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Topic: Possibility Of A380 & 747 As Tri-Jets?
Username: AA77W
Posted 2013-03-19 14:11:21 and read 11988 times.

Would it be possible and/or cost effective to design a 4-engine aircraft (ie. 747 or A380) as a Tri-Jet (2 wing-mounted engines and 1 tail-mounted engine)? Many airlines point to the number of engines as an area of significant cost, so seemingly if 4-engine aircraft could be redesigned as a 3-engine aircraft that would reduce maintence and potentially fuel cost?

I realize that there are many factors involved: weight, effiency, fuel-burn, maintence, range, etc. Certainly Airbus and Boeing must have considered a Tri-Jet option when designing the aforementioned aircrafts and deemed it not cost and/or mission effective, but why?

Facts, figures, and productive thoughts are welcomed.

Topic: RE: Possibility Of A380 & 747 As Tri-Jets?
Username: eljonno
Posted 2013-03-19 14:23:39 and read 11937 times.

I seem to remember this coming up in a thread a few years ago.
I think the consensus was that since ETOPS, most twinjets can't do anything that a quad can't : so trijets are pretty much obsolete.

Quoting AA77W (Thread starter):
I realize that there are many factors involved: weight, effiency, fuel-burn, maintence, range, etc. Certainly Airbus and Boeing must have considered a Tri-Jet option when designing the aforementioned aircrafts and deemed it not cost and/or mission effective, but why?

Additionally having tail-mounted engines makes regular maintenance and checks more complicated from an access perspective (in contrast to wing-mounted engines).

[Edited 2013-03-19 14:25:26]

[Edited 2013-03-19 14:33:06]

Topic: RE: Possibility Of A380 & 747 As Tri-Jets?
Username: Stitch
Posted 2013-03-19 14:27:57 and read 11901 times.

Back in the early-1970s, Boeing considered a three-engine 747-300 as a competitor to the DC-10 and L-1011.

747-300 Tri-Jet Concept

Topic: RE: Possibility Of A380 & 747 As Tri-Jets?
Username: eljonno
Posted 2013-03-19 14:30:30 and read 11868 times.

Quoting Stitch (Reply 2):
Back in the early-1970s, Boeing considered a three-engine 747-300 as a competitor to the DC-10 and L-1011.

And it looked totally AWESOME!!!! Thanks for sharing!

Topic: RE: Possibility Of A380 & 747 As Tri-Jets?
Username: mjoelnir
Posted 2013-03-19 15:09:31 and read 11673 times.

I think tri-jets have gone the way of inside the wing mounted engines, obsolete.

The only reason for a trijet was the 60 or 90 minutes rule for twins.

Topic: RE: Possibility Of A380 & 747 As Tri-Jets?
Username: American 767
Posted 2013-03-19 15:17:33 and read 11625 times.

Quoting Stitch (Reply 2):
Back in the early-1970s, Boeing considered a three-engine 747-300 as a competitor to the DC-10 and L-1011.

It looks awesome I agree. Did any airline show an interest in it? I'm sure Pan Am has, or at least Boeing approached them with the proposal. Back then Pan Am was one of Boeing's most important customer. It would have looked neat in Pan Am's then blue cheat line livery.

Ben Soriano

[Edited 2013-03-19 16:12:15]

Topic: RE: Possibility Of A380 & 747 As Tri-Jets?
Username: SpaceshipDC10
Posted 2013-03-19 15:19:05 and read 11604 times.

Quoting AA77W (Thread starter):
as a 3-engine aircraft that would reduce maintence and potentially fuel cost?

Although a trijet may reduce fuel costs, regarding maintenance, an engine in the tail could represent more costs than benefits with the way it has to be installed there and all the tooling necessary to reach it compared to under-wing engines. At least that's what I think.

Topic: RE: Possibility Of A380 & 747 As Tri-Jets?
Username: Stitch
Posted 2013-03-19 16:40:55 and read 11323 times.

Quoting American 767 (Reply 5):
Did any airline show an interest in it?

That I do not know. However, Boeing determined that the proposal required too much re-design of the 747 platform and did not go forward.

[Edited 2013-03-19 16:41:59]

Topic: RE: Possibility Of A380 & 747 As Tri-Jets?
Username: RussianJet
Posted 2013-03-19 17:09:17 and read 11206 times.

Quoting mjoelnir (Reply 4):
The only reason for a trijet was the 60 or 90 minutes rule for twins.

Is that really true? Were there powerful enough engines around when the DC-10 and L1011 came into being that could reasonably power a craft of that weight and size as a twin?

Topic: RE: Possibility Of A380 & 747 As Tri-Jets?
Username: Stitch
Posted 2013-03-19 17:39:38 and read 11118 times.

Quoting RussianJet (Reply 8):
Were there powerful enough engines around when the DC-10 and L1011 came into being that could reasonably power a craft of that weight and size as a twin?

Douglas did consider a twin-engine DC-10 called the "DC-10 Twin" as a response to the Airbus A300. Development studies continued through the 1970s, but the program was shelved when the Japanese (who were approached to help fund the project in exchange for work-sharing) chose to support Boeing's 767 proposal.

Topic: RE: Possibility Of A380 & 747 As Tri-Jets?
Username: LH707330
Posted 2013-03-19 20:14:28 and read 10907 times.

The maintenance issues are a big problem, and the design is also suffers from a structural inefficiency by having the engine back there. The #2 engine is heavy, which means you need to move the wings further aft to keep it balanced. Now you've got to have a larger stabilizer to maintain authority because your lever is shorter.

Topic: RE: Possibility Of A380 & 747 As Tri-Jets?
Username: American 767
Posted 2013-03-19 20:32:23 and read 10844 times.

Quoting LH707330 (Reply 10):
Now you've got to have a larger stabilizer to maintain authority because your lever is shorter.

I believe that was the issue with the design of the MD-11. The stabilizer was a bit too small thus it would be more difficult to control the airplane on landing. I recall two or three Fed Ex MD-11F's that crashed upon landing, one of them in Tokyo because of a wind shear, another in Newark NJ.

Topic: RE: Possibility Of A380 & 747 As Tri-Jets?
Username: LH707330
Posted 2013-03-19 20:41:12 and read 10810 times.

Quoting American 767 (Reply 11):
I believe that was the issue with the design of the MD-11. The stabilizer was a bit too small thus it would be more difficult to control the airplane on landing. I recall two or three Fed Ex MD-11F's that crashed upon landing, one of them in Tokyo because of a wind shear, another in Newark NJ.

That and the high wing loading meant they had high approach speeds and lower margins for error.

Topic: RE: Possibility Of A380 & 747 As Tri-Jets?
Username: thegeek
Posted 2013-03-19 21:44:06 and read 10664 times.

Quoting LH707330 (Reply 10):
The #2 engine is heavy, which means you need to move the wings further aft to keep it balanced. Now you've got to have a larger stabilizer to maintain authority because your lever is shorter.

I've actually never thought of this limitation. I guess that applies to rear engined aircraft as well.

Topic: RE: Possibility Of A380 & 747 As Tri-Jets?
Username: windy95
Posted 2013-03-20 05:50:57 and read 9790 times.

If it was feasible or cost effective do you not think that they would of done it?

Topic: RE: Possibility Of A380 & 747 As Tri-Jets?
Username: JBirdAV8r
Posted 2013-03-20 05:57:00 and read 9707 times.

Quoting eljonno (Reply 1):
Additionally having tail-mounted engines makes regular maintenance and checks more complicated from an access perspective (in contrast to wing-mounted engines).

Not to mention it's "expensive" from a weight/structure standpoint to have all that ducting through the tail.

Topic: RE: Possibility Of A380 & 747 As Tri-Jets?
Username: strfyr51
Posted 2013-03-20 06:09:33 and read 9556 times.

If you'd ever had to Change that center engine? You'd know what it's like to NOT have to change one of those "Bad Oscars" the wing engines can be down and up in 6 hours, the Tail engine?? add 6-10 hours. they're not only Not Fun but Dangerous. The "S" Duct robbed efficiency. Not to mention the maintenance of it. better to have the GE90-115 or a GE90-150 than a tail mounted Engine

Topic: RE: Possibility Of A380 & 747 As Tri-Jets?
Username: softrally
Posted 2013-03-20 07:07:10 and read 8774 times.

Very unlikely. As Stitch said, Boeing did consider a tri-engined 747. However, it turned out that too much re-engineering would be necessary that it would not be a viable option. If Airbus or Boeing wants to build a trijet, they will probably start from scratch.

Topic: RE: Possibility Of A380 & 747 As Tri-Jets?
Username: penguins
Posted 2013-03-20 07:19:08 and read 8608 times.

The Tri-Jets will not make a revival. The necessity for a third engine is no longer present. I even foresee the age of the quad-jet coming to an end as we see huge leaps in engine technology.

Topic: RE: Possibility Of A380 & 747 As Tri-Jets?
Username: CanemedicRN
Posted 2013-03-20 10:14:45 and read 6718 times.

What about the Dassault Falcon 7X or 900LX? I know it's a business jet, but it's still being produced. Wonder what the advantage is over a twin such as a Gulfstream IV, etc. A pilot I talked to at PDK in Atlanta compared it to flying a Learjet and said it was like driving a sports car vs family sedan.

I just think they're sexy!


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Photo © Stewart K. Marshall

Topic: RE: Possibility Of A380 & 747 As Tri-Jets?
Username: dergay
Posted 2013-03-20 10:46:43 and read 6326 times.

Quoting Stitch (Reply 2):
Back in the early-1970s, Boeing considered a three-engine 747-300 as a competitor to the DC-10 and L-1011

Thankfully they copied the elegant Lockheed for the third engine and not the ugly Dougling method of sticking it in as an afterthought! Fully agree though that beauty is in the eye of the beholder and others may not concur.....

Topic: RE: Possibility Of A380 & 747 As Tri-Jets?
Username: RussianJet
Posted 2013-03-20 10:46:48 and read 6326 times.

Quoting strfyr51 (Reply 16):
. they're not only Not Fun but Dangerous

Can you elborate on that? Do you mean dangerous to work on, or dangerous in terms of operating? Which ever one, how exactly is it dangerous? Not disbelieving you at all, just curious for more detail on that.

Quoting strfyr51 (Reply 16):
The "S" Duct robbed efficiency

I can well believe that. What sort of percentages are we talking?

Topic: RE: Possibility Of A380 & 747 As Tri-Jets?
Username: SpaceshipDC10
Posted 2013-03-20 10:52:01 and read 6261 times.

When McDonnell Douglas was still around, they first proposed a trijet MD-12. But around 1992, they changed their mind and proposed a whole new doubledecker quadjet resembling to today's A380. At least part of the reason for that change was the structural and weight problems regarding the rear engine that would have been of course larger than on their previous two trijets. With the MD-11 they used the same banjos as for the DC-10 to fit a larger engine.

Topic: RE: Possibility Of A380 & 747 As Tri-Jets?
Username: AA77W
Posted 2013-03-20 12:29:45 and read 5109 times.

Quoting Stitch (Reply 2):
Back in the early-1970s, Boeing considered a three-engine 747-300 as a competitor to the DC-10 and L-1011.

I didn't know about this. Thanks for sharing!!

Quoting windy95 (Reply 14):
If it was feasible or cost effective do you not think that they would of done it?

Posting this topic is an effort to educate myself, not question the decisions or logic of the manufacturers. Per my original post, I said that Airbus and Boeing must have considered the tri-jet option but decided it wasn't practical -- but what I was really wanting to know was why . . . I certainly appreciate any information you may have to share on the topic!  
Quoting penguins (Reply 18):
The necessity for a third engine is no longer present. I even foresee the age of the quad-jet coming to an end as we see huge leaps in engine technology.

I agree. In fact, I was thinking while writing the OP that engine technology has advanced quite far, and could foresee a 747/A380 design with two engines down the road.

Quoting SpaceshipDC10 (Reply 22):
When McDonnell Douglas was still around, they first proposed a trijet MD-12. But around 1992, they changed their mind and proposed a whole new doubledecker quadjet resembling to today's A380.

I've read about these proposed designs, and that's what sparked my question. Although, I didn't know the full details. Thanks for the info!



Thanks for everyone for the information and the productive/informative responses!

Topic: RE: Possibility Of A380 & 747 As Tri-Jets?
Username: ajhYXE
Posted 2013-03-20 13:23:26 and read 4519 times.

Quoting Stitch (Reply 7):
Boeing determined that the proposal required too much re-design of the 747 platform and did not go forward.

  

I recall hearing one major issue had to do with the "hump" interfering with the air intake of the tail-mounted engine. To keep the entire project from going to waste Boeing took what they could of it and developed the 747SP.

If someone chooses to develop a BWB aircraft perhaps there will be a future for trijets. Otherwise I would honestly consider it a "dead" design.

Topic: RE: Possibility Of A380 & 747 As Tri-Jets?
Username: JakeOrion
Posted 2013-03-20 13:39:52 and read 4467 times.

Quoting ajhYXE (Reply 24):
If someone chooses to develop a BWB aircraft perhaps there will be a future for trijets. Otherwise I would honestly consider it a "dead" design.

Just thinking the same thing:



Also, there could be a possibility the next super sonic jet could be a trijet:

Topic: RE: Possibility Of A380 & 747 As Tri-Jets?
Username: Darksnowynight
Posted 2013-03-20 19:28:37 and read 3888 times.

Quoting penguins (Reply 18):
The Tri-Jets will not make a revival. The necessity for a third engine is no longer present. I even foresee the age of the quad-jet coming to an end as we see huge leaps in engine technology.

There are limited plausibilities for a trijet out there, but yes, I do not see any further development along quad-jet lines.

Quoting AA77W (Reply 23):

I agree. In fact, I was thinking while writing the OP that engine technology has advanced quite far, and could foresee a 747/A380 design with two engines down the road.

I'm sure that whatever replaces the 748i/779 will be a twin, yes.

Topic: RE: Possibility Of A380 & 747 As Tri-Jets?
Username: LH707330
Posted 2013-03-21 23:27:14 and read 3440 times.

Quoting CanemedicRN (Reply 19):
What about the Dassault Falcon 7X or 900LX? I know it's a business jet, but it's still being produced. Wonder what the advantage is over a twin such as a Gulfstream IV, etc. A pilot I talked to at PDK in Atlanta compared it to flying a Learjet and said it was like driving a sports car vs family sedan.

On the Falcons they're low enough that access isn't too much of a problem, and they help with short field ops because of OEI issues (lose 33% vs 50% thrust). I think the Falcon 7 is the largest allowable bizjet out of London City that can make it to New York.


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