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UA857
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Why did the 777 become a twin jet instead of a trijet?

Sat Feb 23, 2019 7:00 am

When Boeing first developed the 777 it was meant to be a trijet why did it become a twinjet?
 
DarkKnight5
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Re: Why did the 777 become a twin jet instead of a trijet?

Sat Feb 23, 2019 7:17 am

So it burned less fuel. What are we doing here????
 
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BWIAirport
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Re: Why did the 777 become a twin jet instead of a trijet?

Sat Feb 23, 2019 7:18 am

I don’t believe so. Boeing had pioneered twin-engine long-haul with the 767 about a decade earlier. The only reason for a trijet over a twin would have been poor engine reliability, which was not a sufficiently sized problem for Boeing to produce a clean-sheet tri.
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JerseyFlyer
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Re: Why did the 777 become a twin jet instead of a trijet?

Sat Feb 23, 2019 7:40 am

Boeing used the designation 777 for a proposed trijet concept design concurrent with those for 757 and 767. It was not proceeded with and the designation was later re-used for the 777 as we know it today.
 
ELBOB
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Re: Why did the 777 become a twin jet instead of a trijet?

Sat Feb 23, 2019 10:33 am

BWIAirport wrote:
The only reason for a trijet over a twin would have been poor engine reliability.


No, it was the absence of high-thrust turbofans that led to the late-70s 777 trijet studies. Initially as a USA-transcon airframe for American Airlines but Boeing preferred to upgauge it to intercontinental size with three JT9D, but it was dropped in 1980 as the TriStar / DC-10 / 747SP had eaten its market.

By the time the twin 767-X studies came around a decade later, big fans had progressed enough that the three manufacturers were confident that the could scale-up their existing engines to the 30-tonnes range to make it a twinjet. It simply couldn't have happened earlier than that due to thrust. That of course became the first-generation 777.

And even that was pushing the limits as the later semi-trijet 777X studies with 'thrusting APU' showed. Again, another series of thrust-bumps were required to make the 200LR / 300ER possible.
 
tonystan
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Re: Why did the 777 become a twin jet instead of a trijet?

Sat Feb 23, 2019 12:11 pm

So they could sell them!
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bikerthai
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Re: Why did the 777 become a twin jet instead of a trijet?

Sat Feb 23, 2019 3:01 pm

At the time the pattent for the third engine may have belonged to someone else. For the 727, Boeing had to pay royalties for the third engine inlet design.

The tail mounted engine is a pain for maintenance. Boeing had significant input from airliners during the 777 development stage. Not sure if any airliners liked the third engine.

This will be an item of issue for any BWB aircraft with top mounted engines.

bt
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justplanesmart
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Re: Why did the 777 become a twin jet instead of a trijet?

Sat Feb 23, 2019 3:13 pm

The airplane that became the 767 began as a Boeing project designated the 7X7. At that time (mid 1970's), three or more engines were needed for intercontinental service. The domestic model was a twin with engines around 40k pounds of thrust. The intercontinental variant would have been a trijet with similar configuration to the Lockheed TriStar, with engines having 30k or so thrust. Over time, the twin did become the 767, and the trijet would have been the 777. This is part of the reason for the 767 having the large wing area (greater than the A300), as the twin and trijet would share the same wing. As engine technology advanced, the idea of ETOPS began, and the need for a third engine lapsed, The eventual 777 came out of a need for a larger aircraft than even the 767 could grow into. By that time, twin-engine planes were the rule.
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NameOmitted
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Re: Why did the 777 become a twin jet instead of a trijet?

Sat Feb 23, 2019 3:36 pm

Why TriStar over DC-10? It's seems a more complete design, what were the advantages?
 
Heinkel
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Re: Why did the 777 become a twin jet instead of a trijet?

Sat Feb 23, 2019 5:03 pm

UA857 wrote:
When Boeing first developed the 777 it was meant to be a trijet why did it become a twinjet?


When we talk about technical stream machines / jet engines, the bigger is always the better. Efficiency grows with engine size, because losses are (relatively) smaller on big engines.

You only use that many engines, as you need to meet the perfomance and safety requirements. If you can do it with two engines, it makes absolutely no sense to use more.

From a technical and an economical point of view. That is the reason, why there are virtually no modern commercial aircraft with more than two engines anymore.

On the A380 four engines were required, because no engines were available, which could provide the required thrust with only two engines. And a trijet makes fuselage design very complicated.

The biggest supertankers at sea today have one single engine. Big nuclear power stations have one single turbine. Nobody uses more engines than required.
 
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Stitch
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Re: Why did the 777 become a twin jet instead of a trijet?

Sat Feb 23, 2019 5:19 pm

ELBOB wrote:
No, it was the absence of high-thrust turbofans that led to the late-70s 777 trijet studies. Initially as a USA-transcon airframe for American Airlines but Boeing preferred to upgauge it to intercontinental size with three JT9D, but it was dropped in 1980 as the TriStar / DC-10 / 747SP had eaten its market.


I believe these were the earliest 757, 767 and 777 studies (I have pictures of the models from 1977 via my uncle, who worked on the program at Boeing at the time). The 757 looked like the final model, except it had a T-tail. The 767 looked like the final model (with a conventional tail) and the 777 looked like the 767, except with the addition of a T-tail and a 727-style inlet feeding into a third engine (also mounted 727-style in the back of the fuselage.


justplanesmart wrote:
The airplane that became the 767 began as a Boeing project designated the 7X7. At that time (mid 1970's), three or more engines were needed for intercontinental service. The domestic model was a twin with engines around 40k pounds of thrust. The intercontinental variant would have been a trijet with similar configuration to the Lockheed TriStar, with engines having 30k or so thrust.


I have a picture of a 757, 767 and 777 model together: the 757 and 767 look like their final forms with the 777 having the third engine mounted in the tail identical to the DC-10. The 7X7 model looks like it was wider than the earlier 777/DC-10 study and it does have the engine mounted in a configuration very similar to the L-1011, as you note.
 
9w748capt
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Re: Why did the 777 become a twin jet instead of a trijet?

Sat Feb 23, 2019 5:22 pm

Nice job OP! Much better thread starter than asking why won't UA de-hub EWR. Baby steps!
 
superjeff
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Re: Why did the 777 become a twin jet instead of a trijet?

Sat Feb 23, 2019 5:24 pm

:
DarkKnight5 wrote:
So it burned less fuel. What are we doing here????

:checkmark:
 
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rikkus67
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Re: Why did the 777 become a twin jet instead of a trijet?

Sat Feb 23, 2019 5:25 pm

Here is an article pdf about the proposed "triple engined" 777X

https://www.flightglobal.com/FlightPDFA ... 201867.PDF
AC.WA.CP.DL.RW.CO.WG.WJ.WN.KI.FL.SK.ACL.UA.US.F9
 
planecane
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Re: Why did the 777 become a twin jet instead of a trijet?

Sat Feb 23, 2019 5:27 pm

Didn't the 777 concept (that became the current model series) begin as a study to grow the 767? If my memory is correct of reading that somewhere then it makes sense that the concept would have 2 engines
 
beechnut
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Re: Why did the 777 become a twin jet instead of a trijet?

Sat Feb 23, 2019 5:28 pm

UA857 wrote:
When Boeing first developed the 777 it was meant to be a trijet why did it become a twinjet?


Better engine technology. Lower fuel consumption, lower maintenance costs, ETOPS

NameOmitted wrote:
Why TriStar over DC-10? It's seems a more complete design, what were the advantages?


https://www.airlinepilotforums.com/749849-post2.html

The L1011 apart from engines was a superior (and arguably safer) aircraft than the DC-10. The AA191 crash for instance, could not have happened on a Tristar due to leading edge slats locking in place. Similarly the 4 hydraulic systems could not be wiped out by an uncontained engine failure as UA232, plus the DC-10 has only three systems instead of 4.
 
AEROFAN
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Re: Why did the 777 become a twin jet instead of a trijet?

Sat Feb 23, 2019 5:39 pm

9w748capt wrote:
Nice job OP! Much better thread starter than asking why won't UA de-hub EWR. Baby steps!


:rotfl: :rotfl: :rotfl: Some of these "why" threads are so darn stupid and tiresome...
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Ferroviarius
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Re: Why did the 777 become a twin jet instead of a trijet?

Sat Feb 23, 2019 5:42 pm

Heinkel wrote:
UA857 wrote:
When Boeing first developed the 777 it was meant to be a trijet why did it become a twinjet?


When we talk about technical stream machines / jet engines, the bigger is always the better. Efficiency grows with engine size, because losses are (relatively) smaller on big engines.

You only use that many engines, as you need to meet the perfomance and safety requirements. If you can do it with two engines, it makes absolutely no sense to use more.

From a technical and an economical point of view. That is the reason, why there are virtually no modern commercial aircraft with more than two engines anymore.

On the A380 four engines were required, because no engines were available, which could provide the required thrust with only two engines. And a trijet makes fuselage design very complicated.

The biggest supertankers at sea today have one single engine. Big nuclear power stations have one single turbine. Nobody uses more engines than required.


Good evening.

I do not know how it is with the biggest Supertankers.
However, while the Maersk E class - ref. https://en.wikipedia.org/wiki/M%C3%A6rs ... ainer_ship - still features one engine, the Wärtsilä-Sulzer RTA96-C - see for example https://en.wikipedia.org/wiki/W%C3%A4rt ... er_RTA96-C and https://www.youtube.com/watch?v=awJIvdzL5dE (impressive!) - the even larger Triple E-Class, ref. Maersk Triple E-class container ship - features twin engines.

It is not only a matter of efficacy, it is also a matter of safety. From an environmental point of view, single engine ships might consume less fuel, but, also from an environmental point of view, super tankers with just one engine might spell - and have done so - environmental disaster if that one engine fails.

I do not know where I read this, but I think I do not remember falsely that both, the Constellation and the Seven Cs had their engine issues. According to a web page in German - I do not remember its address now - passengers were frequently told by the captain of Lufthansa Constellations that they switched one engine off to save fuel. The intention was, of course, to calm down passengers by not letting them know, that one engine had stalled.

Best,
Ferroviarius
 
COSPN
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Re: Why did the 777 become a twin jet instead of a trijet?

Sat Feb 23, 2019 6:47 pm

People are more concerned about the environment and carbon footprint. These days . so 2 engline planes just make sense

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