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An S-duct (or '' serpentine inlet'') is a type of
jet engine A jet engine is a type of reaction engine, discharging a fast-moving jet (fluid), jet of heated gas (usually air) that generates thrust by jet propulsion. While this broad definition may include Rocket engine, rocket, Pump-jet, water jet, and ...
intake duct used in several types of
trijet A trijet is a jet aircraft powered by three jet engines. In general, passenger airline trijets are considered to be second-generation jet airliners, due to their innovative engine locations, in addition to the advancement of turbofan technology. ...
aircraft An aircraft ( aircraft) is a vehicle that is able to flight, fly by gaining support from the Atmosphere of Earth, air. It counters the force of gravity by using either Buoyancy, static lift or the Lift (force), dynamic lift of an airfoil, or, i ...
. In this configuration, the intake is in the upper rear center of the aircraft, above or below the stabilizer, while the exhaust and engine is at the rear of the aircraft. The S-duct is located in the tail, or
empennage The empennage ( or ), also known as the tail or tail assembly, is a structure at the rear of an aircraft that provides stability during flight, in a way similar to the feathers on an arrow.Crane, Dale: ''Dictionary of Aeronautical Terms, third ed ...
, of the aircraft. The shape of the S-duct is distinctive and easily recognized, and was used in several aircraft, beginning in 1962 with the
Hawker Siddeley Trident The Hawker Siddeley HS-121 Trident (originally the de Havilland DH.121 and briefly the Airco DH.121) is a British airliner produced by Hawker Siddeley. In 1957, de Havilland proposed its DH.121 trijet design to a British European Airways (BEA ...
. The
Dassault Falcon 8X The Dassault Falcon 7X is a large-cabin, range business jet manufactured by Dassault Aviation. Unveiled at the 2001 Paris Air Show, its first flight took place on 5 May 2005 and it entered service on 15 June 2007. The Falcon 8X, first delivered ...
and
Dassault Falcon 900 The Dassault Falcon 900, commonly abbreviated as the F900, is a French-built corporate trijet aircraft made by Dassault Aviation. Development The Falcon 900 is a development of the Dassault Falcon 50, Falcon 50, itself a development of the e ...
business jets are the only aircraft in production that use the S-duct design.


Benefits and drawbacks

The S-duct was invented as a solution for positioning the central engine on trijets. The S-duct is easier to service than alternative trijet designs. Most trijet designs opted for the S-duct layout. Only the
McDonnell Douglas DC-10 The McDonnell Douglas DC-10 is an American trijet wide-body aircraft manufactured by McDonnell Douglas. The DC-10 was intended to succeed the Douglas DC-8, DC-8 for long-Range (aeronautics), range flights. It first flew on August 29, 1970; i ...
and MD-11 trijets' designers chose not to use the S-duct and go with a "straight-through" layout. The straight-through layout leaves the engine high above the ground, making access difficult. The straight-through layout also increases total
aerodynamic drag In fluid dynamics, drag, sometimes referred to as fluid resistance, is a force acting opposite to the direction of motion of any object moving with respect to a surrounding fluid. This can exist between two fluid layers, two solid surfaces, or b ...
by 2–4%. Compared to the straight-through design on the DC-10 and MD-11, the S-duct allows a shorter fin and a rudder closer to the longitudinal axis. On the
Lockheed L-1011 TriStar The Lockheed L-1011 TriStar (pronounced "El-ten-eleven") is an American medium-to-long-range, wide-body trijet airliner built by the Lockheed Corporation. It was the third wide-body airliner to enter commercial operations, after the Boeing 747 ...
, engineers were able to maintain engine performance comparable with straight-through designs by limiting the curve of the S-duct to less than a quarter of the radius of the engine intake diameter. The S-duct design also reduced the total empty aircraft weight. The research undertaken during the design of the L-1011 indicated that losses of using an S-duct were more than compensated for by the savings. The S-shaped duct is a complicated and costly design. Since modern jet engines have more power and reliability than those of the 1970s and can safely power the aircraft with only two engines, the trijet design is no longer used for large commercial airliners but is used on the latest
Dassault Falcon 7X The Dassault Falcon 7X is a large-cabin, range business jet manufactured by Dassault Aviation. Unveiled at the 2001 Paris Air Show, its first flight took place on 5 May 2005 and it entered service on 15 June 2007. The Falcon 8X, first delivered ...
and Falcon 8X business jets in order to provide more total thrust while enabling the continued use of smaller engines in the class with which Falcons have historically been designed. In international aviation regulations the triple engine layout is also considered inherently safer for the increased redundancy, which allows the use of certain airfields at high elevation only for aircraft with more than two engines. The S-duct is also used on combat aircraft, where it has the advantage of blocking the front of the jet engine from radar. The spinning compressor blades would produce a strong radar return, compared to the smooth sides of the duct.


List of S-duct aircraft

Aircraft built with S-ducts are the: *
Chengdu J-20 The Chengdu J-20 (), also known as Mighty Dragon (, NATO reporting name: Fagin), is a twinjet, twin-engine Night fighter, all-weather Stealth aircraft, stealth fighter developed by China's Chengdu Aircraft Corporation for the People's Liberatio ...
*
Dassault Falcon 7X The Dassault Falcon 7X is a large-cabin, range business jet manufactured by Dassault Aviation. Unveiled at the 2001 Paris Air Show, its first flight took place on 5 May 2005 and it entered service on 15 June 2007. The Falcon 8X, first delivered ...
*
Dassault Falcon 900 The Dassault Falcon 900, commonly abbreviated as the F900, is a French-built corporate trijet aircraft made by Dassault Aviation. Development The Falcon 900 is a development of the Dassault Falcon 50, Falcon 50, itself a development of the e ...
*
Dassault Rafale The Dassault Rafale (, literally meaning "gust of wind", or "burst of fire" in a more military sense) is a French Twinjet, twin-engine, Canard (aeronautics), canard delta wing, Multirole combat aircraft, multirole fighter aircraft designed and ...
*
Eurofighter Typhoon The Eurofighter Typhoon is a European multinational twin-engine, supersonic, canard delta wing, multirole fighter. The Typhoon was designed originally as an air-superiority fighter and is manufactured by a consortium of Airbus, BAE Syste ...
*
Lockheed Martin F-22 Raptor The Lockheed Martin/Boeing F-22 Raptor is an American Twinjet, twin-engine, Jet engine, jet-powered, Night fighter, all-weather, supersonic Stealth aircraft, stealth fighter aircraft. As a product of the United States Air Force's Advanced Ta ...
*
Lockheed Martin F-35 Lightning II The Lockheed Martin F-35 Lightning II is an American family of single-seat, single-engine, supersonic Stealth aircraft, stealth strike fighters. A multirole combat aircraft designed for both Air superiority fighter, air superiority and att ...
*
Rockwell B-1 Lancer The Rockwell B-1 Lancer is a supersonic variable-sweep wing, heavy bomber used by the United States Air Force. It has been nicknamed the "Bone" (from "B-One"). , it is one of the United States Air Force's three strategic bombers, along with t ...
Aircraft previously built with S-ducts, but no longer in production, are the: *
Boeing 727 The Boeing 727 is an American Narrow-body aircraft, narrow-body airliner that was developed and produced by Boeing Commercial Airplanes. After the heavier Boeing 707, 707 quad-jet was introduced in 1958, Boeing addressed the demand for shorter ...
*
Dassault Falcon 50 The Dassault Falcon 50 is a French super-midsize, long-range business jet, featuring a trijet layout with an S-duct air intake for the central engine. It has the same fuselage cross-section and similar capacity as the earlier twin-engined Falco ...
* Epic Victory *
Hawker Siddeley Trident The Hawker Siddeley HS-121 Trident (originally the de Havilland DH.121 and briefly the Airco DH.121) is a British airliner produced by Hawker Siddeley. In 1957, de Havilland proposed its DH.121 trijet design to a British European Airways (BEA ...
*
IAI Lavi The IAI Lavi (, "lion") is a single-engined fourth-generation jet fighter, fourth-generation multirole jet fighter developed in Israel, by Israel Aircraft Industries (IAI), during the 1980s. The decision to develop the Lavi was controversial, bot ...
(inverse S-duct, with underside intake and the nozzles on upper fuselage) *
Lockheed L-1011 TriStar The Lockheed L-1011 TriStar (pronounced "El-ten-eleven") is an American medium-to-long-range, wide-body trijet airliner built by the Lockheed Corporation. It was the third wide-body airliner to enter commercial operations, after the Boeing 747 ...
*
Lockheed Martin F-22 Raptor The Lockheed Martin/Boeing F-22 Raptor is an American Twinjet, twin-engine, Jet engine, jet-powered, Night fighter, all-weather, supersonic Stealth aircraft, stealth fighter aircraft. As a product of the United States Air Force's Advanced Ta ...
*
Lockheed YF-22 The Lockheed/Boeing/General Dynamics YF-22 is an American single-seat, twinjet, twin-engine, stealth aircraft, stealth fighter aircraft, fighter technology demonstrator prototype designed for the United States Air Force (USAF). The design te ...
* Mikoyan Project 1.44 *
Northrop Grumman B-2 Spirit The Northrop B-2 Spirit, also known as the Stealth Bomber, is an American Heavy bomber, heavy strategic bomber, featuring low-observable stealth aircraft, stealth technology designed to penetrator (aircraft), penetrate dense anti-aircraft war ...
*
Northrop YF-23 The Northrop/McDonnell Douglas YF-23 is an American single-seat, twinjet, twin-engine, Stealth aircraft, stealth fighter aircraft, fighter technology demonstrator prototype designed for the United States Air Force (USAF). The design team, wit ...
(inverse S-duct) *
Short SC.1 The Short SC.1 was the first British fixed-wing vertical take-off and landing (VTOL) jet aircraft. It was developed by Short Brothers. It was powered by an arrangement of five Rolls-Royce RB.108 turbojets, four of which were used for vertical ...
*
Sukhoi Su-47 The Sukhoi Su-47 ''Berkut'' () (NATO reporting name Firkin), also designated S-32 and S-37 during initial development, was a Russian experimental supersonic fighter aircraft, jet fighter developed by the Sukhoi, JSC Sukhoi Company. A distingui ...
*
Tupolev Tu-154 The Tupolev Tu-154 (; NATO reporting name: "Careless") is a three-engined, medium-range, narrow-body airliner designed in the mid-1960s and manufactured by Tupolev. A workhorse of Soviet and (subsequently) Russian airlines for several decades, ...
* Tupolev Tu-154M *
Yakovlev Yak-40 The Yakovlev Yak-40 (; NATO reporting name: Codling) is a regional jet designed in Soviet Union by Yakovlev. The trijet's maiden flight was in 1966, and it was in production from 1967 to 1981. It was introduced to service in 1968, with export mo ...
*
Yakovlev Yak-42 The Yakovlev Yak-42 (; NATO reporting name: "Clobber") is a 100/120-seat three-engined mid-range passenger jet developed in the mid 1970s to replace the technically obsolete Tupolev Tu-134. It was the first airliner produced in the Soviet Union ...
The Boeing 747-300 Trijet (not to be confused with the later 747-300) was designed with an S-duct layout, but never built.


See also

*
Components of jet engines This article briefly describes the components and systems found in jet engines. Major components Major components of a turbojet including references to turbofans, turboprops and turboshafts: *Cold section: **Air intake (inlet) — For subsoni ...
*
Variable cycle engine A variable cycle engine (VCE), also referred to as adaptive cycle engine (ACE), is an aircraft jet engine that is designed to operate efficiently under mixed flight conditions, such as subsonic flight, subsonic, transonic and supersonic. An advan ...


References

{{Reflist Aircraft propulsion components Aircraft configurations