Bjorn’s Corner: New aircraft technologies. Part 24. Single or dual aisle fuselage?

By Bjorn Fehrm.

August 4, 2023, ©. Leeham News: This is a summary article to Part 24P. Single or dual aisle fuselage. It summarizes the drag and weight simulations we have done on fuselages for the next generation Heart of the Market airliners. When it comes to drag and weight for a 250-seat airliner that shall replace today’s Single Aisle, is a Single Aisle fuselage lower in drag and weight than a Dual Aisle fuselage?

Figure 1. The drag components of a Single Aisle airliner at 35kft and M0.78. Source; Leeham Co.

Drag and Weight of a 250-seat Single or Dual Aisle fuselage

We looked at what type of fuselage will be needed for the replacement aircraft for the Single Aisle segment during the 2030s in Part 5 and how to manufacture such fuselages in high volumes in Part 6.

The conclusion was that the expected further growth of the Heart of the Market, which presently is around 200 seats, will continue to grow and be at 250 seats during the first half of the next-generation aircraft lifespan. What we didn’t do is a drag and weight comparison of the two fuselage types to understand the consequences of using one or the other.

With the capabilities of our Aircraft Performance and Cost Model (APCM), we can make such a comparison. In order to make it “Apples to Apples,” we have designed the two different aircraft types so that:

  • They are classical tube-with-wing designs with identical engines, wings, tails, and systems.
  • We use the fuselage types we discussed in Part 5, except that we use a more optimal fuselage type for the single-aisle than the 40-year-old A320 cross-section, Figure 2.
  • They have the same passenger capacity in a typical Us Domestic two-class cabin with 24 First class seats at 36-inch pitch and 226 Economy slim line seats at 29-inch pitch.
  • The single-aisle is four abreast in First class and six abreast in Economy. The twin-aisle is six abreast in First class and seven abreast in Economy.
  • All weight data and limits are held as identical as is reasonable to let the two different fuselage types demonstrate their difference in drag contribution and weight.

The fuselage types we compare are a wide aisle single-aisle fuselage that has the same drag and wetted area and lower weight than the A320/A321 fuselage type to an elliptical dual aisle fuselage that has the same outer skin circumference per seat as the single-aisle fuselage, Figure 2. Both fuselages are manufactured using a Thermoplastic composite material system

Figure 2. The compared wide aisle Single Aisle fuselage to the elliptical Dual Aisle fuselage. Source; Leeham Co.

The results when we apply these fuselages in a representative “ Heart of the Market airliner” with 250 passengers capacity and 3,000nm range is:

  • Both fuselages are pretty close in drag and weight.
  • The single-aisle fuselage has a slightly lower wetted area (which decides the dominant friction drag, Figure 1) and lower pressure drag factor (due to lower cross-sectional area), but this is a minor drag.

On the other hand, the long fuselage, with a fineness ratio well over 10, is not structurally optimal (too long and thin). The weight of the fuselage employed in a real aircraft is, therefore, higher than for the shorter and more optimal dual aisle fuselage.

We can see that both fuselages will result in aircraft with similar characteristics. The major difference will be the development potential for the aircraft as it goes from its first 25 years of its 50 years life into the second half.

The dual aisle alternative will have ample stretch potential where the single-aisle is stretched to the limit. At a 50 meters fuselage, the single-aisle gates in airports are already in danger due to the tail of the aircraft stretching into the taxi area at many airports.

37 Comments on “Bjorn’s Corner: New aircraft technologies. Part 24. Single or dual aisle fuselage?

  1. Great approach to choose the critical items such as materials, engines, wings etc the same. Though if the basic airframe is lighter it drips through everywhere and adds up.

    IMO choosing 4 abreast for First on the NB is remarcable. If choosen 5 abreast (middle seat visibly wider like on A220) that seens acceptable & the seats are wide enough. It has significant impact on CASM.

    Anyway taking 250 seats all economy to determine aircraft lenght is more representative IMO, as most NB’s globally fly single class configuration today.

  2. An elliptical fuselage cross section introduces out of plane bending loads that will require more structural weight to resist. Also, wider fuselages have higher pressurisation loads (assuming the same differential pressure). For these two reasons you cannot assume an apples-to-apples comparison. And stemming from this, the engine, wing and tail surfaces are likely to be bigger on the widebody, perhaps the landing gear too.

    The widebody has more 7/6 more passengers’ luggage per pallet to cope with than the narrowbody, and it will carry fewer pallets in total since it is shorter.

    The decision to have four first class on the narrowbody (down from six) is flattering to the widebody (six down from seven). Five first on a narrowbody is achievable.

    Picking pitches of 36″ and 29″ is flattering to the narrowbody. Larger pitches are likely which would reduce the passenger load well below 250. I’d agree with the verdict that 250 pax is the maximum or more than the maximum a narrowbody should be designed to take.

    I have no idea weather or not a simple extrapolation is sufficient to determine the middle of the market in 30 years time. I do think that to exist in the 30 years time the aircraft would have to be competitive at EIS in five years time (say) and every year thereafter. So my question would be “Can a new build 250 seat seven abreast widebody (with new engines I’d think) be competitive with a derivative 220 seat narrowbody in the year 2030?”

    And though all the above is relevant, no aircraft ever went broke. It’s airlines that go bankrupt not their equipment. The issue will be decided at the airline economics level, not the aircraft economics level.

    • JetBlue has a fantastic, and very popular 1-1 Business Class seat configuration on the A-321

      Qatar Airways has a 2-2 Business Class layout on the A-320.
      Many Airlines are planning a 1-1 seat layout on their A-321 for medium-haul flights.

      Passengers are vey happy with this design

    • “The issue will be decided at the airline economics level, not the aircraft economics level”

      So we have Lleeham News analysts and readers dreaming of “more aisles; wider aisles, etc.”, while airlines managers force Airbus to give up their “18 in. standard” and squeeze everything to give them a 10 abreast economy class on an existing 9 abreast aircraft.

      A couple of problems for a new middle-of-the-market wide body aircraft:

      – the 6 abreast narrow body competitors are so efficient. I doubt any airline would jump into a 7 abreast wide body of same pax capacity and range of their existing narrow bodies. So the thing is gonna have to be bigger than the 767 to accommodate a tight 8 abreast budget economy class and make the economics work more favorably, not smaller. So it needs to sit somewhere between the 767 and the A330, more like a 532 cm wide fuselage, to make sense. Not 492 cm, at all.

      – the 747 (the original wide body) has its roots in the substitution of cargo ships and ocean liners and much of the wide body, long haul business have it as a measure to this day. Clearly, there is room in the market now for even a long haul aircraft that won’t take trunks, jet-skis, home appliances in its cargo bay. just reasonable passenger luggage – just as it’s done with regional and narrow body aircraft. The success of Airbus’ LD3-45 for their narrow bodies might show the way to go, a new “middle-of-the-market” container size that can be both, larger than the LD3-45 to cope with all the extra checked baggage and smaller than the std. LD3 to respect the overall size constraints of the new class of airplanes.

      Now, the A220 and the A320/321 are some 10% taller than wide. Are they elliptical? They are some of the most efficient aircraft in their respective markets. If manufacturers can go the other way around and make planes wider than tall, maybe will see some nice new wide body airplanes arriving soon enough – just don’t expect wider seats or aisles or any gain in space-per-pax in basic economy, cos’ airlines say 17 is the new 18.

  3. Locking at the differences between 757-300 vs 767-200. The empty mass goes from 64 470kg to 80 310kg, MTOW from 123 830kg to 142 880 kg for a similar pax load of approx 240 in 2 class. So a difference of approx 15 – 19 tons. That hurts drag due to lift as well as form drag from a wider cross section. Still from cargo handling and pax comfort the widebody wins. I doubt a full carbon narrowbody is needed, wings yes, but only parts of the body is cost effective in carbon vs. Al-Li alloy. On the narrowbody you want boarding from both front and rear doors. I think I know which one Ryanair would pick…

    • much of that extra weight is due to the 767 being designed from day 1 to carry the structure & fuel and wing for the 7000 nmi range 200ER vs 3400 nmi range of the 757-300.

      if the 767 were designed from the ground up for a 3400 NMI max range it would be much lighter, perhaps within a few tons of the 757-300

      • Wow, that is interesting. Thanks Bilbo, I thought so, even at 80 tons OEW for the 767-200 its impressive. Especially compared to today’s widebodies.

  4. Interesting. Thank you, Bjørn.

    I think we can expect further growth in aircraft size, but you have chosen quite a large aircraft as an single aisle replacement. With 250 seats two-class, this is approximately 50 % larger than the A321neo.

    Another trend is the move to single class aircraft, with US domestic as an outlier.

    It would be even more interesting to see the same analysis with a 250 seat single class aircraft.

    Today I was on a Wizzair flight, an A321neo with 239 seats. My first flight in a high density A321neo. It didn’t feel any different from sitting in a 737-8 with 186 or 189 seats, the most common type I fly with.

    What I’m wondering is where the crossover point is. At how many seats (single class) is a twin-aisle (7 abrest) a better choice (efficency) than a single aisle aircraft (6 abreast)? Aisle width, seat width and pitch should be the same on both aircraft, in order to compare apples to apples.

    • Wizzair has 3 toilets for 230 passengers
      For a medium-haul flight, this is not acceptable

      Passengers have to wait a long line
      There is no room, to wait in line, comfortably
      There is no privacy

      Passengers seated in the last 5 rows, have to endure people standing in line, throughout the flight

  5. 7 abreast 2-3-2 would be a big plus for passengers.
    I miss the 767.. too bad Boeing is not doing a new
    plane any time soon.

  6. I did an extrapolation stretching the A321XLR to a A322NEO, trading range for capacity.

    I took 30 inch pitch for economy and fixed galley and lavatory rates and current technology.

    I ended up with a 249 seat cabin, aircraft lenght 49.3m and a remaining range of 5-6 hours, the XLR’s MTOW of 101t. And most importantly, a 55.5t OEW, short time to market, strong A321 commonality and limited investment.

    • Yes, the A322 is in Airbus computers and would get the new carbon “wing of tomorrow”. Boeing has its versions in its computers as well hence they need to make someting much better/cheaper to make as Airbus can quickly get started with 1000 orders from IndiGo/Wizz and a few others. Airbus is busy getting more A321neo FAL’s up and running and will not move until Boeing decides the 737-10 needs a replacement.

      • I think the improved A321XLR wing, with strenghtenings, new flap system and 35k lbs ready cowling is ok for an A322NEO.

        Enventually there will be an A300/A310/A330 replacement, but that might take years.

        P.S. I think the figure 1. in the OP cannot be assumed to be similar for a flat 7 abreast cross section.

        And the seats of 2-3-2 abreast cabin should be as wide as on the 3-3 NB.

        • It seems in the figure the Dual Aisle is getting an extra seat, an extra aisle and stronger/thicker side fuselage in just 0.89m.

          Not sure we aren’t benefitting the dual aisle here a bit, by using different seat/ aisle specifications, instead of the same ones.

          Either the Dual Aisle cross section could/should be wider or the Single Aisle narrower to ensure an apples to apples comparison.

          • the dual aisle alleviates the need for a wide aisle in the single aisle (not that it wouldn’t be a nice to have, but the wide aisle in the single aisle is more necessary)

  7. Bjorn,

    have you investigated the side by side double bubble?

    taking an A-320 sized fuselage section and slicing off one lane of seats and joining them resulting in 10 wide (3-4-3) economy, 2-4-2 domestic first and two lanes of LD3-45 in the cargo bay, you get 24 first class and 230 economy in 26 rows for a very short fuselage.

    you also get significant lift from the fuselage to offset some of the skin friction, while maintaining the structural benefits of a cylindrical fuselage with only tension members down the fuselage “keel” and the opportunity to fair in and use the unpressurised “valleys” outside the keel area for aircraft systems that do not require pressurization.

    here is an example crossection of the resulting fuselage:

  8. The aisles can be discussed, the SA seems to have a very wide one (24 inch), the DA uncomfortably narrow ones (18 inches). But you have to make sure the 3 seaters on both aircraft have the same width, 61 inch, 1,57m. And the 2 seaters should have the same seat pan and armrest widths.

    Applying that, the fuselage difference of the proposed SA and DA (0.89m) seems very small.

  9. You are way to much driven by an eliptical cross section.
    For a 230 – 270 pax aircraft, dual line is more or less pre-given if an OEM wants to distinct himself from what flies today. And the most logic barrel is a round barrel with 188 inch in diameter and a 2-3-2 seating. And I think you are also putting way to much into the LD3-45 in the lower deck. Have a look at your Figure 2 and how much useless space is next to the LD3-45 shape in the tunnel areas. And with the above size, it would fit nicely in a Cat. C aircraft at a max. weight of 135 ton (300 klbs)

    • Theoretically you could consider a 1.5 aisle cross section. Making the aisle wide enough for passengers to by-pass each other or crew with trolleys. And to have a 1-2-1 dual aisle in front (everybody aisle access).

      Might migitate some of the disadvantages (fuselage stiffness / inflight moving around) of high capacity/ single aisle configurations. While retaing SA advantages (weight).

      • The argument against the a dual aisle is the second aisle is wasted space. Isn’t the .5 exta width also wasted space?

        Is the eliptical or dual aisle as punitive as some opine against the single aisle?

        Someone is going to build this, be it Airbus or Boeing, someone is going to build a domestic people mover. The day of the WB domestic has returned.

        • William, an 1.5 aisle gives the opportunity to overtake people doing their luggage, during (de)boarding. Speeding up the disembarking process.

          Something impossible using an 18 inch aisle. Also if there are 2 of them..

          And it saves ~10 inch cross section, while creating extra luggage space per passenger.

          • @ Keesje

            I understand the premise of the extra wide aisle. I think you are stating an extra wide aisle saves 10 inches in width. I am stating is it enough difference to really matter. Of course it up to the airlines and not the pax, but I think they would prefer two standard aisles and 2-3-2 seating compared to 3-3. Its only 10inches, yes I know that brings extra weight, but is it enough to make a difference? I think these articles by Bjorn are showing it does not.

            On a similar note, I prefer the A330’s 2-4-2 compared to the 787’s 3-3-3

    • Seems you are describing the 767-400, though I believe its OEW is lighter than 135 tons.

      How the heck did Boeing design in the late 70s an 80 ton OEW 767-200 and today every WB skipped the gym?

      • 767-400 OEW is 229klbs, A330-200 OEW is 265klbs.

        but the A330-200 is the smallest of the 330 family, vs the “simple double stretch” 767-400 (I know, I know, not really “simple”)

        the A330-200 has 1700NMI greater range (30%) and carries a wing & landing gear designed not only for the A330-300 mission but also the A340, so it has extra structure for the higher MTOWs and outer 2 engines.

        A330 also has a much larger diameter fuselage designed for LD3 containers where the 767 has a much smaller lower lobe for LD2s

    • New container dimensions to snugly fit an oval fuselage cargo hold is doable if it becomes popular. Guess LD3-45 height and wider. Still the 2-3-2 seating is only 1 more compared to 3-3. A 3-2-3 seating gives you 8 in a row and the popular middle twin seats can sell for a higher price and be for pax with special needs.

  10. This is what I think, from an Airline perspective:

    A narrow-body aircraft, with more than 190 seats is not very efficient
    Boarding and disembarking is very slow

    There is not enough space for hand luggage
    This delays the boarding, and stresses passengers and crew
    Sometimes, some luggage can not make it, because the holds are already full

    Organising the passenger service, in a narrow-body aircraft, with more than 35 rows is complicated, because there are only 2 galleys (Fwd-Aft)

    Passengers flying a narrow-body aircraft, for a medium-haul flight, feel more stressed, than on a wide-body aircraft. They feel cramped and confined. Specially when flying over the sea, or when severe turbulence happens.

  11. What passengers love, when flying medium-haul flights?

    All passengers love flying a wide-body plane
    Frequent flyer passengers love small wide-body aircrafts
    There is always room for the luggage

    Boarding and disembarking is super fast, and seamless
    The inflight service is more personalised
    The cabin crew can have a direct contact with all passengers

    When severe turbulence happens, wide-body aircrafts have more inertia
    This makes the event less stressful for passengers

    On a wide-body aircraft, the crew has more options, to respond in a medical emergency. Specially when flying over the sea, or inhabited areas.

  12. In the comparison based on weight and drag, there is one factor mentioned in the article that seems to me to introduce a bias.
    If fuselage fineness can be equated with the lift/drag (CL/CD) ratio, then an elliptical fuselage at a given AOA may have a different CL. However, we’ve learned that the fixed wing rigging angle AOI (between the wing chord line and the fuselage symetrical axis) is generally determined by an AOA that give max cruise CL and minimum fuselage CD. Isn’t there a form of aerodynamic bias ( before any structural impact ) in this fuselage comparison, by making it independent of cruise AOA ? In other words both aircraft ( with same wing and different fuselage) will have a different AOI setting to fly at the same AOA?

    • This is what you fix with the micro-movable flaps on the 787 and A350. You can, at all weights, set the wing and fuselage AoA to the optimum values. A new heart-of-the-market airliner will certainly use this almost-for-free technology.

  13. Boeing will have to consider Airbus could launch an A322NEO, capacity for range variant tomorrow.

    Based on the 102t MTOW XLR, wings, fuel , engine and landing gear. A 4-5 row stretch. With enhanced GTF’s and LEAP’s.

    It seems this prospect & back of envelope calculations prevented Boeing from moving on with an NMA through the years.

    It would not be as capable as a new NMA, but unbeatable on economics on 90% of foreseen routes.

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