re L-1011 duct S-bend. at least the three engines are co-planar. s-bend did have good ram efficiency by all accounts. on the ground the co-planar middle engine allows the designer to use it for reverse thrust if he wants. on a dc-10 it isn't an option... it will lift the nose.

enicalyth,

You will find the center engine on a DC10 / MD11 does in fact have a translating cowl/blocker door style thrust reverser.

Interesting idea though since I saw the results of a Citation X, aft cg tests and thrust reverser deployment...messy!

DC-10 #2 reverser
 

#2 DOES have a reverser, although it is inhibited until the nose gear squat switch is closed.

Related subject: One astute F/E wrote a paper that the DC-10 should cruise more efficiently with #2 at a reduced power setting, because trim drag would be reduced.

He would have been correct except for the fact that thrust falls off faster than fuel flow as the golever is retarded (i.e. SFC deteriorates).

fan duct
 

The real reason for the fan duct is to get the fan pressure ratio up so it creates some real velocity going out the fan nozzle. An unducted fan (all the engine guys have experimented with open fans) is great at lower mach, but its propulsive efficiency drops off pretty fast as aircraft speed increases.

Black Baron asked about RPM - for the 747-size engine it's around 3500 RPM on the fan (called N1 in the commercial world, or NF or NL in military markets). Bigger engines turn slower, little ones turn faster.

TriStar S duct.

I saw an explanation about the design of the S duct from Lockheed. When originally concieved the L10 was going to be the proverbial 'bus' and so quick on and off loading was required. To make use of the centre doors with airstairs or bridges would have brought the wing engines very close to equipment using those centre doors.

The solution was to move the engine farther out on the wing which incidentally also improved wing bending relief. The downside was the need for a bigger rudder to keep the Vmcg/Vmca speeds under control.

Lockheed considered a DC-10 type arrangement but settled on the S duct to give a full length rudder. As has been mentioned before this also minimised the pitching effect of a high mounted #2 engine.

At the time of the L10 line closure they were also considering using the space above the S duct as an extra fuel tank to give the balance effect now often achieved with a tailplane tank.

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