Underfire,

I think the disagreement is about math, not engineering. No one's disputing that removing the windows makes the skin 25% lighter. And I think we agree that the weight of rest of the fuselage structure is static, or even decreases slightly without windows. But if the skin is 20%, or even 50%, of the fuselage weight, making the skin 25% lighter doesn't make the fuselage 25% lighter.

(I think you agree that even without the windows, a fuselage still has to have things like stringers and bulkheads. None of those go into the 1560 KG in the second formula.)

It's OK John, underfire just earned his way onto my ignore list. I guess 39+ years experience in aircraft and aircraft system engineering design just doesn't carry weight with some people.

OK, some actual numbers:

The paper is based on the centre section of an A320 fuselage. Assuming that corresponds to the production joins, it's 5.98 m in length. The O/D of the fuselage is 3.95 m. So, assuming a generous skin thickness of 1.6 mm, the total volume of material is approximately 0.118 cu m.

Assuming around 2,700 kg/cu m for aluminium alloy, that gives a total skin weight for the centre section of 320 kg. For the entire 36.6 m fuselage, skin weight would be around 2,000 kg (the figure in the paper is 2155.4 kg, so we won't argue with that).

So clearly the numbers quoted in the paper for "weight of windowless/traditional fuselage" are the skin weight.

It is pretty clear from the article being criticised that the weight gains quoted are their estimate of the savings in their estimate of the skin weight based on their understanding of the static design loads on a typical central section of an A320 fuselage.
It is pointless to talk of percentage gains in fuselage weight when the baseline used is just a fraction of the total fuselage structural weight. Typically, fuselage structural weights lie in the range 21 to 23% of the aircraft empty weight. For an A320, that is a lot more than the 2155 kg quoted, so to talk of a 25% reduction in fuselage weight is a nonsense.
More to the point is that almost none of a typical transport fuselage is designed by the static loads. Depending on what part of the fuselage you are discussing, manufacturing (minimum gauge) limits, fatigue, damage tolerance and crashworthiness considerations dominate. Any actual weight gain will be the difference between the windowed design and the windowless weight that meets these more severe criteria, not the theoretical weight that satisfies the static load condition.
Nobody is disputing that removing the windows would save weight, although I am with tdracer in doubting whether it would be anything like the number claimed, neither would one suggest there was any real technical risk, but the commercial risks would be very large.
I'm not holding my breath waiting for an announcement that the next design to surface will be windowless!

Not opaque
 

No, the 787 windows, even when darkened to the max, still let light pass thru, allowing the pax to see the outside world in all its glory. Really a brilliant system.

Fascinating discussion.

Perhaps more important is not the presence of Windows, but the shape of the hole they occupy? Especially the corners? Or the strength of the skin above? (deHavilland, Boeing).

Adios Comet, Aloha, Aloha.

Read the article, total weight savings 750kg. According to the formula total airframe weight with windows 3000kg.
Empty weight around 40.000kg. Very heavy wings, engines and landing gear........

Removing Windows is a money saver, yep. But along with the glass and pertinent structure, a line could also remove all but perhaps twenty five seats; these for the remaining people who would purchase fares from this "lighter" line?

Two days ago embarked on a 12.5 hour flight, midnight departure, about five minutes after take off CC came around asking for the shades to be closed, very politely, but with a measure of insistence. Asked why, it's so pax are not disturbed by the aircrafts external lighting, or raising/setting sun. Entirely new one on me.