Bjorn’s Corner: Aircraft technology developments. Part 1.

By Bjorn Fehrm

February 24, 2023, ©. Leeham News: In the Sustainable Air Transport series we finished last week, we described new aircraft technologies developed to reduce Green House Gas (GHG) emissions.

There was one area we didn’t discuss, the progress on conventional technologies to reduce the fuel burn of an airliner.

We now start a series, digging deeper into what we can do with conventional technologies to reduce the fuel burned per passenger kilometer and, thus, GHG emissions.

Figure 1. Truss braced wing, a way to reduce aircraft drag. Source: Boeing.

Airliner technology to reduce fuel burn and by it emissions

The emissions from burning fossil fuels are at the root of our GHG emissions and, therefore, climate problems. In the Sustainable Air Transport series, we covered what new untried techniques could do in addition to Sustainable Aviation Fuel, SAF, to help lower GHG emissions.

We found only SAF can give short-term relief. But the continuous work on lowering fuel consumption per passenger-km also helps, and we didn’t cover this development.

Now we go through conventional areas that all target a more economical and environmentally friendly future airliner. We divide it on the aircraft-level into airframe and propulsion improvements.

Airframe improvements

We will cover new structural and aerodynamic developments that aim to lower mass and drag, thus reducing fuel consumption.

Boeing’s truss braced wing work, Figure 1, is an excellent example of such developments. Why does it decrease airplane drag and lower mass? What are the positives and negatives? What are Airbus and other players doing in these fields?

Propulsion improvements

We also look at developments on the propulsion side. What’s the advantage of the geared turbofan? Can direct drive designs continue to evolve?

What about Open Rotor and CFM’s Open Fan called RISE? What are the similarities and differences between Open Fan and Open Rotor? Which solution is better?

If the Open rotor/Open fan is a better way to drive the aircraft forward, why hasn’t it happened to date? The technology was flight-trialed 35 years ago! What has changed that now makes it more likely to find a commercial application?

Deeper than normal discussions

We will go through all this in depth. To dig deeper, we will use the Leeham aircraft design and performance model to analyze the airframe improvements and GasTurb to look at the propulsion developments. We start with airframe developments next week.

30 Comments on “Bjorn’s Corner: Aircraft technology developments. Part 1.

  1. For turbofan engines it is complex to design a counter rotating fan with movable blades like a constant speed prop, especially the fan stage driven by a rotating frame in the hot section, the open fan with only stators movable is a much simpler solution. It looks ugly but with the right of color of spinner, blades and stators one can get used to it. Nothing stops an ultra high bypass ratio turbofan design to have moveable Fan Exit Guide Vanes to get a similar effect by using a huge nacelle including a thrust reverser. You loose some efficiency by not having the inlet ramm effect but you gain by having a larger bypass ratio as free air is sucked along the fan air flow.

  2. What if we make the truss braced wing a bit shorter and wider, to make it stronger, so we can leave out the truss brace, put in fuel (good weight distribution) and put it where where landing gear fit in efficiently. Maybe it prevents crash required bottom structure (weight) of a high wing too. 😉

    Did this graphic years ago, not everybody liked it..

    • You want a slender wing to have as high % of laminar flow as possible to reduce drag. Hence the brace should be slender as well and as it helps lift at alfa it needs decing as well.
      I have a feeling that 2+3 seating is not optimal as it makes a long aircraft for 200pax. You can start with 2+3 and make the fuselage wider pretty quick to take 3+3 seating and after the wingbox start taper off to 2+3 seating again if its makes big difference in fuselage drag.

      • The 2+3 seating maybe just because they are using a DC-9/717 based fuselage for the technology demonstrator.
        It would also be better aerodynamically for the short length they will be using. Im sure the digital version has already been flown inside Boeing software tools to study airflow and structural loadings

      • Airliner fuselages did have lots of taper in the past, but they kept on pushing the parallel section longer so that they could fit in more seats.
        Evolution is hard to beat.

    • They are not going with TWB on a whim.

      Clearly it has to be a significant jump and the TTBW offers at least the possibility of that as well as having folding wing tips.

    • Keesje, You might want to label the images of the alternative configurations. Otherwise the small image are hard to see and harder to decifer.

      • no need.
        look at start on the left and end on the right side.
        everything said.

  3. You have a transparent ad button over “read more” again. This sort of junk is making the site unreadable. Sad to see.

  4. the new “MIT” elliptical propeller concept (which is not really new, there is a company who has been making boat propellers using the same concept for years) is extensible to the open fan turbine, provides significant improvements in efficiency and greatly reduced noise.

    noise is one of the largest factors holding back open fan systems. if you can get a double dose of efficiency improvements and a significant noise reduction vs conventional open fans, it would seem like they ought to be investigated.

    @bjorn, it would be excellent if you could speak to this during this series. very much looking forward to reading your whole series.

    • Yes, the results on boats were very impressive. I would like to see Bjorn investigate as well.

    • Ducted boat propellers operate in a very different medium, water.
      The rpm is also very different.
      Without knowing the numbers but I can see from the images that it would never work in air at the speeds required.
      It might also be an example of the ‘lab scale’ concept which works at a small scale and shorter duration amoung other optimum only parameters.

      • These are not ducted. They are a variant of the so called “MIT” elliptical quadcopter propellers (that predate the MIT ones)

        The concept is proven in air and water at multiple different size and speed regimes.

        And air and water behave in very similar manners, most concepts are transferrable with appropriate scaling for the different fluid properties of air and water

        • Scalable doesnt apply. Just look at modern high speed propellers for say warships (30kts) compared to say any turboprop.(250 kts)
          Theres no comparison or anything to scale, no plane is going to fly at such low speeds, for goodness sake the takeoff speed will always be much greater than any ‘fast boat’

          • speed has (almost) nothing to do with it. fluid mediums are fluid mediums. obviously the specific shape would need to be tailored to the particular regime, but the basic concept of dual offset airfoils linked at the ends to control vortex shedding will absolutely have similar benefits.

            look at the fan of any modern turbofan and compare it to a high speed ship propeller (or say, the prop on a 688 class sub).

            now look at the ducted propulsor of a submarine and a turbine compressor stage….

            ooh look, they are _very_ similar.

            given the real world demonstration of noise reduction both in the boat and drone regimes with the “MIT” style propeller, it is much more than likely that similar benefits would accrue in an open rotor turbofan.

          • Pump jets for subs, thats scaling down from close cowling and cooling fans used in WW2 radial engines…which had a normnal propeller to provide the thrust.
            Have you seen the propellers for diesel subs that dont have the higher speeds on the nuclear boats. Totally unviable in the air.

        • Don’t see how. Compressible fluid dynamics and incompressible are different animals.

    • Thanks for the pointer. I guess this is what you’re referring to:

      The Boxprop, a vaguely similar concept from Chalmers University and GKN in Sweden, was presented at Farnborough 2018. It was similar to the contra-rotating open rotor concept flight-tested by GE and Allison in the 1980s, but on the front rotor, the blade tips were joined together in pairs and were swept forward to reduce or eliminate the tip vortexes.

      When combined with an intercooled core and pulsed detonation combustion, it was estimated to have an SFC of 10.12 g/kN/s (0.357 pounds/pound-thrust/hour) at Mach 0.73. More specs here: (see page 10)

    • They work well on outboard engines as they are designed for a certian size prop and if you want more power a larger prop have space problems and this is a good solution. They have already solved the efficiency and noise problems of UDF props thanks to LEAP fan blade technology and advanced software. So to compete on an aircraft you need to be cheaper, more efficient, longer life ion wing and lighter. That is pretty hard. Also fan blades are hard to design together with its attachment and fan disk.

      • The issue is blade ends go supersonic at even WWII engine speeds.

        You can design shorter and flatter blades but in the end you wind up with a Turbo Prop which the so called RISE is a variation of (it has a jet core and it has a gearbox).

        You then have the mounting issue (as well as blade off).

        A lot of the talk on the RISE is just like the Mitsubishi and Embraer talk about the scope clauses going away.

        In this case you have technical issues and you have perception issues and neither one goes away.

        Horizon is a great case in point. Prior it ran the highly efficient Q-400s and Dash 8 before that.

        Now all of the Q-400 are being retired and replaced with the E-175.

        The RISE is nothing more than Lipstick on a Pig.

        GE/Safran want to delay anyone and any project so they can catch up to GTF. The EU gave them free money and they get a new core and a gearbox out of it.

        And its 2035, yea its a nice corporate move aided and abetted by Calhoun.

        • Horizons changeover isnt an exemplar as mentioned before. US has 76 seater regional jets which practically dont exist elsewwhere, because its dumb way to use a regional jet

          • Duke:

            If that is not true why are not ATR sales through the roof? They no longer have the -8 to compete with.

            Yes there is a market area for a TP, but its not huge and as soon as an airline can swing it, they move to jets, be it the various Embraer Products of the A220.

            Frankly the Western US is natural Q400 territory with small cities the right distance from each other, high and hot in the areas East of the Sierra/Cascade Ranges.

            ATR will have an easier market situation now, but the move is to jets whenever they can.

            Yes the scope clauses are unique to the US but the desire to get to jets as soon as possible is not.

        • > The EU gave them free money and they get a new core
          > and a gearbox out of it.

          Plus the EU gambit created angst at Boeing which now can’t bring itself to launch a new aircraft because it fears RISE will be real in 10 years and any aircraft they design using existing engines will be obsoleted.

          • I think the 737MAx isn’t good enough for the next 10 years.

            Airbus will ramp up A320 & A220, the Chinese C919. Apart form UA and SW few 737 loyalists.

  5. Innovation in all areas for reduced drag, and increased efficiency of the motors with a full life cycle benefit is interesting, and we only need to consider that whatever the technology pathway it has also to be economic.

    This is the great thing about your work Bjorn, these corners look at the feasibility of the business case, and technology and when we consider carbon taxation – and SAF, the abatement cost of Carbon needs to be raised – and we know Carbon Capture Utilisation and Storage is key – but also not efficient.

    So we need to consider the viability of a new fuel, and we know from your thoughts Gaseous Hydrogen will only go so far, liquid Hydrogen has more range, and payload due to the volumetric efficiency – but the jury seems to be out on the use of Gas Turbines burning Hydrogen with NOx.

    Do we see any ways to reduce NOx, and perhaps the question – do we really need combustion when we could leverage the liquid hydrogen with the motor, and by using superconducting – resize the powertrain – wing, weight of the overall airframe and still carry the passengers. Whilst the aim is for an overall lower drag, lighter weight aircraft that can have a higher range with a reduced Direct Operating Cost – all enabled by an optimized power train. What are your thoughts?

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