science

S-duct

S-duct is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand S-duct rather than just read about it. In short: An S-duct (or serpentine inlet) is a type of jet engine intake duct used in several types of trijet aircraft. 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.

Key takeaways

S-duct belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect S-duct to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of S-duct from memory before moving on to harder problems.

Reference excerpt

An S-duct (or serpentine inlet) is a type of jet engine intake duct used in several types of trijet aircraft. 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, 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 Dassault Falcon 8X and Dassault Falcon 900 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 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 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, 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 and Falcon 8X business jets in order to provide more total thrust while enabling the continued use of smaller engines in the 15–30 kN (3,400–6,700 lbf) 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.

On stealth aircraft The S-duct is also used on combat aircraft, where it has the advantage of blocking the front section of the jet engine from radar beams increasing the stealth capability. The spinning compressor blades would produce a strong radar signature return, compared to the smooth sides of the duct.

List of S-duct aircraft

Currently produced aircraft with an S-ducts:

Chengdu J-20 Dassault Falcon 7X Dassault Falcon 900 Dassault Rafale Eurofighter Typhoon Lockheed Martin F-35 Lightning II Rockwell B-1 Lancer Shenyang J-35 Previously produced aircraft with an S-ducts:

Boeing 727 Dassault Falcon 50 Epic Victory Hawker Siddeley Trident IAI Lavi (inverse S-duct, with underside intake and the nozzles on upper fuselage) Lockheed L-1011 TriStar Lockheed Martin F-22 Raptor Lockheed YF-22 Mikoyan Project 1.44 Northrop Grumman B-2 Spirit Northrop YF-23 (inverse S-duct) Short SC.1 Sukhoi Su-47 Tupolev Tu-154 Tupolev Tu-154M Yakovlev Yak-40 Yakovlev Yak-42 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 Variable cycle engine

References

Illustrations

Worked examples

Example 1 — a first encounter with S-duct

Start with the simplest possible case. Write down what S-duct claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to S-duct before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about S-duct ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of S-duct

In research: S-duct appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses S-duct in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: S-duct is common in secondary-school and first-year university syllabi. It links to neighbouring topics Aircraft configurations, Aircraft propulsion components, so understanding it makes those chapters shorter.
In everyday life: Look for S-duct outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study S-duct in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what S-duct means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain S-duct out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is S-duct in simple terms?

Why does S-duct matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study S-duct?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on S-duct.