Bjorn’s Corner: New aircraft technologies. Part 17. Airframe with lower induced drag

By Bjorn Fehrm

June 16, 2023, ©. Leeham News: This is a summary of the article Part 17P. Airframe with lower induced drag. The article analyzes Boeing 737 MAX 8-sized airliners with Truss Braced Wings versus wings with folding wingtips and the standard MAX 8 wing. We use our Aircraft Performance and Cost model to get the drag data and overall efficiency improvement for the concepts.

Figure 1. Boeing concepts of a next-generation airliner with Truss Braced Wing. Source: Boeing.

Truss Braced Wing or Wider Wing with folding wingtips?

We run a Boeing 737 MAX 8-sized Truss Braced Wing (TBW) airliner, with a configuration like in Figure 1, against today’s MAX 8 and against a MAX 8-sized aircraft with an extended cantilever wing with folding wingtips in our Aircraft Performance model.

All three aircraft have a similar fuselage size and cabin capacity, using the MAX 8 data for all three. The TBW version has a 52m wide wing and a T-tail, like in Figure 1. Other Aerodynamic data was described in last week’s articles.

The Wide Wing (WW) version has the same fuselage and tail as a MAX 8 but has a 35.8m base wing with two 4m folding wingtips for a 43.8m wing. The wing keeps the wingarea of the MAX 8 by virtue of composite construction and advanced aerodynamics.

The MAX 8 keeps its 35.92m wing with a split winglet.

We have kept other aerodynamic data the same for all three except for a slight increase of Interference drag for the TBW. Transonic drag is the same for all three as they fly at long-range cruise speeds.

The WW has a heavier wing due to increased span and fold, whereas the TBW adds more weight due to wing flutter stiffness requirements, the truss brace mass, the unfavorable main gear placement, and the need for a larger T-tail to control the wide wing.

We flew the aircraft on the typical single-aisle average stage length of 800nm and note the differences in performance. The Truss Braced Wing showed a high single-digit percent improvement over today’s MAX 8, whereas the folding wing stayed in the mid-single digits. All aircraft used today’s MAX 8 LEAP-1B engines.


Our target was to see the ballpark improvements one can expect from technologies like folding wings using modern composite techniques and a TBW using the same techniques but combining them into a wider and more advanced concept.

We described all the challenges a TBW must overcome in last week’s Corner. The WW is less challenging. The Boeing 777X has introduced folding wingtips to civil airliners (they have been on Naval fighters for 80 years). The main challenge will be increased wing mass due to the fold and increased wing bending moment. At the same time, both aircraft get less sensitive to increased empty weights as the drag due to weight, induced drag, is significantly reduced.

Which concept will prevail? We will have to see. What we know is that the folding wing will be present on the next generation of Heart-of-the-Market airplanes. If it will be truss braced or not is too early to tell.

22 Comments on “Bjorn’s Corner: New aircraft technologies. Part 17. Airframe with lower induced drag

  1. If the fuel burn advantages of TBW compared to WW are so slim, then one can seriously question whether TBW is worth all the extra design/use hassle.

    Starting to look as if the whole NASA/BA X-66A collaboration is just a PR exercise.

    • Also, I’m not yet sure the TBW could match a more conventional wing in cruising speed. Net effect on efficiency = ?

      • Hello Vincent,

        Re: ” Net effect on efficiency = ?”

        Mr. Fehrm’s summary in the above post of his more detailed analysis in the paywall version of this post, using current generation Leap 1B engines for all the aircraft being compared, was as follows.

        “The Truss Braced Wing showed a high single-digit percent improvement over today’s MAX 8, whereas the folding wing stayed in the mid-single digits. All aircraft used today’s MAX 8 LEAP-1B engines.”

        According to a 1-19-19 ‘The Points Guy” article summarizing a paywall Aviation Week article, in January 2019 Boeing estimated fuel consumption savings for their transonic trussed braced wing concept vs current aircraft of 8%, not including savings from future developments in engine technology.

        “Boeing thinks the aircraft will be 8% more fuel efficient than traditional cantilevered wing jets, Aviation Week reports.”

        Recently Boeing CEO Calhoun has been stating that a total efficiency improvement of 20% to 30% over current aircraft would be needed to justify development of a new aircraft. If the Truss braced wing improves efficiency by 8%, then engines with 12% to 22% lower fuel consumption would be needed to meet Calhoun’s threshold of 20% to 30% total improvement to justify development of a new aircraft. PW says WET GTF will be 10% more efficient than the current GTF, and GE RISE project is aiming for a 20% reduction in fuel burn vs, current engines.

        “I have been clear that we need something that delivers in the neighborhood of a 20-30% improvement over today’s airplanes. That is what will justify airlines making a move. ”

        • Hello AP,

          Vincent said “more conventional” … not “conventional”.
          I think we can agree that the WW design alluded to in the article is a (much) “more conventional” design that the TBW design — in that a WW design merely requires a longer wing with a folding tip, as opposed to the trussing, modified tail, more-forward engine placement, modified fuel storage, etc., of the TBW.

          The article makes clear that the advantage of a TBW design relative to a WW design is only about 5%. Since both designs can be fitted with a RISE engine (if that ever materializes), the difference remains 5%. One can legitimately ask if such a small difference justifies all the hassle of a TBW design relative to a WW design.

          • Yes, these were the points I was thinking of.
            Also, the likely reconfiguration costs of airport layout and such for a TBW aircraft. TBW looks like hopium to me.

          • Airliners almost kill for a 5% improvement. They invest $ millions per engine to get 1% improvement in a well designed PIP package. Just look how they invested in 737NG winglets that only give you a benefit after x nm. With these big UDF fans you get shorter T-O, steeper climb that can quickly become the norm besides lower emissions. With a DC-7 size UDF prop on a narrowbody you are looking at high wing aircrafts as an under wing on a narrowbody will have extreme tall landing gears.

          • @ Claes

            Yes, the *airlines* love a 5% fuel saving.
            But the question is whether the *airframers* have a similar appetite when it can only be achieved via such a challenging and expensive development path.

            Coming back to the *airlines*…not at all sure that they’ll like the various (potential) practical issues associated with TBW designs.

      • Hello Vincent,

        Re: “Also, I’m not yet sure the TBW could match a more conventional wing in cruising speed”.

        The design cruise speed for Boeing’s current Transonic Trussed Braced wing concept is Mach 0.8, about the same as today’s single aisle airliners. A320neo’s and 737 MAX’s both typically cruise at Mach 0.78 to 0.79. Long range wide bodies, typically are designed for cruise around Mach 0.85. The truss braced wing concept that Boeing was studying with NASA from 2010 to 2019 had a design cruise speed of Mach 0.70 to 0.75 (SUGAR Phase 1 to 3); however, in 2019 the concept was refined and revised (SUGAR Phase 4) with increased wing sweep to enable a cruise speed of Mach 0.8 at the cost of less savings in fuel consumption. With this redesign “truss braced wing” became “transonic truss braced wing”. SUGAR = Subsonic Ultra Green Aircraft Research program.

        See the excerpt below from Wikpedia’s X-66A article.

        “By early 2019, following extensive wind tunnel testing at NASA Ames Research Center, an optimized truss and more sweep for the 170 ft (52 m) span wing allowed flying higher and faster, up from Mach 0.70–0.75 to Mach 0.80 like current jetliners.”

        Cruise speed of A319, 320 and 321 neo’s according to Wikipedia.
        “Cruise: Mach 0.78 (450 kn; 833 km/h), Max.: Mach 0.82 (473 kn; 876 km/h)”

        Cruise speed of 737 MAX’s according to Wikipedia.
        “Mach 0.79 (453 kn; 839 km/h)”

        MAX Mach is same for MAX’s as it is for A32X neos, i.e., Mach 0.82.

        Wind tunnel testing to date has verified computer modeling that predicts 8 to 9% lower fuel consumption vs conventional configurations when the same engines are used. Next step = verification of wind tunnel results and exploration of handling characteristics with a full size demonstrator. See the excerpt below from a January 2020 paper by Boeing and NASA Langley Research Center staff.

        “A TTBW concept has been developed for a design cruise Mach number of 0.80. Detailed high-speed aerodynamic design shows that the TTBW concept is well suited for operation at this condition. High aerodynamic efficiency predictions at cruise have been validated in a high-fidelity wind tunnel test in the NASA ARC 11-Foot TWT. Highlift system design of the TTBW has yielded low-speed performance in-line with expectations. Results from
        computational studies and wind tunnel testing in the NASA LaRC 14- by 22-Foot Subsonic Tunnel have demonstrated the necessity of differential Krueger rigging on the outboard wing panel. ”

  2. Fly by wire flutter control will help reduce mass on the truss brazed wing.

    • Claes – do you mean fly-by-wire eliminating the need for outboard ailerons? A combination of spoilers, inboard ailerons/flaps and even differential movement of the elevators providing adequate roll control instead, which would leave the wing free of the twisting forces from the wing tip imposed by a traditional aileron. Or are you suggesting something different – an active flying control surface to dampen out wing flutter?

      • I think he meant “automated flutter control” as deployed in the 747-8 — where a control-loop performs constant mini-corrections to the aileron settings so as to cancel flutter effects…

        • Yes, similar high speed moving hinged horizontal surfaces to dampen out any flutter initiation at smart locations along the wing. Don’t know if aileron actuators are that fast and precise to do this function on a very thin and short chord wing. I suspect electrical small ballscrew actuators at several locations along the wing trailing edge to actuate its dedicated control surface. We will see what they choose and what FAA allows to be certified.

          • Its an X plane for NASA development purposes only . Doesnt need a standard commercial FAA certification

          • @Dukef. The antiflutter system will be a key development and eventually they have to choose and test a system that has a chance of getting FAA/EASA certified.

          • Thats not the X-66 TBW project issue. You were suggesting that the FAA ‘needs’ to certify the current development.
            ‘We will see what they choose and what FAA allows..’

            If and when a new plane is produced thats when that issue will arise and FAA might have to come up with some new standards for the design.

            The last comparable project that might have commercial outcome was the USAF/Lockheed X-55.
            Advanced Composite Cargo Aircraft

  3. Reductions in fuel burn won’t even come close to satisfy the Greens.

    Even SAF won’t satisfy the Greens, because, as I have read elsewhere, “SAF is still emitting CO2 and pollutants while it burns”

    The whole industrial sector is still trying to placate the Greens, while the Greens keep moving the goalposts.

    The provisional end game for the Greens is for everyone to stay home and eat their garbage.

    But their ultimate end game is to rid Gaia of the human infestation.

    • Reduced fuel burn also benefits the airlines (which have to pay fuel bills) — not just the environment.

      However, I do agree with your point that the Greens are constantly moving the goalposts.

  4. Clearly an updated GTF engine (non compliant with current) would get a 5-10% improvement, but at best we are talking 15% right now with a TBW FW (that is fuselage and engine combined)

    I would do some modeling at slower cruise speeds. That would seem to be a route to easy improvement (and I assume there are speeds too low and you get more drag that do not work)

  5. Folding wings goes back more than 80 years . The Short S.41 Seaplane used by the RN from 1912 had backward folding wings.
    The Douglas Devastator for USN in 1935 was the first with hydraulic folding wings in the modern sense
    Boeing 377 Strato cruiser had an unusual folding vertical tail – to fit in the then smaller hangars -for maintenance only ( copied by B52 and KC-135)

  6. Ducofourl

    I had seen an interesting documentary on the design of the 777 almost 25 years ago now.

    There were synthetic images where we already saw a 777-200 with folding wings long before the 777-X. ANA was already interested and former CEO P. Condit even spoke of detachable winglets because JAL took them off and put them back on his 747s when needed, he said.

    I think this one was canceled because Boeing at the time was already working on the “Raked Wingtip” that we will see on the 767-400ER, 777-LR programs up to the 787 Dreamliner and these “Raked Wingtip” which ended up folding on the 777-X…

    So it’s a culmination of 30 years with the folding wings and the entire 777 programs


    • The 777-200 with folding wing images were synthetic because that ‘option’ never was built. It would have reduced wingspan from gate code E to D which was that used by DC-10. The 747-400 was the larger span Code F
      The raked ‘winglet’ was included in original design

  7. My ushanka wearing friends inform me that a new group is formed in Russia to develop a “Широкофюзеляжный” class airliner. Classified thus, only to place it in the category of long haul airliner it is in fact, of unconventional layout and therefore not a wide-body. It was described to me as “летающее крыло” which is not a BLB but a flying wing although blended wing designs are also considered. PD35’s will suck it through the heavens with hybridisation also proposed.

    The Zhukovsky institute is developing the airframe with end of decade time frames.

    This is a project distinct from the C929 in which Russia’s participation would seem to evolve into something similar to that of Spirit AeroSystems with other OEM’s.

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