S-duct

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. Currently, the Dassault Falcon 7X and Dassault Falcon 900 business jets, and the Tupolev Tu-154M narrow-body airliner in limited production, are the only aircraft in production that use the S-duct design.

The S-duct was invented as a solution for positioning the central engine on trijets. The S-duct was 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 layout also increases total aircraft aerodynamic drag by 2–4%.

Compared to an "in tail" design like 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 straight-through engine performance 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 above 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.

...
Wikipedia