flipdewaf
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Did/Does the 779X need a new wing? and does it need to be so big?

Wed Jul 10, 2019 2:38 pm

A simple question I suppose but one that probably has more to do with specific customer requirements than we will or can ever know.

From a relative lay-persons perspective I can see that the extremely successful 777-300ER program delivers an aircraft which on the face of it has significant compromises in terms of geometry and wing sizing giving a less than ideal initial cruise altitude and often limiting runway performance but yet airlines wanted to buy...

Without muddying the waters too much with the complexity so oft used for obfuscation purposes around these parts the takeoff distance requirements are driven by two major factors.
1.Wing loading (W/S) Lift required per unit area of wing which effectively drives how fast you have to go before you can take off
2.Thrust to weight ratio (T/W) The amount of thrust per unit of weight of mass of the aircraft which drives how fast you get to the required speeds.
With this in mind an increase in weight has a square law effect on the takeoff distance and this can be counteracted by an increase in wing area and/or thrust.
2 drivers but 3 variables. We see this scenario play out time an time again, we see an MTOW increase and a corresponding thrust increase as planform is normally hard to change.

For attainable altitudes then the drivers are based on:
1. Excess thrust available
2. Excess lift available to the buffet margin.

Excess thrust availability comes from either increased thrust or reduced drag.
Excess lift availability is a more complex situation with the buffet margin being determined by the compressability affects over the wings. In essence the higher the mach the greater the compressability affects but also the higher the lift coefficient is the higher the effects of the buffet.
In order to have excess lift available one can do several things.
1. reduce the lift required (make it lighter, this effect can be seen over the course of a flight where the step climbs occur.)
2. Increase the lift available
a.Make the wing bigger
b.Make the wing thinner
c.Design the wing to handle transonic flow better (this isn't low hanging fruit)

When we look at the above observations we can see that for an aircraft like the 77W (or indeed the A321) there is a mismatch between what the aircraft has to do to be able to get off the ground vs what it is then capable of when it gets in the air, i.e. the excess thrust required to be able to get the thing in the air is wasted when it gets up to cruise levels as the wing is more limiting on its ability to reach altitude than the engine (why we talk about wing limited vs engine limited altitude) in an ideal world the engine and wing would be matched.

With the upgrade of the 777 offering from the 77W/L going to the 778/9 X then the approach Boeing has taken has been to:
1. Update/upgrade the engine offering
2. Lengthen the fuselage (contour the walls as well)
3. Update the wing including a major change to its overall dimensions.

Whilst I agree that the engine upgrade was a no-brainer the part I have been thinking a lot about recently is the overall impact of the new wing and what it brings.
In very simple terms we can look at it like this.
77W
OWE = 168t
MTOW = 351.5t
Spec pax = 365
Spec range = 7370nm
OWE + pax = 168 + 36.5 = 204.5
Leaving 147t for the fuel (used or not)

779X
OWE = 184t
MTOW = 351.5t
Spec pax = 410
Spec range = 7525
OWE + Pax = 184+41 = 225t
Leaving 126t for fuel

Whilst I wouldn't argue that the 779X is a significant step forward in this regard what part of that step is due to the engine change and what is due to the wings.
Again, a very simple analysis of the problem would be to calculate the maximum range one can simply use the improvement in SFC as a crude guide to increased range so if we simply updated the SFC the spec range would become ~8100nm for a 10% improvement in SFC.

So the issue comes down to effectively a tradeoff: Does the reduced induced drag from a lower span wise loading and the allowable gain in altitude outweigh the increase in induced drag due to increased weight and increase in drag due to larger surface area.

So to give a bit more meat to the the issue and potential discussion I ran a bit of a model with my homemade excel warrior software on a combination of factors. there are essentially 3 varients in question the standard 77W with GE90-115b, the 779X with GE9x and a 77W with GE9X engines.

I chose what I thought was a pretty representative route for this bird SIN-LHR with a 20kt headwind and 50t payload.
Image
There are certain limitations to the model but it indicates that there is very little between the 77W with GE9x and the 779X in terms of total fuel use. Of course tthere are differences in the air frames beyond wings and engines (particularly in the fuselage length) but the "closeness" is of interest.

For reference the GE9X is shown as adding an extra 3t per aircraft and has a slightly higher diameter for aero purposes. The reference SFC that Ihave used is shown.

Because the data was so close I looked a bit more closely at the raw data my model spits out rather than just the summary and the first point of call is the cruise altitude.
Image
Here we can see difference that the wing is providing on the 779X with it being able to consistently be 2-4000ft higher than the 77W. The difference the engine makes here is more subtle but because of the lower fuel usage the initial weights are lower allowing higher initial levels that converge toward the end of the flight.

If we compare weights over the course of the flight we can see how this changes.
Image
The stark difference the engine makes to the slope of the graph made by the engines is easy to see. The Range scale represents distance left to run and can be used as a relatively good proxy for Required takeoff weight to travel that range. The thicker line shows where the data was actually generated for this flight with the thinner line being and extrapolation of that data. I added a line of 348000kgs as the data showed about 3.5t of fuel to initial climb so somewhat shows effective MTOW range at this payload.

The slope in the previous graph is effectively showing fuel flow per unit of distance, as we are mostly used to seeing this in terms of fuel per time I have shown this below.
Image
The data itself looks a bit "choppy" but that is showing the cruise phases at each step. There is data available for the intermediate climbs as well as for the initial climb but these are so spiky at the times of climbing that they detract from the useful cruise data and cause trend lines to fall off. There is a clear difference between the engine types but much less distinction between the wings effect in terms of fuel flow required.

The last chart is showing the total fuel use for the trip.
Image
Similar to the previous charts this really shows that the engine makes the difference here with the 77W+GE9X having very close to the same fuel usage as the 779X.

The wing of the 779X allows the aircraft to effectively add the stretch to the 77W for zero fuel cost, certainly not an insignificant factor but by far the bulk of the fuel savings are coming from the engines.

The new wing does not make the aircraft any more effective at the heavy loads at long range and arguably the simple engine change would allow a higher margin for fuel capacity at ranges above MZFW.

The elephant in the room is Takeoff performance. If the fuel use is effectively identical between the 779X and 77W with GE9x then the Empty weight difference of ~13t can remain. Would the [email protected] 337T be able to cope with 105klb engines?, when does it need the full 115klb? could the GE9X produce this?

I found this quite interesting to look at if somewhat indulgent. Hopefully you will critique and discuss.

Fred
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tommy1808
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Re: Did/Does the 779X need a new wing? and does it need to be so big?

Wed Jul 10, 2019 3:09 pm

flipdewaf wrote:
Whilst I wouldn't argue that the 779X is a significant step forward in this regard what part of that step is due to the engine change and what is due to the wings.
Again, a very simple analysis of the problem would be to calculate the maximum range one can simply use the improvement in SFC as a crude guide to increased range so if we simply updated the SFC the spec range would become ~8100nm for a 10% improvement in SFC.


The A351 promissed, and apparently delivered around that, a 25% fuel burn advantage vs. the 77W. With just 10, or even 15%, better SFC the 77X would have been stillborn. Even with the new wing they stretched it to improve per seat fuel burn.

Best regards
Thomas
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Re: Did/Does the 779X need a new wing? and does it need to be so big?

Wed Jul 10, 2019 5:57 pm

Fred

You have put a lot of work into that post well done.

Yes I do think the GE9X engine could power the 77W, likewise I think the GE90-115 could power the 779 to start flight testing.

The only issue with the GE9X on a 77W I do not know about is ground clearance. Not sure if they changed the mounting to get more clearance.
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Stitch
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Re: Did/Does the 779X need a new wing? and does it need to be so big?

Wed Jul 10, 2019 7:18 pm

It has been often commented that the high wing loading of the 777-300ER requires it to spend more time at lower, less-efficient altitudes as it burns off fuel. So the lower wing-loading of the 777X should have some positive effects in and of themselves.
 
Eyad89
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Re: Did/Does the 779X need a new wing? and does it need to be so big?

Wed Jul 10, 2019 7:42 pm

Interesting. That's a great effort.

In other words, the almost identical fuel burn of 779 vs 77W+GE9X indicates that the higher L/D of 779 compensates for the 13t higher DOW.

Your numbers are in line with how engine SFC imprvements are the leading factor when it comes to fuel saving improvements that we have seen over the years.
 
LH707330
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Re: Did/Does the 779X need a new wing? and does it need to be so big?

Wed Jul 10, 2019 10:51 pm

Couple questions:

1. Are those fuel burn numbers trip burn or trip burn per pax? If it isn't normalized, then you get 410/365=1.12, so 12% more pax for free. Add that to the 10% SFC gain, and you're about in the right ballpark.
2. How are you modeling takeoff performance? If you remove 13t in DOW to get to 337t MTOW, you've got 96% of the weight and 91% of the thrust vs a 77W, for a total 5% T/W penalty. No-drag acceleration is thus 5% lower, and distance traveled to reach the same speed should go up roughly quadratically, meaning you've got a ~10% penalty before you factor in thrust lapse (worse on the higher BPR engine) and drag. the 779 gets around this by having lower runway speeds thanks to the bigger wing.
 
Armadillo1
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Re: Did/Does the 779X need a new wing? and does it need to be so big?

Thu Jul 11, 2019 1:04 am

I still wonder what exactly bad in a345/6 compared to 77w. What exactly too heavy? Wing or fuselage?

Thank for post, very interesting.
I think t/o performance can be solved in 321xlr way by different flaps.

Also, for me , 77x still looks like the only new plane being heavier than previous, or at least equal compared to capacity. Very different from 787 and 350.
 
Armadillo1
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Re: Did/Does the 779X need a new wing? and does it need to be so big?

Thu Jul 11, 2019 11:51 am

Time to return takeoff busters to airliners! (Trident 3B )
 
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Re: Did/Does the 779X need a new wing? and does it need to be so big?

Thu Jul 11, 2019 12:41 pm

Armadillo1 wrote:
I still wonder what exactly bad in a345/6 compared to 77w. What exactly too heavy? Wing or fuselage?

Thank for post, very interesting.
I think t/o performance can be solved in 321xlr way by different flaps.

Also, for me , 77x still looks like the only new plane being heavier than previous, or at least equal compared to capacity. Very different from 787 and 350.


The A345/6 were heavy for their capacity. A logical development given the A340 "classic" was already there and development costs could be kept down, but not what would have come out as a clean sheet aircraft at the time. Four engines mean twice as much engine maintenance, more oil consumption, more parts that can break.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
Armadillo1
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Re: Did/Does the 779X need a new wing? and does it need to be so big?

Thu Jul 11, 2019 1:18 pm

(sigh)
A346 was heavy. yes. thats what i wrote by myself. no need to repeat it as new info.

because its a cheap wing and fuselage stretch. yes. thank you. i wonder how i cant saw this by myself.

but my question is what part of frame exactly was heavy compared to 77w
yes, may be no one know it or cant say because of NDA.
but i hope one day i found it.
 
flipdewaf
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Re: Did/Does the 779X need a new wing? and does it need to be so big?

Thu Jul 11, 2019 1:29 pm

LH707330 wrote:
Couple questions:

1. Are those fuel burn numbers trip burn or trip burn per pax?
Trip fuel burn, its in Kilos
LH707330 wrote:
If it isn't normalized, then you get 410/365=1.12, so 12% more pax for free. Add that to the 10% SFC gain, and you're about in the right ballpark.
Fuel burn over a specific sector doesn't scale linearly with SFC on a given trip (using fuel to carry fuel) the reduction in fuel burn from the 0.55->0.49lb/hr/lb is about 12.6% giving a total fuel reduction per pax as~25% (same as A35K)
LH707330 wrote:
2. How are you modeling takeoff performance?
I'm not moedelling it in as much detail, I simply have a baseline ClMax and derive the distance to get to the speed to lift off at that weight.
LH707330 wrote:
If you remove 13t in DOW to get to 337t MTOW, you've got 96% of the weight and 91% of the thrust vs a 77W, for a total 5% T/W penalty. No-drag acceleration is thus 5% lower, and distance traveled to reach the same speed should go up roughly quadratically, meaning you've got a ~10% penalty before you factor in thrust lapse (worse on the higher BPR engine) and drag. the 779 gets around this by having lower runway speeds thanks to the bigger wing.
I don't currently do anything with thrust lapse in the calculation but its also worth pointing out that at reduced weights you don't have to be going so fast, lower weight increases acc. and decreases Vlo, a double whammy.

Eyad89 wrote:
Interesting. That's a great effort.

In other words, the almost identical fuel burn of 779 vs 77W+GE9X indicates that the higher L/D of 779 compensates for the 13t higher DOW.
Basically, yeah.

Stitch wrote:
It has been often commented that the high wing loading of the 777-300ER requires it to spend more time at lower, less-efficient altitudes as it burns off fuel. So the lower wing-loading of the 777X should have some positive effects in and of themselves.
I'm wrestling this at the moment as its very difficult to untangle the wing loading from the rest of the data, the phrase "less-efficient altitude" is a slightly loaded one and I guess its why I wanted to do this analysis, less efficient for a particular aircraft at a particular weight; Yes, in general? It's tying my brain in knots as to how to clarify and present this.

tommy1808 wrote:
The A351 promissed, and apparently delivered around that, a 25% fuel burn advantage vs. the 77W. With just 10, or even 15%, better SFC the 77X would have been stillborn. Even with the new wing they stretched it to improve per seat fuel burn.
Agreed (kind of) I guess this is an ROI thing, cheap upgrade (engine only) vs expensive upgrade 779X with added capacity.

As another example I have done what I have called a 774er with GE9X. The ~3m Stretch adding 6t to the DOW. Admittedly this will be eating in to the available Payload weight.
Image '774er' Added to the table.
Approximately 3t of fuel used for the mission.
Image
As expected the '774er' site between the other GE9X varients.
Image
Total fuel used changes as expected with the 774er using more for the longer distance (heavier and more wetted area so its to be expected).
Image
The final chart of instantaneous fuel use shows the line for the 774er to be more or less a shift of the 77W+GE9X.

The interesting part for me when looking at this last chart is the fact that the slope of the instant fuel flow chart seems to show that the 779X compared to the smaller winged but re-engined 777 variants is more susceptible to weight variations in terms of fuel flow. We would expect the larger winged Variant to be better at higher weights by a larger amount than it is a low weights. Granted that difference is right in the weeds so might not mean anything but it definitely is not pointing in the expected way.

Fred
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LH707330
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Re: Did/Does the 779X need a new wing? and does it need to be so big?

Thu Jul 11, 2019 6:49 pm

flipdewaf wrote:
LH707330 wrote:
Couple questions:

1. Are those fuel burn numbers trip burn or trip burn per pax?

Trip fuel burn, its in Kilos

LH707330 wrote:
If it isn't normalized, then you get 410/365=1.12, so 12% more pax for free. Add that to the 10% SFC gain, and you're about in the right ballpark.

Fuel burn over a specific sector doesn't scale linearly with SFC on a given trip (using fuel to carry fuel) the reduction in fuel burn from the 0.55->0.49lb/hr/lb is about 12.6% giving a total fuel reduction per pax as~25% (same as A35K)


Ok, so if it's the same trip burn and 12% more pax, can't you argue that the new wing gives you ~12% benefit over the old wing? Your 774ER chart seems to show this.

flipdewaf wrote:
I'm not moedelling it in as much detail, I simply have a baseline ClMax and derive the distance to get to the speed to lift off at that weight.

LH707330 wrote:
If you remove 13t in DOW to get to 337t MTOW, you've got 96% of the weight and 91% of the thrust vs a 77W, for a total 5% T/W penalty. No-drag acceleration is thus 5% lower, and distance traveled to reach the same speed should go up roughly quadratically, meaning you've got a ~10% penalty before you factor in thrust lapse (worse on the higher BPR engine) and drag. the 779 gets around this by having lower runway speeds thanks to the bigger wing.

I don't currently do anything with thrust lapse in the calculation but its also worth pointing out that at reduced weights you don't have to be going so fast, lower weight increases acc. and decreases Vlo, a double whammy.

Ok, so your target speed scales as sqrt(weight), I ignored that in my initial assessment. That should get you back to a ~5% penalty overall, which is more manageable in many situations. I threw thrust lapse out as an idea, IIRC it's about 20% by the time you get to v1, but the change in lapse rate between the engines based on the delta BPR likely won't have an overall impact of more than 1 pp (e.g. 20% vs 21%).

Regarding the A345/346 being too heavy, there were a couple reasons:

1. Tube too long->strengthening weight out of control
2. Wing chord increase added substantial structure in a suboptimal manner (they tried to do it on the cheap)

On the oil consumption front, I think the CFM56-5Cs actually burned more than the T500s, the T500 was pretty good on oil.
 
flipdewaf
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Re: Did/Does the 779X need a new wing? and does it need to be so big?

Fri Jul 12, 2019 8:57 am

LH707330 wrote:

Ok, so if it's the same trip burn and 12% more pax, can't you argue that the new wing gives you ~12% benefit over the old wing? Your 774ER chart seems to show this.

The mythical 774er +GENX burns about 3% more than the the 779X suggesting that at a similar capacity the 779X wing gives a 3% advantage. The added empty weight of the 774er+GENX would increase and so reduce the payload which would have to be accounted for in additional structure so there is probably a bit more than that but it wouldn't drive it up beyond 4% I wouldn't think.

LH707330 wrote:
Ok, so your target speed scales as sqrt(weight), I ignored that in my initial assessment. That should get you back to a ~5% penalty overall, which is more manageable in many situations. I threw thrust lapse out as an idea, IIRC it's about 20% by the time you get to v1, but the change in lapse rate between the engines based on the delta BPR likely won't have an overall impact of more than 1 pp (e.g. 20% vs 21%).

I have been meaning to add a better model of takeoff to my simulation to understand the relevant parameters but want to understand the relevant levers that are affecting it and where they drive to. I might spend my lunch break today trying to understand the thrust lapse in a bit more detail and the relevant parts of modelling it. I have heard 20% at v1 and 20% at v2 but aI also need to make sure I understand Braking and decision times and the climbing and accelerating at the same time (its easy to model one or the other but what the realistic mix between the two is, is a decision I have to understand).

Fred
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Faro
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Re: Did/Does the 779X need a new wing? and does it need to be so big?

Fri Jul 12, 2019 10:00 am

flipdewaf wrote:

For reference the GE9X is shown as adding an extra 3t per aircraft and has a slightly higher diameter for aero purposes.



Very interesting...I know GE have said that the GE9X will be "slightly heavier" than the GE90-115B, but did not quote any figures...do you have a reference for the extra 1.5T per engine or is this an informed estimate?...


Faro
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RJMAZ
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Re: Did/Does the 779X need a new wing? and does it need to be so big?

Fri Jul 12, 2019 10:06 am

Excellent discussion.

With any aircraft size a manufacturer can choose between the following design compromise:

Small wing = lower empty weight
Medium wing = medium empty weight
Larger wing = higher empty weight

The bigger the wing the longer the average flight needs to be for it to be the best design choice. Boeing has clearly optimised the 777X for a longer average flight length than the 777W.

This longer optimised flight is a bad decision for the 777X freighter. In the freighter role I have seen members here question if the 777-8 freighter can match the 777F freighter. The 777F can probably carry 25T more fuel than the 777-8F with the same payload weight and hitting MTOW. In the passenger role 777-8 vs 777LR with the same payload fuel loads would be 140T vs 165T on a long haul flight. That is 17% more fuel in the old model. The newer engines and bigger wing can make up that 17% easily. But when the freighter has a 100T payload the fuel load is now 100T vs 75T. The 777-8F carries 25% less fuel. The big wing and engines can not make up that difference. The 777-8F will need to reduce payload to fly the same distance. The fuel burn per kg will then take a big hit.

In the freighter role Boeing would have been better off using the older wing with the new engines.
 
flipdewaf
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Re: Did/Does the 779X need a new wing? and does it need to be so big?

Fri Jul 12, 2019 10:08 am

Faro wrote:
flipdewaf wrote:

For reference the GE9X is shown as adding an extra 3t per aircraft and has a slightly higher diameter for aero purposes.



Very interesting...I know GE have said that the GE9X will be "slightly heavier" than the GE90-115B, but did not quote any figures...do you have a reference for the extra 1.5T per engine or is this an informed estimate?...


Faro

It's more absorbed information, I think it came from the emirates 779X thread but the weight is a difficult thing to find.

Fred
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flipdewaf
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Re: Did/Does the 779X need a new wing? and does it need to be so big?

Fri Jul 12, 2019 10:22 am

RJMAZ wrote:
Excellent discussion.

With any aircraft size a manufacturer can choose between the following design compromise:

Small wing = lower empty weight
Medium wing = medium empty weight
Larger wing = higher empty weight

The bigger the wing the longer the average flight needs to be for it to be the best design choice. Boeing has clearly optimised the 777X for a longer average flight length than the 777W.

This longer optimised flight is a bad decision for the 777X freighter. In the freighter role I have seen members here question if the 777-8 freighter can match the 777F freighter. The 777F can probably carry 25T more fuel than the 777-8F with the same payload weight and hitting MTOW. In the passenger role 777-8 vs 777LR with the same payload fuel loads would be 140T vs 165T on a long haul flight. That is 17% more fuel in the old model. The newer engines and bigger wing can make up that 17% easily. But when the freighter has a 100T payload the fuel load is now 100T vs 75T. The 777-8F carries 25% less fuel. The big wing and engines can not make up that difference. The 777-8F will need to reduce payload to fly the same distance. The fuel burn per kg will then take a big hit.

In the freighter role Boeing would have been better off using the older wing with the new engines.


I totally agree on the freighter point, with the new heavy wing you are squeezing the point between OWE and MTOW giving a smaller window in which to put your payload and fuel combination.
With regard to the "longer optimised flight" part I think there has developed a synonym between longer range and heavier weight (understandably) A long flight could almost be described as two flights, a heavier one and a lighter one. in the example I used with 50t payload the aircraft was hitting FL410 at the end of the flight, if the payload was much reduced it would be flying at a height not selected for its efficient operation but limited due to its certification. I don't think the wings relative benefit is so much on the long flights but the heavy ones, the MZFW limited type...

I'll do some analysis on this today. (the beauty of having the model is that it takes 2 mins to set up and then 20 mins to do its thing while I get on with my real job).

Fred
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Eyad89
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Re: Did/Does the 779X need a new wing? and does it need to be so big?

Fri Jul 12, 2019 4:42 pm

LH707330 wrote:


Ok, so if it's the same trip burn and 12% more pax, can't you argue that the new wing gives you ~12% benefit over the old wing? Your 774ER chart seems to show this.



The official 2-class passenger count for 77W is 396 in a 10-abreast configuration (Boeing's website). 779's longer fuselage added 20 more seats, that's 5% more seats. The 12% seating difference can be obtained by assuming a 9-abreast 77W vs the 779. This has nothing to do with the wing.

This means the weight of the longer fuselage and bigger wing cancels out the benefit of the higher L/D. As a result, the wings of 779 got 5% more seats for free.

---

Flipdewaf,

If we assume a fuselage weight of 2.3 ton per meter, can we see the performance of a hypothetical 77X that kept the fuselage length of 77W? In other words, a 779 with a DOW of 177t (assuming it would be 2.9m shorter than 779) and a shorter fuselage (for less parasitic drag).

I guess the fuel burn here would illustrate the the difference the newer wing can bring.
 
Sokes
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Re: Did/Does the 779X need a new wing? and does it need to be so big?

Fri Jul 12, 2019 7:59 pm

flipdewaf wrote:
...
Similar to the previous charts this really shows that the engine makes the difference here with the 77W+GE9X having very close to the same fuel usage as the 779X.

The wing of the 779X allows the aircraft to effectively add the stretch to the 77W for zero fuel cost, certainly not an insignificant factor but by far the bulk of the fuel savings are coming from the engines.

The new wing does not make the aircraft any more effective at the heavy loads at long range and arguably the simple engine change would allow a higher margin for fuel capacity at ranges above MZFW.


You say "The new wing does not make the aircraft any more effective at the heavy loads at long range". As I read that the B777-300ER close to MTOW hangs around in rather low altitude, I struggle to believe that. But then I'm not qualified enough to say you are wrong. Also for some reason my computer doesn't show your graphs. Anyway how does it fit to the observation above, that the wing gives the stretch for zero fuel cost?

I believe the idea behind B777X is to increase MTOW and engine thrust massively after some years.
The B777-F is 63,73m long. Fully loaded it can fly 4970nm. That's pretty exact Seattle-Shanghai, but then one needs reserves.
The B777-8 will be 69,8m.
The B777-9 has OEW of 181,4 t and max payload of 73,5 t. What MTOW is required if one wants to fly with full belly from Seattle to at least Shanghai?
A330 and B777 got a lot of improvements over the years. What about a B777-8F with 380t MTOW?
Quite likely you are right that for 351 t MTOW Boeing takes unnecessary trouble.

flipdewaf wrote:
...
The elephant in the room is Takeoff performance. If the fuel use is effectively identical between the 779X and 77W with GE9x then the Empty weight difference of ~13t can remain. Would the [email protected] 337T be able to cope with 105klb engines?, when does it need the full 115klb? could the GE9X produce this?
Fred


I don't know if engine thrust would be enough. As stated earlier I believe the new engine will get higher thrusts later on.
I remember a a-net discussion that in Dubai heat the B777-300ER gets maximum tyre speed problems. Therefore the plane sometimes has to start at reduced MTOW.
"At the November 2013 Dubai Airshow ... 259 orders and commitments for US$95 billion at list prices. This was the largest commercial aircraft launch by dollar
value with Emirates ordering 150, Qatar Airways 50, and Etihad Airways 25, in addition to the September 2013 Lufthansa commitment for 34 aircraft."
https://en.wikipedia.org/wiki/Boeing_777X
So beside having a high payload transpacific plane the B777-X may have been planned as a Middle Eastern plane. They needed a new wing more than a new engine.

If it will be impossible to fit landing gears/ tyres for a strongly increased MTOW, please let me know.
Why can't the world be a little bit more autistic?
 
OldAeroGuy
Posts: 3858
Joined: Sun Dec 05, 2004 6:50 am

Re: Did/Does the 779X need a new wing? and does it need to be so big?

Fri Jul 19, 2019 6:29 pm

flipdewaf wrote:
The elephant in the room is Takeoff performance. If the fuel use is effectively identical between the 779X and 77W with GE9x then the Empty weight difference of ~13t can remain. Would the [email protected] 337T be able to cope with 105klb engines?, when does it need the full 115klb? could the GE9X produce this?

I found this quite interesting to look at if somewhat indulgent. Hopefully you will critique and discuss.

Fred


777-300ER takeoff performance with the GE9X certainly is the elephant in the room with regard to your study. Takeoff performance is much more complex to calculate than mission fuel burn. I've done some estimates based on the Boeing Airport Compatibility Document D6-58329-2 and the Takeoff Performance data shown in Fig 3.3.10.

From this chart for S.L., IAS + 15deg C:
Flaps 20
TOGW: 337t
F.L.: 9,800 ft (GE90-115)
F.L.: 10,750 ft (GE9X) estimate based on the lower T/W.

MTOGW: 351.5t
F.L.: 10,500 ft (GE90-115)

Unfortunately, it looks like the -300ER (GE9X) at Flaps 20 would be 2nd Segment Climb limited at 328.7t. Takeoff at 337t would mean a Flap 15 takeoff. It appears this would add about 800 ft to the takeoff distance.

Flaps 15
TOGW: 337T
F.L.: 11,550 ft (E9X) estimate based on the lower T/W and lower Flap setting.

so the -300ER(GE9X) combination @ 337t would mean a 1050 ft takeoff distance increase compared to the -300ER(GE90-115B) @ 351.5t.

For the 777-9, the 10.8% span increase moves the 2nd Segment Climb limit beyond 351.5t and the 18% wing area increase lowers the takeoff speeds at 351.5t.

Takeoff distance for the 777-9X is estimated to be:
Flaps 20
MTOGW: 351.5t
F.L.: 9,750 ft.

so the -9(GE9X) combination @ 351.5t would mean a 750 ft takeoff distance decrease compared to the -300ER(GE90-115B) @ 351.5t.

These data show why Boeing grew the 777-9 wing area.

With the -300ER wing, a GE9X powered airplane is probably uncompetitive in terms of takeoff performance.

With the -9 wing, a GE9X powered airplane has better takeoff performance than a -300ER wing airplane with the GE90-115.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis

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