S-duct

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.

[1] 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.

[citation needed] 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.

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.