Is there any rule of thumb for weight savings using composite materials. For example, is there an early model large airliner like the B747 or even B737 that later was modified with composite wings or fuselage that shows weight savings?
1$\begingroup$ composite is just as diverse as alloy. if you say alloy it could be iron-carbon aka iron or steel or aluminium-magnesium aka light alloy. composite is basically A (some kind of fiber, for tensile strength) reinforced B (some kind of solid, for compressive strength) ranging from rebar reinforced concrete aka concrete to fiber glass reinforced epoxy aka fiber glass or carbon fiber reinforced polymer aka carbon fiber. This is just like "flavor difference between gain powder and flour pasta" e.g. if you use the right thing do it right it's great or if wrong it's awful... $\endgroup$– user3528438Dec 15, 2017 at 2:57
1$\begingroup$ With about 50% of composites, Boeing and Airbus with the A350 and B787 claim to have gained more than 20% of the empty mass, but I have no reference. The use of composite materials is associated with other improvements, so it will be difficult to answer. $\endgroup$– minsDec 15, 2017 at 21:51
(Own work via boeing.com) Overlayed scaled drawings.
Above you see similar sized planes with the same 3-class seating (± 1 seat). The newer-more-composite 787 is 16 tonnes heavier, yet delivers up to 20% fuel saving (route dependent). 3.4 of the 16 tonnes are the newer and bigger engines. And the rest is primarily bigger wings (which can be built bigger more easily thanks to composites).
The aim is building bigger wings, not losing weight. The other main goal is the reduced maintenance cost (better fatigue tolerance, no rust, etc.)
The result is an airframe comprising nearly half carbon fiber reinforced plastic and other composites. This approach offers weight savings on average of 20 percent compared to more conventional aluminum designs.
Based on that quotation, the answer is -20% (given the same wing, etc.). Also note the 787 also saves weight from the no-bleed architecture, yet as the above shows, it's not a lighter plane than the one it replaces. (No-bleed also improves engine fuel efficiency, not just weight.)
1$\begingroup$ Exactly the answer I was looking for - thanks! $\endgroup$ Dec 25, 2017 at 5:57