Bjorn’s Corner: New aircraft technologies. Part 11. Airframe.

By Bjorn Fehrm

May 5, 2023, ©. Leeham News: In our series on the different technologies available when developing next-generation airliners, we have covered the fuselage configuration and engine options.

Now we turn to airframe technologies. We will look at different airframe architectures, their advantages, and disadvantages. To support the discussion, we will model the different variants in our Airliner Performance and Cost model to understand their characteristics.

Figure 1. The Boeing truss-braced wing is a candidate for the next Heart of the Market airliner. Source: Boeing.

Airframe alternatives

The classical tube and wing have been the configuration for airliners for 100 years. It has seen small variations, the main one being the move of the engines to the back of the airplane, first by the French Caravelle (Figure 2).

Figure 2. The Sud Aviation (now Airbus) Caravelle with Rolls-Royce Avon engines. Source: Wikipedia.

This configuration, which was adopted by Douglas for the DC-9 and BAC for the BAC 1-11, was driven by the very noisy straight jet engines of the 1950s. When put behind the cabin, very little noise and vibrations remained in the cabin, a major change from the wing-placed piston-prop, turbo-prop, and jet-engined airplanes of the day.

As the design is inherently heavier than a design with the engines on the wing, once the engines changed to quieter turbofans, the now ubiquitous tube and wing with engines-on-wing configuration became the standard.

Its advantage is that the high forces from the wing, the engines, and the landing gear are concentrated in a limited area around the wing roots. It limits the amount of structural beefing to handle these loads. The mass of the engines also acts as a counterweight to the wing’s wing-root bending moment.

The introduction of open-rotor propulsion can change the facts so that rear-mounted engines can return, Figure 3.

Figure 3. The integration of open-rotor engines can favor new design configurations. Source: CFM.

Boeing has, together with NASA, studied a further development of the tube and wing concept, where truss braces are reintroduced to lower the weight for extremely wide wing configurations, Figure 1.

The concept is not new. The French Hurel-Dubois HD-31 program developed ultrawide-wing airliners in the early 1950s using truss braces for the same reasons, Figure 4. We will analyze this configuration and discuss its advantages and disadvantages.

Figure 4. Hurel-Dubois HD-31 ultra-wide wing to reduce induced drag. Source: Wikipedia.

Is the time ripe for the more radical Blended Wing Body configuration? A team of specialist BWB pioneers that formed JetZero think it is. The CVs of the company’s management are impressive, and they have NASA support to develop a subscale demonstrator that will fly later this year.

The design for a 250-seat airliner, Figure 5, could use the present 200-seat CFM LEAP and Pratt & Whitney GTF engines as it has lower drag and weight than an equivalent capacity tube and wing design.

Figure 5. JetZero’s 250-seat Heart of the Market airliner concept. Source: JetZero.

We will model this aircraft as well and see if the claims regarding weight and drag are plausible.

So, will the ubiquitous tube and wing with wing-mounted engines be the sensible choice, or is it time to stir the pot a bit? We will know more when we have some data to analyze on top of rendered graphics.

33 Comments on “Bjorn’s Corner: New aircraft technologies. Part 11. Airframe.

  1. Regarding blended wing designs, it will be interesting to see what regulators say about numbers/placement of emergency exits and number of aisles (3 or more?). Such designs do, after all, have a relatively large passenger area volume relative to the surface area along the sides of the plane, which may/will complicate the process of emergency evacuation.

  2. I’ve been waiting for blended wing body my whole career – it was scheduled to be the exciting new design challenge when I was studying 30 years ago…

    Regarding exits, I expect there will be rear exits and some system using roof and floor hatches around the centre of the body. But they’ll have to think of some way to assist getting up and out – ejection seats? 😀

    • The most relevant issue for BWB from what I can tell is the amount of movement passengers outboard would feel. Further out you get the more the distance you move in a turn.

      They do talk about Windows but I think that is simply wrong. Only the people on the fuselage seats can see decently and often those people close the shade. That I do not get, waste of a good window seat for those of us who like to look at the world go by.

      The TBW also has regualrory challenges so it will be interesting to see how they work that out if its produced.

  3. I think the engine configuration such as on the JetZero, means that it should be certified as a single engine aircraft. Because an uncontained failure of #1 engine could easily take out #2.

    Compared to a conventional tube & wing, the JetZero design seems to have more wetted area, frontal surface and is probably much heavier. Among many other constrains.

    IMO there’s a reason these BWB designs for passenger transport remained graphics for 70 years.

  4. BWB designs might fit the military first that need them for stealth tankers. To make a commercial airliner from them is harder, lots of small problems to solve like anti-ice fluid collection onto the upper flat surfaces after lots of ice has melted, procedures for engine change and new rules after DC-10 aft engine failures. Letting the military discover all problems and have tax payers foot the bill to fix them during the first 10-12 years of operation is a smart way to go.

    • There is the problem of having pax spaced further out so when a BWB banks the movement for those pax are much more severe.

    • FAA has received a report indicating cracks on the end stringers of two model 747/8 at station 2285, left and right sides, during a foreign object inspection in preparation for airplane modification. Boeing has also reported similar cracking on other 747/8 airplanes. See Boeing alert bulleting 747_53A2909 RB or AD -2022-01309 docket no FAA 2023-0157 no wonder Air Force One is delayed and Boeing is blaming the contractor for the delays in the project instead of themselves.

      • Really??? This is a door 5 frame end issue found in 2022 and has an AD note that says if it passes eddy current inspection.
        How you can stretch that into being the cause for Air Force Ones problems is an incredible leap in logic. If you want to talk about the AF1 issues, lets start with the electrical system removal, plugging all the adel clamp holes, documenting that process AND creating a completely new electrical system with explosion proof wires switches and terminations. Then you need to produce all the PI drawings for the routing and clamping of the new bundles and revise every component drawing to show thw locations of all the routing and clamping components. Thats just the electrics……. Pontificating as you do isnt even remoely accurate… BA is doing a horrible job on a huge package that their young inexperienced managers didnt understand the scope of work, but you need to be factual instead of providing comic relief……

        • Then you have to prove EMD tolerance and have separate routings for battle hardness. Any late change or added requirment/replacement of boxes and you redo it again with analysis and testing together with documentation that it all meet USAF specs…

          • I was barely scratching the surface. Lets not forget all the plumbing for A2A refueling….. People outside the industry have little understanding of the scope of work and just how many moving parts are here…..

          • And the missile warning systems with countermeasures all integrated into a wires an pulleys aircraft that is nuclear EMD proof that also has a gasoline heavy car “The Beast” on board (interesting how they will contain any leaks/fire from this gasoline fuel inside the “flying garage”). Most have been done before on the existing VC-25A still with 2023 type USAF specs it is not a simple carry over unless there is severe arm twisting to get the old specs approved. Not many engineers want to be “the project engineer” on this one I assume. Maybe the one responsible for the B-52 reengine/upgrade USAF certification to the same 2023 USAF spec’s can be forced to take over it.

          • The 747-8 AF1 doesnt have the air to air refuelling its predecessor had. It has of course a much longer un-refuelled range

    • I wonder if it helps to “blend” the engine into the wing, or body also. Like the early French jet, or Chuck Yeager’s Ride – The Fastest Man Alive…

      • That was an early assumption by the Brits (not familiar with the French side of things) . Boeing followed with a semi blended engine on the 737, but that was a ground clearance aspect (the 737 was small and for smaller airfields)

        It has structure aspects that don’t pay off as well as taking up space and access for maint.

        • Hanging the engines out there has advantages for uncontained failures, cooling, flutter mngt, MRO, optimal wing shape, stowing of fuel and landing gears..

      • The Caravelle , the first french jet airliner had rear fuselage engine pods- itself a first. They werent blended.
        The first jet airiner prototypes to fly the Comet and the Canadian C102 had wing engines with the Comets integral to the wing root while the C102 had the twin engines below and forward of the wing but more integrated to the wing surfaces than the similar early B737

        Boeing had the first below wing podded engines on struts in the B-47 alongside the highly swept wing as they had their own large scale wind tunnel built in early 40s at then huge cost to refine and confirm their aerodynamic calculations.

  5. The BWB design will be inherently quieter in terms of community noise, because the fuselage is a noise barrier between the engines and the ground. I have a feeling that the TBW will win out, since it’s more compatible with the existing aviation infrastructure (terminal design, jet bridges, etc.).

    • On the other hand: we already have an increasing number of ground collisions during towing/taxiing, and that problem’s likely to get significantly worse if planes have much greater wingspans…

      • Hello Bryce,

        Re: ” … that problem’s likely to get significantly worse if planes have much greater wingspans…”

        That is why Boeing’s TBW design studies include folding wings to allow the use of ICAO class C gates. The problem with larger wingspans is not just towing and taxi. The vast majority of gates are designed for 737/A32X class aircraft, and airports would require expensive redesign just to let larger numbers of larger wingspan aircraft park next to each other. Either airports would have to be significantly enlarged, or the the number of gates would have to be significantly reduced if 737/A32X replacements had larger than type C wingspans.

        “The design was presented at the January 2019 AIAA conference and the wing folds outboard of the truss to use airport gates for the 118 ft (36 m)-span 737.[3] (ICAO aerodrome code C).”

        • Yes, folding wing tips are the topic du jour at the moment, but they’re not free of controversy.
          In the case of the 777x, only a relatively small portion of the wing has to fold, and its hinge base is relatively wide compared to its length. In a TBW design, the folding portion will probably be a lot longer, and will also have a relatively narrow (and thin) hinge base. This presents mechanical challenges, which will need very careful consideration…

          • Hello Bryce,

            Re: “In the case of the 777x, only a relatively small portion of the wing has to fold, and its hinge base is relatively wide compared to its length.”

            This is true; however, since before World War 2 most US Navy carrier based aircraft have had wings in which major portions fold to allow aircraft to be packed close together in the limited deck and hangar space of aircraft carriers. In transport or patrol aircraft with wing mounted engines, the wing fold hinge has typically been just outboard of the engines (including A3D, C-2, E-2, S-2 and S-3).

            In the carrier deck video at the link below an EA-6 Prowler (MTOW 61,500 lbs) is seen folding it wings and an E-2 Hawkeye (MTOW 57,500 lbs) is seen unfolding its wings. For comparison the MTOW of a CRJ-200 is 53,000 lbs, and that of a F-28 Mk 1000 or 2000 is 65,000 lbs.


            I believe that that the heaviest US Navy carrier aircraft with folding wings was the Douglas A3D Skywarrior (in service 1956 to 1991), a subsonic jet bomber with two jet engines mounted in pods under a high swept wing, in which the wing fold hinge was about halfway between the engines pods and the wingtips. This aircraft had a MTOW of 82,000 lbs, similar to a BAC-111-200 (78,500 lbs) or a DC-9-10 (82,000 lbs).


          • Hello AP,
            Yes, all true.
            But what is the air time of the average navy plane on an aircraft carrier compared to a commercial airliner?
            Commercial airliners are almost constantly in the air, whereas navy planes spend most of their time on the ground.
            Such a difference matters when dealing with statistical failure processes/rates.

          • the navy has had thousands of supersonic jet aircraft pulling 7-9G with folding wingtips on very, very thin wings for what, 70 years now? all while doing carrier landings (think of the worst commercial landing you have ever experienced multiplied by 3) in corrosive salt water environments being maintained by 18 year olds with 12 weeks of tech school training.

            folding wings are a solved problem.

          • And, yet, the FAA has a whole list of points that need to be satisfied as regards folding wingtips in the 777X:


            One can assume that the EASA will have even more.

            Might have something to do with the fact that commercial airliners have a LOT more people on board than navy carrier aircraft…and commercial airliners don’t have ejection seats.

          • Conditions for EASA and FAA will be met. The gist of the previous comments its not rocket science and plenty of data to go back to.

    • Cascadian:

      I have not seen a blended wing for the Single Aisle sector.

      TBW is the best I have seen for what Bjorn calls the heart of the market. But they do have gate width issues to deal with.

      777X is one solution for too long wings though I don’t know you can make it work for a TBW (and have not seen it addressed other than note its an issue)

      We could go to using the raised passenger transports from the terminal to the aircraft.

      Dual loading would not be a bad thing.

      • Yes, with blended-wing body being more suited to larger capacities, and TBW maxing out at M0.80 in cruise (which works for narrowbodies/shorter-range aircraft but is marginal for larger/longer-range ones), both airframe types theoretically could coexist in their separate niches. That would be cool, but I’d expect one to win out over the other.

  6. I’m afraid that if Airbus would put a state of the art 15% longer wing span (~42m, ICAO code D) on a A321 and stretch it up to 250 seats, an all new TBW could be short lived. Specially with enhance GTF and LEAP engines.

    • @keesje
      In order for ICAO code D to be worthwhile, there should be 6 more rows instead of the current 244 max pax. The A321neo should be raised anyway. A ~170kN engine is also required.

    • Hello keesje,

      I believe that any contender for 737/A32X replacement that requires code D (767/A310) gates, taxiways and runways, will be at a massive disadvantage to contenders that can use code C gates. To be a serious contender, I believe that the stretched A321 that you propose would need folding wings to allow it to use code C gates. Without folding wings, it would largely be limited to use on flights that currently use code D gates at airports with taxiways and runways wide enough for code D aircraft. Code C gates far outnumber code D gates, and airports with taxiways and runways designed to accommodate code C aircraft far outnumber those designed to accommodate code D aircraft.

      The problem is not just gates, it is also taxiway and runway spacing and widths. For instance, the FAA requires a runway width of 100 ft for wingspans of 24 to 36 m (ICAO code C/FAA Type III) and 150 ft for wingspans of 36 to 52 m (ICAO code D/FAA Type IV, i.e. 767/A310). There are many airports served by 737/A32X type aircraft that have 100 ft wide runways that would have to be widened to 150 ft to accommodate code C/Type IV wingspans. Widening runways is not cheap. Widening a runway and then having to relocate taxiways to allow adequate spacing between the runway edge and taxiway is even more expensive. Move the terminal building to allow adequate clearance between relocated taxiways and the tails of parked aircraft is even more expansive. Enlarging the terminal building to avoid having to reduce the number of gates to allow for larger spacing between gates due to increased wingspans is even more expensive.

      This is why Boeing’s TBW studies have included folding wings to allow the aircraft to use airports that have been designed for 737/A32X type aircraft (ICAO code C/FAA Type 3).

      “The design was presented at the January 2019 AIAA conference and the wing folds outboard of the truss to use airport gates for the 118 ft (36 m)-span 737.[3] (ICAO aerodrome code C).”

      • AP:

        I had not seen those referenced about folding the TBW wing though it makes sense, almost a have to.

        I remember the big issues with the A380 and all the airport mods. Good news is not wasted as the 777 ER type had the longer wings.

        A bit of irony that FedEx in Anchorage built two gates for A380F and when that got cancelled they were the fist of choice use for 777F. From memory I think a total of 7 gates could be used for the 777F when the striping was redone.

        Interesting side bit is the MD-11 is going away fast now with FedEx and how that affects Anchorage is uncertain though it will no longer be a base. Pilots can live here of course but staffing flights per a designated base has them actually based. What happens to the Flight simulator Building here is ??? as well. Must have had 50 million plus invested in that over the years (building, simulator, upgrades to it and the mechanical systems that support it)

  7. @Bjorn

    nowhere do I see under consideration Canard tube and wing, which would allow a low mount main wing with overwing engines far to the rear (reducing cabin noise like a rear fuselage mounted engine, ground noise by shielding the engine with the wing and retaining the structural benefits of wing mounted engines) and completely eliminating the vertical stab in favor of enlarged winglets that serve dual roles as vertical stabs and wingtip devices.

    additionally, this would allow for short (and therefore lighter) landing gear with the knock on benefit of easier boarding at airports still using stairs rather than jetways..

    finally, with FBW controls, the canard has lower drag than wing and tail.

    why are you not discussing this option?

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