Why are longer wings with foldable wingtips used on the B777X instead of lighter ones with original size?

Question

The new Boeing 777X has longer wings to improve efficiency, but has to adopt foldable wingtips to "fit" into all original airports that host its predecessor.

Tendentially, longer wings would have an higher weight and foldable mechanism surely add extra weight too, but thanks to new materials, new wings should be almost as heavy as previous ones.

For me, in absence of further information and comparisons, it's not so easy to understand why this solution is surely better of simply have lighter wings with original length, that would allow benefits too.

Answer 1

This explanation is given here :

The wing of the 777X is based upon the design of the 787’s wing. It has less of the sweep of the 787 but 10% more surface area (increased from 4,702 to 5,562 sq. ft.). The re-shaping has led to a higher lift-to-drag ratio, which(and) in turn increases usable fuel bulk from 320,863 to 350,410 lb. As a result, the type will have a range of over 14,000 kilometers (8,699 miles).

Boeing’s motivation for its including folding wingtips within the design was not wholly steered by fuel economy. With a fully-extended wingspan of 71.8 meters, the new 777X falls within the ICAO’s aerodrome Code F. But with raked and folded wings (and a span of 64.8 meters) the type can comfortably arrive at gates designed for Code E. Aircraft in this group includes the current 777.

So not only are they looking for more surface area and aerodynamic efficiency, they need to house more fuel to give it an extended range.

Answer 2

There's a trade-off between saving weight and improving lift-to-drag ratio.

In the case of the 77X, the efficiency gain of the improved lift-to-drag ratio of the longer wing outweighs the potential weight saving of the shorter wing.

In other words: the drag reduction results in more fuel savings than the weight reduction would.

For long-range aircraft, it typically pays off to add some weight to gain better fuel burn efficiency (heavier but more efficient engines) or lower drag. Both result in lower fuel burn in cruise, and thus in weight savings on the fuel you have to take along.

Short-haul aircraft on the other hand spend relatively more time climbing, so they optimize a bit more towards low weight and less towards cruise efficiency.