Number Two engine and 'S' duct explained by Lockheed

Number Two engine and 'S' duct

The Number Two engine is installed in the aft fuselage. Like the wing engines it is a 50,000 pound thrust Rolls Royce Engine. Its inlet is a curved "S" duct which provides performance equivalent to that of a straight through duct,

(There is no performance penalty)

Aside from improved looks, there are several advantages to using an "S" duct. First, this lowers the #2 engine, which gives a low center engine thrust line. This reduces pitch up tendencies, and permits both wider lateral spacing of the wing engines, and a wider flap span.

The aft fuselage does not taper as much as other wide body aircraft because of #2 engine. This allows a longer fuselage constant section and more passenger room.

The lower engine allows for a one piece rudder which gives improved handling by increasing rudder effectivity.

Surprisingly, the "S" duct design also reduces aircraft weight. The engine is mounted in the aft fuselage. This eliminates the need for heavy structural supports (800 lbs.) which would otherwise be required, Those supports, if used, would reduce the passenger cabin by 65 square feet, or 11 seats.