Bjorn’s Corner: New aircraft technologies. Part 4. Fuselage trades

By Bjorn Fehrm

March 17, 2023, ©. Leeham News: This is a summary of the article New aircraft technologies. Part 4P. Fuselage trades. In the article, we discuss the trade-offs involved in designing a fuselage of an airliner with 250 seats using different architectures.

We examine what parameters decide the performance of an aircraft and how fuselage changes like single aisle versus dual aisle affect these parameters.

Figure 1. induced drag comes from the global air circulation from the bottom to the top of the aircraft. Source: Leeham Co.

Single aisle or dual aisle fuselage

In last week’s article, we discussed that it might be better to design the next generation ”Heart of the Market” airplane as a dual aisle airplane when conventional wisdom says it should be a single-aisle plane.

It’s all about the time period of its operational use and how much the heart of the market has moved by then.

The design parameters we look at to understand if it should be a single or dual aisle are:

  1. Airframe drag, which decides fuel consumption
  2. Airframe mass, that affects several parameters, such as wing and engine size and, indirectly, through induced drag, total airframe drag.
  3. The turnaround time for the aircraft, that influences the daily productivity of the airplane.

We show that the cross-section area of a fuselage has a minor influence on airframe drag. Air is very supple and easily curves around a bulbous shape, only causing small amounts of what’s called pressure drag.

The dominant drag for an airliner is the skin friction drag. It’s the drag force created when the air rubs the aircraft’s skin. This drag can also be labeled drag due to size.

The second major drag component is induced drag or drag due to weight. It’s created by the global circulation of air from under the wing when it circulates to the top of the wing, Figure 1.

Induced drag only depends on the wing’s span and how careful the spanwise lift distribution is controlled (it shall be close to elliptical in the spanwise direction). It does NOT depend on wing aspect ratio, a widespread misunderstanding.

This misnomer comes from when induced drag is divided by wing area to normalize the drag to a lift coefficient,so that drag values from different size aircraft can be compared.

Drag changes a lot with speed. Figure 2 shows that at low speeds, like liftoff, induced drag is dominant. It’s typically 85% of the total drag at rotation.

Figure 3. How drag components change with speed. Source: Leeham Co.

At cruise, when we fly at high speed, the friction drag is the dominant drag. The air friction decreases with thinner air at altitude. Even though induced drag increases with thinner air, it’s beneficial for an airliner to fly high, to reduce the dominant friction drag.

It’s why airplanes that can climb high, like jet airliners, fly between 30,000ft and 40,000ft, where the air density is one-third of the density at sea level.

Friction drag depends on the skin area that rubs with the air, the so-called Wetted area. If we develop the parameters that drive wetted areas of different fuselage types, we find the drivers for the dominant drag. But we also find the driver for fuselage mass and weight (mass is mass, weight is a force).

It comes from airliners being stressed skin designs. The skin of the aircraft creates the dominant drag and, as it’s the premier carrier of the loads, also the mass of the plane.

So if we can find which fuselage construction has the least wetted area per carried passenger, we understand which fuselage type is best for our next-generation heart-of-the-market aircraft.

This is the task for next week’s Corner.

94 Comments on “Bjorn’s Corner: New aircraft technologies. Part 4. Fuselage trades

  1. It’s easy to model different cross-sections. Taking a width of 20 inches per seat (including armrest) or aisle plus 10 inches for the skin thickness for similar comfort levels (it’s comparable to current models), multiplying by Pi and dividing by the number of seats across, you get the inches of skin per seat:

    6-abreast NB: 78,5 in per seat
    7-ab. WB: 85,3″/seat
    8-ab. WB: 82,5″/seat.

    The extra skin have to be taken advantage of for paying freight.

    • Of course an 7 or 8 abreast fusealges can be shorter reducing squared inch skin per seat for the fixed 250 economy class seat reference. But I agree cargo capability also comes in.

      Single or twin aisle, single aisle seems best to limit airframe mass (that affects several parameters, such as wing and engine size and, indirectly, through induced drag, total airframe drag) under maybe 250 passengers.

      A single aisle starts causing problems, not only boarding/deboarding, but also with lavatory use, catering service, etc. People/ crew are not able to bypass each other, or trolleys. This doesn’t go away if you have two narrow aisles, people / trolleys still fully block movement during flight and (de)boarding.

      Easyjet now uses 25 inch aisles on their A320/321s, to enable bypassing. Adding 5 inch for a 178 inch (outside) fuselage might offer significant lighter fuselages than going for a twin aisle, for the same capacity. And maybe better cabin mobility then 2 narrow aisles.

      Personally I think Airbus and Boeing will end up with a big single cross section for 200-275 seats. It seems the airlines thought the same (objecting MOM, NMA, 797 twin aisle business cases since 2013).

      • That A370-900 concept is quite a long, mnarrow tube.. not sure I’d want to be in that thing.

        • It feels as long as a 757-300 but with a cabin that’s nearly 25″ wider. That looks bearable…

      • Keesje,

        Nice aircraft. I like the wider middle seat.
        The other could have been 18″ wide seat.

        An additional width of 0.8″, has no negative impact on the weight of this concept…

      • One can argue how many additional rows and A322 can have vs. A321neo. If you add 3+2 rows fwd and aft wingbox you get 6×5=30 more seats and some more cargo space. You can argue it does not need more range than the A321XLR. If you fit engines with 3-5% better SFC you do not need to increase MTOW from 101T. Moving to a carbon wing and AlLi fuselage might be enough with promised 35k engines. Still to keep the same rotation angle you might to modify the MLG. You need an updated APU and Air cycle machines and most likely revise emergency exists. Getting better engines besides pushing to get the promisied 35k engines might be hard now, hence you need to reduce empty mass and reduce fuel consumption with aero mods. How much more can you charge for an A322? 270/240= 1.125 so 12.5% more seats, maybe you can charge 15-20% more ? Does it pay for a new wing and all of above including at least one new FAL? It forces a Boeing move with a major investment for a later delivery date.

        • Given it is impossible to fly for more than 2,700nm within Europe, China or the bottom 48 states + something like 95% of narrow body routes are 1,500nm or less, wouldn’t it make more sense to range the aircraft for (say) 3,000nm and cut a lot of structural weight by not having to carry all that fuel for the 1-2% of routes that it can’t do? Particularly as the XLR typically has 140 seats or so and has only sold something like 15% of current A321 backlog – sure nice to have, but (say) 5% better fuel burn by having lower structural weight would be nicer, I think.
          I’m not convinced that the shibboleth that airlines buy range holds true – I’m sure it is the case when all else is equal, but for a chunky margin boost for all domestic carrier’s routes and pretty much all the other NB routes too could be the killer app.

          • I’d argue that the A321 doesn’t address this issue as the A321neo empty weight is pretty close to the A321neo – the XLR has the extra plumbed in ACT, but everything else is pretty much the same. These numbers are ball-park, but the empty weight of the A321 is is probably around 40 tonnes, plus 24 tonnes of passengers/luggage, plus 29 tonnes of fuel to reach 93 tonnes MTOW.
            However, we know the structure can carry 8 tonnes more as it is the same as the XLR. If you sized the plane for 93 tonnes, you could make the structure 8% less, or 3.2 tonnes less. That would make the MTOW to carry 240 pax full range 89.8 tonnes, which would allow a further 3.5% lighter, or 1.3 tonnes lighter at 35.5 tonnes. A few more iterations and you probably end up at 34-35 tonnes empty weight, or 12-15% less, or 5-6% less at MTOW, which would give an average weight saving through the flight of 8-10%. Even though we know that induced drag is less important at cruise, that 8-10% saving would probably save 2-3% fuel burn. Add in a new wing (with the weight saving from carbon) and you’ve probably got 5% – I’m sure most plane salesmen would sell their grandmother for that…

          • US major Delta has around 190 seats ( 3 classes) in its A321s
            Air Portugal which does fly North Atlantic to US with A321LR has 170 seats

            I think 140 seats on an A321XLR is way too low

          • You are right about 140 seats sounding far too low, I thought the same. However, I’m pretty sure I read it somewhere a couple of days ago and was somewhat amazed, but I guess it is more space constrained and very heavy on the premium class.
            it has 40,000l fuel capacity, which weighs 34 tonnes, plus 51 tonnes (wiki says 50.1 tonnes empty for the 321, the xlr will be a bit more, so that is 85 tonnes, leaving 16 tonnes pax, or 160 pax at full fuel load. Below that, it is a yield argument, not a weight one. However, I’d suggest there is little point of going above that 160 as you might as well save a bit of cash and weight and buy the LR instead, as you won’t be able to use the range

          • Yes Charles. I think for north Atlantic both ways and in Winter to get to central Europe and northern Med fringe your are passenger limited. Portugal is quite easy.
            Thus the XLR with a higher weight and some other tweaks will allow a more usual 170 or so seating.
            The US Europe routes are so seasonal you can have a widebody for peak season only….outside the major US and Europe hubs…but it’s single aisle level traffic for a good part of the year.

          • @ Charles Armstrong
            It’s a bit difficult to discuss seats in any objective way since, as you point out, an airline can elect to make a plane “premium heavy” on certain routes.

            For example, the AA Transcon A321s have far fewer seats (102) than the airline’s regular A321s (181) — not for reasons of fuel restriction, but because they can carry more premium passengers on those routes.
            Same with JetBlue, with its large Mint class section (159 vs 200)
            Same with Aer Lingus (184 vs 212)

            Emirates also has a “premium light” A380, with fewer premium seats than its regular A380s: they use it on routes with low premium demand.

          • Hello Dukeofurl,

            Re: “US major Delta has around 190 seats ( 3 classes) in its A321s
            ….. I think 140 seats on an A321XLR is way too low.”

            To date Delta’s A321neo’s have been delivered with 194 seats (20 First Class, 42 Comfort Plus, 132 Main Cabin); however, Delta announced last year that 21 of the 155 A321neo’s that it has ordered will have a premium configuration with 148 seats (16 Delta One Lie Flat, 12 Delta Premium Select, 54 Comfort Plus, 66 Main Cabin). For an A321 with a premium section of lie flat seats, and galleys to support efficient service of large number of hot meals, instead of primarily cold peanut or pretzel distribution, about 140 seats is actually about what you would expect.

            See the excerpts below from 3-28-22 “The Points Guy” post at the link after the excerpt.

            “The premium A321neos will replace a sub fleet of 18 aging Boeing 757s that were originally delivered to TWA and will fly transcontinental routes, according to Skift Airline Weekly’s Edward Russell.”

            “The premium-configured aircraft will feature 148 seats: 16 lie-flat seats in Delta One, 12 Delta Premium Select premium economy seats, 54 extra-legroom Comfort+ seats and 66 economy seats, according to the report. That’s 20 fewer seats than the 757s that these aircraft will be replacing, though those 757s also have 16 lie-flat seats at the pointy end of the plane and feature a fuselage that’s about 10 feet longer than an A321neo.

            Most Delta A321neos will feature a domestic configuration that includes a new first-class recliner seat. Those aircraft will feature 194 seats: 20 first-class seats, 42 extra-legroom Comfort+ seats and 132 standard coach seats.”


            What a pleasant surprise it is to see a comments section that is not dominated by discussion of Boeing’s free cash flow, the KC-46, COVID-19, or US/China/Russian geopolitics. It is almost as if I am reading a blog that is about aviation instead of corporate finance, virology, or politics.

          • All the more surprising as I am a stock analyst! (albeit European aerospace & defense)

          • We will see sales of A321XLR vs A321neo in the future. History from 767-200 thru 777-200 show the -300ER being the big seller. A bit similar with the A330 that it becomes more and more popular as MTOW and range improved. Once can argue that the bigger 787-9 with better range sell better than the 787-8. The A321XLR can do regular routes during the day and then fly long range over night (like transatlantic) and I suspect it will very popular if it can handle the 18-20hrs flying/day and 5-6 cycles/day

          • Jet Blue
            A-321 Neo

            Nº Seats:
            12 Business
            41 Extra Space
            102 Economy

          • AP:

            Yea my head spins when the finances start to pop up.

            None of the seating max really makes a difference as each Airline will select the best combo they think they can get (Delta thinks is can get more revenue on lie flat seats).

            Bottom line is its a range and paylo0ad and can you make money?

            Singapore was (is?) going to fly Singapore to New York with 170 seats or so with an A350-900. If it does not work they will do something else!

          • @ claes
            “The A321XLR can do regular routes during the day and then fly long range over night (like transatlantic) and I suspect it will very popular if it can handle the 18-20hrs flying/day and 5-6 cycles/day”

            Aer Lingus is currently doing this with (some of) its A321 LRs: a frame arrives in DUB early morning after a transatlantic flight, then does 1-2 European cycles, before being back in DUB again for an afternoon/evening transatlantic flight. This gives business class travelers on the European flights the advantage of “suites” with lie-flat seats.

          • A-321 NEO
            Range: 4.000 NM

            Using a narrow body aircraft, to fly a long, medium-haul route, like LIS-JFK-LIS

            Stopover in LIS 2 Hours
            LIS-JFK Flying Time 08:15
            JFK-LIS Flying Time 07:00
            Stopover in JFK 2 Hours

            If there is not a delay, due to weather conditions, technical problems etc, flying LIS-JFK-LIS, plus the stopover times, takes 19 hours. There are 5 hours left, to use this aircraft in a short-haul flight.

            If everything goes well, you can potentially use this narrow-body aircraft to perform a 1 hour, short-haul, return flight, and have this aircraft ready in LIS, to fly to JFK again

            Maximising the daily flying hours of an aircraft, is the main goal of an Airline, but in Aviation, there are always unexpected delays to take into account


          • I don’t think I made my point clearly – I totally understand the potential for the A321neo XLR – it can be low risk medium range route developer and then be used on short haul to get the utilisation up. More to the point, it didn’t cost very much to develop and makes the investment case for Boeing’s next plane more difficult by taking some of the orders.
            The point I was trying to make was for the next gen aircraft and the simple question is this:

            Which would airlines buy more of:
            (1) a 240 seater that can fly 4,700nm
            (2) a 240 seater that can fly 3,000nm but burns 5% fuel than (1)?

            My supposition is that, given well over 95% of NB flights are under 2,700nm, (2) would sell much better than (1)

            The XLR doesn’t really give the counter argument because it offers optionality of 4,700nm range without burning 5% more fuel than the A321neo. Indeed, I could possibly make the argument that it supports my thesis as there have been non-XLR A321 orders since the XLR has been offered – if the XLR is offering a very low cost option, why would anyone buy any other A321?

          • @ Charles Armitage
            Your argument is perfectly clear — but it just doesn’t seem to hold sway in the aviation world, where various practical trade-offs have to be balanced against one another. The closest approach seems to be to have standard-range and long-range versions of a single aircraft type.

            Incidentally, your question also applies to widebodies — a large portion of which seem to be used on segments up to about 4000 nm (7400 km).

            As regards the A321 XLR orders: AB allows customers to switch between variants after initial ordering, so the numbers can still change — and may very well so so once the XLR is certified and in actual use. We’ll just have to see. Some operators just aren’t interested in long range, whereas others are (e.g. Ryanair vs. Wizz).

      • Kessje

        I just realized that it is possible to fit a 7-abreast cabin for an additional 11″ inches in width than your suggestion.

        (162″ wide 6-abreast cabin vs 173″.m /7-abreast)

        In product design and industrial design, when I was a student, we always had to criticize our concept and put ourselves in a point of view of the competition for example…


    • inches of skin per seat per inch of fuselage length maybe. now multiply by 30 to account for seat pitch, then additions for non-revenue space, cockpit, bathrooms, entry and exit doors, kitchen, tailcone etc….

      • @keesje
        This is independent from the seating length. Try to wrap your head around that!

        • Salamis dont fly. You need a complete fuselage -given they have a similar nose and tail features its the section called ‘passenger cabin ‘ that matters

        • @scott and @bjorn: apologies for schooling this guy…

          @Marc: you should actually learn math before you try to teach it to people.

          you calculated the length along the circumference of a simple cylinder per column of seating for single and twin aisles, assuming the widest part of the cylinder is at the height of the armrests.

          that is only the first element in calculating square feet of surface area per passenger, and to calculate that, you need to account for not only the pitch of passenger seating, but also all the non-revenue length.

          also, the A320’s widest spot is the floor, not the armrests. the 737 is egg shaped not cylindrical (as are many aircraft)

          try and wrap your head around that.

          • @bilbo

            Outstanding! Thank you. I needed a good dose of common sense.

          • @bilbo: it’s not so hard. Try to figure it this way: skin surface is Circumference * Length. Capacity is (Seats-abreast) * Length / pitch. So Skin per seat is Circ * Length / [ (seats-ab.) * Length / pitch ] = Circ / seats-ab. * pitch. (pitch can be ditched as it’s the same).

            skin per seat is independent of Length! and is a linear number, not a surface.

            Sorry if I’m not good at conveying this. Maybe someone better than me will explain it more easily.

            BTW, the A320 widest part is not the floor but at armrest level:

          • @Marc,

            it is not linear, because as you add more passengers, you by necessity need to add more non-revenue space for more bathrooms, more stewardesses (which is a step function, not linear), more exit rows (again a step function) more kitchen space, crew rest space etc.

            you want it to be simple, but it isn’t.

          • “A320’s widest spot is the floor,”

            Armrest lower window sill ~60cm off the floor.
            ( afair there is a drawing in the ACAPS too.)

            The 737 actually is a double lobe with the floor as intersection of the circles. ( and taking the deformation strain vs full circle.)
            The x-sections from 707,737 to 757 differ in the lower lobe. separate for front and rear fuselage.

    • Hello Marc Lacoste,

      Re: :”multiplying by Pi and dividing by the number of seats across, you get the inches of skin per seat”.

      Only if the fuselage is a simple circular cylinder. For a given cabin width, using a double bubble configuration with a smaller diameter under the cabin floor, or an elliptical cross section with a height less than the cabin width, will reduce the circumference per seat below that for a simple circular cross section with the same width. Boeing’s abandoned NMA would likely have used an elliptical fuselage cross section to reduce circumference per passenger seat compared to a conventional wide body fuselage cross section.

      Here are the free bullet points from an April 6, 2017 paywall article on this blog by Bjorn Fehrm titled “Could an NMA be made good enough, Part 3. It will be interesting to see if his conclusions the present series, 6 years later, have changed.


      It’s possible to design a dual aisle fuselage with the same perimeter per seat abreast as a single aisle fuselage.

      This will make the central, cylindrical, section have competitive weight and drag characteristics.

      The larger diameter of the dual aisle fuselage will increase the size of the tapered front and rear sections however.

      It’s still possible for an NMA fuselage to be as weight-efficient as a single aisle fuselage, measured per transported passenger.”

      • Double lobe fuselage with lower lobe less than the circular of the upper lobe (flat bottomed) puts the floor in compression. Going with Euler’s structure failure cases parts in compression require more material than those in tension.

        • It might be wise to give a look to “Breguet deux ponts” layout
          Mini A380 with two single aisle levels

          lots of benefits compared to “pencil” long single aisle frames
          – very short plane, no tail strike risk.
          – very low floor for an easy access front and rear, quick rotation
          – internal stairs as in 747
          – High wing allowing high by pass engines
          – Easy palette access at the rear of lower level (A400M style)
          – extremely weight efficient structure

        • Uwe…
          With respect to the floor being in compression……. On BA aircraft, I’m not sure about AB birds because I’ve never dig around in them.
          There are significant keel beam structures in the lower lobes of Boeing aircraft. These structures change the narrative quite a bit. The floor is actually secondary structure as it is made up of light weight composite honeycomb panels attached to the upper flanges of the floor beams with floating nutplates and clearance fit fasteners. The fact that the floor doesn’t tie anything together fore/aft leaves the floor beams loaded in bending between the stanchions connecting the floor beam to the lower skin panels. There are virtually no fuselage bending loads reacted through the floor because it is a discontinious structure composed of dozens of individual composite panels situated between the seat tracks running fore/aft. They are not sized to carry any loads beyond the live load from passenger weight. The floors dont even support the seats as the seats attach to the seat tracks running fore/aft along the tops of the floor beams. Thats why a cargo conversion plan always talks about upgrading the floor beams, not the floor, because the floor is all secondary structure. There is a good case to be made that the floor beams are actually loaded in tension as the hoop tension load in the skins reacts through the floor beams as the structure pressure cycles. This tension load also helps add fatigue life to the floor beams as it reduces actual bending by preloading the structure.

      • See below for the perimeter per passenger for each of the four fuselage designs that Bjorn Fehrm included in an illustration in the free summary of his 3-18-15 paywall article: “Redefining the 757 replacement: Requirement for the 225/5000 Sector, Part 6”, and also for a Boeing 767.

        Listed from lowest perimeter per passenger (highest efficiency) to highest perimeter per passenger (worst efficiency). Efficiency will also be subject to the fuselage not becoming too stubby or too slender for near optimal streamlining, structural stiffness, and control surface effectiveness.

        MOM7-200: 194 x 176 inch ellipse, 7 abreast.
        Perimeter = 581.5 inches.
        Perimeter per Passenger = 581.5 inches/7 abreast = 83.1 inches/passenger

        A321LR: 165 x 155.5 inches near circular, 6 abreast.
        Perimeter = 500.3 inches.
        Perimeter per Passenger = 500. 3 inches/6 abreast = 83.4 inches/passenger

        NSA6-200: 163.4 inch circle, 6 abreast.
        Perimeter = 513.3 inches.
        Perimeter per Passenger = 513.3 inches/6 abreast = 85.5 inches/passenger

        NLT7-200: 194 inch circle, 7 abreast.
        Perimeter = 609.5 inches
        Perimeter per Passenger = 609.5 inches/7 abreast = 87.1 inches/passenger

        Boeing 767: 198 x 213 inch near-circle, 7 abreast.
        Perimeter = 645.6 inches.
        Perimeter per Passenger = 645.6 inches/7 abreast =92.2 inches/passenger

        Significant points:

        I) The perimeter per passenger of the MOM7-200 and A321LR are within 0.4%.

        II) The perimeter per passenger of the 767 is 11.1 % greater than that of the MOM7-200 and A321LR.

        I calculated the perimeter of circles according to Pi x diameter, the perimeter of near circles according to Pi x (Average Diameter), and for the elliptical MOM7-200 I assumed that the cross section was close enough to a true ellipse for the ellipse perimeter calculation tool at the link below to give an acceptable answer.

        • – MOM7-200: 194 x 176 inch ellipse, 7 abreast.
          – A321LR: 165 x 155.5 inches near circular, 6 abreast.

          I) The perimeter per passenger of the MOM7-200 and A321LR are within 0.4%.

          How did Bjorn slip in an extra seat+ and extra aisle within 29 inch? Are we using the same seat and aisle specifications here? Specially for flights > 5 hours or are we making some far reaching assumptions to make a flat oval MoM work?

          • Hello Keesje,

            Re: “How did Bjorn slip in an extra seat+ and extra aisle within 29 inch?”

            Bjorn uses for the following outside dimensions for the A321LR and MOM7-200 fuselages.

            A321LR: 163 inches high by 155.5 inches wide. In my post above I incorrectly cited the height used by Bjorn as 165 inches; however, I used the correct height in my perimeter calculations. See the end of this post for the details.

            MOM7-200: 176 inches high by 194 inches wide.

            This, the MOM7-200 is (194 inches – 155.5 inches) = 38.5 inches wider than the A321LR, according to Bjorn’s dimensions, not 29 inches wider.

            For the MOM7-200, Bjorn used 18 inch seat widths and 18 inch aisles. You can take a look for yourself at the link below. Fuselage cross section diagrams are part of the free introduction to a paywall article.


            A321LR Perimeter Calculation
            Perimeter = Pi x (155.5 + 163) inches / 2
            Perimeter = 500.3 inches
            Perimeter/Passenger = 500.3 in/6 abreast = 83.4 in/passenger

  2. Regarding skin friction it increases as the laminar boundary layer become turbulent boundary layer. Hence as the wing becomes longer and slender the % of laminar flow increases for the same wing area. It is not easy to keep it laminar all the way and over the “hump” of the wing upper side. Lot of research has been done since before WWII to increase laminar flow area of both wings and engine nacelles (Just look at the 787 inlet lipskins). Maybe Leeham can split comments into different folders (technical vs. political).

    • Super critical wing shapes , which all modern airliners have don’t have a hump anymore . It’s flattish.
      The CRJ wing is a quite extreme example of flat top and even a concave lower wing surface

      • In practice the lamniar boundary layer is just a few inches along its streamlines from the leading edge then becomes turbulent. Like on the Gulfstream 650 where the leading edge is polished and wing design influenced alot by Prof. Anthony Jameson.

  3. I really hope you examine aircraft layouts other than traditional tube and wing.

    I would like to see side by side double bubble for a 10 wide twin aisle and a couple of the BWB types (lockheed’s semi-BWB, McBoeing’s BWB where they graft a 767 section 42 on a flying wing, and a real, northrop style flying wing)

    I’ve done some modeling of a 10 wide side by side double bubble based on the A320 tube joined such that you have 5 seats either side with cable tension members down the keel and unpressurized fairings (like wing boots) in the valleys and an optimized section 42 and beaver tail. the fuselage produces significant lift in this configuration and results in a very compact aircraft with only 23 rows for 250 pax and two rows of LD3-45 containers in the cargo hold.

    another major factor in the economics which you don’t mention here is gate restrictions. anything that isn’t C gate on the ground is going to have a very hard time becoming the core NB replacement.

  4. It will be interesting to see how advanced materials can benefit the “flying pencil” 250 seat narrow body with regards to preventing hogging (presumably on landing) – I suspect they will be very good on the top part of the fuselage which is in longitudinal tension, but less incrementally good on the bottom, which is will be in compression. I understand that the additional weight required to prevent hogging is one of the major constraints in extending narrow bodies beyond ~240 seats.

    • Hogging is a ship term, I have never seen it applied to aircraft and the dynamics of the two are wildly different.

      • it was shorter and I felt more descriptive than “longitudinal bending forces”. I’m also an ex-aerospace engineer who sails…
        There are three causes of hogging – 1) dynamic, caused by the waves – a big wave in the middle of the ship will cause a longitudinal bending stress pushing the bow and stern down, which is pretty akin to the aircraft fuselage stresses with the wings in the middle supporting the fuselage, (waves at either end will cause the opposite effect, sagging); 2) time induced hogging, where the greater bouyancy in the middle gradually causes the ship to hog over a very long time, 3) unloading the ship incorrectly, so the bouyancy in the middle pushes up, while the laden stem and stern are pushed lower. The second and third are unlikely to have an impact on planes, but the first does.
        Interestingly, the Egyptians and Greeks knew about hogging and had a rope in tension going stem to stern above the centre line of their galleys to stop it (I thought the Vikings did too, but I can’t find any references)

        • Thank you for this info on hogging, CA- very helpful (I’m a sailor, too).

        • “Hogging” is quite a generic term for the ending effect you describe. I have seen it used for railway underframes or chassis. In this case the underframes were built with a pronounced hump n the middle, which was levelled out by the weight of the carriage body when that was lowered onto the frame.

          • The Aircraft Horizontal Stabilizer takes weight

            Equally there are not swells in the air.

            Huge difference between designing an arch into something and have it flatten when loaded than the dynamics of a ship fro flat calm to waves 40 feet or more (crest to trough, 40 being the highest most likely, not the highest possible).

            Floor trusses do not hog but we arched them so that when the weight of the floors, walls and furnishings were in the house it was level. Permanent deformation.

          • TW: “..Huge difference between designing ..”

            You may not be aware of it but the design cases aren’t too different.
            You have some “tubish” body that gets some bending forces introduced.
            in an A340-600 it was said that the fuselage bending was visible.

          • Aircraft flying at these speeds have several aero modes that are trigged by small fuselage and rudders deflections. Like phugoid (a slow oscillation between altitude and speed up and down), dutch roll (oscillatory combined roll and yaw motion, needing a yaw damper to cancel) and “The short-period mode” (is a usually heavily damped oscillation with a period of only a few seconds). The motion is a rapid pitching of the aircraft about the center of gravity and most like your “hogging”.

    • Narrow fuselage helps alot in form drag and mass, but you want something bigger than the old 707 cross section. Still every inch cost in addedd mass and drag so maybe max 1-2 inches bigger o.d. than the A320 fuselage but smaller than the Russian MC-21.

  5. So if I reading this right, the extra width for a 7 or 8 WB does not incur that much of a drag penalty as once thought?

    • I am somewhat surprised these types of film appliques have not caught on in aerospace. they have been shown to do very good things for laminar flow maintenance at the cost of requiring some extra effort keeping the aircraft clean. I too would like to see some discussion of this.

      • retention if desired properties is an issue.

        is 4 years of best use enough?

        Do you put it under or over the paintcoat or is it a replacement. ( colored plastic should not be an issue)

        • typically, as when you wrap a car, it is the topcoat. no reason it couldn’t be colored.

  6. Keesje,

    I just realized that it is possible to have two options/alternative to what those who call your concept A370 a “flying pencil”

    1 – Cabin width: 150.5” /
    6 abreast

    2 – Cabin width: 173” /
    7 abreast,

    VS your 162″ cabin width/ 6/ abreast option.

    (1) We find that “162in” is enclosed by two fairly aggressive solutions. The width 150.5” /
    6 abreast is nearly 12″ less wide, which will necessarily result in a (rather) drastically lighter aircraft.

    (2) While the option
    7-abreast for an additional 11″ inches of width over your suggestion. 173″.m /7-abreast you nob only get greater seat density but also “real” traveler transition through a twin aisle.

    I remind you that this is only an 11″ wider cabin..

    If you have an up and down competitor and the market is tight your “A370 concept” can very quickly become a proven industrial waste.

    In product design and industrial design and transport, when I was a student, we always had to criticize our concept and put ourselves in a point of view of the competition.
    Then when you “hold” your concept/bias, you will have to be creative to make improvements and add more value to your product…


  7. Commercial Aviation

    Range 4.500 Nm
    Nº Pax: + 200
    Medium Haul Routes

    I believe, a single aisle Aircraft, with more than 180 seats, is not practical in Commercial Aviation, specially for longer routes

    A Wide body aircraft, handles severe turbulence better, than a single aisle one
    Passengers feel more confident, flying a small Wide body, on longer routes
    Boarding and disembarking is usually done, using 1 door, only
    This slows down the process, and stresses the passengers

    There is not enough space for the hand luggage
    Business Class passengers are bothered during the boarding process
    In many medium-haul flights, there is not enough space for the cargo
    Airlines can not offer a good in-flight service, on longer flights
    There are usually only 2 toilets, for 170 economy class passengers

    Airlines, and passengers, love a small wide body Aircraft, for medium haul
    There are many city-pairs perfect for that type of plane:


    • I am trying to wrap my poor brain around those statements.

      People fly transatlantic all the time in single aisle aircraft. Hawaii is heavily served by single aisles as well. I don’t like being over water with only two engines myself but I have done it more than once.

      The 757 did provide 229 and 295 seats (max). I don’t know how many used that.

      Lot of trade studies on load and unload involved.

      Its all got to do with how much it cost and is there a savings and that is into some serious math for each airline and how it averages out over all buyers.

      Some previous discussion about how Asian buyers did not like the lack of Freight space for a NMA. US Freight is not a serious factor on routes where as Asia it is a much bigger factor.

  8. This might be interesting, its a power point presentation at a conference for
    advanced fuselage design methods for a new aircraft. Theres a bit of jargon and reference to digital design tools.

    However its for small light aircraft and in composite, but basic principles are still relevant- nice not to have to worry about passengers ! The G loading is very high as its aerobatic.

  9. This is all very interesting, but the launch of a new Boeing airplane program is not contingent on technical considerations such as minimizing drag. What it is contingent on is finding a way to eliminate the financial drag a new program would exert on cash flow.
    In this brave new world, casf flow is all that really matters.

    • Its more a matter of bean counters in the way of that decision not cash or even debt.

      Boeing current Management is bean counters and would use any excuse. They could even be right for the wrong reasons.

    • @ John

      You’re correct as regards BA: if there’s no funding (and there isn’t), then there’ll be no new programs. That’s why finances are discussed so often on LNA (to AP’s distress): because they can be (and are) a total showstopper to innovation.

      However, the considerations in the article apply to new programs from OEMs who *do* have access to sufficient funding — not just AB, but also others.

    • “.. finding a way to eliminate the financial drag a new program ..”

      They did that with the 787 Dreamliner.
      ( externalize all investment )

      You know the outcome.
      There is no way around major and competent investment up front.

      • Some may have missed it a few days back , but it was a surprise to many and has long flown under the radar that Airbus uses program accounting for its new aircraft developments , from A380 onwards- according to a story by Bjorn back in 2016 – but still relevant today.
        Their tweak is that its used for the ‘launch orders’ only, however many that may be, as its well known that launch orders recieve special pricing too.

        The exact details arent clear to me but Airbus also is capitalising research and development spending and says so in its accounts. Thats also a means a spreading the costs over a long ‘program’ period, as its expensed at a slow rate
        The A400M following on the A380 development/financial debacle had Airbus in the full scale financial engineering crisis mode in that period.
        But clearer skies ….now

        • That is a nice idea but completely cold. unsurprisingly.

          Airbus started per customer cost spread accounting
          on the A350.
          During the original learning phase each customers frames rang up higher cost for the first frame and notably less for the last frame of a customers ordered batch.
          Accounted per frame was the average cost per frame manufactured. Effect was slightly less outlay early on. magnitudes removed from Boeings $30++b deferred cost mountain.

          • Not so. Bjorn said the program accounting for Airbus started earlier with the A380.
            There is also one ‘customer’ that has its planes built at huge cost. Thats Airbus who still own the first 5 A350-900s. maybe 1 or 2 on top of that sold for giveaway prices .
            Qatar of course was A350 launch airline with 80! and again with the launch of A350-1000 with 42 orders
            Im thinking for Airbus the ‘launch program accounting block’ was over 100
            Well have to see from the accounts period how much the deferred production cost was

            I ve noticed how some are uncurious about this some are . For the A400 financials it seems that Airbus was thinking very creatively and considered ‘selling’ the IP for the program to the buyers and booking that as revenue. It was too dodgy

        • “according to a story by Bjorn back in 2016”

          could you help with a link to that posting by Bjoern?


            He also mentions that Airbus capitalises some development costs over the production run – which are allowed under IFRS but not under US GAAP.
            Which demolishes another shibboleth about some peoples understanding of the Airbus accounting being ‘clean’.

            Airbus also doesnt publish financial numbers for its different airliner types , which Boeing does ( hence the 787 only costs which fascinate some)

            This is also a revelation

            The key take-away is not the figures but that fact that Program Accounting gives the external person a chance to understand the true production costs of an aircraft whereas unit accounting doesn’t.

            So unit accounting hides the true cost of a program. Interesting

          • You can spare the crowing imho.
            It is not the equivalence you make it out to be.

            “So unit accounting hides the true cost of a program. Interesting”

            ROFL. Quarterly reports are supposed to represent the _current company situation_.

            I’d deem it acceptable if programme accounting numbers were used to separately give information on each programme.

            mandatory accounting reports are supposed to give a realistic assessment of the current state of company affairs.
            programme accounting munges todays data with future outlook. The 787 was the perfect example of how much missing the future outlook taints the “today’s numbers” picture.

            IMU your views are formed from living in a society that is deeply invested in giving false witness all around.

          • Uwe Boeing’s individual types are separated out. Not so with Airbus it lumps it commercial aircraft together, are you complaining . Not so apparently, it’s European so all is sweetness and light

            Boeing even displays it’s deferred production costs by program. Try getting any of Airbuses deferred costs separated out, no luck

            It’s incredible that you don’t seem to have read Bjorns analysis properly at all, the obvious conclusion is you have a closed mind

      • “…There is no way around major and competent investment up front.”
        Agreed…insofat as one is committed to a sustainable long term business plan.
        But in the case of BA management, it’s only commitment is to reward the shareholders as much as possible. Since all shareholders are effectively short term investors, long term considerations become irrelevant. Therefore, no Boeing CEO with this GE financialist mindset, will ever justify the launch of a new commercial airplane, as this would put negative pressure on cash flow and the stock price. Therefore, the most likely time for Boeing to launch a new commercial program is….never.

    • You have summed up the situation well.
      It is for the moment for them to recover the cash flow through the 787 and 737MAX. Establish certified 737MAX10, then Boeing’s “other” widebody (the 777-X) that will reap less cash flow must also be certified,

      CEO Calhoun was honest, No new aircraft before 2035!
      If its commercial launch could intervene around 2030, it must be kept in mind that 5-6 years (2023-2028) of progressive cash flow, is a small thing and a short time, it is a long cycle. Because the aerospace industry is different than any other industry.

      As you know, a civil or military aircraft program can last for decades unlike the production of bus cars, or other vehicles which are also in the field of transport. On the other hand, listed companies in general have been plagued by these practices in wanting to attract shareholders. Even the entertainment industry, such as on-demand film/series platforms such as Netflix or Disney+, which were at half mast during the Corona Circus pandemic, is seeing a loss today for having produced, according to them, several programs but not enough return on investment, so they raise prices and incorporate advertising to be more competitive.

      The direct consequence was also the share price for Disney, among other things, always with the aim of offering the most programs in no time. Make no mistake, this is the world today.

      To respond to shareholders is to respond with mediocrity. The world has changed.

      By canceling its NewMid-Size Aircraft (NMA), Boeing has reduced its risk of producing an all-new aircraft which would also be likely to be expensive, and not bring enough cash-flow while they failed between 2018-2019, to close the business case at 3 recovery.

      From 2020 and until 2022, D. Calhoun tried to launch an all-new aircraft several times but in vain.

      At the end of 2022, he did not see it either as an aircraft that inspired them with confidence in terms of production cost and cheap sale. More down to earth, the Boeing CEO is implying to us that a lot of water has to flow under the bridge before something is done around 2030-2035, and they will, because the Boeing Commercial Aircraft (BCA) branch, or any other civilian branch as well at Airbus, Embraer and others, are the ones that bring more cash flow…

  10. Lots of people have flown the narrowbodies DC-8 and 707 on long range flights back then. Of cause if you have the option you pick a fast and modern widebody like 787/A350 instead of an A321XLR. Still we live in a money world and if the flight is cheaper in an A321XLR with the same cruising speed most will fly it. A new aicraft needs to be 12-15% more ecomomical than the existing, while being quieter, a tad faster and more reliable. The production needs to be more robotized with 23-63% less manhrs to build. Not that easy to get everything right and look beautiful.

  11. Claes,

    …”The production needs to be more robotized with 23-63% less manhrs to build. Not that easy to get everything right and look beautiful…”

    Your percentage range is too large. Be that as it may, the higher it is, the less it is possible, and the more it is utopian.


    …”Still we live in a money world and if the flight is cheaper in an A321XLR with the same cruising speed most will fly it…”

    Do you really think that an A321XLR flies at Mach 0.85?…

    • The Better! Faster! Cheaper! advocates (or something..) – in an era of obviously worsening resource constraints- are cause for wonder.

    • Successful automation is hard (just ask Airbus Hamburg), still easier when you design for automation from the beginning. Germany has an advantage of lots of skilled automotive robotic line design experience but will maybe don’t do too much of it on older aircraft designs. We will see how much the new Toulouse A321neo line get robotized by French suppliers.

  12. John,

    …”Therefore, the most likely time for Boeing to launch a new commercial program is….never…”

    What about Airbus? Only the A321XLR remains to be certified and the A350 was launched 18 years ago.

    Boeing launched its last new program almost 10 years ago with the 777X, they must certify it, as well as the 737MAX10. Boeing is unable to launch anything,

    But Airbus, yes…

      • I’m fully aware about
        an Airbus ZeroE development since 2020, but it’s for a flight in 2035, just like what Boeing plans to do for his 737 replacement at “the same time.”
        This is not an Airbus -Boeing / Boeing -Airbus comparison.

        It’s just in response to a weird claim that Boeing wouldn’t have launched an aircraft between 2004 and 2030 (26 years) because they include the 787 without
        the 777X.
        This requires a major overhaul of the same order as a new aircraft launch, both in terms of aerodynamics and the integration of advanced materials that go beyond a simple re-engined aircraft, we agree with that…

        This was the flawed argument of journalists and analysts to express their disappointment after CEO Calhoun canceled all new aircraft development efforts at the end of 2022.

        However, a question comes to mind concerning the Airbus Zero-E,

        Don’t you find that many people are skeptical about the hydrogen solution? Didn’t LNA write an article where a former engineer such as P. Condit (it doesn’t matter that he worked at Boeing) had proven that hydrogen was not the right solution by exposing multiple constraints?
        If you believe it, that’s fine, but what arguments do you think would make the Airbus Zero-E the real way forward for the future and that it will be a success at a time when Boeing does not does not seem (officially) to join?

        It’s not a Boeing VS Airbus war, I’m just trying to figure out which way the industry is really heading…

        • Checklist:

          Great summation at the end. Condit his a lot of my points and thoughts and some I had not (that iTs the difference between someone who does it for a living as an engineer and an engineer in other fields).

          Airbus went with the free Hydrogen Money and Boeing went with the free TBW money.

          And pontificators don’t (won’t) address a split solution if the EU mandates Hydrogen and all other AHJ do not.

          Does that in turn mean a single aisle aircraft from say Algeria can no longer fly to Franforth?

          Obviously wide body are excluded as they would not have the range to get a few hundred miles with Hydrogen (more stuff devoted to Hydrogen fewer passengers).

          Does this in turn shift travel to cars and trains in the EU and the single aisle is no more? Or do you subsidize the single aisle mfg?

          It all gets pretty whacky. Condit said it and Bjorn has supplied his estimates of both range and passengers for a Hydrogen single aisle and they are half of what is current.

          You look at RISE and the EU has had a thing for open rotor and that is where they put the money.

          Make it and the passengers don’t come?

          TBW is also a gamble even if it works its an odd looking aircraft that people are not used to.

          A lot up in the air (if you will pardon the obvious pun)

          • Even more convoluted that normal — impressive 😉

          • TransWorld,

            Hydrogen doesn’t just revolve around Europe. Also look at Embraer’s projects…
            They give an indication of the position of the engines or fuel cell


            Airbus says -> 1000nm / 100 passengers with 2 fuel cells, is just an example

            They can work very well in several directions

            We know this good old law in aeronautics:

            “fewer passengers fly further”

            they can also adapt very well ~2000nm / 50 passengers with 2 fuel cells.
            2000 nm gets quite interesting but 50 people a bit less.

            Airbus claims that a BWB would be the most optimal solution to accommodate hydrogen tanks, but not only, because a BWB would bring more aerodynamics to fly further for example

            In addition, a small 1-class 100-seater BWB would, in my opinion, make its feasibility easier and make its wingspan shorter than a “large” BWB on airports, and would solve the certification problems with doors that would be sufficient for the numbers of 4. Again, its small size would also solve the problems of the sick passenger sitting at the end because the cabin would be only 10 abreast spread over 3 wings while the middle of the cabin would have a horizontal aisle with doors 2 and 4.

            If boarding is through door 2, on the left there is the cabin with 3 rows 3-3 like a narrowbody.

            the 4th row and 5th row are 10 abreast wide, but they leave room for two lavatories in each end with an aisle on each side for access to the 2 lavatories on each side near doors 2 and 4. Finally it is (Aisle -lavatory/3-3/lavatory -aisle)

            Above this part there is the cockpit and doors 1 and 3. This part is as wide as an A320 fuselage and has a lavatory near the cockpit. Gates 3 and 4 are for chartering catering carts.

            On the right when you go through gate 2, on boarding the cabin opens exponentially and reveals 7 rows 10 abreast (Fret/aisle/2-3-3-2/aisle/fret) no more so that passengers at the back of the cabin can quickly evacuate to the upstream side of the cabin. This configuration would (normally) meet the certification requirements, while the cargo area continues to extend to the 4th row. The last rows 5, 6 and 7 are “embedded” in the front part of the fenders.

            It’s beautiful, it’s the dream aircraft, but I can’t say how far it would fly…
            But one thing is sure is that they can size it for 100 passengers with for example 4 fuel cells. If it flies 2,000nm, and even 2,500nm when it takes advantage of an aerodynamic BWB. Just my 2 cents 👍

            TransWorld I am convinced that a hydrogen BWB 100-seater would do the job which would be something other than “weaker than a train in Europe”…


    • One of the differences between AB and BA is how easy it is to reduce their workforce, including engineers.
      BA can do that easily but AB cannot.
      So while BA can decide to reduce their workforce since they don’t want to design a new airplane in the next few years, AB will have to find something for their engineers to do.

      While recent events might have reduced the number of older engineers working for BA, I suspect they will still have a higher average age, so they will be losing more experienced engineers than AB as time goes on.

      So I do expect AB to be continuously improving their products even if no moonshot is available. A220 cost reductions possibly, leading to a profitable A220-500 ?

  13. @Claes

    Boeing had done the same for its 777X. Tested for KUKA Systems on non-X 777s, this was to extend to his all-New Triple 7-X program.

    In 2019 (a nightmarish year for Boeing)
    Boeing announced the withdrawal of these robots, which were a failure.

    I would add that in this disappointment, this one could (among other things) harm the plans of the NMA.
    Remember that there were several failures to close the Boeing New Mid-Size Aircraft business case between 2018-2019…

    Even for Airbus, for the moment it’s only theory on paper IMHO…

    • Checklist:

      The automated riveting system was an interesting try, but I would not call its a failure, just a painful lesson. What works or does not and can the state of the art deliver?

      A technician will tell you he learns from his failures not his success. The failure can be an miner as a test sequence that was wrong. You re-think and try again. Success just means you knew the system well enough to diagnose it or the right path to fixing it.

      With Boeing of course that means the people that learned the lesson need to stay with Boeing. That is why continuity (electrical pun) is more important to that sort of work than the work I did.

      I saw a number of failed diagnosis by others and then they just replaced the whole thing only to find out the new one had all the same issues and they needed to address the issues. The one 90 deg conveyor they finally found 6 different issues with (the guy was tenacious and the management was not going to let them swap in another conveyor). It took them 3 years. It was not my area and when certain parties got involved I made sure not to be around, like talking to a rock and I had lots I could fix.

      • Yes it is hard to automate old aircraft designs not designed for robots. It is much easier if the design is new and designed for robots. Like in the car industry. I know the size and tolerances are different, still when volumes go up and the fuselage is standardized for a few years it can be done. Just like th e Ford built B-24 Liberators requiring a standardized fuselage, great program on History channel.

  14. @TransWorld,

    I think that Airbus would also have resources for an alternative aircraft to Zero-E in 2035. They will be ready to launch a response to Boeing whatever the choice of Boeing around 2030…

    There is also nothing preventing Airbus from doing both.

    Everything leads us to believe that it is preferable for a hydrogen aircraft to be small in size because the logistics and the infrastructure on the ground to transport the hydrogen is quite limited and more complicated, as we also know, hydrogen takes plenty of room in the tanks,
    But do you ever know…

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