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Posts: 2121
Joined: Sun Sep 16, 2001 11:31 am

S- Duct

Sat Dec 22, 2001 12:25 pm

In the case of the L1011/727 where the no. 2 engine is mounted on an "s-duct" does the power setting of the no.2 engine need to be higher than engines 1 @ 3 to match their thrust output? That is the reason McDonnell Douglas chose a "straight-through" engine duct for the DC-10 as the performance of no. 2 engine on a DC-10 matches 1 and 3 exactly. Anyone know how much thrust is in fact lost on the L1011/727 center engine vs engine/fuselage mounted??

RE: S- Duct

Sat Dec 22, 2001 12:52 pm

As far as power settings, all engine power settings are identical for both the 727 and the L1011. When you do the performance calculations, you will be given an Engine Pressure Ratio number. This number is bugged on all three EPR gauges.

I do not believe there is any noticeable engine performance loss due to the S-turn. If there is even a loss. I will ask this of my "mentor" who has been teaching me 727 and L1011 systems the past few months to get a more definite answer.

Anywho, when you're setting EPR, they are all the same.
Posts: 293
Joined: Sun Jun 17, 2001 1:25 am

RE: S- Duct

Mon Dec 24, 2001 7:20 pm

This has been discussed before, though it is not easy to search and find the older posts, since s-duct does not appear in them in exactly that syntax. In any case, try these 2 threads. There is some interesting info on this subject in each.

Posts: 313
Joined: Sun Jun 18, 2000 12:49 pm

RE: S- Duct

Mon Dec 24, 2001 10:29 pm

Main reason to have a "straight" tail mounted center engine versus the "S" shape is that the "S" takes up room from the body of the a/c, so the bulkhead has to be ~10 feet further up in the plane, taking up interior space. However the the straight through engine has the disadvantage that full power pushes the nose down.

RE: S- Duct

Tue Dec 25, 2001 11:01 am

It's been over 15 years since I've flown a 727 and all of my manuals are packed away, but I'm almost positive that the #2 engine required a higher EPR for takeoff. The difference was to compensate for the S-duct loss. It got kind of interesting when you had different "dash numbers" mounted on the engine pods - since you would have different EPR settings for that also.
Posts: 1788
Joined: Mon Nov 12, 2001 10:34 am

RE: S- Duct

Tue Dec 25, 2001 3:50 pm

There is no correction for the EPR setting on the number two engine on the L1011 due to the 'S' duct. In addition, in the design of the TriStar, the curve of the 'S' duct was kept at less than one quarter the radius of the intake diameter to achieve the desired objective, ie: straight thru performance. On the -500 model, the AOM recommends that the number two throttle movement on takeoff in high velocity crosswinds be slightly slower than normal to avoid intake disturbance.
TriStar....a truly fine machine.

RE: S- Duct

Wed Dec 26, 2001 11:42 pm

If I remember correctly, the higher EPR setting on the #2 engine on the 727 was due to the fact that you did not bleed #2 engine on takeoff. Therefore the EPR was .02 higher on the engine.
Posts: 136
Joined: Mon Aug 21, 2000 5:48 pm

RE: S- Duct

Fri Dec 28, 2001 12:03 pm

I just got a pretty techie L1011 book...Mentions nothing about a higher EPR for the no2 engine. The S duct is actually pretty slight. Infact it compares the Tristar S-duct to the 727 S-duct, which is more severe. I don't believe the higher EPR is required for the Tristar's no2 engine. As for the engine placement, both the L-1011 and DC-10 flew with a slight nose-up attitude in cruise.

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