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ElroyJetson
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Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Mon Jan 15, 2018 6:55 am

I have been researching various sources on the question which frame will be more efficient, the A351 or the 779?

Airbus claims the A351 will be 25% more efficient than the existing 77W, and they are strongly touting the 351 as a 77W replacement. Most analysis I have seen think the 25% number is too high but think a 20% improvement is realistic.

However, Airbus avoids talking directly about any efficiency improvement of the 351 versus the 779 and usually cites the fact the 779 is a heavier frame. Of course, this avoids the fact the 779 is a bigger plane and seats roughly 40 more pax.

The GEX9 is supposed to be 5-8% more efficient than the RR Trent engines that will power the 351. Also the larger wing of the 779 is supposed to be roughly 12% more efficient than the A351 wing on missions of 4000nm or more.

One analysis I saw predicted the 779 will have at least a 5% lower CASM than the A351 on missions of 4000nm or more and will be virtually identical in terms of fuel burn.

Does this sound realistic? If airlines can fill the additional seats on the 779 is it's better choice? Or is the A351 truly as efficient in terms of CASM as the 779 even though it seats about 9% fewer pax?

I know there are a lot of knowledgeable people here and I am curious regarding an honest comparison between the 779 and the A351.

Thanks
 
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LAX772LR
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Mon Jan 15, 2018 7:08 am

Since only one of them is an actual plane, that's not a question that can effectively at this point be answered....

When the 779 is (1) built and (2) is in testing, then we'll know. Until then, it's talking/selling points, computer simulations, and configuration tweaks that will vary.
 
jubguy3
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Mon Jan 15, 2018 7:16 am

I'll get the needless pedantic stuff out of the way so people can answer your question... :-)

The A350-1000's code is designated as A35K.
 
jubguy3
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Mon Jan 15, 2018 7:21 am

KingOrGod wrote:
jubguy3 wrote:
I'll get the needless pedantic stuff out of the way so people can answer your question... :-)

The A350-1000's code is designated as A350K.


Yes, lets LOL.

It's actually A35K if you are going to go out of your way to correct somebody...


I noticed my error, corrected it, and then thought to myself "I wonder if somebody replied to my comment before I fixed it..." I guess I was right ahaha.

So yes, you are correct :P

Edit: comments are posting out of order
 
KingOrGod
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Mon Jan 15, 2018 7:24 am

jubguy3 wrote:
I'll get the needless pedantic stuff out of the way so people can answer your question... :-)

The A350-1000's code is designated as A350K.


Yes, lets LOL.

It's actually A35K if you are going to go out of your way to correct somebody...

EDIT: Lol no worries :)
Last edited by KingOrGod on Mon Jan 15, 2018 7:27 am, edited 1 time in total.
 
parapente
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Mon Jan 15, 2018 7:52 am

One is a paper plane at the moment so it's not a question that can be answered.
However what is known and stated by the thread starter is that one aircraft is markedly bigger than the other (40 plus seats).
So it's likely to be more of a question about optimum size for the particular customer in question.
I believe that the 2 A350 sizes pretty much match the 772 and 773.The 779 is more 747 size I believe.
 
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seahawk
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Mon Jan 15, 2018 8:23 am

Impossible to answer.We know that the A350 is better than what was guaranteed in the contracts, we have no idea how the 777-9 will perform, nor if RR manages to improve the Trent XWB until the 777-9 is delivered to airlines.
 
Casablanca
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Mon Jan 15, 2018 8:56 am

On the internet the 350 seems like a dream airplane......in reality talking to station managers/ and staff travelers being left behind when there are close to 100 seats available I don't get the impression it is a 777 killer as it seems to be quite maxed out if flight time more than 12 hours. I would bet a beer the 779 may win this one. I personally I don't really care just surprised as I see a disconnect between what I keep reading and what Im always hearing on line. Time will tell: but Boeing and Airbus never will!
 
flipdewaf
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Mon Jan 15, 2018 9:13 am

First off I think the issue is what does one mean by efficiency? There are many different metrics that one can use and it will make a huge difference.

Second there is route choice including payload requirements including passenger expectations and how that might have a bearing.

A Eurofighter typhoon is more efficient then either but you already knew that.

The issue isn't that there is so much bias but that there is no standard to compare the two on an equal footing, there isn't for basically any aircraft.

Fred


Sent from my iPhone using Tapatalk
 
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keesje
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Mon Jan 15, 2018 10:02 am

the fact the 779 is a bigger plane and seats roughly 40 more pax.

That sounds about right for a 9 abreast A35k and 10 abreast 779.

The GEX9 is supposed to be 5-8% more efficient than the RR Trent engines that will power the 351.

5-8% sounds very high, the A35K's engine hasn't enetered service. I wouldn't assume 5-8%.

Image

the larger wing of the 779 is supposed to be roughly 12% more efficient than the A351 wing on missions of 4000nm or more.

Equally 12% is a huge number. I would hesitate believing that. A bigger wing has more drag too & Airbus doesn't have a reputation for bad wings.

Image

One analysis I saw predicted the 779 will have at least a 5% lower CASM than the A351 on missions of 4000nm or more and will be virtually identical in terms of fuel burn.

Hmm. I can't see the numbers behind that, to be frankly. Who's analyses is that?

Does this sound realistic? If airlines can fill the additional seats on the 779 is it's better choice? Or is the A351 truly as efficient in terms of CASM as the 779 even though it seats about 9% fewer pax?


It doesn't sound very realistic to me me. If the 777-9 is really build 30t + (?!) heavier than the A350-1000 it would be a challenge to compensate that with higher capacity. https://static.seekingalpha.com/uploads/2014/12/9932311_14186689180693_0.png
 
kurtverbose
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Mon Jan 15, 2018 10:18 am

keesje wrote:
Hmm. I can't see the numbers behind that, to be frankly. Who's analyses is that?


He has a history of quoting stuff without a single source.

It's an interesting topic, but pointless arguing about made up numbers.

For the record, it's likely that the engine and wing will be better on the 777. The engine is quite a few years newer and so is the wing. The wing on the A35k is also a lower aspect ratio than the A359 so is a little compromised. But as you point out - the 777 has 30 tonnes extra weight, which is a lot for 40 extra seats.

My view is for most missions it will be a wash, but airlines will prefer, as always, a smaller model apart from niche routes. And if the 777 does become a sales success I can imagine a response from Airbus that will be difficult for Boeing to respond to.
 
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keesje
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Mon Jan 15, 2018 10:29 am

Aspect Ratio's is important, but there are reasons aircraft have AR lowering sweep. Drag, fuel volume, load distribution come in.
 
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scbriml
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Mon Jan 15, 2018 10:45 am

ElroyJetson wrote:
I have been researching various sources on the question which frame will be more efficient, the A351 or the 779?


Aside from the fact one hasn't even flown yet, providing the sources of your research would be helpful.

Otherwise, everything you posted is pure conjecture:

ElroyJetson wrote:
Most analysis I have seen think...

ElroyJetson wrote:
The GEX9 is supposed to be...

ElroyJetson wrote:
the larger wing of the 779 is supposed to be...

ElroyJetson wrote:
One analysis I saw predicted...
 
ap305
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Mon Jan 15, 2018 11:23 am

Casablanca wrote:
On the internet the 350 seems like a dream airplane......in reality talking to station managers/ and staff travelers being left behind when there are close to 100 seats available I don't get the impression it is a 777 killer as it seems to be quite maxed out if flight time more than 12 hours. I would bet a beer the 779 may win this one. I personally I don't really care just surprised as I see a disconnect between what I keep reading and what Im always hearing on line. Time will tell: but Boeing and Airbus never will!


What you hear is simply wrong... The a350 performs better than spec and there are no sectors barring the westbound sfo-sin where there has been any passenger payload hit. We have a senior a350 pilot here who has confirmed that the a350-900 lifts more payload over very long sectors than the 777w.

http://www.flightglobal.com/news/articl ... -a-438555/
"We're very happy with the A350," says Gebremariam. "It's been close to a year that we've been operating the aircraft. It's exactly up to the specification, if not a little bit more."


http://www.airbus.com/newsroom/press-re ... -xwb-.html

“China Airlines is happy to be receiving the 100th A350 XWB,” said Nuan-shuan Ho, Chairman of China Airlines. “This remarkable new aircraft has not only met, but exceeded our expectations at every level.
Last edited by ap305 on Mon Jan 15, 2018 11:28 am, edited 1 time in total.
 
kurtverbose
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Mon Jan 15, 2018 11:25 am

scbriml wrote:
Aside from the fact one hasn't even flown yet, providing the sources of your research would be helpful.

Otherwise, everything you posted is pure conjecture:


Not all conjecture, there's also the analysis he saw. No idea who wrote it though as there's no source.

ElroyJetson wrote:
One analysis I saw predicted the 779 will have at least a 5% lower CASM than the A351 on missions of 4000nm or more and will be virtually identical in terms of fuel burn.


Anyway, another analysis I saw (or made up) says the reverse.
 
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ElroyJetson
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Mon Jan 15, 2018 1:31 pm

kurtverbose wrote:
keesje wrote:
Hmm. I can't see the numbers behind that, to be frankly. Who's analyses is that?




Sorry ladies....the only source I could find is seeking alpha which is questionable I know. That is why I said an honest discussion because there does not seem to be a lot of information out there. I did not post the link because I did not want to hear the usual 10 or so ladies bitch about the source.

I am looking for information or ideas other than seeking alpha. But since some like to complain no matter what you do here's the link.


https://seekingalpha.com/article/276143 ... 350-part-1


***Edit*** To avoid the usual off topic crying I would like the discussion focused on facts or opinions regarding the two aircraft in question. I hope this is clear. Thanks. :)
Last edited by ElroyJetson on Mon Jan 15, 2018 1:43 pm, edited 1 time in total.
 
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zeke
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Mon Jan 15, 2018 1:41 pm

This is my take on this discussion

Block fuel over a given city pair, the A350-1000 will be ahead

CASK/CASM, both airframes can sit 440 passengers, whichever airframe burns less fuel over that route would generally win, as fuel is a major fraction of the costs. If you look at anything except maximum then it becomes subjective comparison based on perceived comfort.

Engines, the improvements like “5%” are normally based upon improvements in TFSC, they are not saying the engine will burn 5% less than the TXWB. The 779 is heavier, and needs more thrust in cruise. It is the thrust by the TSFC that gives the fuel flow. The 779 burn more fuel per hour than the A350-1000.

Weight : this is a majot area of work. The A350-1000 is lighter than the 77W, and the 779 is heavier. You can almost load the A350-1000 up to a full load of passengers and you would be at the 779 empty weight.

Drag, the wing on the 779 has been extended which should give a good reduction in the induced drag, they are claiming around 12%, but this is 12% of the induced drag which is normally around 30% for an airliner (12% of 30%), they have also been very quiet on where that 12% is based upon, I think it is based on the 77W. I would expect the A350-1000 to have a better wing than the 77W by a fair margin due to the new design that incorporates an adaptive wing section. The major component of drag in cruise is skin friction, they have been very quiet on how the extra wing area, extra engine nacelle area, and extra fuselage area is going to increase skin friction drag.

Revenue : The 779 is ahead in revunue potential. More floor area which means more seats, of the same number of seats with higher premium cabin. the 779 can also uplift more total payload, up to a a certian range.
 
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ElroyJetson
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Mon Jan 15, 2018 1:57 pm

zeke wrote:
This is my take on this discussion

Block fuel over a given city pair, the A350-1000 will be ahead

CASK/CASM, both airframes can sit 440 passengers, whichever airframe burns less fuel over that route would generally win, as fuel is a major fraction of the costs. If you look at anything except maximum then it becomes subjective comparison based on perceived comfort.

Engines, the improvements like “5%” are normally based upon improvements in TFSC, they are not saying the engine will burn 5% less than the TXWB. The 779 is heavier, and needs more thrust in cruise. It is the thrust by the TSFC that gives the fuel flow. The 779 burn more fuel per hour than the A350-1000.

Weight : this is a majot area of work. The A350-1000 is lighter than the 77W, and the 779 is heavier. You can almost load the A350-1000 up to a full load of passengers and you would be at the 779 empty weight.

Drag, the wing on the 779 has been extended which should give a good reduction in the induced drag, they are claiming around 12%, but this is 12% of the induced drag which is normally around 30% for an airliner (12% of 30%), they have also been very quiet on where that 12% is based upon, I think it is based on the 77W. I would expect the A350-1000 to have a better wing than the 77W by a fair margin due to the new design that incorporates an adaptive wing section. The major component of drag in cruise is skin friction, they have been very quiet on how the extra wing area, extra engine nacelle area, and extra fuselage area is going to increase skin friction drag.

Revenue : The 779 is ahead in revunue potential. More floor area which means more seats, of the same number of seats with higher premium cabin. the 779 can also uplift more total payload, up to a a certian range.



Thank you. This seems very fair and is exactly what moves the topic along. The 12% wing improvement is versus the 77W not the A351 and a reduction in induced drag is the primary reason cited. The new GEX9 is supposed to be 6-8% more efficient than the existing 77W engine. How any of this compares to the A351 I have no idea.

The claims in the seeking alpha article seem far-fetched because as you say, the A351 is a much lighter frame. However, from what I've seen in a standard configuration the A351 seats 366 pax and the 779 will seat 400. Airbus claims Boeing made the 779 larger to make the CASM numbers look better which I am sure is true. Is the difference in pax capacity and revenue potential of the 779 enough to be competitive with a smaller and lighter A351? That I do not know. :)
 
flipdewaf
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Mon Jan 15, 2018 2:14 pm

A351 779X
OWE 155 181
MTOW 308 351
Pax 366 414
Range 7950 7600
Pax weight36.6 41.4
MTOW-Pax 271.4 309.6
Fuel 116.4 128.6
Fuel per
pax 0.318 0.311
Fuel per
pax per
mile @max
range 4.00E-05 4.09E-05

Weights in tonnes and distance in nm.

if my numbers are wrong let me know which I should change and excel will be unbiased with the maths.

Fred

PS. sorry for the table being badly formatted, I really tried to get it right this time with a fixed width text editor and it didn't help.
 
ap305
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Mon Jan 15, 2018 2:46 pm

One factor not to be forgotten is that by the time the 777-9 enters service the 316t a351 will likely be available. This will cut into any medium range payload advantage the 777x has. It will also extend the ultra long range performance of the a351.
 
frmrCapCadet
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Mon Jan 15, 2018 3:05 pm

Almost invariably the better question is where each model will excel - which ranges and payloads. the -9 will excel at the big end, the 351 at the lower end. The relative success of each plane will be determined by the crossover points which will define the niche of each plane. At certain loads and ranges it will be a wash.
 
kurtverbose
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Mon Jan 15, 2018 3:08 pm

zeke wrote:
The major component of drag in cruise is skin friction, they have been very quiet on how the extra wing area, extra engine nacelle area, and extra fuselage area is going to increase skin friction drag.


Won't that be partly offset by the higher altitudes the 777X will be capable of?
 
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ElroyJetson
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Mon Jan 15, 2018 5:02 pm

flipdewaf wrote:
A351 779X
OWE 155 181
MTOW 308 351
Pax 366 414
Range 7950 7600
Pax weight36.6 41.4
MTOW-Pax 271.4 309.6
Fuel 116.4 128.6
Fuel per
pax 0.318 0.311
Fuel per
pax per
mile @max
range 4.00E-05 4.09E-05

Weights in tonnes and distance in nm.

if my numbers are wrong let me know which I should change and excel will be unbiased with the maths.

Fred

PS. sorry for the table being badly formatted, I really tried to get it right this time with a fixed width text editor and it didn't help.



This is excellent thanks. Based on your numbers it looks like (by my unbiased math :)) that the A351 will have about a 2% better fuel burn per passenger than the 779. That makes sense to me and throws water on the seeking alpha article.

So it will boil down to the basic aeronautics of each plane (i.e. propulsive efficiency, laminar flow, induced drag, etc) along with any potential revenue advantage the larger 779 will have.

If your numbers hold true then this is my assessment. If you are an airline that thinks you can consistently fill 400 + seats (i.e. Emirates) and will fly most missions 4000nm or more the revenue potential of the 779 should win out. If 366 seats is your sweet spot and you are looking for slightly better fuel burn go with the A351.

This dynamic seems to mirror the 789 versus the A359. The 789 is lighter and has better fuel burn, but the A359 has more revenue potential if you can consistently fill the seats. Like the A359 vs the 789, both the A351 and the 779 look pretty evenly matched and will it will ultimately depend on the mission requirements of each airline.
 
OldAeroGuy
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Mon Jan 15, 2018 5:17 pm

ElroyJetson wrote:
The new GEX9 is supposed to be 6-8% more efficient than the existing 77W engine. How any of this compares to the A351 I have no idea.


The GE9X is planned to be 10% more efficient than the GE90, according to GE, who manufactures both engines.

https://www.geaviation.com/commercial/e ... aft-engine

From the above source:

"A blend of evolutionary and revolutionary designs will enable the GE9X to be the most fuel-efficient jet engine GE has ever produced on a per-pounds-of-thrust basis. It’s designed to deliver a 10% improved aircraft fuel burn versus the GE90-115B-powered 777-300ER"
 
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ElroyJetson
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Mon Jan 15, 2018 5:47 pm

[photoid][/photoid]
OldAeroGuy wrote:
ElroyJetson wrote:
The new GEX9 is supposed to be 6-8% more efficient than the existing 77W engine. How any of this compares to the A351 I have no idea.


The GE9X is planned to be 10% more efficient than the GE90, according to GE, who manufactures both engines.

https://www.geaviation.com/commercial/e ... aft-engine

From the above source:

"A blend of evolutionary and revolutionary designs will enable the GE9X to be the most fuel-efficient jet engine GE has ever produced on a per-pounds-of-thrust basis. It’s designed to deliver a 10% improved aircraft fuel burn versus the GE90-115B-powered 777-300ER"



Thanks. The saving alpha article claimed a 6-8% advantage for the GE9X versus the 77W GE90-115B. If the true number is 10% that is impressive.


Image
 
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zeke
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Mon Jan 15, 2018 5:50 pm

OldAeroGuy wrote:
"A blend of evolutionary and revolutionary designs will enable the GE9X to be the most fuel-efficient jet engine GE has ever produced on a per-pounds-of-thrust basis. It’s designed to deliver a 10% improved aircraft fuel burn versus the GE90-115B-powered 777-300ER"


That says the GE9X engine fuel burn is 0.82 times the fuel burn of the GE90, do you actually believe that ?

(105000/115000)-0.1*(105000/115000)
 
OldAeroGuy
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Mon Jan 15, 2018 6:28 pm

zeke wrote:
OldAeroGuy wrote:
"A blend of evolutionary and revolutionary designs will enable the GE9X to be the most fuel-efficient jet engine GE has ever produced on a per-pounds-of-thrust basis. It’s designed to deliver a 10% improved aircraft fuel burn versus the GE90-115B-powered 777-300ER"


That says the GE9X engine fuel burn is 0.82 times the fuel burn of the GE90, do you actually believe that ?

(105000/115000)-0.1*(105000/115000)


No, I believe exactly what the statement says. At the same cruise thrust level, TSFC (Thrust Specific Fuel Consumption) for the GE9X will 90% that of the GE90.

Takeoff thrust has nothing to do with cruise TSFC levels.
 
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Matt6461
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Mon Jan 15, 2018 7:46 pm

There are two published, sophisticated independent analyses on this topic.

One of them, by AirwaysNews, has since been withdrawn - probably because it argued that the A380NEO was a great idea: http://airwaysnews.com/blog/2015/01/21/ ... s-for-neo/.

The others, by Leeham, are behind a paywall: http://leehamnews.com/2014/12/17/a380-a ... titivness/
They ran a couple comparisons besides the A380NEO discussion; all concluded the 777-9 is more efficient.

Both analyses conclude that the 777-9 is more efficient than A35K on both fuel and CASM. IIRC the fuel delta is only 2-4% per seat, meaning the 779's is entirely due to SFC edge.

Zeke wrote:
CASK/CASM, both airframes can sit 440 passengers, whichever airframe burns less fuel over that route would generally win, as fuel is a major fraction of the costs. If you look at anything except maximum then it becomes subjective comparison based on perceived comfort.


Subjective schmubjective. The 777-9 is significantly bigger.

Zeke wrote:
That says the GE9X engine fuel burn is 0.82 times the fuel burn of the GE90, do you actually believe that ?

(105000/115000)-0.1*(105000/115000)


You do realize that engines don't run at max thrust throughout a flight, right? Of course you do. So why even make this argument?
 
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ElroyJetson
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Mon Jan 15, 2018 9:10 pm

Seeking alpha claimed the 779 is more efficient than the 35K and said engine SFC was a major factor. They also claimed a number of aerodynamic elements were more efficient. Both Airbus and Boeing tend to slightly underestimate fuel burn and aerodynamic factors before launch in order to give themselves a little wiggle room. Once a full accounting of real world data comes in on both frames it will be interesting to see how each actually compares to the other.
 
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Matt6461
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Mon Jan 15, 2018 9:52 pm

ElroyJetson wrote:
Seeking alpha claimed the 779 is more efficient than the 35K and said engine SFC was a major factor. They also claimed a number of aerodynamic elements were more efficient. Both Airbus and Boeing tend to slightly underestimate fuel burn and aerodynamic factors before launch in order to give themselves a little wiggle room. Once a full accounting of real world data comes in on both frames it will be interesting to see how each actually compares to the other.


There's really no debate here outside of Airbus marketing material (and of course a.net Airbus fanboys). Even the post by Flipdewaf shows a lower fuel burn for 777-9; you misread the figures in concluding the A35K is more fuel-efficient.
 
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ElroyJetson
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Mon Jan 15, 2018 11:02 pm

Matt6461 wrote:
ElroyJetson wrote:
Seeking alpha claimed the 779 is more efficient than the 35K and said engine SFC was a major factor. They also claimed a number of aerodynamic elements were more efficient. Both Airbus and Boeing tend to slightly underestimate fuel burn and aerodynamic factors before launch in order to give themselves a little wiggle room. Once a full accounting of real world data comes in on both frames it will be interesting to see how each actually compares to the other.


There's really no debate here outside of Airbus marketing material (and of course a.net Airbus fanboys). Even the post by Flipdewaf shows a lower fuel burn for 777-9; you misread the figures in concluding the A35K is more fuel-efficient.



So I did. My mistake. His figures show a .318 fuel burn per pax for the A35K versus .311 for the 779. So maybe seeking alpha isn't so incompetent after all? Still the difference is only 2% per seat. But this might explain why Airbus only talks about the OWE difference and not real figures regarding true efficiency.
 
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keesje
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Tue Jan 16, 2018 12:33 am

10 abreast 777-300ER operators like EK (354 seats), United (366), AF (381) don’t get close to the “typical” 777-9 414 seats used here.

If we look the “odd” 9 abreast operators BA (309) JAL(246), SQ(264) they hardly go above 300.

It shows the “typical” seatcount of a two rows bigger -9 at 414 is “ambitious” at best, deceptively chosen more likely.

Claiming it is “2% better per seat” before wings are installed based on it, is IMO no more than candy for hungry believers.

I see it often these days; big, unsupported assumptions early on and 3.56% difference based decisions 13 slides later. Most decisionmakers by then being overwelmed by numbers and cool graphs. If the outcome isn’t ok, slightly tune the assumptions in slide 2. “Love” it..
 
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zeke
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Tue Jan 16, 2018 2:10 am

OldAeroGuy wrote:
No, I believe exactly what the statement says. At the same cruise thrust level, TSFC (Thrust Specific Fuel Consumption) for the GE9X will 90% that of the GE90.

Takeoff thrust has nothing to do with cruise TSFC levels.


I am sorry where is that written in that comment, specifically “cruise thrust level” ? They have also made a a similar statement that the engine is the quietest ever built on a “per-pounds-of-thrust basis”.

Takeoff and cruise TSFC are related, they come from the same engine under differ thrust demands and different atmospheric conditions, still the same engine. That is like saying the fuel economy of the same car engine in Mexico and Miami are not related.

The only thrust level that is fixed and published is takeoff thrust, why make the disclaimer “per-pounds-of-thrust basis” for all claims if it is not something that is known to all ?

The other factor is GE make engines, not aircraft. Your statements seem to suggest the 779 will burn 10% less fuel than the 77W, GE has made no comment if the 779 will need the same, more, or less thrust in cruise compared to the 77W.

In my view is a clickbait comment, like their previous claim that the GE9X was lighter than the GE90, only later to say that it is actually heavier, the fan is lighter but the case to contain it is heavier. They are still around 11 months away from having it certified.
 
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Matt6461
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Tue Jan 16, 2018 3:10 am

ElroyJetson wrote:
So maybe seeking alpha isn't so incompetent after all?


They're basically incompetent. Total clickbait; they don't demonstrate even a basic understanding of aerodynamics.

Zeke wrote:
Your statements seem to suggest the 779 will burn 10% less fuel than the 77W, GE has made no comment if the 779 will need the same, more, or less thrust in cruise compared to the 77W.


GE says 10% lower SFC, Boeing claims ~10-15% lower trip fuel burn, depending on the seating comparison.
Given that thrust=drag in cruise, and that cruise is ~90-95% of fuel burn for the >4000nm stage-lengths compared, you should easily see that the OEM's are saying the 777-9 will cruise at a slightly lower average thrust output than 77W, ceteris paribus.

Keesje wrote:
I see it often these days; big, unsupported assumptions early on and 3.56% difference based decisions 13 slides later. Most decisionmakers by then being overwelmed by numbers and cool graphs. If the outcome isn’t ok, slightly tune the assumptions in slide 2. “Love” it..


...which is why we should be comparing cabin lengths, widths, areas. Getting to hard numbers as much as possible. We definitely shouldn't be looking at nominal airline seatcounts, as the variations in these samples are enormous.
 
OldAeroGuy
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Tue Jan 16, 2018 3:11 am

Zeke, your comments surprise me.

In the past, you've used TSFC (Thrust Specific Fuel Consumption) and have been quite precise about its definition. What GE states is consistent with your use of TSFC.

Please quote where I've stated in this thread that the 779 will burn 10% less fuel than the 77W. My comments were related to the GE9X vs the GE90.
 
Newbiepilot
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Tue Jan 16, 2018 3:38 am

keesje wrote:
10 abreast 777-300ER operators like EK (354 seats), United (366), AF (381) don’t get close to the “typical” 777-9 414 seats used here.

If we look the “odd” 9 abreast operators BA (309) JAL(246), SQ(264) they hardly go above 300.

It shows the “typical” seatcount of a two rows bigger -9 at 414 is “ambitious” at best, deceptively chosen more likely.

Claiming it is “2% better per seat” before wings are installed based on it, is IMO no more than candy for hungry believers.

I see it often these days; big, unsupported assumptions early on and 3.56% difference based decisions 13 slides later. Most decisionmakers by then being overwelmed by numbers and cool graphs. If the outcome isn’t ok, slightly tune the assumptions in slide 2. “Love” it..


The 777-9 is over 9ft longer than the 777-300. It also removed a pair of exits. Assuming the extra space goes with economy and some of the newer generation smaller lavatories and galleys are used (compare to Airbus Airspace/Spaceflex), that can result in 4-5 rows of seats. 50 more seats than a 77W is feasible.
 
OldAeroGuy
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Tue Jan 16, 2018 3:44 am

keesje wrote:
10 abreast 777-300ER operators like EK (354 seats), United (366), AF (381) don’t get close to the “typical” 777-9 414 seats used here.

If we look the “odd” 9 abreast operators BA (309) JAL(246), SQ(264) they hardly go above 300.

It shows the “typical” seatcount of a two rows bigger -9 at 414 is “ambitious” at best, deceptively chosen more likely.


Of course there is also the EK 427 seat 77W in a two class layout.

Is a 366 seat A3510 in a two or three class arrangement?
 
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zeke
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Tue Jan 16, 2018 5:22 am

OldAeroGuy wrote:
Zeke, your comments surprise me.

In the past, you've used TSFC (Thrust Specific Fuel Consumption) and have been quite precise about its definition. What GE states is consistent with your use of TSFC.

Please quote where I've stated in this thread that the 779 will burn 10% less fuel than the 77W. My comments were related to the GE9X vs the GE90.


Where does GE state TSFC or cruise thrust ?

This thread is suppose to be a HONEST DISCUSSION, not a discussion hidden behind marketing double speak.

So where does the TXWB come in ?

From AW&ST

“In fact, GEnx will achieve a 15.4% cut in SFC over the CF6-80C2B6F, the powerplant of the 767-300ER, Brisken says. GEnx also should have a 6.9% SFC advantage over the GE90-94B, which powers the 777-200ER. That comparison is interesting because Boeing says a planned second stretch of the 787--to be called the 787-10 and likely launched this year--will have a seating overlap with the 777-200ER. In most observers' view, that makes the first-generation 777 models obsolete to the newer technology 787s. “

However the “Trent XWB-84 has ~2.4% better cruise efficiency than GEnx-1B75/P2” https://seekingalpha.com/article/324298 ... difference

That would make the Trent XWB-84 in the circa of 10% better than the GE90-94B, how that translates to the Trent XWB-97 has not been said. A 10% better TSFC improvement over the GE90-115 does not seem that impressive in the context of other relevant developments.

If two engines have similar TSFC, the engine with the bigger engine fan size creates more drag and weight, which in turn requires more thrust and carries a fuel penalty in the end.
 
flipdewaf
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Tue Jan 16, 2018 7:13 am

ElroyJetson wrote:
Matt6461 wrote:
ElroyJetson wrote:
Seeking alpha claimed the 779 is more efficient than the 35K and said engine SFC was a major factor. They also claimed a number of aerodynamic elements were more efficient. Both Airbus and Boeing tend to slightly underestimate fuel burn and aerodynamic factors before launch in order to give themselves a little wiggle room. Once a full accounting of real world data comes in on both frames it will be interesting to see how each actually compares to the other.


There's really no debate here outside of Airbus marketing material (and of course a.net Airbus fanboys). Even the post by Flipdewaf shows a lower fuel burn for 777-9; you misread the figures in concluding the A35K is more fuel-efficient.



So I did. My mistake. His figures show a .318 fuel burn per pax for the A35K versus .311 for the 779. So maybe seeking alpha isn't so incompetent after all? Still the difference is only 2% per seat. But this might explain why Airbus only talks about the OWE difference and not real figures regarding true efficiency.

Indeed, the trip fuel per pax for the 779 is ~2% lower than the A351 but then the A351 trip is 4.6% further...

Fred


Sent from my iPhone using Tapatalk
 
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keesje
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Tue Jan 16, 2018 1:08 pm

Always those efficiency numbers turn out to be per seat. Setting the seat numbers is the unholy partgame of marketing people.

But less knowledgeable blindly quote them, if they like the outcomes.

Sending the marketing folks Rolling On Laughing Floor
 
Newbiepilot
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Tue Jan 16, 2018 2:37 pm

keesje wrote:
Always those efficiency numbers turn out to be per seat. Setting the seat numbers is the unholy partgame of marketing people.

But less knowledgeable blindly quote them, if they like the outcomes.

Sending the marketing folks Rolling On Laughing Floor


Adjusting seat counts to make CASM numbers look more appealing is a trick both manufacturers have been using for decades. Comparing seat counts and seat density between Airbus and Boeing marketing figures is not particularly useful. Going from 777 to 777x or A330 to A330neo is also a challenge since the manufacturers want to include space saving lavatories, galleys etc in their seat counts for the new model despite the fact that the smaller lavs are available on the older airplane models as well including retrofits.
 
OldAeroGuy
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Tue Jan 16, 2018 4:28 pm

zeke wrote:
This thread is suppose to be a HONEST DISCUSSION, not a discussion hidden behind marketing double speak.

So where does the TXWB come in ?

From AW&ST

However the “Trent XWB-84 has ~2.4% better cruise efficiency than GEnx-1B7

That would make the Trent XWB-84 in the circa of 10% better than the GE90-94B, how that translates to the Trent XWB-97 has not been said. A 10% better TSFC improvement over the GE90-115 does not seem that impressive in the context of other relevant developments.

If two engines have similar TSFC, the engine with the bigger engine fan size creates more drag and weight, which in turn requires more thrust and carries a fuel penalty in the end.


As this is an honest discussion, let's bring some data to discuss.

Overall Pressure Ratio is an indicator of engine efficiency although, as you point out, increases in OPR may be offset by engine weight increases.

Engine: Overall Pressure Ratio
GE CF6: 30.5
GE90: 42
Genx: 44.7
RR Trent XWB: 52
GE9: 60

These data are taken from the following references:
https://en.wikipedia.org/wiki/Overall_pressure_ratio
http://large.stanford.edu/courses/2016/ph240/ginsberg2/

The second reference points out that the GEnx fuel burn is 15% better than the CF6 for an OPR increase of 44.7 vs 30.5.

Given the GE9 OPR of 60 vs the GE90 of 42, a 10% fuel burn improvement seems achievable. This would explain why GE has devoted 5-7 years and several billions of dollars to GE9 development. The OPR improvement is hardly "marketing double speak".

With an OPR of 52, the Trent XWB fuel burn level will lie between the GE90 to GE9 increment.
 
Armadillo1
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Tue Jan 16, 2018 6:04 pm

dont stop people from inventing perpetum mobile
 
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zeke
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Tue Jan 16, 2018 7:03 pm

OldAeroGuy wrote:
Given the GE9 OPR of 60 vs the GE90 of 42, a 10% fuel burn improvement seems achievable. This would explain why GE has devoted 5-7 years and several billions of dollars to GE9 development. The OPR improvement is hardly "marketing double speak".

With an OPR of 52, the Trent XWB fuel burn level will lie between the GE90 to GE9 increment.


Top of climb the OPR for the GEnx-1B is just iunder 60, the TXWB over 60. The numbers you were quoting were at sea level takeoff.

The TXWB-97 is 1200 kg lighter than a GE90-115 (per TCDS) , it doesn’t have the same drag either.
 
kurtverbose
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Tue Jan 16, 2018 7:51 pm

Don't OPR efficiency gains diminish the higher you go?
 
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keesje
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Tue Jan 16, 2018 7:52 pm

If the sfc of the GE9 and TXWb are about similar and I would assume the same for the wing (no miracles), capacity would be the main difference between the two aircraft. Specially if the A350s are specified 9 abreast and the 777s 10 abreast in economy. I would be interested to see how the 30t OWE difference comes back in total cost.
 
OldAeroGuy
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Tue Jan 16, 2018 10:01 pm

zeke wrote:
OldAeroGuy wrote:
Given the GE9 OPR of 60 vs the GE90 of 42, a 10% fuel burn improvement seems achievable. This would explain why GE has devoted 5-7 years and several billions of dollars to GE9 development. The OPR improvement is hardly "marketing double speak".

With an OPR of 52, the Trent XWB fuel burn level will lie between the GE90 to GE9 increment.


Top of climb the OPR for the GEnx-1B is just iunder 60, the TXWB over 60. The numbers you were quoting were at sea level takeoff.


Just a few replies ago, you were arguing that takeoff TSFC was indicative of cruise TSFC. Are you now arguing that the TXWB will have an OPR improvement over takeoff and the GE9 won't?
 
OldAeroGuy
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Tue Jan 16, 2018 10:04 pm

keesje wrote:
If the sfc of the GE9 and TXWb are about similar and I would assume the same for the wing (no miracles), capacity would be the main difference between the two aircraft. Specially if the A350s are specified 9 abreast and the 777s 10 abreast in economy. I would be interested to see how the 30t OWE difference comes back in total cost.


I don't think that it's proven that the GE9 and the TXWB are about similar.
 
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zeke
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Wed Jan 17, 2018 2:57 am

OldAeroGuy wrote:
Just a few replies ago, you were arguing that takeoff TSFC was indicative of cruise TSFC. Are you now arguing that the TXWB will have an OPR improvement over takeoff and the GE9 won't?


No argument at all, I stated fact in your quote from GE the basis was “per-pounds-of-thrust basis‘, and the only thrust level they have advertised is takeoff thrust. GE made no reference to TSFC or cruise thrust like you did. Additionally you previously said the 10% was the improvement in cruise, and later you said the 10% improvement was for takeoff, the statements are contradictory. I can see the GE9X having lower fuel flow on takeoff as the induced drag and additional wing area will help takeoff performance.

OldAeroGuy wrote:
I don't think that it's proven that the GE9 and the TXWB are about similar.


Of course not, only one of them is certified and in production. Same with the 779, as everyone knows the first one is yet to be built, let alone flown.
 
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lightsaber
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Re: Honest Discussion. Which Plane is More Efficient.....The A351 or 779?

Wed Jan 17, 2018 5:16 am

Please recall the GE9X will have variable cycle technology. There will be reduced turbine cooling at cruise, already in the LEAP engines.
It is extensively using fuel cooling to improve fuel burn (instead of air cooling).
The first CMC turbine blades.
I'm hearing rumors the GE9X will have one more turbine clearance control valve.

The TXWB and GE9X are two very different technology engines.

Both airframe's will do well. It will depend on demand, cargo, and payload at range requirements. In general, it is easier to sell smaller airframe's as long as the cost per flight is less.

There is an incredible amount of weight in the 779 to improve efficiency at range (wing, engines). This biases the optimal mission to a longer mission.

Both will sell. Everything I'm seeing is GE has an incredible engine design. This isn't to say the TXWB is bad, but it is a much more conservative design. I'd bet the GE will have a higher RPM high spool too (which is more efficient).

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