Benefits and Drawbacks
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 DC-10 and MD-11 trijets 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 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 L-1011, engineers were able to maintain straight-through engine performance by limiting the curve of the S-duct to less than 25% 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 has currently fallen out of favour.
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