Why didn’t 3 engine planes push out 4 engine planes like 2 engines did?

globalwings · Mon Jan 27, 2020 4:12 am

I’d be curious on the perspective of those with technical knowledge and are able to properly analyze and discuss a topic comparing 4, 3, and 2 engine commercial jets. It’s pretty obvious that the market has spoken on the efficiency, value, and savings of commercial jets that have been designed with twin engines (777, A350, 787) over quad engines (747, A340, A380). During the era when commercial jets were introduced with three engines (DC-10, MD-11, L-1011, 727) how did they not also evolve to become more efficient than jets with 4 engines? With one less engine, how did that not result in savings compared to quad engine commercial aircraft and do to quad jets what twin engine jets eventually did?

carlokiii · Mon Jan 27, 2020 4:20 am

Twins and quads both have symmetry going for them vs the lone and different Number 2 engine in the trijets, which had always been not easy regular maintenance-wise. While in theory, trijets would be more efficient, and a natural stepping stone from quads to twins, having an engine which is remarkably setup differently than the rest was a huge consideration.

Antarius · Mon Jan 27, 2020 5:09 am

Engine reliability. As these improved, the 3rd onwards engine became redundant, which allowed the jump to twins.

Channex757 · Mon Jan 27, 2020 5:18 am

How soon they forget...

The original 4-pot planes in the USA were pushed out by the 3-holer 727. Boeing's original trijet replaced planes such as the 720 or 707-120 for those trips where transcontinental range was needed. Sales numbers bear this out as the 727 sold like crazy.

strfyr51 · Mon Jan 27, 2020 5:22 am

carlokiii wrote:
Twins and quads both have symmetry going for them vs the lone and different Number 2 engine in the trijets, which had always been not easy regular maintenance-wise. While in theory, trijets would be more efficient, and a natural stepping stone from quads to twins, having an engine which is remarkably setup differently than the rest was a huge consideration.

the 727 was pretty easy to maintain as all 3 engines had the same Pain to get to, the airplane and the engine were straight forward and simple with the exception? the center engine coukd not be upgraded to a larger turbofan design aside from the rolls BR715 which was just an advanced version of the JT8D

Channex757 · Mon Jan 27, 2020 5:28 am

strfyr51 wrote:
carlokiii wrote:
Twins and quads both have symmetry going for them vs the lone and different Number 2 engine in the trijets, which had always been not easy regular maintenance-wise. While in theory, trijets would be more efficient, and a natural stepping stone from quads to twins, having an engine which is remarkably setup differently than the rest was a huge consideration.

the 727 was pretty easy to maintain as all 3 engines had the same Pain to get to, the airplane and the engine were straight forward and simple with the exception? the center engine coukd not be upgraded to a larger turbofan design aside from the rolls BR715 which was just an advanced version of the JT8D

I think you'll find that was actually the RR Tay engine, an updated version of the Spey.

The BR715 would be hard to shoehorn in that s-duct!

Pacific · Mon Jan 27, 2020 5:48 am

In the 1970s, available Trijets didn't have the range or the size of the 747-200.

The only large Trijet that came afterwards was the MD-11, made by a cash-strapped company and had a 20 year old wing.

Pure speculation but the MD-11 would have to be 10% larger (A333/772 size) and flown an hour further (744 range) to have killed the quads.

AKL321NX · Mon Jan 27, 2020 6:25 am

Because the only good one didn't sell well

questions · Mon Jan 27, 2020 6:34 am

AKL321NX wrote:
Because the only good one didn't sell well

L1011?

SEPilot · Mon Jan 27, 2020 10:31 am

The center engine causes many problems, and the structure necessary to support it adds a lot of weight. Engines on the wing are always the most efficient structurally, as their weight is supported by the wing directly and is not cantilevered in any way. Tail mounted engines require stronger wings, stronger wing/fuselage joints and a stronger fuselage to transfer the engine weight to the wings. So part of the efficiency gain of going from four engines to three is lost because of the extra structure required. And then you have the difficulty of accessing the center engine for maintenance.

JerseyFlyer · Mon Jan 27, 2020 10:35 am

There is more to efficiency than number of engines. SQ ordered 3-engined MD11s which failed to deliver on spec. They cancelled and ordered 4-engined A340s, which did, but in turn were rapidly were rapidly replaced by 2-engined 772s which were better.

seahawk · Mon Jan 27, 2020 10:46 am

Because they could be even more easily turned into twins, as the combined thrust requirement is lower.

Amiga500 · Mon Jan 27, 2020 10:55 am

SEPilot wrote:
The center engine causes many problems, and the structure necessary to support it adds a lot of weight. Engines on the wing are always the most efficient structurally, as their weight is supported by the wing directly and is not cantilevered in any way. Tail mounted engines require stronger wings, stronger wing/fuselage joints and a stronger fuselage to transfer the engine weight to the wings. So part of the efficiency gain of going from four engines to three is lost because of the extra structure required. And then you have the difficulty of accessing the center engine for maintenance.

+1

Also - right when you need the thrust the most take-off and climb-out, the air into engine is highly disturbed leading to reduced engine efficiency. So in reality, the two wing engines were supplying more than 1/3rd (each) of the thrust anyway.

Bhoy · Mon Jan 27, 2020 11:32 am

In the 70s/80s, when the Trijet could have pushed quad jets out, the engines weren’t powerful enough to be able to power those (relatively for the era) ULH Routes that the 747 was bought for.
The 340 was devised before ETOPS really went mainstream, and once that had been achieved, the concept of the 4 holer was on it’s deathbed.

scbriml · Mon Jan 27, 2020 11:36 am

If the DC-10/MD-11 or TriStar had been larger, they might well have pushed the 747 out. But they weren't.

Max Q · Mon Jan 27, 2020 11:54 am

Amiga500 wrote:
SEPilot wrote:
The center engine causes many problems, and the structure necessary to support it adds a lot of weight. Engines on the wing are always the most efficient structurally, as their weight is supported by the wing directly and is not cantilevered in any way. Tail mounted engines require stronger wings, stronger wing/fuselage joints and a stronger fuselage to transfer the engine weight to the wings. So part of the efficiency gain of going from four engines to three is lost because of the extra structure required. And then you have the difficulty of accessing the center engine for maintenance.

+1

Also - right when you need the thrust the most take-off and climb-out, the air into engine is highly disturbed leading to reduced engine efficiency. So in reality, the two wing engines were supplying more than 1/3rd (each) of the thrust anyway.

Not correct, the #2 engine inlet design on the 727 was carefully designed to ensure adequate airflow and no loss of thrust, in very strong crosswind conditions you sometimes had to be a little slower increasing power during take off on the center engine to avoid compressor stalls but
you still weren’t losing thrust, each engine bore the same ‘load’

No reason to believe that was different on any 3 engine jet transport

rbavfan · Mon Jan 27, 2020 11:56 am

globalwings wrote:
I’d be curious on the perspective of those with technical knowledge and are able to properly analyze and discuss a topic comparing 4, 3, and 2 engine commercial jets. It’s pretty obvious that the market has spoken on the efficiency, value, and savings of commercial jets that have been designed with twin engines (777, A350, 787) over quad engines (747, A340, A380). During the era when commercial jets were introduced with three engines (DC-10, MD-11, L-1011, 727) how did they not also evolve to become more efficient than jets with 4 engines? With one less engine, how did that not result in savings compared to quad engine commercial aircraft and do to quad jets what twin engine jets eventually did?

3 engine planes were harder more costly to do engine maintenance due to the location of the third engine for one thing.

Jonne1184 · Mon Jan 27, 2020 12:06 pm

How should have the Trijets replaced the 747? They (the DC-10 and L-1011) used the same engines as the 747. So they just had to be smaller and thus targeted at a different market. Same for the first two engined widebodys, which also used those engines. It was with the A330 and 777, that the first engines appeared large enough to replace a larger number of smaller engines.

VirginFlyer · Mon Jan 27, 2020 12:12 pm

Jonne1184 wrote:
How should have the Trijets replaced the 747? They (the DC-10 and L-1011) used the same engines as the 747. So they just had to be smaller and thus targeted at a different market. Same for the first two engined widebodys, which also used those engines. It was with the A330 and 777, that the first engines appeared large enough to replace a larger number of smaller engines.

Indeed, it is probably worth pointing out that the A300 and 767 effectively replaced the 4-engine 707 and DC-8 in terms of payload range performance - so this part of the market went directly from 4 to 2 engines.

V/F

Amiga500 · Mon Jan 27, 2020 12:37 pm

Max Q wrote:
Amiga500 wrote:
SEPilot wrote:
The center engine causes many problems, and the structure necessary to support it adds a lot of weight. Engines on the wing are always the most efficient structurally, as their weight is supported by the wing directly and is not cantilevered in any way. Tail mounted engines require stronger wings, stronger wing/fuselage joints and a stronger fuselage to transfer the engine weight to the wings. So part of the efficiency gain of going from four engines to three is lost because of the extra structure required. And then you have the difficulty of accessing the center engine for maintenance.

+1

Also - right when you need the thrust the most take-off and climb-out, the air into engine is highly disturbed leading to reduced engine efficiency. So in reality, the two wing engines were supplying more than 1/3rd (each) of the thrust anyway.

Not correct, the #2 engine inlet design on the 727 was carefully designed to ensure adequate airflow and no loss of thrust, in very strong crosswind conditions you sometimes had to be a little slower increasing power during take off on the center engine to avoid compressor stalls but
you still weren’t losing thrust, each engine bore the same ‘load’

No reason to believe that was different on any 3 engine jet transport

So carefully designed they had an engine surge on the very first flight and eventually had to add vortex generators to the S-duct to make it manageable?

Which also required further rework when the JT8D was refanned, and a redesign again when they went to the Tay.

All of which retained appreciable inlet distortion onto the fan face.

-------------------------

This was all with low BPR engines as well - increase the fan diameter and packaging the thing becomes a nightmare.

DIJKKIJK · Mon Jan 27, 2020 12:43 pm

questions wrote:
AKL321NX wrote:
Because the only good one didn't sell well

L1011?

DC-10?

rbavfan · Mon Jan 27, 2020 12:44 pm

strfyr51 wrote:
carlokiii wrote:
Twins and quads both have symmetry going for them vs the lone and different Number 2 engine in the trijets, which had always been not easy regular maintenance-wise. While in theory, trijets would be more efficient, and a natural stepping stone from quads to twins, having an engine which is remarkably setup differently than the rest was a huge consideration.

the 727 was pretty easy to maintain as all 3 engines had the same Pain to get to, the airplane and the engine were straight forward and simple with the exception? the center engine coukd not be upgraded to a larger turbofan design aside from the rolls BR715 which was just an advanced version of the JT8D

The BR715 is not just an advanced version of the JT8D.

VSMUT · Mon Jan 27, 2020 12:55 pm

Trijets are a mess. The have to contend with the wing bending relief of a twinjet as well as a heavy, beefed up tail structure. I'd say they are potentially a worse concept than a quad.

Quad jets have the wing bending relief solved by the outer engines, so wings can be made lighter.

In practice, it was the MD-11 vs A340-300 vs 777-200ER that put the principles to test:
The MD-11 was a mess, a poorly reheated DC-10. Even if it had been a twin or a quad, chances are it would have failed anyway. They didn't put enough into making it what it should have been.

The A340-300 was actually not as bad as it is often portrayed. Most numbers indicate that it exceeds the 777-200ER. In terms of total orders it lagged, but that was because Boeing bagged the big orders. Airbus on the other hand netted more individual customers for the A340-300. It was however at the limits of what it could do, and the attempt at stretching it was an abject failure.

The 777-200ER was successful despite being a bit on the heavy side for its size, but it was the only one that was really designed with growth in mind. In the end that was where it really excelled.

rbavfan · Mon Jan 27, 2020 12:56 pm

DIJKKIJK wrote:
questions wrote:
AKL321NX wrote:
Because the only good one didn't sell well

L1011?

DC-10?

I agree with the first post the L1011 was far better built and more stable.

steeler83 · Mon Jan 27, 2020 12:57 pm

Would it be worth noting here that Boeing initially was designing the 777 to be a trijet? I believe ETOPS along with technological advancements rendered the 3rd engine as redundant (as has been discussed). IIRC, the MD11 was a little larger than the DC10 and was produced (by Boeing, when they acquired McDonnell-Douglas) around the same time as the 777. The 777 sold; the MD11 not so much.

rbavfan · Mon Jan 27, 2020 1:00 pm

Amiga500 wrote:
Max Q wrote:
Amiga500 wrote:

+1

Also - right when you need the thrust the most take-off and climb-out, the air into engine is highly disturbed leading to reduced engine efficiency. So in reality, the two wing engines were supplying more than 1/3rd (each) of the thrust anyway.

Not correct, the #2 engine inlet design on the 727 was carefully designed to ensure adequate airflow and no loss of thrust, in very strong crosswind conditions you sometimes had to be a little slower increasing power during take off on the center engine to avoid compressor stalls but
you still weren’t losing thrust, each engine bore the same ‘load’

No reason to believe that was different on any 3 engine jet transport

So carefully designed they had an engine surge on the very first flight and eventually had to add vortex generators to the S-duct to make it manageable?

Which also required further rework when the JT8D was refanned, and a redesign again when they went to the Tay.

All of which retained appreciable inlet distortion onto the fan face.

-------------------------

This was all with low BPR engines as well - increase the fan diameter and packaging the thing becomes a nightmare.

The Tay was only used on the side engines. The middle engine never got it. So no it was not reworked to fit the Tay in the middle slot.

rbavfan · Mon Jan 27, 2020 1:02 pm

VirginFlyer wrote:
Jonne1184 wrote:
How should have the Trijets replaced the 747? They (the DC-10 and L-1011) used the same engines as the 747. So they just had to be smaller and thus targeted at a different market. Same for the first two engined widebodys, which also used those engines. It was with the A330 and 777, that the first engines appeared large enough to replace a larger number of smaller engines.

Indeed, it is probably worth pointing out that the A300 and 767 effectively replaced the 4-engine 707 and DC-8 in terms of payload range performance - so this part of the market went directly from 4 to 2 engines.

V/F

There were also a lot of 707 & DC-8's on domestic runs that were replaced by 727-200's.

milhaus · Mon Jan 27, 2020 1:48 pm

We should divide topic to two, one for narrowbody and one for widebody. On narrowbody line it was straight, four engined B707, DC8, CV880/990 were substituted by 727 as soon as performance allowed it. Then 727 was replaced by MD80/737/320. One exception is TU154 which replaced twin engine TU104 and four engine Il18. On widebody line we should take in account that aircraft are built around available engine, so first generation of widebody trijets DC10 and Tristar were much smaller than 747 due to engine size. Performance of Queen of the sky was also much better. There was only one second generation trijet MD11, which was just warmed up DC10 with old inadequate wing. It was not succesfull against A340 which was state of the art aircraft at this time.

GalaxyFlyer · Mon Jan 27, 2020 2:37 pm

rbavfan wrote:
Amiga500 wrote:
Max Q wrote:

Not correct, the #2 engine inlet design on the 727 was carefully designed to ensure adequate airflow and no loss of thrust, in very strong crosswind conditions you sometimes had to be a little slower increasing power during take off on the center engine to avoid compressor stalls but
you still weren’t losing thrust, each engine bore the same ‘load’

No reason to believe that was different on any 3 engine jet transport

So carefully designed they had an engine surge on the very first flight and eventually had to add vortex generators to the S-duct to make it manageable?

Which also required further rework when the JT8D was refanned, and a redesign again when they went to the Tay.

All of which retained appreciable inlet distortion onto the fan face.

-------------------------

This was all with low BPR engines as well - increase the fan diameter and packaging the thing becomes a nightmare.

The Tay was only used on the side engines. The middle engine never got it. So no it was not reworked to fit the Tay in the middle slot.

The Tays were in all three positions, it was the Valsan conversion that used -217s in 1 & 3 with a -17 in #2 position.

JerseyFlyer · Mon Jan 27, 2020 2:42 pm

steeler83 wrote:
Would it be worth noting here that Boeing initially was designing the 777 to be a trijet? I believe ETOPS along with technological advancements rendered the 3rd engine as redundant (as has been discussed). IIRC, the MD11 was a little larger than the DC10 and was produced (by Boeing, when they acquired McDonnell-Douglas) around the same time as the 777. The 777 sold; the MD11 not so much.

I don't think that is right. Boeing did a generation earlier propose three concepts, of which it would build one or two. These were tentatively designated 757, 767, 777, the last being a trijet concept that they chose not to proceed with - nothing directly related to the 777 we recognise today.

frmrCapCadet · Mon Jan 27, 2020 2:58 pm

Channex757 wrote:
How soon they forget...

The original 4-pot planes in the USA were pushed out by the 3-holer 727. Boeing's original trijet replaced planes such as the 720 or 707-120 for those trips where transcontinental range was needed. Sales numbers bear this out as the 727 sold like crazy.

Per wikipedia, the 727 was a big seller 1960 to 1980, and last commercial flight was January of 2019!! Tri-jets continue to soldier on with cargo, but think we can see the end, at least in the US. And that spot in the tail has been reserved for some time by those pesky APs which are also a jet.

ikramerica · Mon Jan 27, 2020 3:03 pm

The premise makes no sense. Tri-jets took over for all mid-sized mid-range quad aircraft. 727, DC10 and L1011 took over the dc8 and 707 routes by and large, except where greater capacity or range was required. The 747 took over there.

airbazar · Mon Jan 27, 2020 3:10 pm

The short answer is, they did push out quads. No new quad engine plane has been built in the the same market segment as a tri-jet. So your assumption to this thread is incorrect. Many airlines continued to operate quads for their range. The L-1011 and even the DC-10 didn't quite have the range that the A340 provided. The new tech applied on the A340 also made them more efficient than the existing tri-jets. However I feel that it's worth pointing out that the A340 window was extremely small so IMO it's no so much that trijets or twins pushed out quads as much as the fast technological advances in the 80's pushed out the quads.

IADCA · Mon Jan 27, 2020 4:03 pm

Channex757 wrote:
How soon they forget...

The original 4-pot planes in the USA were pushed out by the 3-holer 727. Boeing's original trijet replaced planes such as the 720 or 707-120 for those trips where transcontinental range was needed. Sales numbers bear this out as the 727 sold like crazy.

ikramerica wrote:
The premise makes no sense. Tri-jets took over for all mid-sized mid-range quad aircraft. 727, DC10 and L1011 took over the dc8 and 707 routes by and large, except where greater capacity or range was required. The 747 took over there.

airbazar wrote:
The short answer is, they did push out quads. No new quad engine plane has been built in the the same market segment as a tri-jet. So your assumption to this thread is incorrect. Many airlines continued to operate quads for their range. The L-1011 and even the DC-10 didn't quite have the range that the A340 provided. The new tech applied on the A340 also made them more efficient than the existing tri-jets. However I feel that it's worth pointing out that the A340 window was extremely small so IMO it's no so much that trijets or twins pushed out quads as much as the fast technological advances in the 80's pushed out the quads.

OP, these are the correct answer. As much as they are receding into the fog of time, the 1970s did occur. For example, there were more than 1,800 727s built. They replaced routes that had earlier, when flown by jets, had been flown by quads (707/720, DC-8, and the two Convair birds). DC-10 and L-1011 largely replaced 707 and DC-8, and in combination with the 727-200 largely killed off those earlier planes.

flyingclrs727 · Mon Jan 27, 2020 4:14 pm

rbavfan wrote:
VirginFlyer wrote:
Jonne1184 wrote:
How should have the Trijets replaced the 747? They (the DC-10 and L-1011) used the same engines as the 747. So they just had to be smaller and thus targeted at a different market. Same for the first two engined widebodys, which also used those engines. It was with the A330 and 777, that the first engines appeared large enough to replace a larger number of smaller engines.

Indeed, it is probably worth pointing out that the A300 and 767 effectively replaced the 4-engine 707 and DC-8 in terms of payload range performance - so this part of the market went directly from 4 to 2 engines.

V/F

There were also a lot of 707 & DC-8's on domestic runs that were replaced by 727-200's.

Those were midcon routes not transcon.

flyingclrs727 · Mon Jan 27, 2020 4:25 pm

airbazar wrote:
The short answer is, they did push out quads. No new quad engine plane has been built in the the same market segment as a tri-jet. So your assumption to this thread is incorrect. Many airlines continued to operate quads for their range. The L-1011 and even the DC-10 didn't quite have the range that the A340 provided. The new tech applied on the A340 also made them more efficient than the existing tri-jets. However I feel that it's worth pointing out that the A340 window was extremely small so IMO it's no so much that trijets or twins pushed out quads as much as the fast technological advances in the 80's pushed out the quads.

As mentioned earlier, the MD-11 got replaced the by A340 by some airlines. The engineering compromises needed to make a trijet made a plane heavier than a quad or twin. The A340-300 had smaller engines than the MD-11. By the 1990's there were more engines available at different thrust levels than in the 50's and 60's. This made it unnecessary for Airbus to build a tri-jet with engines developed for the 747 in order to offer a jet of MD-11 size and range.

KrisJ · Mon Jan 27, 2020 5:19 pm

The first widebody trijets did not pushed out the 4 engine widebodies because... 1) there was only one 4 engine widebody outthere and 2) it was optimized for different markets.
The initial request made by AA the Mdd and Lockheed replied to was to supply USA continental routes.

PHLBOS · Mon Jan 27, 2020 6:11 pm

globalwings wrote:
I’d be curious on the perspective of those with technical knowledge and are able to properly analyze and discuss a topic comparing 4, 3, and 2 engine commercial jets. It’s pretty obvious that the market has spoken on the efficiency, value, and savings of commercial jets that have been designed with twin engines (777, A350, 787) over quad engines (747, A340, A380). During the era when commercial jets were introduced with three engines (DC-10, MD-11, L-1011, 727) how did they not also evolve to become more efficient than jets with 4 engines? With one less engine, how did that not result in savings compared to quad engine commercial aircraft and do to quad jets what twin engine jets eventually did?

Another item that many here have overlooked is that the majority of the above-3-holer planes (the later-produced MD-11, IIRC was the sole exception) required a 3-man cockpit crew (Pilot, Co-Pilot & Flight Engineer). Although the first 767s were initially designed with a 3-man cockpit crew in mind; it either debuted requiring only a 2-man cockpit crew or changed over to such shortly after it went into production. The combination of both cost efficiency of a 2-man cockpit crew (Pilot & Co-Pilot), along with fuel efficiency, IMHO was what pushed the phase-out of 4-engined planes further than the fore-mentioned 3-holers.

mxaxai · Mon Jan 27, 2020 6:20 pm

airbazar wrote:
The short answer is, they did push out quads. No new quad engine plane has been built in the the same market segment as a tri-jet.

The A340-300 covered almost exactly the same market that the MD-11 was supposed to be in, as did the Il-96. The A340 was, of course, designed as a sister aircraft to the twin-engined A330, so a quad-engine configuration was a lot easier than integrating a third engine into the fuselage or tailplane. And the Il-96 was partially derived from the older Il-86, which was forced to use four engines due to the lack of powerful engines in the USSR.

Then there is the BAe 146, first flight 1981, when two engines would have easily sufficed. In its largest variant it had almost the same performance as the older tri-jet Hawker Siddeley Trident.

There are very few tri-props built after WW2 - only the Trislander comes to mind - but a variety of smaller and larger quad-props. It is a PITA to mount a propeller on the fuselage, though, so that's hardly surprising.

MIflyer12 · Mon Jan 27, 2020 6:31 pm

3-engine aircraft did push out 4-engine aircraft. How many 707s and DC-8s were sold after DC-10s and L-1011s were introduced?

VSMUT · Mon Jan 27, 2020 6:52 pm

MIflyer12 wrote:
3-engine aircraft did push out 4-engine aircraft. How many 707s and DC-8s were sold after DC-10s and L-1011s were introduced?

About 2700, across the 747, A380, A340, Avro RJ, Il-86 and Il-96. If we are pedantic, Boeing sold another 153 707s between 1972 and 1994.

KlimaBXsst · Mon Jan 27, 2020 6:59 pm

We seem to be deep into the belief one size or design fits all.... just tighten or loosen the Velcro.

I am not so convinced.

KlimaBXsst · Mon Jan 27, 2020 7:01 pm

The Boom Overture designers are also open-minded fortunately too.

Jonne1184 · Tue Jan 28, 2020 7:12 am

VSMUT wrote:
MIflyer12 wrote:
3-engine aircraft did push out 4-engine aircraft. How many 707s and DC-8s were sold after DC-10s and L-1011s were introduced?

About 2700, across the 747, A380, A340, Avro RJ, Il-86 and Il-96. If we are pedantic, Boeing sold another 153 707s between 1972 and 1994.

You could even add another 220 Il-62 to that list.

Max Q · Tue Jan 28, 2020 8:46 am

Amiga500 wrote:
Max Q wrote:
Amiga500 wrote:

+1

Also - right when you need the thrust the most take-off and climb-out, the air into engine is highly disturbed leading to reduced engine efficiency. So in reality, the two wing engines were supplying more than 1/3rd (each) of the thrust anyway.

Not correct, the #2 engine inlet design on the 727 was carefully designed to ensure adequate airflow and no loss of thrust, in very strong crosswind conditions you sometimes had to be a little slower increasing power during take off on the center engine to avoid compressor stalls but
you still weren’t losing thrust, each engine bore the same ‘load’

No reason to believe that was different on any 3 engine jet transport

So carefully designed they had an engine surge on the very first flight and eventually had to add vortex generators to the S-duct to make it manageable?

Which also required further rework when the JT8D was refanned, and a redesign again when they went to the Tay.

All of which retained appreciable inlet distortion onto the fan face.

-------------------------

This was all with low BPR engines as well - increase the fan diameter and packaging the thing becomes a nightmare.

Thats what flight testing is for, a problem was discovered and resolved

You’re trying to make a point that’s not true, that the center engine thrust was significantly less than numbers one and three due to compromised airflow

That’s just nonsense, and I flew the 727 for five years, I can attest to it personally

Tristarsteve · Tue Jan 28, 2020 2:11 pm

JerseyFlyer wrote:
steeler83 wrote:
Would it be worth noting here that Boeing initially was designing the 777 to be a trijet? I believe ETOPS along with technological advancements rendered the 3rd engine as redundant (as has been discussed). IIRC, the MD11 was a little larger than the DC10 and was produced (by Boeing, when they acquired McDonnell-Douglas) around the same time as the 777. The 777 sold; the MD11 not so much.

I don't think that is right. Boeing did a generation earlier propose three concepts, of which it would build one or two. These were tentatively designated 757, 767, 777, the last being a trijet concept that they chose not to proceed with - nothing directly related to the 777 we recognise today.

You are thinking of the studies on the B777X in 1998, before the GE90-110 was invented.

https://www.flightglobal.com/boeing-studies-triple-engined-777x/21900.article

1239

Guy Norris/LOS ANGELES

Boeing is considering the use of a third, tail-mounted engine in the 777-200X/300X that would also double up as an auxiliary power unit. The virtually unprecedented use of a thrusting APU is one of a wide range of changes being studied to improve the field performance of the aircraft as the Seattle-based airframe builder attempts to find a launch customer for the aircraft after being beaten to several key orders by the latest versions of the Airbus A340.

The study of what is dubbed an "auxiliary power and thrust unit" (APTU) is considering dual-role powerplants covering the 31-66.7kN (7,000-15,000lb) thrust range, including the Allison AE3007, BMW Rolls-Royce BR710 and General Electric CF34-8. GE has also proposed a civil version of the F414 fighter engine, but Boeing says noise considerations will make it "more likely to go with an engine of a higher bypass ratio".

The added thrust of the tail engine would augment the main engines for take-off as well as for initial climb, but would otherwise "provide the capabilities of today's APU" says 777 preliminary design chief engineer Mike Burtle.

Amiga500 · Tue Jan 28, 2020 4:15 pm

Max Q wrote:
Thats what flight testing is for, a problem was discovered and resolved

You’re trying to make a point that’s not true, that the center engine thrust was significantly less than numbers one and three due to compromised airflow

That’s just nonsense, and I flew the 727 for five years, I can attest to it personally

The (large body of) evidence of T/W ratios for Tri-jet compared to twins simply does not back up your assertion.

The average installed T/W of a tri-jet is 13% lower than a twin - when compared across a broad range - and its more or less the same regardless of MTOW. If everything else was equal, then that figure would be nearer 25% lower as the thrust requirement is defined by one-engine-inoperative condition.

Of course, everything else isn't equal - as the middle engine doesn't deliver the same max thrust as the two wing engines at takeoff/climb-out.

727-100 T/W: ~0.244
B737-100 T/W: ~0.255

For the Trijet to be effective, that 727 T/W needs to be nearer 0.192.

aeromoe · Tue Jan 28, 2020 9:21 pm

Capacity and range.

strfyr51 · Tue Jan 28, 2020 9:46 pm

th three engine airplanes did what they did with the technology available to them. Had Lockheed had the tech available to them for a more efficient wing like today's wing or McDonald Douglas had a wing as advanced as the 777-200 or -300ER? they both might have been Monster Airplanes with all manner of Range. I think the twin concept cane about when both Pratt and GE went to the Monster fans with 40+ inches of fan Blades. You couldn't even install one of those monsters on the tail of a DC10-030, MD-11 or L1011 . But? Had they had the wing? It's probable they would not have needed the Huge Fans we see today. That's why I believe Airbus was so late to the Game in ETOPS.
By the time they jumped in with both feet? the 777 Was already in production while they were fielding the A340 and A330. Even Virgin ws aghast at ETOPS and they were late to the party as well. I used tp watch the A340 climb out of ORD and It always looked like it was Laboring just to stay aloft while the 777 climbed out like a Saturn 5. I still think were they of a mind to? a well designed 3 engine airplane with a Late technology wing could still be a viable airplane with advanced PW2040's or Rolls RB211's or Trent engines... 0000000

strfyr51 · Tue Jan 28, 2020 9:47 pm

[quote="strfyr51"]th three engine airplanes did what they did with the technology available to them. Had Lockheed had the tech available to them for a more efficient wing like today's wing or McDonald Douglas had a wing as advanced as the 777-200 or -300ER? they both might have been Monster Airplanes with all manner of Range. I think the twin concept cane about when both Pratt and GE went to the Monster fans with 40+ inches of fan Blades. You couldn't even install one of those monsters on the tail of a DC10-030, MD-11 or L1011 . But? Had they had the wing? It's probable they would not have needed the Huge Fans we see today. That's why I believe Airbus was so late to the Game in ETOPS.
By the time they jumped in with both feet? the 777 Was already in production while they were fielding the A340 and A330. Even Virgin ws aghast at ETOPS and they were late to the party as well. I used tp watch the A340 climb out of ORD and It always looked like it was Laboring just to stay aloft while the 777 climbed out like a Saturn 5. I still think were they of a mind to? a well designed 3 engine airplane with a Late technology wing could still be a viable airplane with advanced PW2040's or Rolls RB211's or Trent engines...

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