Bjorn’s Corner: New aircraft technologies. Part 16. Airframe with lower induced drag

By Bjorn Fehrm

June 9, 2023, ©. Leeham News: This is a summary of article Part 16P. Airframe with lower induced drag. It discusses the Truss Braced Wing type of airframe that increases the practical wing span of an aircraft and thus reduces induced drag.

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

An airframe for lower induced drag

Last week we laid the foundation for how to reduce induced drag. Here are the fundamentals in repeat:

The induced drag of aircraft is dependent on the parameters in the induced drag formula;

Induced drag = Lift^2 divided by 0.5 * Air density * Speed^2 * Pi * Wingspan^2

In the formula:

  • Lift is squared. It means if we double the cruise weight of the aircraft, induced drag increases four times.
  • Wingspan is squared. It means if we double the wingspan, induced drag reduces four times.
  • Speed is squared. It means if we double the speed, induced drag reduces four times. It also means induced drag is at the maximum at low speed during takeoff and landing, where it constitutes 70%-90% of total drag.

For more than 10 years, Boeing and NASA have looked into the Truss Braced Wing to build an airframe that has reduced induced drag through a very wide wing. Figure 1 shows how this technology could be used to produce the next generation of single-aisle airliners.

If you build a very wide wing as a normal cantilever wing, the wing bending moment will force you to make the inner wing beefy to take high tensile and compression loads, making the wing heavy. The truss brace in Figure 1 reduces the wing bending moment to a fraction of the cantilever wing for an equal span.

The TBW is very challenging to realize, especially flutter-wise, as the wing is long and has a narrow chord. Figure 2 shows all the challenges Boeing must overcome to realize a TBW airliner.

Figure 2. The challenges to a TBW that Boeing must solve. Source: Boeing.

NASA and Boeing will build a demonstrator to prove that it has solved these challenges. It’s based on an MD-90 fuselage and will fly in 2028, Figure 3.

Figure 3. The MD-90 Boeing TBW demonstrator concept (engines are not representative, it’s an early rendering). Source: Boeing.

What drag gains can be achieved?

Article 16P starts calculating the gains from a TBW for an airliner the size of a 737 MAX. The induced drag is reduced, but at the same time, the wing with its struts has an increased wetted area, which increases the dominant air friction drag.

To see how this all plays out in practice, we put the standard 737 MAX-sized aircraft with a normal wing and then with a TBW in the Leeham Airliner Performance and Cost model and fly a typical route in next week’s Corner.


40 Comments on “Bjorn’s Corner: New aircraft technologies. Part 16. Airframe with lower induced drag

  1. I recall reading that for much of the flight, the wing of an airliner has suboptimally low angle-of-attack. A wing with less area and flying at a greater angle-of-attack would be more lift-to-drag efficient. Hence, a slender wing. Then obviously sophisticated slats and flaps are needed for low speed characteristics.

    • In regards to eVTOL: One of the factors that partially compensates for the heavy investment in lift engine weight is that a much higher wing loading can be used since a take off roll is not required. This will all depend on safety considerations: are the wings to allow an unpowered landing or ditching or are they their purely for cruise and a minor ability to glide to an area where a ballistics parachute can be deployed. I believe Lilim has allowed for landing on a runway for when battery power is below the minimum for a Vertical Landing.

  2. Thank you Mr Ferhrm on the chanllenges pictures. Many issues to solve before its becomes a reality.

  3. I wonder why they are using a MD-80 fuselage. On that plane the wings were mounted well back on the fuselage to balance the weight of the engines at the rear of the plane.
    The drawing shows the engines on the wing which would seem to require a complete redesign of the fuselage.

    • Because MD-80s could be had cheap and the aircraft will essentially have a residual value of zero after sitting in storage for close to 10 years, that way they do not add to the quarterly losses. Never mind the additional/borderline impossible structural design tasks that have nothing to do with the premise of this project. TTBW demonstrators may very well fail because of the issue you raise. If they were smart, they would have started with a BAE-146 (cert base 1972). A much more believable starting point, but alas, expecting Boeing PD/BR&T to use and apply common sense is an exercise in futility.

      • Well NASA is on the same stupid path.

        A nice cheap airframe for a demonstrator is just fine. All you are interested in is how the wing performs. The rest is not an issue.

        I thought it was a great move. Wing position vs a 737 might well have been a factor with the MD wing just happening to be in the right location.

          • when you factor in wing sweep it matters. looks like the TTBW is very low sweep relative to the DC-9 moving the center of lift far forward relative to the base wing.

            also, they are not using the original wing box as a wing box, they have to build a new wing box section on the top of the fuselage (so location of the original wing box is irrelevant) and would only potentially be using the legacy wing box as a tension member for the struts (which it would be comically over built for)

          • MD90 wing sweep is 24.5 deg very close to say the A320 with 25 deg

            As they are wanting similar cruise speeds as today the sweep will be in the same range

          • lower Mach and much lower sweep angle.
            for the desired effect low sweep is mandatory.

          • Hello Uwe,

            Re: “lower Mach and much lower sweep angle.
            for the desired effect low sweep is mandatory.”

            The X-66A is designed to cruise at about 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 (SUGAR 1 to 3 – depicted in Mr. Fehrm’s figure 3) had a design cruise speed of Mach 0.70 to 0.75; however, in 2019 the concept was refined and revised (SUGAR-4 – depicted in the first picture in Mr. Fehrm’s post) 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 A320neo’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 A320 neos, i.e, Mach 0.82.

      • Think it was a shortened MD-90 they will use. The key technology is the wing and software to control it and the anti-flutter system. The body just need to be big enough to fit a high wing that eventually will have room and strength for the RISE demo engine. The V2500-D5 engines can be set to thrust up to 35K and I think it has a beefier landing gear. If they get it right they need to design a 240pax version with folding wingtips as this size is quickly becoming standard for narowbodies.

    • Observe the change from low wing to high wing.

      They will cut out the wing box section, rotate it by 180° (along the wing tip to tip axis )
      and then fit it back between tail and nose 🙂

      • no they won’t. they will graft a new wing box and structural reinforcements onto the top.

        the landing gear, central fuel tank and all kinds of other systems are in the wingbox and moving them is idiotic for a prototype that doesn’t care at all about weight.

      • “They will cut out the wing box section, rotate it by 180°…”

        Cant happen. The DC9 was a double bubble fuselage, with the lower lobe -below floor- sized for baggage, wing box. etc.
        If you reuse the existing wing box its going to take up a lot of the upper ‘passenger lobe’ of the double bubble. Its designed for a a wide thicker wing anyway.
        It will be a new wing and wing box to work with the new location

        • Humor … a difficult concept for some.

          (superior) wings:
          strangely A320 family craft have much better runway performance than any 737.
          Installed thrust is dependent on respective certification requirements.
          737 has to meet much laxer Single Engine Out
          requirements ( framework was upgraded just short of the A320 certification. Think about tilting tables )

          • A320 aircraft have ‘better runway performance’ because they use higher thrust ratings than Boeing 737
            The 737 NG introduced a newer wing design which gave more internal fuel from larger area and better lift at takeoff- hence the lower TO thrust
            Theres not like for like as thrust ratings are a choice made by manufacturer when buying the engine- incl paying more for the top thrust ratings
            Also consider that the -8 and max 8 are bigger planes than the lower capacity ( both passengers and range) A320 – which retains it same wing from the late 70s design period.

            The A321 uses the same wing but required double slotted flaps, different to the A320 single slotted, to achieve the higher lift for takeoff. The A321XLR has changed a 3rd time to have high lift but single slotted. The so called commonality is lost of course

    • They want to test on what has the lightest and narrowest fuselage with the smallest wing. The MD80 fuselage was the right candidate…

    • Boeing is probably just starting with a basic fuselge they have sitting around. But as far as weight distribution. It seems this prototype will not use the wings for fuel storage. Instead it apprears fuel will be stored in new tanks at the roof and floor of the fuselage in the wing box area. This changes the Center of Gravity significantly. I expect they will have to mout the engines to the fuselage rather than the wings on the actually prototype. This will be interesting to watch. Wish they could get Burt Rutan onto this. He was always an innovator who could think outside the box

  4. The Boeing TTBW is the end result when inexperienced people design a commercial airplane for the sake of differentiation, without actually understanding why every successful commercial aircraft uses a low-wing configuration. Even if there are aerodynamic advantages with no structural/flutter/weight compromises (dubious for many reasons), a high-wing airplane (any high-wing airplane, including TTBW) presents many serious safety and certification failures that must be utterly ignored when building a high-wing airplane. Can it fly? Sure. Can a similar airplane become a commercial product? No, due to fundamental safety flaws. Is there any real return on the ~ $1B that Boeing+NASA would be burning on this altar? Highly doubt it. Perhaps the biggest loss here is time. The valuable time that should be spent on relevant & useful design studies, tests, and workflow improvements, will instead be spent on building and flying a white elephant, with nothing useful to be learned that isn’t already known to the qualified practitioners.
    If the industry’s track record of cost overruns is any indication, I’d be very surprised if they can do this for the earmarked amount.
    A sad (but true) story.

    • That would be totally wrong.

      A standard tube and underlung wing has no aerodynamic advantages. The top mounted TBW does.

      When a 737 designed in the 60s can be as economical to run as an A320, then your aerdfynanmics improvement are in the minuscule range.

      this breaks at least potentially breaks that log jam. It may not work, but its better than repeating the same old same oh for minuscule gains and expecting all improvements from the engines which have reached limits as well

      Well other than the GTF and P&W made the same bold Leap, and it works and has far more upside than the Leap does.

      • The other advantage is that this configuration offers a lot of ground clearance for an engine such as the future R.I.S.E. of Safran.
        Combine the two and we could get 30% efficiency.
        This is what D. Calhoun had honestly said.


      • “When a 737 designed in the 60s can be as economical to run as an A320,”
        The engines are the biggest factor and the current 737 max has little else in common outside the fuselage with the original very short 737-100.

        Remember the NG version had a complete new wing design, which actually gives advantage for the Max – more internal fuel and better lift requiring a lower max TO thrust required from its engines. The small old aerodynamics wing of the A320 series is at a disadvantage just rolling down the runway.

  5. An obvious issue with a cantilever high wing configuration is that the wing carry-through structure has to pass above the fuselage tube to provide an uninterrupted cabin. Strut braced, the carry-through structure can be much less obtrusive, reducing frontal area relative to a high cantilever wing, and probably wing-body interference drag.
    And regarding wetted area, Figure 2 alludes to a 20% increase in design (ie cruise) lift coefficient, which probably translates to a 20% reduction in wing area. That may more than compensate for the wetted area of the struts.

  6. There is something else I don’t understand here. In many conversations, wing span comes up as a limiting factor when it comes to gate design and airport utilisation. What is the impact here on the design of the gates, taxiways and other airport elements? Boeing went to the foldable wing for 777X to make them fit in existing gates and here we plan to replace 10,000’s narrow body aircraft with TBW with a much larger wing span. Doesn’t this conflict with existing airport design? And isn’t this a showstopper?

    The second point is engine mounting: I always thought that high wing caused major issues with engine maintenance due to issues with engine access and this was commercially a showstopper.

    • Hello NdB,

      Re: “Boeing went to the foldable wing for 777X to make them fit in existing gates and here we plan to replace 10,000’s narrow body aircraft with TBW with a much larger wing span.”

      Boeing’s transonic truss braced wing design concept includes wings that fold outboard of the truss, which meets the wing at the wing at the wingspan of a 737. See the excerpt below from Wikipedia, or the paywall version of Mr Fehrm’s post.

      “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.”

  7. Interesting design. Scratching my head reviewing figure 2 especially about fuel volume. Thin wing design has advantages for efficiency but that fuel, whatever it is, has to go somewhere. This wing doesn’t have the strength to carry the loads like a traditional cantilever wing with full fuel. NdB makes an excellent point about those super long wings and gate space.

    If you worked for an airline like I did, the maintenance and engineering crews will be working overtime repairing ground damage from bag smashers running carts into those trusses. Guaranteed it will happen. With the load those trusses will support, damage tolerance will be critical. Figure 2 does point this out.

    Good series Bjorn.

    • A long thin wing still can carry enough fuel especially if they use wet outer folding wing. You can also fill the wingbox, struts, vertical tail and horizontal tail with fuel.

      • I think we can rule out a wet wing beyond the fold. Thats a lot of weight for the folding mechanism and complications to filling it when parked and the wing is vertical.
        Remember this is a prototype X plane not a version ever expected for production.

        • It will not be on the prototype but when you do the 240pax production version and are getting desperate for more fuel having a thin short chord wing you start looking everywhere for fuel tanks and trim tanks. A little bit like the Concorde with its thin delta wing. Having RISE engines burning 20% less fuel will help. Flying a tad slower also helps. The advantage with the big UDF fan is that you get really short TO distance and good climb performance, then at top of climb having a very good glide ratio you can fly continuous decent all the way to your destination

          • The delta wing is especially suited to being ‘thin’ -in aerodynamic terms as aspect ratio- and having a large fuel volume within the large wing area.
            B707-320C – 90,000 litres fuel volume
            Concorde -120,000 litres

  8. Studying figure 1 and noticing those high mounted engines located so far forward. The CG has to be way out of skew, but then Boeing has a lot of experience with skewed CG don’t they?

    Engines near first class and close to the cabin….. noise!!
    Then, certification of fan blade failure and containment!

    • @Airdoc

      Lol! Your Boeing bashing only exposes / reveal the frightening possibility that one day Boeing will sell so many of them that it would be comparable to the low A350 order vs 787 Dreamliner when Airbus will have copied Boeing as usual

      My point is that it will no longer be A320 vs 737 from a 20 year gap. It will be a real “fair” head to head as we saw during the launch of the 787 and A350 two years apart…

      When Boeing gets ahead,… it gets ahead unfortunately, I grant you, you have every reason to be worried……

      • @Airdoc

        Your comment is still quite valid though. The engines are slightly forward as well as the wing implant.
        But the centering is good, nothing asymmetrical from the moment the air friction (aerodynamics) surrounds the long wingtips much further out and at the back than a smaller more “traditional” wing
        The law is quite different than the “traditional” low wings. Staggering the wings at the rear is like corrupting the centering in this case.

        In the case of tri engines for example, like the 727, L-1011 TriStar, DC10/MD11 and many others, the weight of the 3rd engines at the back forced the placement of the wings further back to obtain a balance.This is the opposite case with the TBW …

        For 1st class passengers, insulators thicker move to be a solution.

  9. The relation between induced drag and wingspan is misleading. The aspect ratio variable makes more sense while assessing the induced drag. It is true the formula initially shows a relation with the wingspan. However, the lift is related to the wing surface ( chord times span for a straight wing). Assuming that increasing the span would necessarily reduce the induced drag is not correct. But looking at the aspect ratio is mathematically more correct.

    • Hi, you have fallen into the normalization trap. The Induced drag formula has no wing area in it; the Induced drag COEFFICIENT has it to normalize the coefficient between different size wings and aircraft. It has nothing to do with the wing area being involved in induced drag, and the fact that the coefficient formula has the constituents for aspect ratio in it doesn’t mean that aspect ratio is involved in induced drag.
      Read McLean’s book “Understanding Aerodynamics”; he debunks more myths than that aspect ratio is involved in induced drag. Probably the best aero book in the last 10 years.

  10. Any thoughts on the noise and vibration effects of the installed RISE engine? That truss will need to counter sonic induced vibration and fatigue. It is just a demonstrator but it does need to work for a few flights while the structure is being developed.

    • They want the strut to help lift at alfa, hence they get similar “help” as the large nacelle on the 737MAX, but with a fly by wire system it is fairly easy to solve. The anti-ice flushing can be a bit more annoying for the struts.

      • FBW is easy to solve ?

        Only if its based on current designs of broad thick wings with ‘thick’ flaps. This isnt. Plus they want to use the FBW to alleviate the poor flight characteristics when it isnt in high altitude cruise and all the other ‘difficult situations’ like one engine out at takeoff or flying through huge updrafts in a storm

  11. According to the excerpts below from the 6-12-23 NASA press release at the link after the excerpts, the Boeing/NASA transonic truss braced wing demonstrator has been assigned the experimental aircraft designation X-66A by the US Air Force.

    “NASA and Boeing said Monday the aircraft produced through the agency’s Sustainable Flight Demonstrator project has been designated by the U.S. Air Force as the X-66A.

    The new X-plane seeks to inform a potential new generation of more sustainable single-aisle aircraft – the workhorse of passenger airlines around the world. Working with NASA, Boeing will build, test, and fly a full-scale demonstrator aircraft with extra-long, thin wings stabilized by diagonal struts, known as a Transonic Truss-Braced Wing concept.”

    “The X-66A is the X-plane specifically aimed at helping the United States achieve the goal of net-zero greenhouse gas emissions by 2050. To build the X-66A, Boeing will work with NASA to modify an MD-90 aircraft, shortening the fuselage and replacing its wings and engines. The resulting demonstrator aircraft will have long, thin wings with engines mounted underneath and a set of aerodynamic trusses for support. The design, which Boeing submitted for NASA’s Sustainable Flight Demonstrator project, is known as a Transonic Truss-Braced Wing.”

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