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Max Q
Topic Author
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The 787 and potential weight increases, why so problematic ?

Fri Mar 08, 2019 1:45 pm

Lots of interesting discussions on the 787
here and a common theme is that the airframe is almost ‘maxed out’ as far as further MGTOW increases are concerned


Not sure if that’s accurate but it seems telling that Boeing is planning a relatively small 2.5 ton boost in the next few years


As a percentage of total aircraft weight this is very small however.


Curious as to the reasons behind this, every conventional, metal construction Boeing 7 series prior to the Dreamliner has seen substantial, sometimes significant
gross weight increases over the years, this capability to grow is designed in to the structure or allows for it during production



So why does that philosophy not seem to apply to the 787 ? Obviously a predominately composite structure that is built completely differently than ‘conventional aircraft’ has different considerations in this respect, but why is that ?


Was the 787 designed to be right at, or very close to its MGTOW limit from day one as part of the drive to ‘ ultimate efficiency’ with no extra structure allowing for growth ?


Or is it that Boeing’s composite manufacturing process itself is so precise and customized for this aircraft that it’s not easily modified to allow these changes, perhaps a combination of both ?
The best contribution to safety is a competent Pilot.


GGg
 
WIederling
Posts: 9580
Joined: Sun Sep 13, 2015 2:15 pm

Re: The 787 and potential weight increases, why so problematic ?

Fri Mar 08, 2019 1:59 pm

Max Q wrote:
Was the 787 designed to be right at, or very close to its MGTOW limit from day one as part of the drive to ‘ ultimate efficiency’ with no extra structure allowing for growth ?


The 787 gained OEW and thus needed MTOW quite early in the design process and long before EIS.
i.e. most of the growing potential was used up before it ever flew.

This is from June 2007 ( just before the roll out.)
https://en.wikipedia.org/w/index.php?ti ... ifications
ver OEW MTOW
-3 110t 165t
-8 110t 219t
-9 115t 244t

2 years earlier ( June 2005 ):
https://en.wikipedia.org/w/index.php?ti ... ifications
ver OEW MTOW
-3 ?? 163t
-8 ?? 216t
-9 ?? 226t

based on early planning the airframe seems to have had ~30t available for growing.
Murphy is an optimist
 
SteinarN
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Joined: Mon Dec 29, 2014 1:26 pm

Re: The 787 and potential weight increases, why so problematic ?

Fri Mar 08, 2019 2:23 pm

As Wledeling says.
The wing loading is high, but not as high as A321 or B77W. The pavement loading is very high, I dont have the numbers at hand but I seem to remember that 254 ton is very near the limit for the current 4-wheel bogie. However, it is still surpricing how much more easy Airbus are able to increase the MTOW of the A350 compared to the B787. One should think that in service data and more anvanced stress analysis and calculations since service entry should have revealed more margins in the airframe to allow more than only one percent increase, especially considering it is, how many years since service entry, 7 years?
 
GalaxyFlyer
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Joined: Fri Jan 01, 2016 4:44 am

Re: The 787 and potential weight increases, why so problematic ?

Fri Mar 08, 2019 2:36 pm

Equally likely, in service data, advanced stress analysis and calculations revealed the limits for growth are close.

GF
 
WIederling
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Re: The 787 and potential weight increases, why so problematic ?

Fri Mar 08, 2019 2:46 pm

GalaxyFlyer wrote:
Equally likely, in service data, advanced stress analysis and calculations revealed the limits for growth are close.

Any body ever had the chance to look:
How much room does the retracted gearleg+boggie+wheels have in its wheel well ?
Murphy is an optimist
 
GalaxyFlyer
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Joined: Fri Jan 01, 2016 4:44 am

Re: The 787 and potential weight increases, why so problematic ?

Fri Mar 08, 2019 4:26 pm

Beefing up the gear may be within the current wells—higher ply tires, different metallurgy in the struts, attachments strength etc. it depends on where the weakest point is.
 
WIederling
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Re: The 787 and potential weight increases, why so problematic ?

Fri Mar 08, 2019 4:42 pm

GalaxyFlyer wrote:
Beefing up the gear may be within the current wells—higher ply tires, different metallurgy in the struts, attachments strength etc. it depends on where the weakest point is.


That does not change wheel loading which is maxed out as it is apparently.
Larger tires, wider track ( see A359 )

Wheel truck on A359: 2times 1.74m x 2.0m spanning 3.5m²
tire: 4 x 400 x 530 R23 42PR 10.6m apart
Wheel truck on 789: 2times 1.52m x 1.51m spanning 2.3m²
tire: 4 x 54 x 21 R23 38PR 9.8m apart
Murphy is an optimist
 
stephanwintner
Posts: 79
Joined: Thu Jan 03, 2019 5:04 pm

Re: The 787 and potential weight increases, why so problematic ?

Fri Mar 08, 2019 5:31 pm

Speaking as someone who briefly worked on it, I was always curious about what Boeings predicted weights vs. actuals were over the program.

I suspect their early weight estimates, which drove structural design, may have underestimated the mass of some items, e.g the lightning protection system, which may have added a lot of heavy copper later on. They may have also underestimated the fastener mass - they may have expected to use more co-curing than they ended up with - and various faying and sealing compounds. Details, but that may explain some of the relative lack of margin they seem to have ended up with. A gram here and two there, over and over throughout the entire aircraft... I also think there were major design changes to the electrical system late in the program, long after main structure was being produced, which may have eaten into those margins. Then again I wasn't involved in weight or structures work, so maybe my speculation is off the mark. I do know Boeing didn't move to composites without doing lots of up-front homework developing materials, fastening approaches, BVID criteria, etc.

A boeing manager said - correctly - "This is the worst composite airplane we will ever make". He meant that they would learn and next time round early estimates of little details like this would be much closer to the mark.
 
WIederling
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Re: The 787 and potential weight increases, why so problematic ?

Fri Mar 08, 2019 5:42 pm

stephanwintner wrote:
He meant that they would learn and next time round early estimates of little details like this would be much closer to the mark.


you have two effects layered:
The planned weights escalated. ( see the historic numbers from wikipedia I posted further up.)
The real existing prototype(+10..20 early ) airframe was massively overweight ( short on 10t ? ) vs planned numbers.

Today reduction to escalated but planned weights have apparently been achieved.
( Though I seem to remember that around frame 100 the targeted MEW was redefined?)
Murphy is an optimist
 
stephanwintner
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Joined: Thu Jan 03, 2019 5:04 pm

Re: The 787 and potential weight increases, why so problematic ?

Fri Mar 08, 2019 8:13 pm

Wiederling - well... kind of. I don't see those as two effects. Two sides of the same coin perhaps.

Any engineering program works through the iterative process, both effects occur - we engineers find that early values were a bit rough, and refine both the current estimate, and possibly the targets (which you called planned). Continual iterations. My current product is some 7% above current target, targets have altered a couple times since day 1, and my team has some design changes coming (this month, revision 11 of my layout) which ought to get us to more like 3%.

The wing designers release parts to manufacturing based on the best data available at that time - both target and current values - and relatively early in the process. ( Clearly, assuming that a program that is currently 3% overweight will achieve targets on airframe 1 is optimistic. How optimistic should one be?). The more accurate (realistic) the earliest values are (both target, and current) the more accurate all subsequent work will be, so the design as released is more likely to achieve the early intent - all of it, including intended margin for later growth.

Ideally, targets should be a touch optimistic, but also realistic and achievable. Current values should be realistic, not optimistic, not overly pessimistic either. To me, thats two sides of one coin.

( I suspect I am telling you things you know. No disrespect intended, just trying to be clear. )
 
GalaxyFlyer
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Re: The 787 and potential weight increases, why so problematic ?

Fri Mar 08, 2019 8:50 pm

WIederling wrote:
GalaxyFlyer wrote:
Beefing up the gear may be within the current wells—higher ply tires, different metallurgy in the struts, attachments strength etc. it depends on where the weakest point is.


That does not change wheel loading which is maxed out as it is apparently.
Larger tires, wider track ( see A359 )

Wheel truck on A359: 2times 1.74m x 2.0m spanning 3.5m²
tire: 4 x 400 x 530 R23 42PR 10.6m apart
Wheel truck on 789: 2times 1.52m x 1.51m spanning 2.3m²
tire: 4 x 54 x 21 R23 38PR 9.8m apart


I was considering the strength of the gear to handle increased weights, not its ACN and you’d be correct if the present max weight is limited by ACN and the PCN at airline airports.


Gf
 
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Stitch
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Re: The 787 and potential weight increases, why so problematic ?

Fri Mar 08, 2019 9:06 pm

During the design process I would expect there are "maximum" values the OEMs define for the operating weights and then plan their initial releases and growth plans around that.

At 7E7 launch, the 7E7-8 had an MTOW of 216,000kg and the 7E7-9 had an MTOW of 227,000kg. So this left about 25,000kg of growth for a future 7E7-10.

There was MTOW growth early on in the 787-8 to account for OEW creep, however the 787-9's growth was driven by customer demand (remember that the 787-9 entered service below spec in terms of OEW thanks to all the effort to pull it out of the 787-8).

Also, remember that the 787 had to not step on the toes of the longer-range 777 family (777-200LR / 777-300ER). So that would have influenced the operating weights and performance the 787 team during the definition stage.

The A350, on the other hand, was designed to do just that, so Airbus would have ensured the frames were capable of higher gross weights even if that meant the A350-800 and A350-900 would be structurally heavier than the 787-8 and 787-9.
 
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Stitch
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Re: The 787 and potential weight increases, why so problematic ?

Fri Mar 08, 2019 9:13 pm

LN1 was just under 10,000kg above spec.
LN6 through LN19 were 6,100 kg above spec.
LN20 was 4,000kg overweight and was the first frame to incorporate the 228,000kg MTOW (a ~8,000kg increase over the LN6-LN19 MTOW).
LN34 was the next block point with even more weight reductions as was LN54.
LN90 was the block point when the frames were at Spec Weight (using weight-optimized parts from the 787-9 design).
 
WIederling
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Re: The 787 and potential weight increases, why so problematic ?

Fri Mar 08, 2019 11:01 pm

Stitch wrote:
LN1 was just under 10,000kg above spec.
LN6 through LN19 were 6,100 kg above spec.
LN20 was 4,000kg overweight and was the first frame to incorporate the 228,000kg MTOW (a ~8,000kg increase over the LN6-LN19 MTOW).
LN34 was the next block point with even more weight reductions as was LN54.
LN90 was the block point when the frames were at Spec Weight (using weight-optimized parts from the 787-9 design).


well done enumeration.
What I do wonder is how the production cost have evolved ( corrected by leaning curve effects.)
Murphy is an optimist
 
tealnz
Posts: 638
Joined: Mon Nov 09, 2015 10:47 am

Re: The 787 and potential weight increases, why so problematic ?

Fri Mar 08, 2019 11:57 pm

To go back to the OP’s question, I’ve seen comment on a.net (sorry, can’t remember who/where) that the 787 is a comparatively ‘tight’ design, leaving minimal scope for further development beyond the 789 (at least without major engineering presumably). On the positive side the tightness/optimisation of the design makes for a very efficient airframe – reflected in its performance and sales record. The corollary of that is that it doesn’t have built-in scope for further development comparable to eg the A350.

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