justloveplanes
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Joined: Thu Jul 08, 2004 5:38 am

Weight Differential 787 vs 777X

Sun May 06, 2018 7:46 pm

Boeing having made the decision to stick with an Al fuselage and Al/Ti wingbox, what weight penalty did this incur versus a all composite (787 type) design? Why not Al/Li fuselage?

Ivan
 
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Stitch
Posts: 25662
Joined: Wed Jul 06, 2005 4:26 am

Re: Weight Differential 787 vs 777X

Sun May 06, 2018 8:37 pm

Main reason was familiarity with the existing material for both Boeing and the suppliers.
 
VSMUT
Posts: 1956
Joined: Mon Aug 08, 2016 11:40 am

Re: Weight Differential 787 vs 777X

Mon May 07, 2018 5:21 pm

justloveplanes wrote:
Boeing having made the decision to stick with an Al fuselage and Al/Ti wingbox, what weight penalty did this incur versus a all composite (787 type) design? Why not Al/Li fuselage?

Ivan


Composites are "better" than Al/Li, but you can't just replace aluminum with composites. The two materials have very difference qualities, and the fuselage design with stringers and ribs would have to be drastically changed. The 777X is an attempt at a cheaper upgrade of the 777, rather than an expensive all-out 777 replacement. Had they gone with a clean-sheet replacement, you can be 99% sure that it would've been composite.
 
RJMAZ
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Re: Weight Differential 787 vs 777X

Sun May 13, 2018 1:24 am

Composites provide a big advantage when it comes to wings but only a small advantage when it comes to the fuselage.

The aluminium lithium alloy narrows that advantage when it comes to the fuselage. So with the 777X wings already in composite i could only be maybe 5% lighter if it used all composite.

The reason why composite fuselages aren't super light is they need to put a metal grid inside the carbon layout. This metal grid acts as a sensor to detect internal delamination and has some other purposes. The wings aren't deemed as critical in terms of cracking so they can wait until normal inspection methods.
 
WIederling
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Re: Weight Differential 787 vs 777X

Sun May 13, 2018 8:54 am

RJMAZ wrote:
This metal grid acts as a sensor to detect internal delamination and has some other purposes. The wings aren't deemed as critical in terms of cracking so they can wait until normal inspection methods.


Aha.
You don't perchance have a link to some learned article on that around?
Murphy is an optimist
 
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DocLightning
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Re: Weight Differential 787 vs 777X

Tue May 15, 2018 6:44 am

justloveplanes wrote:
Boeing having made the decision to stick with an Al fuselage and Al/Ti wingbox, what weight penalty did this incur versus a all composite (787 type) design? Why not Al/Li fuselage?

Ivan


A change in materials would lead to a major change in structural behavior, which would lead to a complete redesign of the fuselage, which would probably mean that the 77X could not be certified as a 777, even if the exterior mold lines are the same.
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
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strfyr51
Posts: 3016
Joined: Tue Apr 10, 2012 5:04 pm

Re: Weight Differential 787 vs 777X

Tue May 15, 2018 10:40 pm

VSMUT wrote:
justloveplanes wrote:
Boeing having made the decision to stick with an Al fuselage and Al/Ti wingbox, what weight penalty did this incur versus a all composite (787 type) design? Why not Al/Li fuselage?

Ivan


Composites are "better" than Al/Li, but you can't just replace aluminum with composites. The two materials have very difference qualities, and the fuselage design with stringers and ribs would have to be drastically changed. The 777X is an attempt at a cheaper upgrade of the 777, rather than an expensive all-out 777 replacement. Had they gone with a clean-sheet replacement, you can be 99% sure that it would've been composite.

You can only make so many modifications to an Airplane and keep it under the same model Certification and Type Certificate.
This is what will have to happen when the B737,B757, and B767 are replaced. I'll bet they're nearly out of stuff they can Do, Especially with the B737 and call it a B737. The B757 will go fully Digital with new flight control architecture and the B767 will go "Bonkers" with new Technology, Thus the B797. (NMA)

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