Many of you might get the impression that the third engine of the Lockheed L1011 and the Boeing 727 is actually located on top of the fuselage, similar to that of the McDonnell Douglas DC-10. However, they are not on top. They are actually located at the very rear of the fuselage, center-mounted. Both the Lockheed Tristar and the Boeing 727 use an S-duct intake from the top of the fuselage that leads to the actual engines.
The DC-10, on the other hand, mounted the whole engine on top of the fuselage for simplicity and easier maintenance.
The S-Duct Role in Trijets
The S-duct, also known as a serpentine inlet, is a special type of jet engine intake duct that has been a key feature in several trijet aircraft. First appearing in 1962 with the Hawker Siddeley Trident, this unique design has its intake located in the upper rear center of the aircraft, either above or below the stabilizer. The exhaust and engine are situated at the rear, and the S-duct itself is part of the tail or empennage. Its shape is easily recognizable and has been a popular choice for aircraft designers.
The Pros and Cons of the S-duct Design
One of the main reasons the S-duct was invented was to solve the problem of positioning the central engine on trijets. This design made it easier to service the central engine compared to alternative layouts. In fact, most trijet designs have opted for the S-duct layout, with the exception of the McDonnell Douglas DC-10 and MD-11.
These aircraft went for a “straight-through” layout, which has its own set of challenges. The straight-through design places the engine high above the ground, making it hard to access for maintenance. It also increases the aircraft’s total aerodynamic drag by 2–4%.
The S-duct, on the other hand, allows for a shorter fin and a rudder that is closer to the longitudinal axis of the aircraft. This was particularly evident in the Lockheed L-1011 TriStar. Engineers were able to maintain engine performance comparable to straight-through designs by limiting the curve of the S-duct. This not only reduced the total empty weight of the aircraft but also showed that the losses incurred by using an S-duct were more than compensated for by these benefits.
Modern Applications and Variations
While the S-duct is a complicated and costly design, it still finds applications in modern aviation. For instance, the latest Dassault Falcon 7X and Falcon 8X business jets use the S-duct to provide more total thrust while still using smaller engines in the 15-30kN class. The triple engine layout is also considered safer according to international aviation regulations, allowing these aircraft to use certain high-elevation airfields.
The S-duct is not just limited to commercial airliners; it also has applications in combat aircraft. One of its major advantages in this context is that it blocks the front of the jet engine from radar detection. The spinning compressor blades of the engine would otherwise produce a strong radar return.
Other Aircraft Featuring the S-duct
The S-duct has been a part of various aircraft over the years, not just the Hawker Siddeley Trident where it first appeared. Other planes that have utilized this design include the Tupolev Tu-154 and the Yakovlev Yak-40. Of course, we also have two of the most popular trijets, the Lockheed L1011 Tristar and the Boeing 727.
The S-duct intake, however, is still being used by some civilian aircraft such as the Dassault Falcon 900X and Falcon 7X.
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