Thrust vectoring

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Thrust vectoring is the ability of an aircraft or other vehicle to direct the thrust from its main engine(s) in a direction other than parallel to the vehicle's longitudinal axis. The technique was originally envisaged to provide upward vertical thrust as a means to give aircraft VTOL or STOL capability. Subsequently it was realised that the use of vectored thrust in combat situations enabled an aircraft to perform various maneuvers not available to conventional-engined planes.

Most currently operational vectored thrust aircraft use turbofans with rotating nozzles or vanes to deflect the exhaust stream. This method can successfully deflect thrust through as much as 90 degrees, relative to the aircraft centreline. However, the engine must be sized for vertical lift, rather than normal flight, which results in a weight penalty. Afterburning (or Plenum Chamber Burning - PCB - in the bypass stream) is difficult to incorporate and is not practical for Take-off/Landing, because the very hot exhaust can damage runway surfaces. Without afterburning it is difficult to reach supersonic flight speeds. A PCB engine, the Bristol Siddeley BS100, was cancelled in 1965.

Thrust vectoring is also used as a control mechanism for airships, particularly modern non-rigid airships. In this application, the majority of the load is typically supported by buoyancy and vectored thrust is used to control the motion of the aircraft. However designs have recently been proposed, particularly for Project WALRUS, where a significant portion of the weight of the aircraft is supported by vectored thrust. The first airship that used a control system based on pressurized air was the Forlanini's Omnia Dir in 1930s.

A fluidic nozzle diverts the thrust via fluid effects. Tests have shown that air forced into the exhaust stream can effect deflected thrust of up to 15 degrees. Currently in the experimental stage, fluidic nozzles are desirable for their lower weight, mechanical simplicity (no moving surfaces) and lower radar cross section and will likely be featured on many 6th generation fighter aircraft.

Tiltrotor aircraft achieve thrust vectoring by rotation of turboprop engine nacelles. The mechanical complexities of this solution are quite troublesome, including the twisting of flexible internal components and driveshaft power transfer between engines.

Most current tilt-rotor designs feature 2 rotors in a side-by-side configuration. If such a craft is flown in a way where it enters a vortex ring state, one of the rotors will always enter slightly before the other, causing the aircraft to perform a rather drastic and unplanned roll.

The best known example of thrust vectoring in an engine is the Rolls-Royce Pegasus engine of the Hawker-Siddeley Harrier brother to the BS100. (with variants built by McDonnell Douglas). Contrary to popular belief, the practice of using sudden changes as a combat manuvre was not applied against conventional Argentine fighters in the Falklands War. The technique has been used in various experimental and development planes, some with vectored thrust in directions other than upwards. Widespread use of thrust vectoring for maneuverability in a Western fighter aircraft would have to wait for the 21st century, and the deployment of the Lockheed Martin F-22 Raptor fifth-generation jet fighter, with its afterburning, thrust-vectoring Pratt & Whitney F119 turbofan.

Lockheed Martin F-35 Lightning II went into production recently. Although this aircraft incorporates a conventional afterburning turbofan (F135 or F136) which facilitates supersonic operation, the variant for the US Marine Corp and RAF also incorporates a vertically mounted, Low pressure shaft-driven remote fan, which is driven through a clutch during landing from the engine. The exhaust from this fan is deflected by a thrust vectoring nozzle, as is the main engine exhaust, to provide the appropriate combination of lift and propulsive thrust during transition.

Rockets or rocket-powered aircraft can also use thrust vectoring. Many missiles use this technique since at launch they are moving so slowly that to be able to steer effectively they would need massive fins, and they would impose a serious drag penalty once they are moving very fast. In addition, rockets often go very high up into the atmosphere or even beyond it, where aerodynamic surfaces are useless, so they need to use gas-dynamic steering. Examples of rockets and missiles which use thrust vectoring include both large systems such as the Space Shuttle SRB, S-300P (SA-10) surface-to-air missile, UGM-27 Polaris nuclear ballistic missile and RT-23 (SS-24) ballistic missile and smaller battlefield weapons such as Swingfire.

[edit] List of vectored thrust aircraft

For VTOL capability:

• Hawker-Siddeley Harrier
• McDonnell Douglas/British Aerospace AV-8B Harrier II
• Boeing V-22 Osprey (Turboprop)
• Boeing X-32
• Lockheed Martin F-35 Joint Strike Fighter (B model)
• Moller Skycar
• Dornier Do 31
• Armstrong Whitworth AW.681
• Yakovlev Yak-141

To enhance maneuverability:

Two Dimentional Thrust Vectoring (pitch axis):

• Lockheed Martin F-22 Raptor
• McDonnell Douglas F-15S/MTD
• McDonnell Douglas X-36
• Sukhoi Su-35
• Super-10
• JF-17 (speculated future addition)

Three Dimentional Thrust Vectoring (pitch and yaw axis):

• Lockheed F-16 MATV
• McDonnell Douglas F-15 ACTIVE
• McDonnell Douglas F-18 HARV
• Mikoyan-Gurevich MiG-35 MFI
• Mikoyan Project 1.44
• Rockwell-MBB X-31
• Sukhoi Su-30MKI
• Sukhoi Su-37
• Sukhoi Su-47
• X-44 MANTA

Other:

• Zeppelin NT - an example of a modern thrust vectoring airship

[edit] See also

• Gimbaled thrust, the most common thrust system in modern rockets
• Pugachev's Cobra a maneuver typical of thrust vectored fighter aircraft. (though some aircraft without thrust vectoring can perform it too)