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 Thrust Reversal

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PostSubject: Thrust Reversal   Mon Mar 01, 2010 11:52 pm

Thrust Reversal

Thrust reversal, also called reverse thrust, is the temporary diversion of an aircraft engine's
exhaust or changing of propeller pitch so that the thrust produced is
directed forward, rather than aft. This acts against the forward travel
of the aircraft, providing deceleration. Thrust reversers are used by
many jet aircraft
to help slow down just after touch-down, reducing wear on the brakes
and enabling shorter landing distances. It is also available on many propeller aircraft through reversing the controllable pitch propellers to a negative angle.

Operation

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Thrust reversers deployed on the outer two of the four turbofans of an Ilyushin Il-62 landing at Munich Airport



Reverse thrust is typically applied immediately after touchdown, often along with spoilers, to improve deceleration early in the landing roll when residual aerodynamic lift and high speed limit the effectiveness of the friction brakes located on the landing gear. Reverse thrust is always selected manually, either using levers attached to the thrust levers,
or by moving the thrust levers into a reverse thrust 'gate'. When
thrust is reversed, passengers will hear a sudden increase in engine
noise, particularly those seated just forward of the engines.

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Thrust reverser deployed on the Pratt & Whitney JT8D-7 turbofan engine of an Aloha Airlines Boeing 737-200 landing at Honolulu, HI



The early deceleration provided by reverse thrust can reduce landing
roll by a third or more. Regulations dictate, however, that a plane
must be able to land on a runway without the use of thrust reversers in
order to be certified to land there as part of scheduled airline service.
Once the aircraft's speed has slowed, thrust reverse is shut down to
prevent the reversed airflow from raising debris in front of the engine
intakes where it can be ingested, causing foreign object damage.
Thrust reverse is effective at any aircraft speed, and, if
circumstances require, can be used all the way to a stop, or even to
provide thrust to push the aircraft backward, though aircraft tugs or towbars are more commonly used for that purpose.
If the full power of reverse thrust is not desirable, thrust reverse
can be operated with the throttles set at less than full power, even
down to idle power, which reduces stress and wear on engine components.
Reverse thrust is sometimes selected on idling engines to eliminate
residual thrust, particularly in icy or slippery conditions, or where
the engines' jet blast could do damage.


In-flight operation

Some aircraft are able to safely use reverse thrust in flight,
though the majority of these are propeller-driven. Many commercial
aircraft cannot use reverse thrust in flight. Exceptions include
Russian and Soviet aircraft which are able to reverse thrust in flight
(mostly before touchdown). In-flight use of reverse thrust has several
advantages: It allows for rapid deceleration, enabling quick changes of
speed; it also prevents the speed buildup normally associated with
steep dives, allowing for rapid loss of altitude, which can be
especially useful in hostile environments such as combat zones, and
when making steep approaches to land.
For example, the ATR 72 turboprop can reverse thrust in flight, should the appropriate control lock be withdrawn. The Hawker Siddeley Trident,
a 120-180 seat airliner, was capable of descending at up to
10,000 ft/min (3,050 m/min) by use of the thrust reversers, though this
capability was rarely used. Concorde
could also use reverse thrust in the air to increase the rate of
descent. Only the inboard engines are used and the engines are placed
only in reverse idle when subsonic and below 30,000 ft. This will
increase the rate of descent to around 10,000 fpm.[citation needed] The US Air Force's C-17A is one of the few modern aircraft that uses reverse thrust in flight. The Boeing-manufactured
aircraft is capable of in-flight deployment of reverse thrust on all
four engines to facilitate steep tactical descents up to 15,000 ft/min
(4,600 m/min) into combat environments (this means that the aircraft's
descent rate is just over 170 mph, or 274kph). The Saab 37 Viggen
(retired in November 2005) also had the ability to use reverse thrust
before landing, enabling the use of many roads constructed in Sweden to
double as wartime runways.
The Shuttle Training Aircraft, a highly modified Grumman Gulfstream II, uses reverse thrust in flight to help simulate the Space Shuttle aerodynamics so astronauts can practice landings.

Types of aircraft

Small aircraft typically do not feature reverse thrust, except in
specialized applications. Conversely, large aircraft (weighing more
than 12,500 lb) almost always have the ability to reverse thrust. Both reciprocating engine and turboprop
aircraft can have reverse thrust, and almost all propeller aircraft
with reverse thrust have the ability to set the propeller angle to flat
pitch (called Beta range) which generates no forward or reverse thrust,
but provides large amounts of drag. This is especially useful in
aircraft with complex reciprocating or turbine engines, as it enables
engine speed to be kept high as the aircraft descends, avoiding doing
damage to the engines by shock cooling them.

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Controllable pitch propeller on one of the four turboprop engines of a United States Air Force Lockheed C-130 Hercules



Propeller aircraft

Propeller aircraft generate reverse thrust by changing the angle of their controllable pitch propellers
so that the propellers direct their thrust forward, instead of aft as
normal. Reverse thrust has been available on propeller aircraft dating
back to the 1930s. Reverse thrust became available due to the
development of controllable-pitch propellers, which change the angle of
the propeller blades to make efficient use of engine power over a wide
range of conditions.


Multiengine

Early multiengine aircraft such as the Boeing 247 and Douglas DC-2
were among the first to feature reverse thrust. As piston aircraft
became heavier and more complex, reverse thrust became more important
to allow them to operate from airports originally configured to handle
the smaller planes of previous years. Additionally, the higher
performance and greater altitude attainable by post World War II piston aircraft like the Lockheed Constellation
made the ability to use flat pitch, or, in extreme cases, reverse
thrust, in order to descend and slow for landing without overcooling
the engines or approaching the runway with excessive speed. Finally,
the advent of turboprops like the Vickers Viscount and Lockheed Electra brought even higher speeds and cruising altitudes to the fleet, as well as increased power that could be used both for improved performance and to provide reverse thrust.


Single-engine

Single-engine aircraft tend to be of such limited size that the
weight and complexity of reverse thrust is unwarranted. However, large
single-engine aircraft like the Cessna Caravan & Pilatus Porter
do have reverse thrust available, and single-engine seaplanes and
flying boats tend to have reverse thrust as well. In other respects,
reverse thrust on single-engine aircraft works much like that on other
propeller aircraft.

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Twin radial engine Canadair CL-215 flying boat used for firefighting by the Minnesota Department of Natural Resources



Seaplanes and flying boats


One special application of reverse thrust comes in its use on seaplanes and flying boats. These aircraft, when landing on water, have no conventional braking method and must rely on slaloming and/or reverse thrust, as well as the drag
of the water in order to slow or stop. Additionally, reverse thrust is
often necessary for maneuvering on the water, where it is used to make
tight turns or even back the aircraft, such as when leaving a dock or
beach.


Jet aircraft

On aircraft using jet engines, thrust reversal is accomplished by causing the jet blast
to flow forward rather than aft. The engine does not run or rotate in
reverse; instead, thrust reversers are used to block the blast and
redirect it forward. Two methods are commonly used: In the target-type
thrust reverser, the reverser blades angle outward, giving the general
appearance of flower petals, and forcing engine thrust to flow forward.
In the clamshell type, two reverser buckets are hinged so that when
they deploy, they intrude into the exhaust of the engine, capturing and
reorienting the jet blast. This type of reverser is usually clearly
visible at the rear of the engine during use.


Turbofan

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Boeing C-17 creating a visible vortex while demonstrating the use of reverse thrust to push the aircraft backwards down the runway.


In addition to the two types used on turbojet and low-bypass
turbofan engines, a third type of thrust reverser is found on some
high-bypass turbofan engines. Doors in the bypass duct
are used to redirect the air that has been accelerated by the engine's
fan section but has not passed through the combustion chamber (called
bypass air) so that it provides reverse thrust.
The Boeing C-17
has a rare form of the above type in which even the exhaust from the
core is redirected along with the main fan's air. This gives the C-17
unrivaled stopping ability among large jet powered aircraft.

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