Thrust reversal

Source: Wikipedia, the free encyclopedia.
Thrust reversers deployed on the CFM56 engine of an Airbus A321

Thrust reversal, also called reverse thrust, is the temporary diversion of an

deceleration. 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
. There have been accidents involving thrust reversal systems, including fatal ones.

Reverse thrust is also available on many

propeller-driven aircraft through reversing the controllable-pitch propellers to a negative angle. The equivalent concept for a ship is called astern propulsion
.

Principle and uses

Half-deployed target-type reverser of a RB.199 engine for the Panavia Tornado, one of very few fighter aircraft with thrust reversal

A landing roll consists of touchdown, bringing the aircraft to taxi speed, and eventually to a complete stop. However, most commercial jet engines continue to produce thrust in the forward direction, even when idle, acting against the deceleration of the aircraft.[1] The brakes of the landing gear of most modern aircraft are sufficient in normal circumstances to stop the aircraft by themselves, but for safety purposes, and to reduce the stress on the brakes,[2] another deceleration method is needed. In scenarios involving bad weather, where factors like snow or rain on the runway reduce the effectiveness of the brakes, and in emergencies like rejected takeoffs,[3] this need is more pronounced.[4]

A simple and effective method is to reverse the direction of the exhaust stream of the jet engine and use the power of the engine itself to decelerate. Ideally, the reversed exhaust stream would be directed straight forward.

pitch of their propeller blades. Most commercial jetliners have such devices, and it also has applications in military aviation.[5]

Types of thrust reversal systems

Small aircraft typically do not have thrust reversal systems, except in specialized applications. On the other hand, large aircraft (those weighing more than 12,500 lb) almost always have the ability to reverse thrust. Reciprocating engine, turboprop and jet aircraft can all be designed to include thrust reversal systems.

Propeller-driven aircraft

Variable-pitch propellers of an E-2C Hawkeye

Propeller-driven aircraft generate reverse thrust by changing the angle of their controllable-pitch propellers so that the propellers direct their thrust forward. This reverse thrust feature became available with 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. Reverse thrust is created when the propeller pitch angle is reduced from fine to negative. This is called the beta position.[7]

Piston-engine aircraft tend not to have reverse thrust, however turboprop aircraft generally do.[8] Examples include the PAC P-750 XSTOL,[9] Cessna 208 Caravan, and Pilatus PC-6 Porter.

One special application of reverse thrust comes in its use on multi-engine 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. In addition, reverse thrust is often necessary for maneuvering on the water, where it is used to make tight turns or even propel the aircraft in reverse, maneuvers which may prove necessary for leaving a dock or beach.[citation needed]

Jet aircraft

A target-type thrust reverser being deployed

On aircraft using jet engines, thrust reversal is accomplished by causing the jet blast to flow forward. The engine does not run or rotate in reverse; instead, thrust reversing devices are used to block the blast and redirect it forward. High bypass ratio engines usually reverse thrust by changing the direction of only the fan airflow, since the majority of thrust is generated by this section, as opposed to the core. There are three jet engine thrust reversal systems in common use:[6]

External types

Target 'bucket' thrust reverser deployed on the Tay engines of a Fokker 100

The target thrust reverser uses a pair of

hydraulically operated bucket or clamshell type doors to reverse the hot gas stream.[1] For forward thrust, these doors form the propelling nozzle of the engine. In the original implementation of this system on the Boeing 707,[10] and still common today, two reverser buckets were hinged so when deployed they block the rearward flow of the exhaust and redirect it with a forward component. This type of reverser is visible at the rear of the engine during deployment.[6]

Internal types

VC10

Internal thrust reversers use deflector doors inside the engine shroud to redirect airflow through openings in the side of the nacelle.

pneumatically operated clamshell deflectors to redirect engine exhaust.[6][5] The reverser ducts may be fitted with cascade vanes to further redirect the airflow forward.[5]

CFM-56 engines of Finnair
Airbus A340-300
Cold-stream type thrust reverser being deployed on a Boeing 777-300

In contrast to the two types used on turbojet and low-bypass turbofan engines, many high-bypass turbofan engines use a cold-stream reverser. This design places the deflector doors in the

bypass duct to redirect only the portion of the airflow from the engine's fan section that bypasses the combustion chamber.[4] Engines such as the A320 and A340 versions of the CFM56 direct the airflow forward with a pivoting-door reverser similar to the internal clamshell used in some turbojets.[11] Cascade reversers use a vane cascade that is uncovered by a sleeve around the perimeter of the engine nacelle that slides aft by means of an air motor. During normal operation, the reverse thrust vanes are blocked. On selection, the system folds the doors to block off the cold stream final nozzle and redirect this airflow to the cascade vanes.[6]

In cold-stream reversers, the exhaust from the combustion chamber continues to generate forward thrust, making this design less effective.[1][6] It can also redirect core exhaust flow if equipped with a hot stream spoiler.[5] The cold stream cascade system is known for structural integrity, reliability and versatility, but can be heavy and difficult to integrate into nacelles housing large engines.[12]

Operation

Reverse thrust levers forward of the main levers, seen on a Boeing 747-8

In most cockpit setups, reverse thrust is set when the thrust levers are on idle by pulling them farther back.

aerodynamic lift and high speed limit the effectiveness of the brakes located on the landing gear. Reverse thrust is always selected manually, either using levers attached to the thrust levers
or moving the thrust levers into a reverse thrust 'gate'.

The early deceleration provided by reverse thrust can reduce landing roll by a quarter or more.[5] Regulations dictate, however, that an aircraft must be able to land on a runway without the use of thrust reversal in order to be certified to land there as part of scheduled airline service.

Once the aircraft's speed has slowed, reverse thrust is shut down to prevent the reversed airflow from throwing debris in front of the engine intakes where it can be ingested, causing

towbars are more commonly used for that purpose. When reverse thrust is used to push an aircraft back from the gate, the maneuver is called a powerback. Some manufacturers warn against the use of this procedure during icy conditions as using reverse thrust on snow- or slush-covered ground can cause slush, water, and runway deicers to become airborne and adhere to wing surfaces.[13]

If the full power of reverse thrust is not desirable, thrust reverse can be operated with the throttle 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, in particular in icy or slick conditions, or when the engines' jet blast could cause damage.[citation needed]

In-flight operation

A vortex made visible as powerback is used on a Boeing C-17 Globemaster III

Some aircraft, notably some Russian and

Soviet aircraft, are able to safely use reverse thrust in flight, though the majority of these are propeller-driven. Many commercial aircraft, however, cannot. In-flight use of reverse thrust has several advantages. It allows for rapid deceleration, enabling quick changes of speed. It also prevents the speed build-up 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.[citation needed
]

The Douglas DC-8 series of airliners has been certified for in-flight reverse thrust since service entry in 1959. Safe and effective for facilitating quick descents at acceptable speeds, it nonetheless produced significant aircraft buffeting, so actual use was less common on passenger flights and more common on cargo and ferry flights, where passenger comfort is not a concern.[14]

The Hawker Siddeley Trident, a 120- to 180-seat airliner, was capable of descending at up to 10,000 ft/min (3,050 m/min) by use of reverse thrust, though this capability was rarely used.

The

subsonic flight and when the aircraft was below 30,000 ft (9,100 m) in altitude. This would increase the rate of descent to around 10,000 ft/min (3,000 m/min).[citation needed
]

The

tactical descents up to 15,000 ft/min (4,600 m/min) into combat environments (a descent rate of just over 170 mph, or 274 km/h). The Lockheed C-5 Galaxy, introduced in 1969, also has in-flight reverse capability, although on the inboard engines only.[15]

The Saab 37 Viggen (retired in November 2005) also had the ability to use reverse thrust both before landing, to shorten the needed runway, and taxiing after landing, allowing many Swedish roads to double as wartime runways.

The Shuttle Training Aircraft, a highly modified Grumman Gulfstream II, used reverse thrust in flight to help simulate Space Shuttle aerodynamics so astronauts could practice landings. A similar technique was employed on a modified Tupolev Tu-154 which simulated the Russian Buran space shuttle.[citation needed]

Effectiveness

The amount of thrust and

spoilers. For aircraft susceptible to such an occurrence, pilots must take care to achieve a firm position on the ground before applying reverse thrust.[2] If applied before the nose-wheel is in contact with the ground, there is a chance of asymmetric deployment causing an uncontrollable yaw towards the side of higher thrust, as steering the aircraft with the nose wheel is the only way to maintain control of the direction of travel in this situation.[1]

Reverse thrust mode is used only for a fraction of aircraft operating time but affects it greatly in terms of

Thrust reversal-related accidents and incidents

In-flight deployment of reverse thrust has directly contributed to the crashes of several transport-type aircraft:

See also

References

  1. ^ . Retrieved 9 July 2013.
  2. ^
    ISBN 978-0-9681928-2-5. Retrieved 11 July 2013.[self-published source
    ]
  3. ^ "How Jet Crews Make Their Go/No-Go Decision During Takeoff".
  4. ^ . Retrieved 11 July 2013.
  5. ^ . Retrieved 11 July 2013.
  6. ^ a b c d e f "Thrust Reversing". Purdue AAE Propulsion. Retrieved 10 July 2013.
  7. ^ "Reverse thrust: Stopping with style". 3 January 2017.
  8. ^ FAA: Airplane Flying Handbook (FAA-H-8083-3B) Chapter 14: Transition to Turbopropeller-Powered Airplanes
  9. ^ "P-750 XSTOL Specifications". Pacific Aerospace. Retrieved 9 September 2013.
  10. ^ "Boeing's Jet Stratoliner." Popular Science, July 1954, p. 24.
  11. .
  12. ^ . Retrieved 10 July 2013.
  13. ^ "Safe Winter Operations". Boeing Corp.
  14. ^ Hamid, Hedayat U.; Margason, Richard J.; Hardy, Gordon (June 1995). "NASA Technical Reports Server (NTRS)" (PDF).
  15. ^ Rogoway, Tyler (31 August 2015). "What It's Like To Fly America's Biggest Jet, The Gargantuan C-5 Galaxy". jalopnik.com. Retrieved 3 April 2018.
  16. ^ "ASN Aircraft accident Douglas DC-8-52 ZK-NZB Auckland International Airport (AKL)".
  17. ^ "Accident Database: Accident Synopsis 02091982". airdisaster.com. Archived from the original on 2 May 2008. Retrieved 3 April 2018.{{cite web}}: CS1 maint: unfit URL (link)
  18. ^ Stokes, Henry Scott. "Cockpit Fight Reported on Jet That Crashed in Tokyo," The New York Times. 14 February 1982. Retrieved on 10 November 2011.
  19. ^ "Troubled Pilot". Time. 1 March 1982. Archived from the original on May 2, 2008. Retrieved 10 November 2011.
  20. Cockpit Voice Recorder
    Database. 2004-09-23. Retrieved 2006-12-14.

External links