So no, they are not foldable today - the headline submitted to HN is total clickbait.
"Over the next three years, Airbus will design, build and flight test full scale wing extensions."
That's a meaningful improvement but not that big. Perhaps the limits to this kind of technology are ~50% fuel gain? https://www.technologyreview.com/2013/01/24/180345/hybrid-wi...
We've got AI, but I'm still waiting on exponential technological gains. So far I've mostly gotten exponential enshittification of my google experience. Is AI going to unlock the detonation engine and bring down the cost of my plane ticket? https://www.technologyreview.com/2012/11/23/181486/exploding...
I've flown on the A321LR a few times, and it's always crazy to think about that tiny plane cruising over the Atlantic without stops.
New fuels (SAF) and the technology to use them have been developed for less.
This is all to say nothing of the continuous work done over decades now to make aircraft more efficient; as these effects compound, intuition says that returns would start diminishing, so 10-20% is even more impressive.
Aircraft wings oscillate during flight, the location (and movement) of mass within them (e.g. fuel) is critical to model.
Also, the location of the fold is likely less flexible (e.g. more rigid) than the remainder of the wing. This will greatly affect the dihedral angle of the remaining outboard portion of the wing.
The goal is longer, thinner wings than usual. Long and thin wings are far more efficient—think gliders and the U2. But beyond a certain width, those wings become incompatible with existing airports and other facilities for handling commercial aircraft. So the idea is to mount folding extensions that are extended for flight, but likely fold when being hangered or navigating taxiways.
It was hard enough to convince a few dozen of the world's largest airports to make the alterations needed to accommodate the A380's wings. Demanding similar changes at the thousands of smaller airports where A320-class planes land would be a non-starter.
E.g. for the 777X it fits in Category E. Without folding tips it would be in Category F. Less gates can accommodate Category F.
https://skybrary.aero/articles/icao-aerodrome-reference-code
The second place was the XB-70:
The XB-70 was designed to use compression lift, a phenomenon also referred to
as Mach wave riding, which was demonstrated with the help of wind-tunnel
tests. It was shown to provide an increase in total lift at supersonic speeds
as wingtips folded downward, and would capture the shock waves generated by
the aircraft. Wing lift could therefore significantly increase by up to 30%
without any drag penalty.
Longer, thinner wings (higher aspect ratios) create less lift-induced drag for a given wing area, so ideally you want a super duper long and skinny wing, which will create less drag for a given amount of lift, and thus improve your fuel efficiency. Fuel is a massive portion of the total cost of operating an airliner, so there is a large financial incentive from the airlines to reduce fuel costs.
Unfortunately really long wings make airliners too wide to park at most airport gates, and in some cases too wide to taxi safely around obstacles, so they're experimenting with ways to make the wings fold and get a "best of both worlds" situation.
I can't say for certain, but I believe Boeing may have explored this concept at some point. The idea would be to equip aircraft with mechanically extendable wingtips that could expand outward by several meters during flight and retract while on the ground.
The primary challenge is that most airport gates cannot accommodate aircraft with a significantly larger wingspan. Retractable wingtips would allow the aircraft to benefit from the improved aerodynamic efficiency and fuel savings of a longer wing in flight, while still fitting within existing gate infrastructure during boarding, deplaning, and parking.
My guess is that they will be folded after landing, and unfolded prior to takeoff, as other planes with foldable wings do, so that they can continue to fit in standard spaces in the parking/gate areas.
XB-70 Valkyrie
... doesn't look like there's any folding being tested here, just the resulting wing shape to see what the benefits might. Full scale foldable wings would be years off.
https://simpleflying.com/boeing-once-offered-the-original-77...
You used to be able to see the qualification test rig mechanism at the Museum of Flight restoration center in Everett, but they scrapped it during the pandemic.