It is the most efficient way of thrust reverser use in terms of fuel consumption and brake wear.
TYPES OF THRUST REVERSERS
Thrust reversers can be differentiated by the types of their subsystems. These subsystems are the
• Airflow deflection system
• Actuation system
• Control system
The airflow deflection system comprises the structural components necessary for the deflection of the airflow during the operation in the reverse thrust position. For the change between the forward thrust operation and
the reverse thrust operation some components of the airflow deflection system are movable. To achieve its movement an actuation system is installed. This is controlled by the pilots via the thrust lever and the reverser
control system. It is designed to move the reverser components into one of the two end positions. These are the forward thrust (stowed) position or the reverse thrust (deployed) position.
