Sponsor Message:
Aviation Technical / Operations Forum
My Starred Topics | Profile | New Topic | Forum Index | Help | Search 
747-9X: "Back Of The Envelope" Ultimate Twin  
User currently onlineMatt6461 From United States of America, joined Oct 2013, 44 posts, RR: 0
Posted (9 months 2 weeks 3 days 6 hours ago) and read 15181 times:

I know I'm not the first person to think of making the 747 a twinjet. But here's my thoughts on why it should be done. This is a very rough exercise and any thoughts on refinement are greatly appreciated.

SUMMARY: Current 4-engine VLA’s present too much capacity risk for little to no average economic advantage versus the biggest modern twins. While VLA demand exists, that market needs a VLA twin, the only platform that can deliver acceptable CASM/capacity risk profile short of perhaps 700 seats. No OEM can launch a clean sheet VLA any time soon, so the only and best solution is a rewinged and reengined 747 twin. This program also promises economically feasible kangaroo-length nonstop. I call it the 747x here [knowing full well that an earlier 747x didn’t fare too well – only similarity is the name].
____________________________________________________________________________
The A388 seems incapable of justifying its enormous capacity by markedly lower CASM than 779x, A351, and even 787-10. Both OEM’s see a large market for VLA’s in next two decades, but perhaps neither has a product that warrants more than a few hundred orders. Outside of severe slot constraints and hot and heavy routes requiring 4 engine lift, airlines will not assume the capacity risk of a VLA without markedly better CASM. An all new VLA is far in the future, while the A380 needs a massive stretch to ~700 seats and reengining to deliver adequate CASM advantage. Even that would still leave a huge gap between 779x and A389neo. Might a radically revamped 747 fill the next decade’s ~450 seat role?
Below is a quick-calc proposal to rewing and reengine the 747-8 as a twinjet, with a small stretch. I believe this will give a ~25% CASM improvement over current 747-8, delivering 20% better CASM than any other plane. Here’s how it works:
Boeing scales the new 777x wing and engine parameters to lift 987,000 lbs – the 748i’s MTOW. This means identical wing loading and weight/thrust requirements, so:
779x has MTOW of 775,000 lbs and span of 233’ 3’’
747x Wingspan= ((987,000/775,000)^.5)*233.25 = 263’ 3’’ [Just inside Code F limit]
Thrust (twinjets) = (987,000/775,000)*102,000 = ~130,000. This project assumes that the GE9X can be scaled up to 125-130k, which doesn’t seem unrealistic given that the GE90-115B reached 115k and had a smaller diameter than GE9X. If this can’t be reached, a new engine would have to developed, which may make the project too costly.
These changes yield SFC and L/D benefits as follows:
SFC: A 10-12% SFC improvement should accrue from 4 --> 2 engine move. Theoretical fuel efficiency increases with the square of blade length, which give a 12+% value for the change [(132/105)^.5]. Additionally, I remember reading that the GE90-115B has the same SFC as the GEnx currently on 748i, and GE is promising a 10% SFC improvement over the -115B.
L/D: Boeing sources have indicated that the 777x’s new CFRP wing contributes a 12% L/D improvement. For the 749X, I will assume that the L/D benefit would be at least as good. Recall that 748i’s wing still retains the “bones” of the original 747, including wing box and main spar. Thus it has more sweep than would a modern airliner that minimizes shock buildup via aft-weighted airfoil design. The 748i thus has an induced drag penalty from suboptimal aspect ratio and I would expect a 749X’s supercritical CFRP wing to yield at least the 12% L/D improvement that it brings to the 777x.
So let’s say a 10% SFC improvement and a 12.5% L/D improvement for our 747x. Would the new wing and engine, however, add weight? Let’s look at components affected:
The new wing’s area would be ~595 m2, ~13.3% greater than the 748i’s wing area. If CFRP’s weight/stiffness ratio is ~18% better than the 748i’s aluminium, it should be a wash. Let’s assume the new wing’s greater cantilever washes about the 5% difference in weight by area. Again, this is a back of the envelope calculation.
The 748i’s 4 engines weigh 12,400 lbs each. The 90-115B weighs 19,040 lbs. Let’s assume a ~130k GE9x has the same strength-weight ratio as the 90-115B, which yields ~21,500. The new engines would save ~7,000 lbs in weight versus the 748i.
The vertical stabilizer would have to extended to deal with takeoff engine-out conditions, which are more challenging for a twin than a quad. Let’s say Boeing takes this opportunity to rebuild the empennage out of CFRP, yielding a larger vertical height but roughly equal weight and aerodynamic traits.
To fit the giant new engines, landing gear would likely have to be lengthened. The new wing could integrate a pylon that ameliorated some of this adjustment, but likely not all. 777w’s airport planning manual diagrams show that its engine nacelles are flush with the wing on the vertical axis, whereas 748i’s nacelles are not. The difference in fan diameter is 27 inches. Pylon positioning could probably cover half of this, landing gear the rest. Let’s assume a 15% landing gear weight penalty, and assume that landing gear is 5% of the 748i’s MOEW. This yields a weight penalty of ~3,500 lbs.
Boeing could use aluminum-lithium for traditional Al, yielding a 5% weight saving in the fuselage. If fuselage weight is 40% of the airframe, and MOEW for 748i is ~473,000 lbs, this yields ~9,500 lbs saving.
Assume for the moment that 749x’s MOEW equals 748i’s. Holding cruise speed constant, the
SFC and L/D improvements yield a 20% increase in range when plugged into the Breguet range equation: (10/9)*(9/8). Given the 748i’s nominal range of 8,100 nmi, the new wing and engine give a new nominal range of 9,720 nmi. Block fuel burn at 8,100 nmi would be more like 22% better by the simple range equation. That’s a huge improvement, but it gets even better.
First, ranges beyond 8,000 nmi reach a niche market, as sales of the 77L and A345 demonstrate. The 749x would do better giving back some range to increase capacity and decrease CASM.
Five-frame stretch and crown space usage:
Stretching by two frames forward and three frames aft gives you about 15 feet in fuselage length – 5 main deck economy rows and one upper deck 2x2 row at modern business class pitch. The stretch is weighted aft because unsweeping the wings changes the center of gravity some. I calculate weight penalty as follows: (1) assume fuselage is 40% of 748i’s MOEW, (2) assume floor beams are 9% of 748i’s MOEW, (3) assume that 90% of fuselage and floor beam weight is in the 175 feet between the exit doors – the area to be stretched. 15 feet stretches this block by 8.6 percent, adding ~18,200 lbs to MOEW at linear expansion. However, because stringers would have to be added to other frames to support the stretch, I increase the penalty by 20% to ~22,000 lbs. In addition, rather than stretch the now 265 foot plane further, let’s put the economy galley in the crown (mostly), take a 3,000 pound weight penalty, and add space for 16 economy seats. Remember, however, that the net weight different of engines, Al-Li use, and landing gear is about -13,000 lbs. So structural MOEW differential from the stretch is plus ~17,000 lbs. 66 seats are gained (5 new economy rows, 16 from galley space, 4 J on upper deck, 4 seats lost to additional lavs). Add 50 pounds to MOEW per seat. At Lufthansa’s V1 configuration (taken from SeatGuru), this gives 432 seats (8/96/328) in a 749x. It carries less capacity risk and ~20% better pax block fuel than a 388 (as summarized below).
Now run the range equation again with MOEW at 487.2k, MTOW still at 987k, and assume 200 lbs per pax. At MTOW, the 749x’s range is ~8900 nmi. Using the range equation to shed fuel to 8100 nmi range, the 749x’s block fuel per seat is about 26-28% lower than 748i’s at their common max range (depending on how you apportion additional seats between J/Y). So the 749x looks like a very attractive aircraft, having block fuel burn probably 20% better than any other plane (even if 779 and 388 have 6% better block fuel than 748i, which seems generous but possible).
There is also a role for an unstretched 748x with ultralong capability. Taking the net engine and alloy weight savings into account (but keeping the crown galley penalty and extra seats), the range equation gives a new range of ~9900 nmi, enough for nonstop kangaroo flights, as well as NYC-SIN/BKK/CGK, LAX-DXB/AUH/BOM, etc. with cargo payload. At 8100 nmi, the 748x still burns over 20% less block fuel per seat than 748i. For these flights especially, the skybunk idea could sell well enough to justify its inclusion. Adding a 5k weight penalty for 16 skybunks only decreases range to ~9750nmi. If potential revenue from these skybunks is ~20 economy seats (a conservative estimate imho), that makes economics even more attractive.
A few possible adjustments to these rough calculations:
First, if the CFRP wing gives a 12% L/D benefit on the 777, I would expect the combo of CFRP and 42 engines to yield significantly greater L/D improvement on the 747. As noted above, the aspect ratio improvement will be greater for 747 than for 777. Relative wing root thickness improvement should be better also, as the 748i’s bones are from older design. Parasite drag from decreased engine/pylon profile should yield benefits as well. Redoing the empennage as CFRP with a blade ending and refined horizontal stabilizers should yield efficiency improvements even with a strengthened vertical stabilizer.
The SFC computation at 10% may not be attainable by scaling up from the GE9X’s core. But not by too much, as the -115B sustained ~128k thrust in certification testing, a number that was needed on th 77W. Maintenance costs would likely rise with the stressed core, but we’d still be easily ahead of 4 hole costs.
However, required thrust might be closer to 125k than 130k. 130k comes from scaling up the 777x, but: (1) the 747x will have a larger wingspan and therefore lower induced drag, which dominates at takeoff speeds; (2) the 747x’s fuselage’s lower side tapers earlier, which reduces cargo capacity but allows the plane to rotate earlier in its takeoff run than a 777.
I am aware that the massive new wing might weigh more than the 748i’s wing, despite CFRP weight savings. Even if this knocked 5% off operating economics, 749x would still swamp the competition in CASM, still present less capacity risk than 388 or 389.
Finally, I haven’t added the skybunk option to economic performance of the 749x. After favorable introduction on 748x ultra-long haul flights, the market may adopt skybunks on 749x as well, and this would further improve its economics.
____________________________________
The 748/9x looks like an attractive proposition for airlines. But is it for Boeing and GE? Development costs for the 777x are estimated to be ~$4 billion, including the new engines. Let’s assume the wing and engine development costs for 747x are similar. 747x will also require a new empennage and landing gear – say that adds $1.5 billion to the bill. How many 747x’s can Boeing expect to sell? The 748x would likely have a monopoly on the ultralong market for some time. This is a niche that will be served by the first mover until a step change in technology unseats it. 150 aircraft in this market seems feasible. USA to Southeast Asia and Australia (BKK, SIN, CGK, SYD) could support 40 aircraft. Kangaroo to Europe another 50 (5,000 daily go SYD-LHR alone). The gulf carriers could easily buy 50 to support western U.S. and Latin American flights, plus EK likes more range than needed to allow payload capacity. Then there are Asia-Latin America flights, such as PEK-GRU. The 748x would continue to be the basis for the freighter version, offering 20% increased range or ~22% better block fuel per ton-mile. It would continue to dominate the freighter market and would change the economics of large freighters, inducing additional sales. 300 sales of the freighter version seems feasible.
Taking an average of the OEM’s forecast for VLA demand gives about 1,000 sales through 2032. The 749x would dominate these sales by offering at least 20% better block fuel and likely 15% better CASM than the 388. Airbus could achieve CASM parity or better by stretching towards a 700-seat capacity, but such a massive 389 would again be a niche aircraft. The 380 is simply too massive for a twin-engine option. It has to go huge to get adequate CASM with four engines and a massive wing. Its wing can’t be slimmed and lengthened without exceeding Code F or hinging the wing. Therefore, the 749x would dominate the VLA market and Boeing could easily sell 700 749x’s. Between all versions, Boeing could easily sell 1,000 747x’s by 2032.
But could they bring the plane to the market in time and produce enough of them before obsolescence? Production capacity on the 747 line is currently woefully underused, so building assembly plant space wouldn’t be an issue. The production of the new wing could be combined with the 777x’s wing facility, harnessing economies of scale. The new empennage could also be brought within this capital investment. The biggest issue is engineering resources. Boeing has the 787-10, the MAX, and the 777x on its plate right now. However, the 777x EIS is constrained by engine EIS (per reports), rather than by engineering work. Somewhere between now and 2019, especially after MAX and -10 work tapers off, Boeing has engineering resources not committed to airframe work. And because the new 747 wing is basically a “scaling up” of the 777x’s wing, presumably much of the learning curve has already occurred, such that creating the new wing would be less work (intellectually) than for the 777x. Give GE another 2 years to scale up the GE9X to ~125-130k, and put EIS at 2021 for 748x, 2022 for 749x. At 6/month with a 2 year ramp-up, Boeing would build 700-800 747x’s through 2032. This number is fewer than demand, allowing Boeing to charge a premium for its airframes. Even a reengined 388 would only close half the CASM gap at most, while still presenting additional capacity risk: Boeing would have substantial pricing leverage.
The A380 program, a ground up program with years of delays, is expected to break even at 500 units. The 747x would have less than half the development costs, so could break even at ~250 units perhaps. It seems likely that 7-800 sales would make Boeing a handsome profit while underwriting further CFRP learning and investment and stealing market share from Airbus.
Towards the end of the 2020 decade, new technologies, such as CNTP, truss-braced wing integration, and perhaps open rotor (though this might not work on long range planes) will begin to obsolete the 747x at last. It is for this reason that Boeing should select the 747x over an all-new VLA. The new VLA’s advantage would basically amount to lighter fuselage with more efficient configuration (e.g. a true double bubble for the “hump”). While that might improve CASM by 5%, it would at least double development costs and push EIS further out, given engineering constraints. Boeing needs to work on the NSA in the early 2020’s, so a clean sheet VLA just doesn’t fit, besides being perhaps less profitable for Boeing. The 747x allows Boeing to recapture share in the VLA market with relatively lower R&D. Short of an efficient 389’s perhaps 700 seats, only a giant twin can offer attractive economics at VLA capacity risk, and only a radically revamped twin-engine 747 is posed to fill this market gap in the foreseeable future.

What do y’all think? Comments are appreciated, I’m no pro and there are potential counterarguments…

[Edited 2013-10-16 17:05:46]

32 replies: All unread, showing first 25:
 
User currently offlinetortugamon From United States of America, joined Apr 2013, 3400 posts, RR: 10
Reply 1, posted (9 months 2 weeks 3 days 6 hours ago) and read 14977 times:

I did not get all the way through your very lengthy post but I think a 777-9x stretch to 81m would get you to 747-8 capacity and it would be a relatively low cost twin especially if it is designed during the 779x design phase. The Wing is big enough and the engine has a bigger fan that the GE90-15Bs. It would't be without its difficulties but make a heavy 747 into a twin would definitely be a lot more work for very similar capacity.

I will continue to read but a 747 would be 20+% heavier than a 777-10x with similar seat capacity while carrying less cargo while costing more to develop so if there is a need (big if) I don't think a twin 747 is realistic.

tortugamon


User currently offlineCentre From United States of America, joined Mar 2010, 488 posts, RR: 0
Reply 2, posted (9 months 2 weeks 3 days 5 hours ago) and read 14813 times:

I believe the 748 variants will be the last 747 around.

Quoting Matt6461 (Thread starter):
The A388 seems incapable of justifying its enormous capacity by markedly lower CASM than 779x, A351, and even 787-10.

How did you come up with such conclusion?



I have cut 4 times, and it's still short.
User currently offlineikramerica From United States of America, joined May 2005, 21476 posts, RR: 60
Reply 3, posted (9 months 2 weeks 3 days 5 hours ago) and read 14557 times:

You need an engine of at least 160k thrust for engine out assuming the design could handle the thrust imbalance on take off


Of all the things to worry about... the Wookie has no pants.
User currently onlineMatt6461 From United States of America, joined Oct 2013, 44 posts, RR: 0
Reply 4, posted (9 months 2 weeks 3 days 5 hours ago) and read 14517 times:

Tortugamon - Thanks for the response. One problem with the 7-10x, I believe, is that it would be a range-constrained aircraft like 787-10. Note that going from 787-9 to -10 costs 1,500 nmi in range - a similar step for the 777-10x would put it at 6,600. MTOW could be increased but then you're using a less than optimal wing size. 749x doesn't have that problem.

The 779x represents a proportionately longer stretch than does the 787-9, this would be true of the 777-10x also. Each stretch of the fuselage increases the weight penalty per marginal seat added. This caused the A346's problem,as I understand it. My 747-9x is proportionately less stretched than would be a 777-10x, which closes some of the gap on relative fuselage weight. 747x uses crown space to increase seating and revenue (skybunks) rather than stretching beyond 265 feet LOA, an option not possible on 777x. Airlines passed on this option on 748i, but I think it would be more likely adopted once the easier stretch capacity fixes are exhausted. (i.e. adding more fuselage plugs gets too heavy, so let's look at that crown space again).

My intuition is that adding increasingly heavier fuselage plugs to 777x and asking its wing to carry greater load to make range acceptable would not be attractive VLA option - or at least not attractive as 749x as sketched here. You're taking on both weight and drag penalties. I don't see, for example, how you could get 20% CASM benefit from 777-10x that I propose here. But maybe I'm wrong on that 20%. Good points, thanks again for discussion.


User currently onlineMatt6461 From United States of America, joined Oct 2013, 44 posts, RR: 0
Reply 5, posted (9 months 2 weeks 3 days 5 hours ago) and read 14449 times:

Quoting ikramerica (Reply 3):
You need an engine of at least 160k thrust for engine out assuming the design could handle the thrust imbalance on take off

I don't see how that's correct Ikramerica. This is a simple scale up from 777x, assuming identical wing loading and thrust/weight ratio. If 777x can take off under those conditions - and with less rotational leeway and more induced drag - then why can't 749x (assuming we strengthen the vertical stabilizer).

If your intuition stems from having 3 GEnx's in engine out versus having 1 GE9x, recall that 749x would take off at at least a 13% lower speed than 748i. Prior to V1, both engines are running by definition. Thus the point at which 749x reaches V1 leaves a lot more runway space for the remaining engine to lift into the air than for 748i.


User currently offlineStitch From United States of America, joined Jul 2005, 30622 posts, RR: 84
Reply 6, posted (9 months 2 weeks 3 days 5 hours ago) and read 14437 times:
Support Airliners.net - become a First Class Member!

Quoting Matt6461 (Thread starter):
What do y’all think?

An interesting thought exercise, but completely impractical in terms of actuality.


The 747 wing is designed for four engines, so you'd need to redesign it to handle the load from two engines. The 747-500X and 747-600X would have used a wing similar to the 777-200s, except designed for four engines and with a 78m span. You'd probably also need to redesign the wingbox for the different loads.

A two-engine 747 would probably break the grandfather clause the current 747 is certified under, which would require changes to the fuselage since the A zone (nose) and the upper deck would both be illegal (modern requirements require an exit forward of the cabin, which neither the A zone nor the upper deck forward of the mid-deck doors has).

As already noted, you'd need engines of much greater power than the GE90-115B because you need to be able to get that 1 million pounds into the air at V1 and then keep her in the air long enough to dump fuel and get her back on the ground. You'd also need to increase the size of the vertical stabilizer and rudder to counter the yaw effect from the thrust of having only one massively-powerful engine operating on one side of the airplane.

The crown area of the 747 is useless outside of cruise as you cannot put passengers or crew up there during take-off or landing due to lack of emergency exits. The crown area of the 747-8 is currently available for galley or lavatory space, but as yet no airline sees it worth the effort / inconvenience.

As for cost, beyond just R&D, you'd have to change the 747 assembly line to build it. Boeing would probably be lucky to get away with what Airbus spent on the A380 program, to say nothing of only half.


User currently onlineMatt6461 From United States of America, joined Oct 2013, 44 posts, RR: 0
Reply 7, posted (9 months 2 weeks 3 days 5 hours ago) and read 14411 times:

Tortugamon - One "global" point. I agree that the 747 fuselage is no longer a good design. The issue, as I see it, is that a true VLA needs a bigger fuselage than 777, and 747 is the only candidate for twin engine. With new wings, empennage, and engines, the 747 fuselage built out of Al-Li seems just good enough to fill the VLA role one last time.

User currently offlinePEK777 From China, joined Jun 2012, 129 posts, RR: 1
Reply 8, posted (9 months 2 weeks 3 days 5 hours ago) and read 14299 times:

The 777X will occupy the large aircraft sweet spot for the foreseeable future - it is just too reliable, flexible, and economical. Boeing needs to focus on a true 757 replacement.

User currently offlinetortugamon From United States of America, joined Apr 2013, 3400 posts, RR: 10
Reply 9, posted (9 months 2 weeks 3 days 4 hours ago) and read 13946 times:

Quoting Matt6461 (Reply 4):
Note that going from 787-9 to -10 costs 1,500 nmi in range - a similar step for the 777-10x would put it at 6,600.

Boeing plays around with these ranges so much is so hard to tell. My own calculation is based on percentages (8,250 787-9 range - 7,000 787-10 range divided by 8,250 and applied to 777-9x range of 8,100nm) and gets me to 6,900nm. The average A380 route is 4,000nm and EK's average is about 3,100nm meaning that without making a MTOW bump it could still do a large majority of A380 routes and the vast majority of EK's (the largest VLA customer by far) and could do many of these routes with full/much more cargo while carrying 450+ seats (compared to A388 at 525).

Quoting Matt6461 (Reply 4):
MTOW could be increased but then you're using a less than optimal wing size.

True, to make it an 8knm beast would take some heavy and not so nice lifting. If 8knm is needed then the 400 seat 779 should do just fine until ~2035. Otherwise, I think Boeing is comfortable with the idea that 450+ seats and 8knm range may be A380 territory and they chose not to compete.

Quoting Matt6461 (Reply 4):
Each stretch of the fuselage increases the weight penalty per marginal seat added. This caused the A346's problem,as I understand it.

I am assuming that when Boeing designs the 77x with AlLi they are going to build it around the 779, I imagine there will be few things left from the original 772 at that point. A 4.5m stretch is rather small if the 779 is optimized. Therefore I would expect weight penalties to be less significant.

Quoting Matt6461 (Reply 4):
747x uses crown space to increase seating and revenue (skybunks) rather than stretching beyond 265 feet LOA, an option not possible on 777x.

As you mention, no airline seems to be using the skyloft now and I am not sure that will change. I hope it does! However, the crew loft on the 77W is surprisingly large and only uses a small part of the crown space. I would like to see them use it on the 77x though I think it is unlikely. I am personally advocating for food cart storage in the crown to eliminate galley space so I can have a legitimate standing room bar area  .

Quoting Matt6461 (Reply 4):
I don't see, for example, how you could get 20% CASM benefit from 777-10x that I propose here. But maybe I'm wrong on that 20%.

Neither do I. However, I am not sure if this 747X could do it either. The added weight would be a killer. To optimize it like you mention would be much more than the cost of the 777x program and I think most of us think the VLA market is not very large. Its also tough to swallow when all of this cost will only amount to less than 11% more seats (450) than the 779x (407). Doesn't that mean that this 747x has to have a lower trip cost than the 779 in order to beat it by 20%?

Thanks for putting in the work. I always enjoy these types of discussions.

tortugamon


User currently offlineikramerica From United States of America, joined May 2005, 21476 posts, RR: 60
Reply 10, posted (9 months 2 weeks 3 days 3 hours ago) and read 13748 times:

Quoting Matt6461 (Reply 5):
I don't see how that's correct Ikramerica. This is a simple scale up from 777x, assuming identical wing loading and thrust/weight ratio. If 777x can take off under those conditions - and with less rotational leeway and more induced drag - then why can't 749x (assuming we strengthen the vertical stabilizer).

See this...

Quoting Stitch (Reply 6):
As already noted, you'd need engines of much greater power than the GE90-115B because you need to be able to get that 1 million pounds into the air at V1 and then keep her in the air long enough to dump fuel and get her back on the ground. You'd also need to increase the size of the vertical stabilizer and rudder to counter the yaw effect from the thrust of having only one massively-powerful engine operating on one side of the airplane.

The 747 was not designed for 2 engines. Could it be modified? Yes, with a new wing, and there will be new stresses of 160k of thrust on one wing dragging the wing spar when the other engine is out. Right now, the design under takeoff loads is for 1 engine out, so an imbalance of 60k or so, not 160k.

Boeing looked at a 3 holer for the 747, where the tail engine would not have equal thrust to the other. Of course, that was at a time when an engine big enough wouldn't even be possible.



Of all the things to worry about... the Wookie has no pants.
User currently onlineMatt6461 From United States of America, joined Oct 2013, 44 posts, RR: 0
Reply 11, posted (9 months 2 weeks 3 days 1 hour ago) and read 13178 times:

Quoting tortugamon (Reply 9):
Its also tough to swallow when all of this cost will only amount to less than 11% more seats (450) than the 779x (407). Doesn't that mean that this 747x has to have a lower trip cost than the 779 in order to beat it by 20%?

If we're using Boeing's seat counts then my 749x would seat 533 (467+66), 31.2% greater than than 779x. Boeing's numbers are always inflated, though I went off Lufthansa's 748i plus 66 seats. I am assuming that the 779x is a 350 seater in practice. But to use Boeing's numbers for 748i and 7779x fro comparison: My breguet range calculation has the 749x taking off at 947k lbs to reach 8100 nmi. 947 is ~22.2% greater than 779x's MTOW of 775. So my 749x lifts 7% less weight/pax for a similar mission based on Boeing seat counts. The 749x also has a 13% greater wingspan, which should give ~6% reduction in induced drag. Plus a bigger plane will have higher Reynolds numbers for less shock formation and lower fractional skin drag. Also the 747's hump is more efficient re pressure drag.

But your point is well-taken: Even if L/D is 6% better (both induced and parasitic), that combined with above relative weight advantage sums to 13% better block fuel burn. Integrating the savings by the range equation gets you perhaps 15% savings (for this to be true, the 749x would have higher landing weight fraction, and thus burn a smaller fraction of initial weight, which may or not be the case.) 13-15% fuel burn improvement would be huge, but still less than my initial estimate of 20%.

Possible source of discrepancy: When I was running this rough calculation, I was surprised that the 779x, which is roughly to the 77W what my 749x is to 748i, predicts only 21% per-pax fuel improvement, whereas the breguet numbers I was using for 748i/749x yielded ~27%. The discrepancy probably lies in the larger CFRP wing being slightly heavier than the old wing.

So perhaps something more like 13-15% better fuel/pax (versus 779x and A388) is best case scenario for a 749x. Using skybunks to get something like the revenue of 40 economy seats at little weight penalty would push the improvement into the mid-teens, perhaps. With market acceptance, that might justify launching a 749x, if development costs were much less than a clean sheet design. If Stitch is right about those costs, however, it wouldn't be launched.

[Edited 2013-10-16 21:34:21]

User currently onlineMatt6461 From United States of America, joined Oct 2013, 44 posts, RR: 0
Reply 12, posted (9 months 2 weeks 3 days 1 hour ago) and read 13101 times:

Quoting ikramerica (Reply 10):
Quoting Stitch (Reply 6):
As already noted, you'd need engines of much greater power than the GE90-115B because you need to be able to get that 1 million pounds into the air at V1 and then keep her in the air long enough to dump fuel and get her back on the ground. You'd also need to increase the size of the vertical stabilizer and rudder to counter the yaw effect from the thrust of having only one massively-powerful engine operating on one side of the airplane.

The 747 was not designed for 2 engines. Could it be modified? Yes, with a new wing, and there will be new stresses of 160k of thrust on one wing dragging the wing spar when the other engine is out. Right now, the design under takeoff loads is for 1 engine out, so an imbalance of 60k or so, not 160k.

Just FYI (and I know the initial post was loooong): I did mention the increased vertical stabilizer and bigger-than-115B engines in the initial post. If Boeing strengthened and extended the vertical stabilizer as part of a CFRP empennage redesign I bet the whole thing comes out as an aerodynamic and weight wash - at worst for the 749x.

I had not considered this issue of imbalanced thrust and its effect at the wing box structurally. Is this really a big issue? I have always thought the max bending moment was the dominant structural strength issue, and that any wing designed to withstand the bending moment of a million pounds would also be able to withstand imbalanced thrust of maybe 130k pounds. A horizontally-oriented force acting on the wing spars would oppose the Young's modulus of the rear spar in a manner analogous to how a truss takes on a wing's loaded bending moment. It seems that if the wing needed to strengthened for thrust imbalance, this could be done with little weight penalty (if needed at all).


User currently onlineMatt6461 From United States of America, joined Oct 2013, 44 posts, RR: 0
Reply 13, posted (9 months 2 weeks 3 days 1 hour ago) and read 13076 times:

Quoting Stitch (Reply 6):
An interesting thought exercise, but completely impractical in terms of actuality.

This is what I thought of the exercise when I started it, then I convinced myself in its midst. Thanks for playing along in the exercise!


User currently onlineMatt6461 From United States of America, joined Oct 2013, 44 posts, RR: 0
Reply 14, posted (9 months 2 weeks 3 days 1 hour ago) and read 12949 times:

Quoting Centre (Reply 2):

Quoting Matt6461 (Thread starter):
The A388 seems incapable of justifying its enormous capacity by markedly lower CASM than 779x, A351, and even 787-10.

How did you come up with such conclusion?

Airbus's CEO admitted recently that a reengine was needed to compete with modern twins. It is no coincidence that Lufthansa cancelled 3 A380 orders while simultaneously placing orders for big modern twins. Aboulafia has been saying for years that the efficient A380 will exceed the 80 meter box. Airbus put a much bigger wing on than is needed for a low-500 seater. At 700 seats and new engines, possibly winglets, the A380 would be better off.


User currently offlinerwy04lga From United States of America, joined Jul 2005, 3176 posts, RR: 8
Reply 15, posted (9 months 2 weeks 3 days ago) and read 12816 times:

Quoting Matt6461 (Reply 14):

Wow, counselor, that intro was one hell of a brief. Welcome to Airliners.net!

The 4-engined 747 is not subject to ETOPS rules, the 2-engined version would be.



Just accept that some days, you're the pigeon, and other days the statue
User currently offlineMax Q From United States of America, joined May 2001, 4381 posts, RR: 19
Reply 16, posted (9 months 2 weeks 3 days ago) and read 12697 times:

Why is it so popular here to take an Aircraft specifically designed as a quad and make it yet another twin
even in theory ?!!!


Don't we have enough boring twins as it is ?


You will have your wish before long as it is !



The best contribution to safety is a competent Pilot.
User currently onlineMatt6461 From United States of America, joined Oct 2013, 44 posts, RR: 0
Reply 17, posted (9 months 2 weeks 3 days ago) and read 12558 times:

Quoting rwy04lga (Reply 15):
Wow, counselor, that intro was one hell of a brief. Welcome to Airliners.net!

Thanks! I didn't realize that my profession would be publicly posted. Outing oneself as an attorney isn't the best way to make friends...  


User currently offlineRonaldo747 From United States of America, joined Feb 2009, 374 posts, RR: 0
Reply 18, posted (9 months 2 weeks 2 days 22 hours ago) and read 11728 times:

Quoting tortugamon (Reply 1):
but I think a 777-9x stretch to 81m would get you to 747-8 capacity

I don't think that the manufacturers will go outside the anticipated 80m x80m box at least for the next 30 years. Going outside it means that huge investment would be needed. The A380 already point to the limits of (some of) todays airports.


User currently offlineFWI747 From France, joined Jul 2007, 71 posts, RR: 0
Reply 19, posted (9 months 2 weeks 2 days 21 hours ago) and read 10659 times:

Welcome to A.NET, and thank you for this thorough study

I think that Stitch raised a valid point when he says that the 747-9x may break the grandfather clause the current 747 is certified under, and with those heavy modification I can't see how the 7479X could retain it...(the 748 has basically the same wing box than de 744).
This only point may raise considerably the certification bill (and I dont even mention a full flight by wire flight control).

But, since you are considering such a massive re-designe of this great airplane, why not add a pair of exit upstairs and repositioning the other at both ends of the cabin, then add a "jettisonable" exit in the nose (like the one in the tail cone of the MD82 series).

Another point is that the tail cone aero is not optimum, it could be redisigned a la 737MAX, and in the same move, the curve of the rear belly of the fuse could be changed to allow one or two more LD3 in the hold. However, There may be an issue with tail clearance at rotation...

At that point we are probably heading to ~$8 Bn...


User currently offlineparapente From United Kingdom, joined Mar 2006, 1548 posts, RR: 10
Reply 20, posted (9 months 2 weeks 2 days 21 hours ago) and read 10220 times:

Phew that must have been a very large envelope!

User currently offlinetortugamon From United States of America, joined Apr 2013, 3400 posts, RR: 10
Reply 21, posted (9 months 2 weeks 2 days 20 hours ago) and read 9144 times:

Quoting Matt6461 (Reply 11):
If we're using Boeing's seat counts then my 749x would seat 533

I have to admit to missing the 5 frame stretch in the intro. I see where you are coming from now.

Quoting Matt6461 (Reply 11):
So perhaps something more like 13-15% better fuel/pax (versus 779x and A388) is best case scenario for a 749x.

Relative to the 779x these would still be phenomenal results for any aircraft, let alone one that is looking for a way to be relevant into next decade. The program manager mentioned last year that a 747-8 stretch has been thought up and was on a drawing board but it 'had to wait its turn'. I am not sure how sincere that was. Although before this conversation I had not envisioned a stretch and a drop of two engines happening at the same time, I hope some creative designs are being considered to give her new life.

Quoting Matt6461 (Reply 11):
I was surprised that the 779x, which is roughly to the 77W what my 749x is to 748i, predicts only 21% per-pax fuel improvement, whereas the breguet numbers I was using for 748i/749x yielded ~27%.

There have been multiple remarks by people that have been briefed that the 779x will be better compared to the 77W than what Boeing is saying. Some of this delta could be sand bagging and conservative.

Quoting Ronaldo747 (Reply 18):
I don't think that the manufacturers will go outside the anticipated 80m x80m box at least for the next 30 years. Going outside it means that huge investment would be needed. The A380 already point to the limits of (some of) todays airports.

3.2 feet will launch a huge investment for airports? I hear what you are saying and I agree its a real hurdle. Span impacts runways, taxiways, and terminal spots while length impacts turn radius at turning points and it may have its butt sticking out 1m into the hallway a little bit but it won't be bumping wingtips.

This 777-10x is a bit of a pipe dream but I personally see airports adapting like they always have if being confronted with such an aircraft. The A380 had a lot of barriers to break down and breaking the 80 x 80 box was one fight they didn't want as they already had enough on there plate. Expanding that box length-wise is not nearly as bad as width-wise. Could you picture an A380 airport rejecting a 777-10? They have already done the heavy lifting.

I personally see rotation angle and over weight MLG as bigger hurdles to a 777-10x then a 1-2m lengthening of the box.

tortugamon


User currently offlineCXB77L From Australia, joined Feb 2009, 2597 posts, RR: 5
Reply 22, posted (9 months 2 weeks 2 days 17 hours ago) and read 6902 times:
AIRLINERS.NET CREW
CHAT OPERATOR

Quoting Matt6461 (Thread starter):

Interesting thought, but I agree with Stitch in that it wouldn't work because:

Quoting Stitch (Reply 6):
A two-engine 747 would probably break the grandfather clause the current 747 is certified under, which would require changes to the fuselage since the A zone (nose) and the upper deck would both be illegal (modern requirements require an exit forward of the cabin, which neither the A zone nor the upper deck forward of the mid-deck doors has).

... and also,

Quoting ikramerica (Reply 10):
The 747 was not designed for 2 engines.

.

For those reasons, I do not believe it is feasible to redesign the 747 as a twin. They may as well design an all new twin larger than the 777. Personally, I think that the "Y3", whenever it emerges, will be such an aircraft - a very large, twin engined widebody.

Quoting Matt6461 (Reply 7):
The issue, as I see it, is that a true VLA needs a bigger fuselage than 777

There appears to be two schools of thought when it comes to defining what constitutes a "true VLA". One is that a VLA is anything the size of the original VLA (747-100) and above, and the other adopts a sliding scale depending on the size of aircraft at the time, which has been gradually increasing over time.

The current 777-300ER is longer, has a larger wingspan and heavier than the 747-100, and carries as near as damnit to the same number of passengers in Boeing's typical three class configuration. In terms of performance, the 777-300ER beats even the 747-400 for payload/range. Going by this school of thought, the 777-300ER is as much a VLA as the 747-100 was.

The second, which adopts a 'sliding scale', as it were, would suggest that the 747-100 is no longer a VLA (or the 747-200, for that matter).

I disagree that the 777 fuselage isn't big enough to be a "true VLA". It is large enough to seat as many per row in economy as the 747, and long enough that it matches the original 747 for seating capacity. The 777-9X will stretch the fuselage and the wingspan even further, making longer and wider (span wise) than the 747-8, which is already longer than the A380-800.

Quoting Matt6461 (Reply 17):
Outing oneself as an attorney isn't the best way to make friends...

I'm in the legal profession myself, so welcome  



Boeing 777 fanboy
User currently offlineseabosdca From United States of America, joined Sep 2007, 5325 posts, RR: 4
Reply 23, posted (9 months 2 weeks 2 days 15 hours ago) and read 6850 times:

Quoting CXB77L (Reply 22):
For those reasons, I do not believe it is feasible to redesign the 747 as a twin. They may as well design an all new twin larger than the 777. Personally, I think that the "Y3", whenever it emerges, will be such an aircraft - a very large, twin engined widebody.

  

I think it would be easier and likely cheaper to design an all-new twin in this size class, liberally using 787 technology, than to use the 747 as a basis. I also think the 777-9X has eaten such a new large twin's lunch. If there is a "Y3," it won't be for quite a long time.

That said, very interesting thought exercise, and something that could have made a lot of sense for Boeing to think about back when they were looking into the 747-500X and -600X.

Quoting Matt6461 (Reply 17):
Outing oneself as an attorney isn't the best way to make friends...

There is a surprising number of us around here...  


User currently onlineMatt6461 From United States of America, joined Oct 2013, 44 posts, RR: 0
Reply 24, posted (9 months 2 weeks 2 days 14 hours ago) and read 6821 times:

Thanks again for the feedback and welcomes all.

Re recertification due to exit doors, I was not aware of this issue. I agree that if the fuse has to be recertified it's undoubtedly better to just do a new VLA and ditch the metal fuse (might be anyway). But Boeing has a ton political clout. If it were on the cusp of launching 749x and certification was the only hurdle to financial feasibility, I would predict it could convince the Obama/Clinton/Rubio administration to extend the waiver. You'd have senators from every state with potential workshare pulling all the stops. It would be like Keystone XL issue but with positive environmental impact. BA could do public relations around safety of 747, portray the exit regulation as just more bureaucratic nonsense...

Quoting CXB77L (Reply 22):
I disagree that the 777 fuselage isn't big enough to be a "true VLA". It is large enough to seat as many per row in economy as the 747, and long enough that it matches the original 747 for seating capacity.

Good point but the argument about true VLA definition is largely semantic. The true issue is the gap between 779x (at 350-370 seats) and A380-900re (at 650-700 seats) for the next decade (I just don't think 388 will play a big role). 777-10x may have a role to play, but won't be a true intercontinental aircraft. What's the use of all those L/D slots if you can't fill them on >4,000 mi hops?

Maybe the 749x could fill the 779x-A389 gap with reasonable development costs. Maybe not.

If not, then launch Y3 as 500 seat VLA twin with ULH capability?   That's for another post and another day...


25 LH707330 : Matt, welcome to a.net! Echoing others, you must have had a large envelope. If this is true, then you just sank the idea yourself. The 748 has similar
26 Matt6461 : The idea is only sunk if 13-15% fuel burn improvement over 779x and 388 are insufficient to justify program launch. For non-fuel reasons, the 749x wo
27 Post contains images Matt6461 : I can't explain this. I remember reading something to this extent somewhere (brilliant citation I know). It may be that efficiency asymptotically app
28 Post contains images airmagnac : People always forget the systems ! Not fair ! OK you did mention the gear, but only in passing. In addition to the weight, you're going to have to fi
29 LH707330 : This I'm not too sure of, perhaps someone else can chime in. This makes sense, because the tip clearance remains roughly constant, so the swept/total
30 Post contains images Matt6461 : Yep. Totally missed that - there wasn't enough room on my envelope. That's a big deal, would push development costs skyward and probably favor a clea
31 LH707330 : Wing loading plays a role too, the higher the wing loading, the higher the induced drag, because you have to hold a higher alpha to maintain level fl
32 Post contains images airmagnac : No, no and 3 times no ! First of all you still haven't defined what you call CASM, and which costs you throw into it. But whatever they may be, those
Top Of Page
Forum Index

Reply To This Topic 747-9X: "Back Of The Envelope" Ultimate Twin
Username:
No username? Sign up now!
Password: 


Forgot Password? Be reminded.
Remember me on this computer (uses cookies)
  • Tech/Ops related posts only!
  • Not Tech/Ops related? Use the other forums
  • No adverts of any kind. This includes web pages.
  • No hostile language or criticizing of others.
  • Do not post copyright protected material.
  • Use relevant and describing topics.
  • Check if your post already been discussed.
  • Check your spelling!
  • DETAILED RULES
Add Images Add SmiliesPosting Help

Please check your spelling (press "Check Spelling" above)


Similar topics:More similar topics...
THe Hole At The Back Of The Plane posted Fri Feb 9 2001 07:53:44 by Bruce
Physics Of The Push-Back posted Sat Dec 25 2010 06:43:38 by faro
What Is The Cone On The Back Of A Jet Engine? posted Sun May 9 2010 08:55:23 by c5load
Vent At The Back Of CFM 56 Engine posted Sat Apr 24 2010 04:39:00 by smartt1982
State-of-the-Art Flight Deck For 747-8 posted Tue Apr 20 2010 00:54:43 by propilot83
The Size Of The 747 Freighter Opening posted Sat Oct 21 2006 11:34:47 by NZ107
Speed Of Air Out Of The GE-90-115's Back End posted Sun Sep 10 2006 01:47:24 by UAL747
Holes On The Cowlings Of The 747? posted Sun Apr 16 2006 02:58:58 by CalAir
The Effects Of The 12 Vmc On A "normal" Light Twin posted Mon Feb 27 2006 15:08:07 by NorCal
Different Lengths Of The 747-8(00) posted Sat Jan 21 2006 06:01:37 by Bohica

Sponsor Message:
Printer friendly format