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747-9X: "Back Of The Envelope" Ultimate Twin  
User currently offlineMatt6461 From United States of America, joined Oct 2013, 80 posts, RR: 0
Posted (1 year 2 months 2 days 19 hours ago) and read 23301 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, 3451 posts, RR: 11
Reply 1, posted (1 year 2 months 2 days 18 hours ago) and read 23097 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 Canada, joined Mar 2010, 490 posts, RR: 0
Reply 2, posted (1 year 2 months 2 days 18 hours ago) and read 22933 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, 21580 posts, RR: 59
Reply 3, posted (1 year 2 months 2 days 18 hours ago) and read 22677 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 offlineMatt6461 From United States of America, joined Oct 2013, 80 posts, RR: 0
Reply 4, posted (1 year 2 months 2 days 17 hours ago) and read 22637 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 offlineMatt6461 From United States of America, joined Oct 2013, 80 posts, RR: 0
Reply 5, posted (1 year 2 months 2 days 17 hours ago) and read 22569 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 onlineStitch From United States of America, joined Jul 2005, 31387 posts, RR: 85
Reply 6, posted (1 year 2 months 2 days 17 hours ago) and read 22557 times:
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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 offlineMatt6461 From United States of America, joined Oct 2013, 80 posts, RR: 0
Reply 7, posted (1 year 2 months 2 days 17 hours ago) and read 22531 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, 176 posts, RR: 1
Reply 8, posted (1 year 2 months 2 days 17 hours ago) and read 22419 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, 3451 posts, RR: 11
Reply 9, posted (1 year 2 months 2 days 16 hours ago) and read 22066 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, 21580 posts, RR: 59
Reply 10, posted (1 year 2 months 2 days 16 hours ago) and read 21868 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 offlineMatt6461 From United States of America, joined Oct 2013, 80 posts, RR: 0
Reply 11, posted (1 year 2 months 2 days 14 hours ago) and read 21298 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 offlineMatt6461 From United States of America, joined Oct 2013, 80 posts, RR: 0
Reply 12, posted (1 year 2 months 2 days 14 hours ago) and read 21221 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 offlineMatt6461 From United States of America, joined Oct 2013, 80 posts, RR: 0
Reply 13, posted (1 year 2 months 2 days 14 hours ago) and read 21196 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 offlineMatt6461 From United States of America, joined Oct 2013, 80 posts, RR: 0
Reply 14, posted (1 year 2 months 2 days 13 hours ago) and read 21069 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 (1 year 2 months 2 days 13 hours ago) and read 20936 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, 4778 posts, RR: 19
Reply 16, posted (1 year 2 months 2 days 12 hours ago) and read 20817 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 offlineMatt6461 From United States of America, joined Oct 2013, 80 posts, RR: 0
Reply 17, posted (1 year 2 months 2 days 12 hours ago) and read 20678 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, 395 posts, RR: 0
Reply 18, posted (1 year 2 months 2 days 11 hours ago) and read 19848 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 (1 year 2 months 2 days 10 hours ago) and read 18779 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, 1662 posts, RR: 10
Reply 20, posted (1 year 2 months 2 days 9 hours ago) and read 18340 times:

Phew that must have been a very large envelope!

User currently offlinetortugamon From United States of America, joined Apr 2013, 3451 posts, RR: 11
Reply 21, posted (1 year 2 months 2 days 8 hours ago) and read 17264 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, 2693 posts, RR: 5
Reply 22, posted (1 year 2 months 2 days 5 hours ago) and read 15022 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, 5828 posts, RR: 6
Reply 23, posted (1 year 2 months 2 days 3 hours ago) and read 14970 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 offlineMatt6461 From United States of America, joined Oct 2013, 80 posts, RR: 0
Reply 24, posted (1 year 2 months 2 days 3 hours ago) and read 14941 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...


User currently offlineLH707330 From United States of America, joined Jun 2012, 848 posts, RR: 0
Reply 25, posted (1 year 2 months 1 day 23 hours ago) and read 14856 times:

Matt, welcome to a.net! Echoing others, you must have had a large envelope.

Quoting Matt6461 (Reply 11):
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.

If this is true, then you just sank the idea yourself. The 748 has similar seat costs as the 77W, but more capacity risk, so if the 74X and the 77X have the same relationship, expect sales for the 74X to mirror those of the 748. I would not put >5 bn into such a project if I were Boeing.

One thing I'm not sure you covered was the need for more strengthening in the wing due to the lack of bending relief from the outboard engines on a quad. I reckon that would add a few tons.

Another one is the factory itself. At present, the 68 m 748 can't have its raked tips attached until it gets to the slant positions because there's only about a meter clearance on either side. If you're stuck with a ~62m span restriction in the wing join phase, you'd have to find a creative way to adjust all of the outboard structure, which will doubtless add weight.

Quoting Matt6461 (Thread starter):
Theoretical fuel efficiency increases with the square of blade length

Could you explain this one? Assuming your tip clearance stays the same, your ratio of swept area/total intake area approaches 100% as you increase diameter, so you get diminishing returns. Am I missing something there?

If the change from 744 to 748 cost Boeing around $2b, I reckon this would be high-single-digits if not $10b, at which point you may as well go clean sheet. That said, kudos for taking a good whack at the ball.


User currently offlineMatt6461 From United States of America, joined Oct 2013, 80 posts, RR: 0
Reply 26, posted (1 year 2 months 1 day 21 hours ago) and read 14810 times:

Quoting LH707330 (Reply 25):
Quoting Matt6461 (Reply 11):
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.

If this is true, then you just sank the idea yourself. The 748 has similar seat costs as the 77W, but more capacity risk, so if the 74X and the 77X have the same relationship, expect sales for the 74X to mirror those of the 748. I would not put >5 bn into such a project if I were Boeing.

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 would improve over 748i more than 779x will over 77W: engine maintenance, a bigger capacity increase (causing lower per-pax non-fuel costs, including acquisition). Furthermore, 748i would move from being slightly worse than its competitor to being substantially better. 77W, by contrast, has solid decade of monopoly in its market space. 779x isn't a slam-dunk over A350-1000, whereas the 749x I sketch here handily beats the 388. So while I wouldn't expect 77W or even 779x sales from the 749x, my project assumes that Boeing can break even without selling 1,000 airframes. Meanwhile it funds CFRP and other R&D by stealing market share from its competitor.

Quoting LH707330 (Reply 25):
Another one is the factory itself. At present, the 68 m 748 can't have its raked tips attached until it gets to the slant positions because there's only about a meter clearance on either side.

This another wrinkle in my plans, damn you! Maybe Boeing could add a wing to the giant 747 factory, I dunno. Or add a bay towards the end of the assembly line, where the outer 40 feet of wing is attached. Or roll the stub-winged aircraft into a new nearby building. There's gotta be a solution there...

[Edited 2013-10-17 14:38:16]

User currently offlineMatt6461 From United States of America, joined Oct 2013, 80 posts, RR: 0
Reply 27, posted (1 year 2 months 1 day 21 hours ago) and read 14841 times:

Quoting LH707330 (Reply 25):
Quoting Matt6461 (Thread starter):
Theoretical fuel efficiency increases with the square of blade length

Could you explain this one? Assuming your tip clearance stays the same, your ratio of swept area/total intake area approaches 100% as you increase diameter, so you get diminishing returns. Am I missing something there?

I can't explain this.   I remember reading something to this extent somewhere (brilliant citation I know). It may be that efficiency asymptotically approaches a theoretical limit as blade length increases. I'm pretty sure that the GE90-115B bests the GEnx at SFC, despite the latter's more advanced technology - and I think this is because of its bigger diameter, that a bigger fan is intrinsically more efficient, ceterus paribus.


User currently offlineairmagnac From Germany, joined Apr 2012, 321 posts, RR: 52
Reply 28, posted (1 year 2 months 1 day 1 hour ago) and read 14530 times:

Quoting Matt6461 (Reply 26):
This another wrinkle in my plans, damn you!

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 fit the longer gear inside the gear wells, could be an issue...
Otherwise :
The main power source on the aircraft are the engines, so the change from 4 to 2 is going to imply a complete overhaul of the energy generation and distribution systems : electric, hydraulic, bleed (removing the bleed system is impossible as it drives several safety critical pumps IIRC)
The ther major system interacting with the engines is the fuel system, which would have to be modified substantially.
The APU would have to be substantially bigger to provide enough pneumatic power to start the engines.
The major aerodynamic and weight distribution changes are going to require a complete overhaul of the flight controls and autopilot, which are mechanical and therefore not easy to change.
The huge change in metallic masses hanging under the wings may impact all the antennas
The new uncontained engine failure conditions will probably require a complete redesign of the center fuselage

And a couple of other limiting factors linked to increased passenger loads :
The air conditioning packs may not be capable of handling the increased passenger loads. Although I think that can be fixed with bigger packs, it may require an redesign of the belly fairing with increased weight & drag.
More mundane but possibly a showstopper : the capacity of the waste water system ! All that sh*t has to go somewhere, and it takes up a lot of space !

Note that most of these items would proabably require re-design of substantial sections of the airplane, and/or penalties on weight and drag


Coming back to structures & aerodynamics, the big issue with your calculations is that you are looking at each factor in isolation to all others, and do not nvestigate the snowball effects. For example, bigger weight implies more induced drag. More drag, ie more force, means more loads on the wings, therfore more structural weight required. Etc...

The Breguet equation holds under hypotheses which are too limiting to consider a whole flight. You'd have to chop the fligt up into several sections with different parameters.
You also need to look at the new performances. The weight and aerodynamics changes will impose new altitudes and speeds, so that the mission profile of the 747x will be different from that of a 747-8. In turn this will affect uel consumption.



More generally, I still don't understand the infatuation of a-net with CASM. It does not mean much in isolation : would you prefer to pay 1$ to gain 2$, or pay 5$ to gain 11$ ?
You need RASM data at the very least, but even then, those numbers are meaningless if not linked to how the airplane is used by the airline within its wider network and within the airline's strategy to tap the market.
CASM is just an indicator : if it is far off the mark, then the designer is in trouble and has to do something about it. If it is on target...it doesn't mean you're safe and sound, as other things could be way out of whack ! Indicators should never ever be used as objectives (something managers and financiers should keep in mind)
I also do not understand where the 20-25% improvement of CASM comes from ? What's your definition of CASM, and how do you calculate it ? After all, you've just designed a wonder aircraft which beats all modern designs by a wiiiiide margin, all that while being based on a 50 year old design...it rings some bells !


Still, this is fun stuff !    and welcome !



One "oh shit" can erase a thousand "attaboys".
User currently offlineLH707330 From United States of America, joined Jun 2012, 848 posts, RR: 0
Reply 29, posted (1 year 2 months 1 day ago) and read 14509 times:

Quoting Matt6461 (Reply 27):
I'm pretty sure that the GE90-115B bests the GEnx at SFC, despite the latter's more advanced technology

This I'm not too sure of, perhaps someone else can chime in.

Quoting Matt6461 (Reply 27):
that a bigger fan is intrinsically more efficient, ceterus paribus.

This makes sense, because the tip clearance remains roughly constant, so the swept/total increases the larger the fan gets. The area of the gap increases linearly with the diameter, while the change in increased swept area scales at twice the rate, so for a doubling in diameter, you get half the tip loss.


User currently offlineMatt6461 From United States of America, joined Oct 2013, 80 posts, RR: 0
Reply 30, posted (1 year 2 months 11 hours ago) and read 14363 times:

Quoting airmagnac (Reply 28):
People always forget the systems ! Not fair !     

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 clean sheet design over 749x even if my sketched CASM were there.

Quoting airmagnac (Reply 28):
More generally, I still don't understand the infatuation of a-net with CASM. It does not mean much in isolation : would you prefer to pay 1$ to gain 2$, or pay 5$ to gain 11$ ?

The VLA mission is to move large numbers of people between mega-airports as quickly as possible, so while you're right about CASM not being everything, in this specific instance it is the main design goal I think. I started my little doodle after reading A and B's market forecasts for VLA, and noting that perhaps no plane existed to fill the role of exploiting economies of scale on mega-mega routes (if CASM are near parity, even a mega-route will favor increased frequency and lower capacity risk where they come basically free).

Quoting airmagnac (Reply 28):
Coming back to structures & aerodynamics, the big issue with your calculations is that you are looking at each factor in isolation to all others, and do not nvestigate the snowball effects. For example, bigger weight implies more induced drag. More drag, ie more force, means more loads on the wings, therfore more structural weight required. Etc...

I actually went through several iterations on a spreadsheet to meet certain important constraints, such as that wing loading and MTOW/thrust had to equal the 779x. This ensures a simple scale up of wing size and thrust, for the most part (I assume?). I worked backwards from those constraints by picking a payload/MOEW and then making tradeoffs back forwards to optimize. It's hardly sophisticated, which is why I called "back of the envelope," but maybe its broad outlines are valid, including reliance on Breguet. Speaking of which:

Instead of directly using Breguet's equation, I used a ratio between the results for the 779x and my 749x at different MTOWs and landing weights. I know the Breguet equation is too gross for full flight calculation, but figured comparison to the 779x would be ballpark. Maybe not...

Thanks for reading, I have another favor to impose: I've read a lot posts assuming that induced drag is inverse to AR only - that it dimensionless. Is this true? I always thought that for two wings of equal AR but different spans, the longer span will have less induced drag. A ring spinning around the earth, for example, would have no wingtip vortices (although still some induced drag from trailing edge vortices). IIRC Ilan Kroo posted a reminder of the dimensionality of induced drag...


User currently offlineLH707330 From United States of America, joined Jun 2012, 848 posts, RR: 0
Reply 31, posted (1 year 1 month 3 weeks 6 days 15 hours ago) and read 13838 times:

Quoting Matt6461 (Reply 30):
Thanks for reading, I have another favor to impose: I've read a lot posts assuming that induced drag is inverse to AR only - that it dimensionless. Is this true? I always thought that for two wings of equal AR but different spans, the longer span will have less induced drag.

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 flight. As to the longer one having less drag, if all other parameters are equal, it will have slightly less drag due to Reynold's Number, but if changing, say, from 60m to 70m span, this effect would be marginal.


User currently offlineairmagnac From Germany, joined Apr 2012, 321 posts, RR: 52
Reply 32, posted (1 year 1 month 3 weeks 6 days 10 hours ago) and read 13796 times:

Quoting Matt6461 (Reply 30):
in this specific instance it [CASM] is the main design goal I think

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 costs will unavoidably depend on the network structure of the airline, and its general strategy. CASM therefore cannot be used as a design goal, because it depends on elements specific to each user.

Second thing is that a design goal must be a necessary and sufficient condition for success : if you don't reach the goal, it's not good, and if you do, it's good.
During the design phase, you can use some necessary conditions (but not sufficient) as "indicators" to be sure you are not straying too far off the track. Such numbers only cover a very specific aspect of the design, so if they are bad, this particular aspect is bad and therefore the design cannot be right. Yet if they are correct, it doesn't mean a whole lot, because other aspects of the design might have been compromised to obtain this "good value".

In this case, CASM is only a necessary condition for a successful airliner, but it is far from sufficient ! So it is to be used as an indicator only : if it is in the red, the design is bad. But a good-looking CASM does not mean that the design is good, because many other things could be bad.

As an example : you go to a restaurant, expecting a good meal. If the dish comes out and smells rotten, then it's bad and that's that. But even if it smells really good, it may only be so because the chef used a liberal amount of condiments and strong flavored ingeredients to cover the fact that the meat is bad. So no guarantee that the dish will be delicious, even if it smells good.
And if you are the chef and decide to cook only to make good-smelling food, regardless of its taste or its looks, you obviously won't be very successful, because diners expect much more than just a smell.
And the design target isn't even fixed, as the definition of "good smell" varies from one eater to another : for example some people love the strong smell of weird cheeses, some people just can't stand it.

Coming back to airplanes, what you have to do is first decide how many passengers, what range, etc.. and design from there. While designing the product you regularly check the expected ball-park CASM, but the design target should always be your initial definition of the product.

Quoting Matt6461 (Reply 30):
if CASM are near parity, even a mega-route will favor increased frequency and lower capacity risk where they come basically free

That's not universally true in the real-world outside of a-net. It's only true if you can plan the equipment on a given route with no outside constraints. But there are always constraints : market dynamics, the airlines strategy, its established network structure, the fiscal constraints it faces, airport limitations etc...

Quoting Matt6461 (Reply 30):
This ensures a simple scale up of wing size and thrust

Supposing everything scales linerarly. With luck, that's true, or at least it's a valid approximation. But it probably is not, because all those parameters are interrelated.
To find out, you'd need some kind of simple loads model, performance model, handling model...to link all the parameters together and calculate the closed loops. See Ferpe's explanations in the aspect ratio thread



One "oh shit" can erase a thousand "attaboys".
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