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The Lockheed L-1011, the largest and only widebody aircraft to feature an S-duct

An S-duct is a unique type of jet engine intake duct, used only on trijet 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 1963 with the Boeing 727. 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.


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.[1] Most trijet designs opted for the S-duct layout.

In fact, 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%.[1]

On the L-1011, engineers were able to maintain "straight-thru" 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.[2]

The S-shaped duct is a complicated and costly design. Since modern jet engines have more power and reliability than those of 1970s, and can safely power the aircraft with only two engines, the trijet design is no longer used on large jet aircraft.

List of S-duct aircraft

Aircraft currently built with S-ducts are the:

Aircraft previously built with S-ducts, but no longer in production, are the:

The Boeing 747-300 Trijet was designed with an S-duct layout, but never built.[1]


See also


  1. ^ a b The Aeronautical Journal. Royal Aeronautical Society. 1974. pp. 392, 398. Retrieved 2008-12-11.  
  2. ^ SAE Transactions. Society of Automotive Engineers. 1970. p. 1436. Retrieved 2008-12-11.  


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