To overcome the
problems of low drag on large aircraft with high momentum, the designers have
introduced a variety of drag inducing devices such as;
- Spoilers
- Lift dumpers
- Speed brakers
- Thrust reversals
When landing the
aircraft, for the different landing distances required are based on variables
such as;
- Headwinds
- Aircraft Weight
- Runway length
- Runway Condition (Dry, Wet, Snow, Ice)
- Reverse thrust (Engaged or not)
In the thrust
reversals, the airflow is reverse the direction of the exhaust gas stream. The
method of redirecting the flow varies with the size, configuration and the
manufacturer of the engine.
When the runway become
icy or snow covered, to stop the aircraft and additional method should be used.
This is where “Thrust reversals” is used. In this manner, the pilot could
reduce the landing distance of an aircraft.
In high by-pass ratio engines a majority of the thrust is derived from
the fan airflow (bypass air from the fan), the core air from the engine will
exits normal. So changing the direction of the fan airflow would create a net reverse
thrust. Thrust reversals is used to change the direction of the fan airflow.
In Turboprop aircraft reverse thrust action is used by changing the
pitch of the propeller blades. The pitch in which the blade provide reverse
thrust is called the “beta range”. Usually, a hydro-mechanical system is used
to change the blade angle, giving a braking response when activated.
Ideally, the airflow
should be directed in a completely forward direction to gain the complete drag
to stop the aircraft; however, this is not possible, mainly due to aerodynamic
reasons. A discharge angle near 45 degrees is usually chosen, resulting in a
proportionally effective reverse thrust than the thrust of the same engine in
its normal direction.
There are several
methods of obtaining reverse thrust on “Turbo-Jet Engines”:
- Clamshell type deflector doors to reverse the exhaust gas stream.
- Bucket Target system with external type doors (Bucket doors) to reverse the exhaust.
- Fan engines utilize blocker doors to reverse the cold stream airflow system.
1. The
clamshell door system is a
pneumatically operated system. When the pilot activate the reverse thrust
system, the clamshell doors rotate to uncover the ducts and close the inner fan
airflow. Therefore, when the direction of flow is changed, the thrust created
by the fans of the engine will be less. However, this will not affect Normal
engine operation. The thrust created by the primary airflow will be same. Since
80% of the full thrust is generated by the fan airflow. We should only affect
that airflow.
2. The
bucket target system is a
hydraulically actuated system that uses bucket type doors to reverse the hot
gas stream (Exhaust airflow). The thrust reverse doors are actuated by a
conventional hydraulic powered pushrod system. The actuator incorporates a
mechanical lock in the extended position. In the forward thrust mode the bucket
doors form the convergent-divergent final nozzle for the engine.
3. The
cold stream reverse system is
actuated by an air motor. The output is converted into mechanical movement by a
series of flexible drives, gearboxes and screw jacks. During normal operation,
the reverse thrust cascade vanes are covered by the blocker doors. On selection
of reverse thrust, the actuation system folds the blocker doors to blank off
the cold stream final nozzle, thus diverting the airflow through the cascade
vanes.
In passenger aircraft,
reverse thrust is only used when on the ground. There are typically interlocks
in the system that prevent the thrust reversals from deploying if the aircraft
does not sense that it is on the ground. Once the aircraft touches down, the
pilot will deploy the reverse thrust. Buy in military aircraft, thrust
reversals can be used in the air. As it will enable them to perform very steep
descents.
Thrust reversals do not
always have to be operational for an aircraft to fly. The thrust reversals can
also break, in which case they will be mechanically locked to prevent them from
deploying until they can be fixed. Many airlines do not use thrust reverse to
save fuel or maintain a higher runway speed after landing to proceed to the
taxiway if it is near the end of the landing runway that has being used.
Operational
Concerns
Older aircraft with
tail mounted Pratt and Whitney JT-8D series engines such as early DC-9 series,
B-727 and B-737-200’s with low hanging wing engines that used hinged cowls for the
reverse thrust. The pilot can delay deploying it, because with the clearance to
ground there is a high probability for debris entering the engines. Moreover,
if the pilot need to abort the landing to gain the full thrust quickly thrust
reversals should not deployed. Modern airlines
no longer use these types of thrust reverse mechanism.
NOTE-
- The brakes do not become effective until aircraft speed is below 100 knots.
- Thrust Reversals are not getting enough air intake to be effective below 80 knots.
So they are used for
the initial deceleration and then closed once the aircraft slowed down.
Stopping
the A380
This aircraft weighing
in a fully loaded at 1,265,000 pounds. With the braking system, thrust
reversers are the least critical components. Only the two inboard engines
on the A380 are equipped with them. Manufacturers have not install
reversers on two outboard engines to save
the weight and lower the chances that those engines, which will sometime
hang over runway edges, would be damaged by ingesting foreign objects.
With these two
reversers, it do help to slow the A380 but not by much. But these do useful
when there is water or snow on the runway which is slippery. Most modern
airliners use reversers to redirect the airflow. The airflow bypassing the
engine core is blocked from exiting and channeled through an assembly of vanes,
called cascade vanes.
In A380, a pilot can
deploy the thrust reversers only on the ground, and can select a range of
thrust reversal from idle to maximum reverse, until the aircraft has slowed to
below 70 knots, or 80.5 mph (1 knot equals 1.15 mph). At that point, the thrust
reversers must be set at idle reverse.
All airliner engines
now have safeguards built in to keep the thrust reversers from accidentally
deploying during flight.
To stop the A380,
enormous composite Honeywell brakes on
16 of the 20 main landing gear wheels do most of the work. As on most new
airliners, the A380’s brakes are anti-skid.
They work like the anti-skid brakes in your car, responding to extreme pressure
by automatically pulsing to prevent brake lockup and skidding. Almost as
important is the aerodynamic braking
of 16 giant wing top spoilers
swinging skyward to create drag and reduce lift. Reducing lift improves
mechanical braking by putting more weight on the wheels.
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Reference – Extracted from
internet
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