MDGLongBeach
Topic Author
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Joined: Sat Jun 24, 2017 6:03 pm

TriJet engine question.

Wed Jul 11, 2018 3:05 am

Hello there,

I’m a huge trijet fan and I still have a lot to learn. My question with the trijets is about the middle engine. If you look at an L1011, or DC-10, the middle engine is smaller than the other wing-mounted engines. My question is, are all the engines the same type, or is the middle engine different than the others, and if they are all the same, how did the manufactures compact the engines to fit in the cowling, and how does compacting them affect performance? (Another thing I’d like to know is what number engine, the middle one is considered.)

Thank you!
-MDG
 
CATIIIevery5yrs
Posts: 86
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Re: TriJet engine question.

Wed Jul 11, 2018 3:07 am

I never flew a trijet but I know the power setting on the 727's middle engine is different from the other 2 engines. I'm sure many people on this forum can elaborate as to why that was. Edited to say the 3 engines are all the same make and model. I came across some new start up in Iowa (via a rabbit hole internet search) who claim to be flying 72's. Maybe some of their folks can explain more. Might be the only ones left flying the bird.
 
GalaxyFlyer
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Re: TriJet engine question.

Wed Jul 11, 2018 3:14 am

The #2 (center) engine has a higher, by .03 EPR, than #1 and #3 due to the outboard engines are bleed for the A/C packs and the center engine isn’t bleed.

The engines are all the same on the DC-10; probably a visual illusion due to the long cowl thru the fin. Although, it could be a rearrangement of the accessory case on #2. The L-1011 engine cannot be seen from outside, it’s buried in the tail.

GF
 
MDGLongBeach
Topic Author
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Re: TriJet engine question.

Wed Jul 11, 2018 3:19 am

That’s interesting that the #2 engine doesn’t provide bleed power, and the dc10 has tricked my eyes then haha!
 
GalaxyFlyer
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Re: TriJet engine question.

Wed Jul 11, 2018 3:21 am

In the normal bleed configuration, #2 isn’t bleed, it can be in an abnormal condition.

Gf
 
BooDog
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Re: TriJet engine question.

Wed Jul 11, 2018 3:36 am

To piggyback on this question: Why is the center engine on a DC10/MD11 not level? Why does it have that slight downward tilt?
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kearnet
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Re: TriJet engine question.

Wed Jul 11, 2018 3:51 am

BooDog wrote:
To piggyback on this question: Why is the center engine on a DC10/MD11 not level? Why does it have that slight downward tilt?


The same reason wing mounted engines are ever so slightly, toed-in toward the fuselage; as the air goes around the plane, it doesn’t parallel the plane, it splits (think of the v shaped wake waves a boat makes going through the water). On some 3 holders (DC-10/MD-11) it was found that that air “arched down” when it got to the tail, so they set the #2 at the best angle to accept the air flow at cruises to the chance of reduce stalls/flameouts. Lockheed and Boeing design didn’t have such affect on air so the kept there fairly in line (straight) with the fuselage.
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AA737-823
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Re: TriJet engine question.

Wed Jul 11, 2018 8:17 am

MDGLongBeach wrote:
Hello there,

I’m a huge trijet fan and I still have a lot to learn. My question with the trijets is about the middle engine. If you look at an L1011, or DC-10, the middle engine is smaller than the other wing-mounted engines. My question is, are all the engines the same type, or is the middle engine different than the others, and if they are all the same, how did the manufactures compact the engines to fit in the cowling, and how does compacting them affect performance? (Another thing I’d like to know is what number engine, the middle one is considered.)

Thank you!
-MDG


The engines on both types you've mentioned are physically identical.
You can actually swap them between positions. Not that you would, of course, but you could.
The TriStar did require a modification to the #2 thrust reverser, to prevent it from blowing reverse thrust air directly onto the elevators. But other than that, the engines themselves are identical.
I'm not sure what you're seeing that makes them appear smaller to you; I've never noticed that visual phenomenon.

The ONE exception to this rule was on the 727-Super27 RE project, which put two updated JT8D-200 engines in the #1 and #3 positions; that larger diameter engine would not fit in the #2 spot, so that engine was left as it was, with, I believe, the thrust reverser removed for noise reduction.
UPS flew some 727-100s that had been re-engined with Rolls Royce Tays; it was a bizarre looking project, but kinda cool!
 
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TurboJet707
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Re: TriJet engine question.

Wed Jul 11, 2018 8:45 am

kearnet wrote:
The same reason wing mounted engines are ever so slightly, toed-in toward the fuselage; as the air goes around the plane, it doesn’t parallel the plane, it splits (think of the v shaped wake waves a boat makes going through the water). On some 3 holders (DC-10/MD-11) it was found that that air “arched down” when it got to the tail, so they set the #2 at the best angle to accept the air flow at cruises to the chance of reduce stalls/flameouts. Lockheed and Boeing design didn’t have such affect on air so the kept there fairly in line (straight) with the fuselage.


Many thanks, I had been wondering about that for years!
I always thought the downward tilt of the no. 2 engine looked rather cool, but already suspected that wouldn't be the only reason ;-)
 
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TurboJet707
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Re: TriJet engine question.

Wed Jul 11, 2018 8:57 am

Most trijets have three engines of the same type. However, there was a modification available on the Boeing 727 where the no. 1 and no. 3 engine were replaced by a more powerful and quieter type, but the centre engine remained the same (as there was insufficient space for the bigger engine in the tail cone). Those 727s were called the 'Super 27s'. The engines are P&W JT8D-217/-219 for the no.1 and 3 position and the 'standard' JT8D-7/-15/-17 for the centre position. Also see viewtopic.php?t=755345
 
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LTU932
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Re: TriJet engine question.

Wed Jul 11, 2018 9:17 am

GalaxyFlyer wrote:
The engines are all the same on the DC-10; probably a visual illusion due to the long cowl thru the fin. Although, it could be a rearrangement of the accessory case on #2. The L-1011 engine cannot be seen from outside, it’s buried in the tail.
The L-1011 has an S-duct (from the looks of it, so does the 727), that is why you cannot see the engine. DC-10 and MD-11 have basically a straightthrough-duct hence why you can see the engine if the aircraft is positioned at a correct angle.
Sometimes the only thing more dangerous than a question is an answer. - Ferengi Rule of Acquisition 208
 
BAeRJ100
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Re: TriJet engine question.

Wed Jul 11, 2018 9:23 am

GalaxyFlyer wrote:
The L-1011 engine cannot be seen from outside, it’s buried in the tail.


The L-1011's #2 engine certainly can be seen from the outside, just like the 727. The rear portion of the engine is easily visible at the back of the aircraft, in line with the passenger windows.

Edit: looks like me and LTU932 had the same idea at the same time.
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deeps001
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Re: TriJet engine question.

Wed Jul 11, 2018 9:27 am

GalaxyFlyer wrote:
The #2 (center) engine has a higher, by .03 EPR, than #1 and #3 due to the outboard engines are bleed for the A/C packs and the center engine isn’t bleed.

The engines are all the same on the DC-10; probably a visual illusion due to the long cowl thru the fin. Although, it could be a rearrangement of the accessory case on #2. The L-1011 engine cannot be seen from outside, it’s buried in the tail.

GF

Yes, this is true. There is nothing particularly interesting about the middle engine #2 though.
 
Bhoy
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Re: TriJet engine question.

Wed Jul 11, 2018 10:03 am

Just for clarity visualizing it, on this cutaway, http://vignette2.wikia.nocookie.net/air ... 0116021224, the #2 engine on the Tristar is 127, showing that the actual engine itself is only a small part of the tail installation, and is in fact the same size as the wing mounted ones, minus their cowlings.
 
BooDog
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Re: TriJet engine question.

Wed Jul 11, 2018 2:28 pm

Here's the full size image of the above link: https://vignette.wikia.nocookie.net/air ... 0116021224
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FatCat
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Re: TriJet engine question.

Wed Jul 11, 2018 3:03 pm

"S" Duct design appears to be the most successful
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smithbs
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Re: TriJet engine question.

Wed Jul 11, 2018 3:17 pm

A tri-jet is also interesting from a structures point of view. There is a lot of load-bearing structure you need in that area, but you need to put in a big air duct through the center of everything along with an engine that's going to be pushing and pulling pretty hard.

Great thread here on DC-10's center engine structure: viewtopic.php?t=776385
 
DarkKnight5
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Joined: Fri Apr 15, 2016 3:36 pm

Re: TriJet engine question.

Wed Jul 11, 2018 3:29 pm

smithbs wrote:
A tri-jet is also interesting from a structures point of view. There is a lot of load-bearing structure you need in that area, but you need to put in a big air duct through the center of everything along with an engine that's going to be pushing and pulling pretty hard.

Great thread here on DC-10's center engine structure: viewtopic.php?t=776385


Also a very interesting Center of Gravity issue caused by strapping an engine to the farthest aft station on the aircraft. It means there’s more mass (i.e. fuselage) forward of the wings than on two-engine planes. I personally think it’s pretty noticeable.
 
Tristarsteve
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Re: TriJet engine question.

Wed Jul 11, 2018 8:55 pm

"S" Duct design appears to be the most successful


The Tristar S duct was designed for the RB211-22 with 42000 lbf. Later the RB211-524 was fitted to the aircraft. This engine was exactly the same size, and used the same S duct and produced 51000 lbf. It was an easy replacement. Gulf Air replaced the -22B engines with -524B in a 24hr input at LHR for each aircraft,.
But there were limitations. Standing still on the ground the Nbr 2 engine thrust was limited to about 85per cent. Crews had to wait until 40 knt ias before applying full thrust , but us engineers were not allowed to run the engine at full power.
We were forced to run the engines on the wing to test them, then fit them to Nbr 2 posn (we had no test bed in BAH). Above 85 pc thrust and the engine started surging. Good fun when you had a nbr 2 engine fail, and no test bed run engine available!!
 
Max Q
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Re: TriJet engine question.

Thu Jul 12, 2018 5:11 am

GalaxyFlyer wrote:
In the normal bleed configuration, #2 isn’t bleed, it can be in an abnormal condition.

Gf



That’s right, it’s coming back now,
#2 could be used for bleed air but it only provided eighth stage unlike 1 and 3 that had 8th augmented by 13th stage


As a result you often needed considerably
more than idle thrust in a descent when using #2 bleed to keep the cabin coming down
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tb727
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Re: TriJet engine question.

Mon Jul 16, 2018 1:30 pm

Max Q wrote:
GalaxyFlyer wrote:
In the normal bleed configuration, #2 isn’t bleed, it can be in an abnormal condition.

Gf



That’s right, it’s coming back now,
#2 could be used for bleed air but it only provided eighth stage unlike 1 and 3 that had 8th augmented by 13th stage


As a result you often needed considerably
more than idle thrust in a descent when using #2 bleed to keep the cabin coming down


Scissor the bleeds!
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GalaxyFlyer
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Re: TriJet engine question.

Mon Jul 16, 2018 2:10 pm

Nr. 2 sometimes had a tendency to stall above F250; there’d be a placard,”Bleed #2 above F250”. At EAL, the placard would have marker written, scratching out F250, F230, F220,F200.

GF

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