You are basically saying the 7W burns 15% more fuel as the tail is bigger/heavier - that just doesn't seem right.
You clearly don't get it.
I will try and explain it.
Lets say you have a wing and engine combo that can have a MTOW of 100t. The 7ab fuselage might add only 1000kg at the same seating capacity. But it is shorter so the tail might be only 500kg heavier. But this is where the loop begins.
That 1500kg of extra empty weight is now taken out of the fuel carried, remember the MTOW is fixed at 100t. If the OEW, payload and fuel is split 50/25/25 then that means that extra empty weight reduces fuel capacity by 6%. The extra drag for the shorter and wider fuselage combined with this reduced fuel would easily see range drop by 10%.
Now to make the comparison fair the 7ab needs to gain back that 10% range. The extra fuel requires a slightly higher MTOW. That requires a slightly bigger wing and slightly more powerful engines. The empty weight has now increased a second time. The extra thrust requires the tail to be enlarged again. The fuel capacity needs to be increased even further but this time by smaller and smaller amounts as the loop continues.
The loop eventually ends up with the 7ab design burning 15% more fuel and a tail that is 4500lb heavier.
The same loop can be used to optimise a cleansheet design for a shorter range.
For instance a 787-8 sized fuselage that was FULLY optimised for 5000nm range would see the empty weight reduced by 20% and the fuel burn per seat would be improved by over 10%. It would take another 20 years for engine technology to get that same 10% gain. So optimisation of a design is huge.
Derating an existing design by reducing MTOW does not reduce fuel burn it simply reduces engine maintenance and landing fees. See the 787-3 as a bad example of of a derated design not an optimised design.
This is like basic engineering 101, same applies on cars, boats, etc. More seats-> heavier design -> bigger engines -> bigger brakes -> bigger suspension -> bigger tires... You cant increase weight on 1 part without impacting other parts. Especially not on something that is so weight sensitive as planes...
I fully understand this principle. I understand the calculations are circular.
However, if you keep your thumb on the scale and ignore certain things yes that is what the result will be. Let me try one more time.
The analysis is coming up with a fuselage that is 1,000KG I agree with that. However the analysis is not giving any credit for shorter cable runs or shorter gear.
So let us take two separate frames and forget the tail for a second. Same wing same engines.
One WB with 300 seat Capacity and 20' shorter than an 300 Seat NB.
The WB has 3% less wetted area and about 19% more frontal area, however the Wetted area calculation I did in tech ops takes the frontal area into account. Same drag off the main engines/ main wing.
Two good articles from NASA here https://www.grc.nasa.gov/www/k-12/airplane/sized.html https://www.grc.nasa.gov/www/k-12/airplane/drageq.html
Basically no one knows what is most important - you have to put them in a windtunnel.
Sitting on the runway the WB could have 1,000KG more fuselage weight. It could be less it could be more. But I will accept 1,000KG
Cable/electrical/servicing runs would be 20' shorter going from the Cockpit to the Tail and 10' shorter from the Wing section to the cockpit or to the tail.
I have no idea how much weight that is - but it would be less - 100-200KG? BTW this article says a typical WB has about 7,400KG in wiring. https://aerospaceamerica.aiaa.org/featu ... on-wiring/
The WB is about 10% less in length. So a savings of 300-400 KG is not unrealistic as they could be talking about a really big WB.
To achieve the same rotation angles as the tail is shorter the WB could have have significantly shorter gear. This could save a bunch of weight.
Let us assume it makes up the difference to the 1,000KG.
So you have two aircraft (sans tails) with identical weights and the WB has less wetted area than the NB, and could be more efficient as more of a teardrop.
Yes the Tail has to be bigger to account for the shorter airplane. But it won't be 50% bigger and weigh 3,500 lbs more. It may just make up for the WB having 3% less wetted area.
There is no way it would burn 15% more fuel.