Moderators: jsumali2, richierich, ua900, PanAm_DC10, hOMSaR
neomax wrote:But twins are a relatively new phenomenon
neomax wrote:but zero pax trijets.
neomax wrote:Trijets are more efficient than similarly sized quad counterparts so what gives?
GalaxyFlyer wrote:The real wonder is why we ever went down the dead end that tris were in the first place. The first modestly successful airliner was the Curtiss Condo, which after the noise and odors of the Ford Tri-motor was a revelation. The first real airliner, the DC-2 followed by the DC-3 were twins. Tris won’t work with props, so the 727 was a reinvention of the idea and novel in 1962. WW II brought quads as an answer to the overwater mission, both range and redundancy with pistons.
GF
GalaxyFlyer wrote:The real wonder is why we ever went down the dead end that tris were in the first place. The first modestly successful airliner was the Curtiss Condo, which after the noise and odors of the Ford Tri-motor was a revelation. The first real airliner, the DC-2 followed by the DC-3 were twins. Tris won’t work with props, so the 727 was a reinvention of the idea and novel in 1962. WW II brought quads as an answer to the overwater mission, both range and redundancy with pistons.
GF
neomax wrote:But twins are a relatively new phenomenon
neomax wrote:But DC10's, L1011's, and 727's by comparison are a thing of the past and almost seem to have disappeared too quickly. Is it just me or have trijets vanished before the true end of their usable lifespan?
BlueberryWheats wrote:Efficiency isn't just about number of engines. Think of all that extra structure (weight) built into the tail to support that heavy third engine. Plus I'm sure someone else will have mentioned how tail engine maintenance is a PITA.
redflyer wrote:I'm sure there are other factors that meant that tail-mounted engine was bastardized.
trnswrld wrote:Someone above mentioned the A300.... Can anyone explain to me why the A300 didn't seem to do anywhere near as well as the 767? I know the A300 came out first, but what made the 767 more successful? I know it was early off in the game for Airbus, did that have anything to do with it? My understanding is that neither the A300 or the early 762 models had spectacular range. Did the A330 come along soon enough to where that was more or less Airbus' answer for their own airplane? Just looking for a little more info on that subject.
NameOmitted wrote:At one point, Boeing was looking at a small third engine on the larger 777s to reduce the required size of the main engines at take off, should one engine fail. This ended up not being necessary. At cruise, the third engine world be shut down, and the aircraft world fly as a twin.
If, in 20 years or so, the market wants a VLA, and Boeing wants to use as much 777 engineering as possible, what is the likelihood of such a plan being revisited?
gwrudolph wrote:At least on the widebody side, I think some of those trijets barely needed three engines for their intended missions even when they were originally launched in the 70s. Some could have been twins but customer requirements for using them at airports that didn't have longer runways (corner cases in my opinion) caused them to go tri vs. twin. I believe, for example, the L1011 was at one point contemplated as a twin. Once the 757/767 family came along and could do takeoff from a reasonably sized runway with a decent range and decent payload, the idea of wb trijets was done.
NameOmitted wrote:If, in 20 years or so, the market wants a VLA, and Boeing wants to use as much 777 engineering as possible, what is the likelihood of such a plan being revisited?
LAX772LR wrote:What do you think the 779 is?
It can physically hold well over 500 pax with no problem, but no customer actually desired it to, so Boeing hasn't offered the amount of doors necessary for such configurations. That could change, if it's what airlines wanted. For now, they don't.
neomax wrote:Trijets are more efficient than similarly sized quad counterparts so what gives?
LAX772LR wrote:The advent of twins had essentially nil to do with runway length.
trnswrld wrote:Someone above mentioned the A300.... Can anyone explain to me why the A300 didn't seem to do anywhere near as well as the 767? I know the A300 came out first, but what made the 767 more successful? I know it was early off in the game for Airbus, did that have anything to do with it? My understanding is that neither the A300 or the early 762 models had spectacular range. Did the A330 come along soon enough to where that was more or less Airbus' answer for their own airplane? Just looking for a little more info on that subject.
reidar76 wrote:If Airbus ever develops an A380neo, could it be a tri-jet? Three GE9X provides significantly more thrust combined than the current four engines.
reidar76 wrote:If Airbus ever develops an A380neo, could it be a tri-jet? Three GE9X provides significantly more thrust combined than the current four engines.
CanadianNorth wrote:Lots of reasons, most already mentioned here.
None of them I don't think on their own would make or break the tri-jet concept, but all put together they have lead us to where we are today.
Fuel burn:
A bit of a trap for logic. Quads vs. tri-jet one would think that 25% less engines would be 25% less fuel, but not really because, assuming range, payload, technology, and engine generation are all equal then a quad design although having more engines each individual engine would be smaller and burning less fuel. So overall fuel burn would go up with engine numbers, but not in direct proportion.
Maintenance:
Tri jet vs quad is one less engine to maintain, but the centre engine is harder to maintain. More ladders and work stands and such to buy, more specialized tooling to have available to hoist the engine and large components in and out, more labour cost for the mechanics to get up there and then come back down every time they drop something or need something, longer delays when something breaks down.
Weight:
The centre engine is usually in or near the empenage. This means you have to design the fuselage to take the weight and thrust loads from back there, you have to run fuel lines, fire extinguishing systems, and all that jazz. All that adds weight to your design.
Engineering:
It's quite the thing to design a spot for that centre engine. Airflow, structure to hold up the tail, all that becomes much more complicated. Lockheed had a heck of a time designing their S duct, and from what I've read the fittings that hold the tail up above the DC-10's centre engine were "quite the thing" to design and engineer.
Re-engine options:
On a quad or a twin as we know them today, if you want to re-engine them as long as you have the ground clearance from the landing gear then basically alls you need is a new engine mount and strengthen the wing structure to accommodate your new engines. Once again, that centre engine though. The MD-11 they had one heck of a time re-engineering the tail fittings to accommodate the larger engine, cost them a ton of money to do. Many 727s were re-engined from their original JT8Ds to the more efficient and quieter JT8D-200 series, however only #1 and #3 engines were replaced with new, the centre engine was left as is because the people doing the mod figured re-designing the whole back end to fit the larger diameter engine would cost more than it would to just live with the older generation engine in there.
Safety:
Basically all the same arguments as having three cockpit crew vs two. The third had a time and a place, but technology and reliability have come to the point where a third engine for safety just isn't needed any more. ETOPS wasn't a thing when the DC-10 and L1011 came about, now we do it every day without thinking twice.
Performance:
Many of the great tri-jets had customers that wanted to use them from relatively short runways and specifically asked for extra engines for that purpose. Major airports around the world have since grown to better accommodate large aircraft so the superior runway performance often doesn't justify the extra cost any more.
Engine availability:
Aircraft are sold in much greater quantities than before. Back in the day a production run of a couple hundred aircraft was considered to be a pretty good go. Nowadays it's perfectly normal, almost expected, for jetliners to break the 1000 order mark. Engines can cost billions to design, so you need a couple thousand sales to make any money. With only a couple hundred of each type engine manufacturers needed to spread their engines out across several aircraft. Produce one basic design and you can use two to power an A300/767 sized airplane, three to power a DC-10/L1011 sized airplane, and four to power a 747 size. Now most of those categories has enough of a market engine manufacturers can produce engines specifically for that category so you don't need to add extra engines to get your required thrust.
To a point though. Engines are only so big, and beyond that size the market is too small at the moment to justify the cost of designing engines big enough to power a stretched 747 or A380 sized aircraft as a twin. Thus the four-holers hang on for now...
gwrudolph wrote:At least on the widebody side, I think some of those trijets barely needed three engines for their intended missions even when they were originally launched in the 70s. Some could have been twins but customer requirements for using them at airports that didn't have longer runways (corner cases in my opinion) caused them to go tri vs. twin. I believe, for example, the L1011 was at one point contemplated as a twin. Once the 757/767 family came along and could do takeoff from a reasonably sized runway with a decent range and decent payload, the idea of wb trijets was done.
Momo1435 wrote:Most quads built in the same period when the DC10's, L1011's were produced are also long retired, so nothing to see there. The MD11 is the only trijet that could be compared to the A340-300 and 747-400 quads. The issues with the MD11 didn't make it a popular plane in the 1st place. With the MD - Boeing merger it was simply replaced by the 777, so it was replaced by a twin, not a quad. In the meantime a very strong conversion market had emerged for the MD11, making it easier for airlines to sell them compared to the A340 which doesn't have a conversion program. Therefor you will see A340s lasting longer as a passenger plane then the MD11. Now the only quads still in production are the significantly larger 747-8 and the A380, they are still too large to be replaced by twins, although the 777-9 comes close.
As for the 727, Boeing replaced it with the 737 and 757, there's no comparable quad, so I don't know why you even mentioned it.
trnswrld wrote:Someone above mentioned the A300.... Can anyone explain to me why the A300 didn't seem to do anywhere near as well as the 767? I know the A300 came out first, but what made the 767 more successful? I know it was early off in the game for Airbus, did that have anything to do with it? My understanding is that neither the A300 or the early 762 models had spectacular range. Did the A330 come along soon enough to where that was more or less Airbus' answer for their own airplane? Just looking for a little more info on that subject.
c933103 wrote:I think the better way to look at this question is that why we are still getting new quads today but not tri. 747-8 inherited the 747 family so it's kinda obvious, but why didn't Airbus use trijet design on A380? Was 3 engines not enough for A380, or was 4 engines being more efficient?
trnswrld wrote:Someone above mentioned the A300.... Can anyone explain to me why the A300 didn't seem to do anywhere near as well as the 767? I know the A300 came out first, but what made the 767 more successful? I know it was early off in the game for Airbus, did that have anything to do with it? My understanding is that neither the A300 or the early 762 models had spectacular range. Did the A330 come along soon enough to where that was more or less Airbus' answer for their own airplane? Just looking for a little more info on that subject.
CanadianNorth wrote:Lots of reasons, most already mentioned here.
None of them I don't think on their own would make or break the tri-jet concept, but all put together they have lead us to where we are today.
Fuel burn:
A bit of a trap for logic. Quads vs. tri-jet one would think that 25% less engines would be 25% less fuel, but not really because, assuming range, payload, technology, and engine generation are all equal then a quad design although having more engines each individual engine would be smaller and burning less fuel. So overall fuel burn would go up with engine numbers, but not in direct proportion.
Maintenance:
Tri jet vs quad is one less engine to maintain, but the centre engine is harder to maintain. More ladders and work stands and such to buy, more specialized tooling to have available to hoist the engine and large components in and out, more labour cost for the mechanics to get up there and then come back down every time they drop something or need something, longer delays when something breaks down.
Weight:
The centre engine is usually in or near the empenage. This means you have to design the fuselage to take the weight and thrust loads from back there, you have to run fuel lines, fire extinguishing systems, and all that jazz. All that adds weight to your design.
Engineering:
It's quite the thing to design a spot for that centre engine. Airflow, structure to hold up the tail, all that becomes much more complicated. Lockheed had a heck of a time designing their S duct, and from what I've read the fittings that hold the tail up above the DC-10's centre engine were "quite the thing" to design and engineer.
Re-engine options:
On a quad or a twin as we know them today, if you want to re-engine them as long as you have the ground clearance from the landing gear then basically alls you need is a new engine mount and strengthen the wing structure to accommodate your new engines. Once again, that centre engine though. The MD-11 they had one heck of a time re-engineering the tail fittings to accommodate the larger engine, cost them a ton of money to do. Many 727s were re-engined from their original JT8Ds to the more efficient and quieter JT8D-200 series, however only #1 and #3 engines were replaced with new, the centre engine was left as is because the people doing the mod figured re-designing the whole back end to fit the larger diameter engine would cost more than it would to just live with the older generation engine in there.
Safety:
Basically all the same arguments as having three cockpit crew vs two. The third had a time and a place, but technology and reliability have come to the point where a third engine for safety just isn't needed any more. ETOPS wasn't a thing when the DC-10 and L1011 came about, now we do it every day without thinking twice.
Performance:
Many of the great tri-jets had customers that wanted to use them from relatively short runways and specifically asked for extra engines for that purpose. Major airports around the world have since grown to better accommodate large aircraft so the superior runway performance often doesn't justify the extra cost any more.
Engine availability:
Aircraft are sold in much greater quantities than before. Back in the day a production run of a couple hundred aircraft was considered to be a pretty good go. Nowadays it's perfectly normal, almost expected, for jetliners to break the 1000 order mark. Engines can cost billions to design, so you need a couple thousand sales to make any money. With only a couple hundred of each type engine manufacturers needed to spread their engines out across several aircraft. Produce one basic design and you can use two to power an A300/767 sized airplane, three to power a DC-10/L1011 sized airplane, and four to power a 747 size. Now most of those categories has enough of a market engine manufacturers can produce engines specifically for that category so you don't need to add extra engines to get your required thrust.
To a point though. Engines are only so big, and beyond that size the market is too small at the moment to justify the cost of designing engines big enough to power a stretched 747 or A380 sized aircraft as a twin. Thus the four-holers hang on for now...
LAX772LR wrote:gwrudolph wrote:At least on the widebody side, I think some of those trijets barely needed three engines for their intended missions even when they were originally launched in the 70s. Some could have been twins but customer requirements for using them at airports that didn't have longer runways (corner cases in my opinion) caused them to go tri vs. twin. I believe, for example, the L1011 was at one point contemplated as a twin. Once the 757/767 family came along and could do takeoff from a reasonably sized runway with a decent range and decent payload, the idea of wb trijets was done.
The advent of twins had essentially nil to do with runway length.NameOmitted wrote:If, in 20 years or so, the market wants a VLA, and Boeing wants to use as much 777 engineering as possible, what is the likelihood of such a plan being revisited?
What do you think the 779 is?
It can physically hold well over 500 pax with no problem, but no customer actually desired it to, so Boeing hasn't offered the amount of doors necessary for such configurations. That could change, if it's what airlines wanted. For now, they don't.
rbavfan wrote:CanadianNorth wrote:Lots of reasons, most already mentioned here.
None of them I don't think on their own would make or break the tri-jet concept, but all put together they have lead us to where we are today.
Fuel burn:
A bit of a trap for logic. Quads vs. tri-jet one would think that 25% less engines would be 25% less fuel, but not really because, assuming range, payload, technology, and engine generation are all equal then a quad design although having more engines each individual engine would be smaller and burning less fuel. So overall fuel burn would go up with engine numbers, but not in direct proportion.
Maintenance:
Tri jet vs quad is one less engine to maintain, but the centre engine is harder to maintain. More ladders and work stands and such to buy, more specialized tooling to have available to hoist the engine and large components in and out, more labour cost for the mechanics to get up there and then come back down every time they drop something or need something, longer delays when something breaks down.
Weight:
The centre engine is usually in or near the empenage. This means you have to design the fuselage to take the weight and thrust loads from back there, you have to run fuel lines, fire extinguishing systems, and all that jazz. All that adds weight to your design.
Engineering:
It's quite the thing to design a spot for that centre engine. Airflow, structure to hold up the tail, all that becomes much more complicated. Lockheed had a heck of a time designing their S duct, and from what I've read the fittings that hold the tail up above the DC-10's centre engine were "quite the thing" to design and engineer.
Re-engine options:
On a quad or a twin as we know them today, if you want to re-engine them as long as you have the ground clearance from the landing gear then basically alls you need is a new engine mount and strengthen the wing structure to accommodate your new engines. Once again, that centre engine though. The MD-11 they had one heck of a time re-engineering the tail fittings to accommodate the larger engine, cost them a ton of money to do. Many 727s were re-engined from their original JT8Ds to the more efficient and quieter JT8D-200 series, however only #1 and #3 engines were replaced with new, the centre engine was left as is because the people doing the mod figured re-designing the whole back end to fit the larger diameter engine would cost more than it would to just live with the older generation engine in there.
Safety:
Basically all the same arguments as having three cockpit crew vs two. The third had a time and a place, but technology and reliability have come to the point where a third engine for safety just isn't needed any more. ETOPS wasn't a thing when the DC-10 and L1011 came about, now we do it every day without thinking twice.
Performance:
Many of the great tri-jets had customers that wanted to use them from relatively short runways and specifically asked for extra engines for that purpose. Major airports around the world have since grown to better accommodate large aircraft so the superior runway performance often doesn't justify the extra cost any more.
Engine availability:
Aircraft are sold in much greater quantities than before. Back in the day a production run of a couple hundred aircraft was considered to be a pretty good go. Nowadays it's perfectly normal, almost expected, for jetliners to break the 1000 order mark. Engines can cost billions to design, so you need a couple thousand sales to make any money. With only a couple hundred of each type engine manufacturers needed to spread their engines out across several aircraft. Produce one basic design and you can use two to power an A300/767 sized airplane, three to power a DC-10/L1011 sized airplane, and four to power a 747 size. Now most of those categories has enough of a market engine manufacturers can produce engines specifically for that category so you don't need to add extra engines to get your required thrust.
To a point though. Engines are only so big, and beyond that size the market is too small at the moment to justify the cost of designing engines big enough to power a stretched 747 or A380 sized aircraft as a twin. Thus the four-holers hang on for now...
On the DC-10/L1011 the original spec request from AA was for a WB trijet to be able to operate from LGA airport. It was not many airlines wanting a trijet. It was an AA RFP.
drgmobile wrote:Two quads -- the A340 and the 747 -- outlasted one trijet -- the MD-11. It all comes down to which companies survived and which didn't. Lockheed had stopped producing commercial jets by the 1990s and McDonnell Douglas was gobbled up by Boeing. Boeing and Airbus had quads and Lockheed and McDonnell Douglas had trijets.
chornedsnorkack wrote:Why were DC-10 and Tristar ever invented as trijets in the first place? Couldn´t a small widebody quadjet like Il-86 (but with better engines!) have worked?
rbavfan wrote:drgmobile wrote:Two quads -- the A340 and the 747 -- outlasted one trijet -- the MD-11. It all comes down to which companies survived and which didn't. Lockheed had stopped producing commercial jets by the 1990s and McDonnell Douglas was gobbled up by Boeing. Boeing and Airbus had quads and Lockheed and McDonnell Douglas had trijets.
Actually the A340 was produced for 20 years. The DC-10 was produced for 20 years and the MD11 for 12 years giving the -10 & -11 stretch 32 year production. So the A340 did not outlast the MD.
Momo1435 wrote:As for the 727, Boeing replaced it with the 737 and 757, there's no comparable quad, so I don't know why you even mentioned it.