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Will There Ever Be A Twin Larger Than A 777?  
User currently offline747400sp From United States of America, joined Aug 2003, 3301 posts, RR: 2
Posted (5 years 2 months 3 days 1 hour ago) and read 11754 times:

Seeing the success that large twin jets has had over the decade, I wonder will we ever see a twin jet that is larger than a T-7? Is it likely, that down the line, when it time to start replacing A388, we will see an ultra wide twin jet with jet engines that has 160-200 inch wide fans? So could we see a bigger twin jet in the future, or is the T-7 likely to be the largest twin jet ever built?

65 replies: All unread, showing first 25:
 
User currently offlineStarlionblue From Greenland, joined Feb 2004, 16908 posts, RR: 67
Reply 1, posted (5 years 2 months 2 days 22 hours ago) and read 11713 times:

Maybe.  Wink

The economic equation is not that complex. "Needs to be cheaper (including purchasing cost and operating cost) than a quad of equivalent size given projected market demand."


The big cost item is engines. The GE90-115B is pretty much the current state of the art. Engines in this class are extremely expensive to design and build. The bigger the more expensive. From the other end, the bigger the aircraft (requiring ever bigger engines), the smaller the demand for said aircraft. That's why 320s/737s sell more than 747s/380s.

So you reach an economic "coffin corner" if you will.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineJoeCanuck From Canada, joined Dec 2005, 5320 posts, RR: 30
Reply 2, posted (5 years 2 months 2 days 21 hours ago) and read 11704 times:

I would think the cost of developing an engine that large for such a limited market would be prohibitive. Much larger than a 777 and, in my opinion, you'd be looking for at least a 3 holer.


What the...?
User currently offlineRheinwaldner From Switzerland, joined Jan 2008, 2198 posts, RR: 5
Reply 3, posted (5 years 2 months 2 days 19 hours ago) and read 11662 times:

Ever? Sure! There is no natural engine size limit.

User currently offlinePagophilus From Australia, joined Oct 2006, 114 posts, RR: 0
Reply 4, posted (5 years 2 months 2 days 19 hours ago) and read 11642 times:

Could you imagine how long it would take for those engines to spool up?

User currently offlineStarlionblue From Greenland, joined Feb 2004, 16908 posts, RR: 67
Reply 5, posted (5 years 2 months 2 days 18 hours ago) and read 11616 times:



Quoting JoeCanuck (Reply 2):
Much larger than a 777 and, in my opinion, you'd be looking for at least a 3 holer.

Nope. If you make it a three holer you can have engines that are about half as powerful. There are many more factors involved but in the roughest terms for a twin you would have, say, 2x 150k engines. For a triplet of the same weight you would have 3x 75k engines. Thrust with an engine out is 150k either way.

In any case three holers are dead for widebody tubes with wings. Makes much more sense to make a quad.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineWildcatYXU From Canada, joined May 2006, 2551 posts, RR: 5
Reply 6, posted (5 years 2 months 1 day 5 hours ago) and read 11226 times:

Considering the current A 388 and B748 sales, the question rather should be: Will we ever see a new jet larger than the 77W?

User currently offlineMetroliner From United Kingdom, joined Jan 2007, 1065 posts, RR: 1
Reply 7, posted (5 years 2 months 1 day 5 hours ago) and read 11223 times:



Quoting Pagophilus (Reply 4):
Could you imagine how long it would take for those engines to spool up?

Well, I'd be more worried about the noise... But engine size and spool time aren't directly related. I'd be willing to bet the GE90 spools up faster than a CFM56...



Set the controls for the heart of the Sun
User currently offlineSeaBosDca From United States of America, joined Sep 2007, 5107 posts, RR: 4
Reply 8, posted (5 years 2 months 1 day 4 hours ago) and read 11214 times:

Quoting JoeCanuck (Reply 2):
Much larger than a 777 and, in my opinion, you'd be looking for at least a 3 holer.

A quad... imagine trying to mount an engine bigger than a GE90-115B in a vertical stabilizer    ... not going to happen.

I think the really big planes (A380 and anything Boeing may develop to compete with it) will continue to be quads.

First, there won't be enough demand to justify the enormous expense of developing an engine significantly bigger than a GE90.

Second, to make the quad decision even easier, the proper engines to power a quad VLA are the same size as those on midsize widebody twins. We've already seen the GEnx get reused on the 748, and the smart money is on the Trent XWB finding its way onto the A380.

[Edited 2009-02-19 17:41:22]


Most gorgeous aircraft: Tu-204-300, 757-200, A330-200, 777-200LR, 787-8
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 9, posted (5 years 2 months 1 day 1 hour ago) and read 11148 times:



Quoting Metroliner (Reply 7):
Well, I'd be more worried about the noise...

Noise from turbomachinery scales with RPM to the 3rd (I think...might be 4th) power...bigger engines have to spin more slowly to keep the tips sonic, so they should get correspondingly quieter as they get bigger.

Quoting Metroliner (Reply 7):
I'd be willing to bet the GE90 spools up faster than a CFM56...

I'm not sure about that...rotating inertia scales with the radius squared and directly with the mass...a GE 90 has twice the diameter and certainly more than twice the rotating mass, so it's got at least 8 times the rotating inertia of a CFM56-7, but only 4 times as much power.

Tom.


User currently offlineXT6Wagon From United States of America, joined Feb 2007, 3321 posts, RR: 4
Reply 10, posted (5 years 2 months 22 hours ago) and read 11091 times:

Yes we will, not soon though.

Mind, you can use a GE115 sized engine to power a plane LARGER than a 773. Use a higher lift profile and you can have a higher MTOW with the same engine. Course it will burn more fuel in cruise then.


User currently offlineNomadd22 From United States of America, joined Feb 2008, 1764 posts, RR: 0
Reply 11, posted (5 years 2 months 16 hours ago) and read 11015 times:

The 90-115 can already be certified for something like 123,000lb thrust bumps. Maybe it wouldn't be that much to rate it for full time 125,000lb. That and a bunch of weight savings from composite and a new wing might let you add a few inches to the tube.


Andy Goetsch
User currently offlineBa97 From Canada, joined Apr 2004, 377 posts, RR: 0
Reply 12, posted (5 years 2 months 2 hours ago) and read 10817 times:

Never say never. I would suspect Mr Wittle never thought a GE90 was possible. Compare a DC7 to a A380. Is it possible in the next 5 or 10? I could not see the interest and market. 15-20 years.... 15 years after the 707 was the 747.


there is economy class, business class, first class...then Concorde..pure class
User currently offlineMarkC From United States of America, joined Apr 2006, 259 posts, RR: 0
Reply 13, posted (5 years 2 months 2 hours ago) and read 10804 times:

I don't think the engines will be any problem at all. More it will be the size of the plane (length and wingspan), along with the height of the landing gear needed to clear bigger fans. But landing gear height is going to have to increase anyway knowing the way engine fans are going.

User currently offlineAcabgd From Serbia, joined Jul 2005, 655 posts, RR: 0
Reply 14, posted (5 years 1 month 4 weeks 1 day 20 hours ago) and read 10720 times:

I also think one of the factors might be the infrastructure at the airports - taxiways, gateways. There was already need for englargments for the B747, then now for the A380.

If anything bigger comes around in, say, 10-15 years, airports will need to adjust again and that's very costly.



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User currently offlineLightsaber From United States of America, joined Jan 2005, 12421 posts, RR: 100
Reply 15, posted (5 years 1 month 4 weeks 4 hours ago) and read 10422 times:
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The engine manufacturers could make a 150k engine today. There have been quite a few advancements since the GE-90-115.

#1: Contra rotation.
#2: Higher mach number compressors
#3: IBR compressors (more efficient)

Note to mention the trend towards greater chord lengths (compare the GEnx low pressure compressor for the 748 vs. the 788. The technology is *that* new.) All allow for greater efficiency than the GE-90. Imagine if the GE-90 had been designed with the latest generation fans. It would improve performance with the bypass ratio.  spin 

Quoting SeaBosDca (Reply 8):
I think the really big planes (A380 and anything Boeing may develop to compete with it) will continue to be quads.

There is a huge integration advantage having the engines out on the wings. Because the weight of the engines pulls down against the lift of the wings, there is less stress on the wings (lighter). Not to mention weight outside of the landing gear reduces the stress at the wing root upon landing or striking a pot hole.

Having to convey the thrust of the tail engine to the wing (primary source of drag) is going to preclude 3-holers from the long haul market from now on. The only 3 engine plane we might ever see would be a BWB.

What should be also noted is the advantage of weight savings. The A350XWB will compete with the 77W with far less thrust. The same would be true of a 450 passenger twin (the largest I current conceive). Its possible to make a 400 seat replacement for the 77W that requires less thrust than the 77W with 8,000nm range!  wideeyed 

Lightsaber



I've posted how many times?!?
User currently offlineJoeCanuck From Canada, joined Dec 2005, 5320 posts, RR: 30
Reply 16, posted (5 years 1 month 4 weeks ago) and read 10336 times:

I will remain skeptical about the weight of the 350 until they are closer to being completed. So far, CFRP hasn't proven to have much in the way of weight savings, though it does have other advantages.

The 787 isn't significantly lighter than the 330. I really don't understand how the 350 will be so significantly lighter than the 777. The -1000 is supposed to be almost the same size as the -300er and yet it is supposed to be a lot lighter. Maybe Airbus has some magical form of composites that Boeing hasn't thought of but I doubt it.

I'm certainly not going to predict that the 350 will be much heavier than advertised but going by the latest 2 composite aircraft, (787 and A400), I won't be very surprised if it is.



What the...?
User currently offlineRwessel From United States of America, joined Jan 2007, 2238 posts, RR: 2
Reply 17, posted (5 years 1 month 3 weeks 5 days 22 hours ago) and read 9935 times:
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Quoting Lightsaber (Reply 15):
Having to convey the thrust of the tail engine to the wing (primary source of drag) is going to preclude 3-holers from the long haul market from now on. The only 3 engine plane we might ever see would be a BWB.

At cruise, parasitic drag is significantly larger than induced drag, and the fuselage is a major contributor to that. Purely from a drag perspective, some thrust on the fuselage will reduce the amount of structure needed because all that fuselage drag is now not being pulled by the wings. Optimally, if one-third of the total drag comes from the fuselage, then a tail mounted engine (plus two on the wing) will essentially zero the fore/aft stress at the wingbox.

The major structural advantage from wing mounted engines comes from reduced bending moment because the fuselage's mass is decreased (by not including heavy engines).

There are some lesser factors as well.

To some extent a completely drag producing tail (including everything behind the wing) can be simpler (and presumably lighter) structurally because all of it's fore-aft load is tensile, which is often easier to deal with than compressive loads, especially in long thin structures. That's complemented by avoiding the added structural requirements for the front end of a three-engine aircraft, because the fore section is becomes longer, and has to carry larger compressive loads.

Another factor is that with a tail mounted engine, the front section of the aircraft will require a stronger structure simply because it'll be longer.

IMO, most of the arguments against tri-jets are unsatisfying, and reflect current fashion and practical reality more than technical issues. It certainly makes sense to build a twin if you reasonably can. But the extreme cost of engine development means that there's an upper limit imposed by the limited market for very large aircraft - you simply don't get enough units to amortize your engine development expenses. But for an aircraft 30-50% larger than a 777, a tri-jet makes sense - except that there's already an aircraft in that niche (747). Even a A388 sized aircraft could be reasonably accommodated with a minor growth version of a GE-90, although anything much larger (A389) starts requiring a rather more substantial growth, and Airbus has availability issues for the big GE-90s anyway.

The "it's too big to fit on the tail" argument is a bit silly too - we're not talking about fitting such a thing on a DC-10, but on a rather larger aircraft. Compare the CF6-80C2 on an MD-11 with 61.5klbs of thrust - it has a fan diameter of 93 inches (overall 106 inches) and weighs about 9800lbs. A GE-90-115, produces 86% more thrust (115klbs), weighs 86% more (18260lbs), and has a fan diameter 38% bigger (128 inches) which works out to a frontal area about 89% larger than the CF6), and a overall diameter increase of 27% (135 inches), for a overall frontal area increase of 62%. IOW, if you scaled up a DC-11 86% (by mass) to GE-90 size, the proportions of the engines to the rest of the plane wouldn't change all that much.

Admittedly newer engines, like the GEnx do increase the fan-diameter to thrust ratio a bit. although not nearly so dramatically as often claimed. The GEnx-2B67 (intended for the 748), with 66.5klbs thrust will have a 105 inch fan, which if we scale it down to match the 61.5klbs of the CF6-80C2, is approximately 101 inches, which is only 8 inches (9%) bigger than the CF6 (18% increase in frontal area).

The difficulty of maintenance of the center engine argument is a bit specious too. First, engines are getting more reliable, which reduces the problem. And then a tri-jet is held up as a reasonable configuration for a BWB, where *all three* engines are difficult to access, not just one of them. Y'all can't have it both ways.

Unless the market changes to drastically increase the likely number of aircraft that can be sold into the segment, someone wanting to build a new aircraft covering the bigger-than-777-to-A380 size range will be significantly tempted by a three engine design.


User currently offlineRheinwaldner From Switzerland, joined Jan 2008, 2198 posts, RR: 5
Reply 18, posted (5 years 1 month 3 weeks 5 days 20 hours ago) and read 9903 times:



Quoting Rwessel (Reply 17):
But the extreme cost of engine development means that there's an upper limit imposed by the limited market for very large aircraft - you simply don't get enough units to amortize your engine development expenses.

I agree with that. I think it was more than good luck that the thrust requirement of new aircraft designs often was set as 2..3..4 times the thrust of existing engines. The 767 having two 747 engines would be an example.


User currently offlineSlz396 From , joined Dec 1969, posts, RR:
Reply 19, posted (5 years 1 month 3 weeks 5 days 20 hours ago) and read 9903 times:

Well, isn't the A350-1000 going to be lager than the 77W???

User currently offlineStarlionblue From Greenland, joined Feb 2004, 16908 posts, RR: 67
Reply 20, posted (5 years 1 month 3 weeks 5 days 20 hours ago) and read 9899 times:



Quoting Rwessel (Reply 17):
The "it's too big to fit on the tail" argument is a bit silly too - we're not talking about fitting such a thing on a DC-10, but on a rather larger aircraft.

I think the lesson learned was that it was too big to fit on the DC-10 as well.  Wink McD had all sorts of issues with the tail engine on larger versions (including the MD-11). The banjo fitting was a tremendous piece of engineering, but it was also tremendously expensive to design.

As I understand it, part of the reason the DC-10 and Tristar became triplets was that the initial version developed for some quite stringent requirements, including transcon from LGA. The gate space requirement made a quad unpractical, while the total thrust and engine out requirements made a twin impossible.

Not saying that the DC-10 and the Tristar weren't great aircraft. They were.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineMax777geek From Italy, joined Mar 2007, 538 posts, RR: 0
Reply 21, posted (5 years 1 month 3 weeks 5 days 19 hours ago) and read 9894 times:



Quoting Starlionblue (Reply 1):
From the other end, the bigger the aircraft (requiring ever bigger engines), the smaller the demand for said aircraft. That's why 320s/737s sell more than 747s/380s.

I would be wondering how, in your equation of "costs/dimensions vs paxcapacity" the fuelburn factor includes. Proportionally a bigger model fuel consumption pro passenger is less than a smaller model with less pax capacity (but with less fuelburning engines, less weight to carry etc) can anyone tell how the fuel consumption is comparable among those 2 classes of airliners ?


User currently offlineRwessel From United States of America, joined Jan 2007, 2238 posts, RR: 2
Reply 22, posted (5 years 1 month 3 weeks 5 days 7 hours ago) and read 9716 times:
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Quoting Slz396 (Reply 19):
Well, isn't the A350-1000 going to be lager than the 77W???

A 77W (777-300ER) maxes out at 775,000lbs (351t), while the last number I saw for the A350-1000 was 657,000lbs (298t). The current plan seems to give the A350-1000 a few more inches length and wingspan.

The 50t less weight shows up in the planned use of 92klbs thrust engines vs. the 115klbs for the 77W.


User currently offlineStarlionblue From Greenland, joined Feb 2004, 16908 posts, RR: 67
Reply 23, posted (5 years 1 month 3 weeks 5 days 6 hours ago) and read 9698 times:



Quoting Max777geek (Reply 21):
I would be wondering how, in your equation of "costs/dimensions vs paxcapacity" the fuelburn factor includes. Proportionally a bigger model fuel consumption pro passenger is less than a smaller model with less pax capacity (but with less fuelburning engines, less weight to carry etc) can anyone tell how the fuel consumption is comparable among those 2 classes of airliners ?

Well sure. The cost per pax on a widebody is typically lower than on a narrowbody.

My point was that there are more routes requiring narrows, so they sell more.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlinePITIngres From United States of America, joined Dec 2007, 1087 posts, RR: 13
Reply 24, posted (5 years 1 month 3 weeks 5 days 5 hours ago) and read 9674 times:



Quoting Starlionblue (Reply 23):
The cost per pax on a widebody is typically lower than on a narrowbody.

To be nit-picky, I'm pretty sure you meant cost per seat. It only becomes cost per pax when there's a butt warming each seat. Which leads directly to your second, entirely inarguable point.



Fly, you fools! Fly!
25 Tdscanuck : I'm not sure how much benenfit you can get from that, given that the fore/aft load on the wing in cruise is about 5% of the vertical load. The wing i
26 XT6Wagon : they are also in the open and easy to access with no structural elements runnign around them like a normal tail mounted engine for a 3 engine plane.
27 Starlionblue : Oops. Yes quite right.
28 Rwessel : I don't disagree. I was replying to the assertion that a major defect of an aft mounted engine is the extra fore-aft load it creates because it now h
29 DocLightning : It needn't be that big, though. Remember, the problem with a twin is that if you have an engine out after v1, the remaining engine needs to provide 1
30 Trijetsrmissed : All sorts of issues? The MD-11 may have had it's shortcomings, but the No. 2 engine was not the cause. Rather, an overweight fuselage that was more d
31 Post contains links and images Starlionblue : Sorry I was unclear. The issues were not in service. They had issues with the cost and complexity of the design. Some argue that the projected cost o
32 Astuteman : Reckon you've hit it right on there, L. Couple of things..... Firstly the smart money is on later versions being lighter than earlier ones. Secondly,
33 Joperrin89 : I honestly think we will. Its just going to be a few years. This economic downturn is not going to last forever. I would be willing to bet the farm t
34 TrijetsRMissed : I seriously doubt this. There was interest in this aircraft from several airlines but the engineers were not given the R&D needed to get the program
35 Post contains links TristarSteve : Well Finnair has produced a brochure on flight in 2093 Check out their 7 engined airliner http://www.departure2093.com/en/main_page/?id=2 Steve
36 Rheinbote : The 788 still burns more than 10% less fuel per seatmile than the A330 - even in the current state of being overweight and with engines that are over
37 Starlionblue : A rose by any other name... Extra cost is extra cost whatever you call it. Apparently today airframers and customers have decided that a quad is chea
38 Alessandro : I doubt it, but I see further stretch of the B777 possible. Not a new airframe though.
39 Rwessel : " target=_blank>http://www.departure2093.com/en/main...?id=2 While I count seven horizontally mounted engines, I think there are also a dozen lift en
40 Max777geek : " target=_blank>http://www.departure2093.com/en/main...?id=2 Nice, but they reminds me more of sci-fi oriented kids draws fantasies than affordable p
41 Post contains links Lightsaber : Technically correct. But when you break down the drag, greater than 50% is at the wing/wingroot. Whenever I've participated in a trade study with tai
42 Jox : Now; Here is a, most likely, stupid thought. Does all engines have to be of equal size? Would it make sense to combine two large wing-mounted engines
43 Tdscanuck : Technically, no. Economically, yes. Having two types of engines instantly doubles your spares inventory. It also doubles your certification and integ
44 Nomadd22 : If you make that two large wing engines and one small tail engine you're talking about the thrusting APU that's been bantered about in a few threads.
45 DEVILFISH : Wouldn't jetblast from the large, wing engines adversely affect the intakes of the two smaller rear engines in such an arrangement, or would it mostl
46 DocLightning : Then why do APU's exist?
47 Zkpilot : Any replacement for the A380 is likely to be a completely different design... ie a BWB That way the aircraft CAN have 3 engines (as they can be mounte
48 Rwessel : So you don't have to run the main engines for a bit of electrical power on the ground, and so that you can start the engines without a ground cart. B
49 Post contains links Keesje : Together with Henry Lam I did some threads. Henry even made a youtube. http://www.youtube.com/watch?v=CVWQ5h5UOfk Someone else made a flight simulato
50 Zkpilot : Not necessarily... When it comes to day to day turnaround maintenance it could be just as easy. The engines would be sitting about the same height ab
51 Tdscanuck : Not if you design it right. APU intakes on current aircraft don't have an issue with jetblast...you ought to be able to place tail engines in a simil
52 DocLightning : Exactly my point. You still have an engine that needs its own parts, its own certification, etc. If you have two big engines and a little engine, the
53 Tdscanuck : That may have been exactly your point, but it wasn't mine. My point was that APU's make very bad propulsion engines, and vice versa, because of the w
54 DocLightning : Ah. I see. I suppose a similar rule holds in biology. You can either be a jack of all trades or a master of one, but you can't be a master of all tra
55 Post contains links JoeCanuck : I would be curious what the burn difference is between an APU and, (for example), A CFM -34 or -56 under a load that a typical APU can expect on a 77
56 SEPilot : I would say the Tristar was. And what advantage have you gained over just having four wing-mounted engines? As to the main point, I have no doubt tha
57 Tdscanuck : The CFM would be appallingly bad...the APU total output on something like a 777 is in vicinity of 250 KW. A CFM-56 generates about 25 MW...so it woul
58 Lightsaber : I'm aware of this on 4 engine freighters (One only needs 3 working hydraulic pumps, 3 alternators, etc.) But last I looked, overwater twins required
59 JoeCanuck : But how much extra weight would be added by installing 2 significantly larger engines? It would seem you could save significantly on development cost
60 Astuteman : I'd agree with that. If large aircraft are still tubes-with-wings then ...... I guess that the absence of thrusting APU's in mainstream aircraft toda
61 JoeCanuck : Until the 777-300, there was not much of a need to worry about it. The only aircraft with the potential need for such are in the 777 family and the -
62 SEPilot : I was thinking of this; but even though a BWB can accommodate three engines more easily than a tube with wings, it would still be more efficient with
63 Tdscanuck : The problem, fundamentally, is that APU's are bad at generating thrust. So, whatever your thrust increment is (say, 20,000 lbs for convenience) you'r
64 Lightsaber : At most 1,000 lbm per engine. If we're talking a new larger twin, its going to need a newly developed pair of engines. The added weight the larger en
65 Post contains images Keesje : Combined thrust / APU is already done on bigger props, "hotel mode'. The thrust a single APTU would offer (e.g. 20 klbs) would have to be added to bo
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