Bjorn’s Corner: New aircraft technologies. Part 7. Propulsion

By Bjorn Fehrm

April 7, 2023, ©. Leeham News: This is a summary of the article New aircraft technologies. Part 7P. Propulsion. The article discusses how developments in the next-generation airliner propulsion system will be the second most important area for improved efficiency and lower emissions after we have decided on the fuselage type.

Figure 1. The CFM LEAP engine gained 15% efficiency compared to the engine it replaced, the CFM56. Source: CFM.


We started the series by saying the most pressing problem for the next generation aircraft that shall replace our most common airliners, the Airbus A320 series and Boeing’s 737 MAX, is the decision on the fuselage type.

Over the 50-year lifecycle of a new aircraft family that comes into the market mid-next decade, the passenger capacity for the heart of the market, i.e., the most produced models, will pass 250 seats, the point where a dual aisle fuselage is a better choice than a single aisle.

The second most important parameter for a new aircraft generation is not how the rest of the airframe is made (including whether it has a composite fuselage or a truss-braced wing) but what advancements we will have on the propulsion side.

The difference in efficiency between the 1980s-designed Airbus A330-300 and 2003 launched Boeing 787 was narrowed from 13% to about 3% when the A330 got re-engined with the engines from the 787 project (the Rolls-Royce Trent 7000, which is a bleed variant of the 787 Trent 1000).

Despite an all-composite airframe and advanced wing aerodynamics, with micromanaged flaps in cruise, the 787 airframe contributed less than 5% to the overall efficiency compared with an A330-300, where the airframe design was set 20 years before.

So the efficiency of a new airliner is likely to depend mainly on propulsion system gains rather than on airframe gains.

We then used GasTurb simulations to compare the propulsive and thermal efficiencies of the previous generation of propulsion, the CFM56 (Figure 2), to the present generation, the CFM LEAP (Figure 1), and Pratt & Whitney GTF.

Figure 2. The CFM56, the engine used on the A320/321ceo and 737NG. Source: CFM.

The overall difference in efficiency is 15%, gained by doubling the bypass ratio and increasing the pressure ratio of the cores by 50%.

Can these techniques be used once more to equip the next generation heart of the market airplane that will appear by 2035, and will the gains be another 15%?

We will look at this in the next Corner.

39 Comments on “Bjorn’s Corner: New aircraft technologies. Part 7. Propulsion

  1. You can always increase bypass ratio, increase pressure and turbine inlet temperatures but it will effect time on wing. LH2 fuel and cooling can help alot in handling the temperatures and also cooling the compressor.

  2. It’s amazing how the a330neo and 787 are so close in terms of efficiency – it does temper the discussions on ‘all new’ vs. ‘continuous weight reduction and larger component upgrades (new wing a320/1/2 AB???) of current models – and consideration given to improvements of the in-service fleet – and inclusion of this concept when designing new models.

    Much to do at the engine makers to get their new engines on-wing performance to where it needs to be, but hopefully what they discover as they do so will help with PIPs and next generation development.

    Happy Easter all.

    • You have to ask what the overall engine cost is though.

      The RR current offerings are not doing well with maint and early wear and tear.

      How often you have to pull the engine is not reflected in fuel burn though if the A330 had the GE engines that would be almost an even match.

      That is hard for me to believe as the composite wing on the 787 should add 5% over an A330NEO.

      • Same as GE but they get better PR

        new durability upgrade ? Thats an ongoing process isnt it for new build engines.

        The story says its for in service engines too
        ‘upgraded hardware to Boeing 787 operators for installation in their in-service GEnx-1Bs ‘

        So the facts lead to the story of the poor wear and tear for Genx current users for some modules of the engine

        • You get new SB’s all time. First to improve durability, then performance and to reduce production cost. A bigger PIP package get introduced and it starts all over again sometimes.

          • Yes. But it clearly says its ‘fixing a problem’ – by avoiding that word- for in service engines.

            It must be more than minor to go to all the time and expense to replace the engine section off wing of all of that type.
            ‘complete retrofitting the upgrade throughout the in-service GEnx-1B fleet’
            In service engines degrade all the time, until the time overhaul restores most of that. GE clearly thinks the degrading and or failure rate is more than thats acceptable.

          • Article in link is from 2018, so there are probably minor PIP’s coming nowdays. With power by the hour contracts GE makes more money the longer the GEnX engine stays on wing. The approx $10M cost of an overhaul moves to a later date and thus you can offer a slightly cheaper PBH rate.

      • The A330 is slightly smaller and i don`t know if that`s accounted for.

        The A330 wing was really good for it`s time, it was always limited by the engine. Just imagine how often Airbus would jinx the day where GE offered to pay for the development of putting a GE90 derivate under the A330.

        I heared numbers depending on the mission, the B789 has about a 5% edge over the A330neo.
        If you would scale up the A330 fuselage to match the B787, i think it would rather be a bit more.

        Surprisingly, for said to be that close, the A339 isn`t seeing much sales.
        I thing the 2-4-2 vs. 3-3-3 Y layout plays a role here. That extra seat in 30-40 Y rows should make a count.

        • I think in the 787 vs A330 comparisons, incomplete marketing figures and personal preferences brought as facts often shine through.

          The A339 is a bit longer than a 787-9 and has leaner, longer wings.

          It’s aerodynamics and engines were finetuned and modified a decade after the 787. Airbus is increasing production rate.

    • Airbus and Boeing are different developement lines.
      IMU and IMHO: Airbus has been a bit ahead of Boeing most of the time.
      787 still is a thin profile and thus heavy wing.
      The observable progress is shown by looking individually at
      A330 to A350XWB ( and looking at what the A330NEO added )
      777 to 787 ( and into the 777X, but that is said to be a 787 scaled copy.)

  3. I think there is a chance of 15% sfc enhancement for a next generation of Engines. GE/CFM, PW and RR each used different technology roadmaps to improve engine performance.

    GE/CFM new materials (ceramic, carbon), high pressure ratios and fuel mix. Pratt invested 25 yrs in getting a working gearbox to optimize rpm’s and Rolls finetuned it’s 3 spool technology while it delayed adjustable fan blades.

    Interestingly those technology roadmaps are not excluding each other and new materials make higher BPR’s possible (& I like the RISE engine configuration much more than the noisy, complicated open rotors of the past).

      • I think GE has pushed the materials end close to as far as they can go.

        You not only have the hard to achieve but the cost of it even if you can do so in the era higher cost and the engine profit models not working like they used to as well as the GTF offering easier improvements and in turn can go the more exotic materials of the LEAP.

        • The next logical step is to see Ceramic materials in the rotating assembly…
          That would be a monumental leap in technology.!!!!
          On the right track with CMC,s in the non rotating core…
          TIAL is being addictive manufactured, in the LPT now, saving a ton of labor and waste over trading machining methods.!!

        • PW has the “advantage” that their core is not being pushed as hard as the LEAP core, they are not using as advanced materials or high temps, and thus through incorporation of those advancements have more inherent room for improvement in the core.

          I’m sure CFM has things up their sleeve, but the PW design has more inherent headroom as with identical technology level cores, the PW design is more efficient, or equally efficient in the current state where PW’s core lags CFM’s.

          • Theres TWO Leap cores . The Boeing one and the Airbus one.
            Its not just a difference in front fan size the core itself is smaller diameter for the Boeing and runs faster and hotter as well as a lower thrust range.[kaching the Max planes need less thrust than the neos]
            The third type the Leap-1C is just a variant on the A

  4. Can someone please explain why an unducted fan is better than a ducted one? Is it just a weight reduction issue or is it something to do with airflow?

    • It is bypass ratio that increase alot, you miss the benefit of the inlet that gives you in pressare recovery for the engine and noice reduction..

      • Is the bypass ratio not determined by the swept diameter of the fan then? I’m having trouble seeing how air beyond the tips of the fan blades can contribute.

        • I could be wrong but I suspect its that props work with a gear reduction to limit tip speeds.

          A Turbo Prop is more efficient than a jet engine. But you can only push them so fast and there is that trade off that starts to fall off at 500-1000 miles (and some comfort factor with a larger jet aircraft, the Dash 8 is a tight seating aircraft. Ok for and 1.5 hours but after that…..

          Mounting location has been an impediment as you have to count fuselage beef-up in the rear against that.

          And the noise.

          No one has actually demosntart6ed a quiet Op on Rotor and tamed the vibrations as well.

          And the Public has a voice and would anyone go there?

          AK airlines is retiring its Q-400 fleet completely despite it having the ideal profile for a turbo prop in the West.

          • Alaska and US turboprop operators in general are a special case as the TP compete against the small scope limited jets which are near to the 70-75 seat size of the current largest TP.
            No where else is that the case , other countries without scope the small jets are usually 90-110 seats only.
            The way the US hub and spoke networks work means that jet service and 2 classes is required for the so called regional routes as well, which doesnt work for the TP cabin.

        • A TBW easily allows integration of RISE engines of Safran where another conventional wing would not do as well
          to be integrated

          What is a 10% efficient conventional Wing aircraft with 150 seats worth in 2035 VS a 30% TTBW/m or BWB efficient aircraft of 120/130 seats in 2035?

          Nothing, the new standards established, the others become obsolete and disappear for a repositioned heart of the market.

          The folding wing is no longer a problem, the
          ICAO wingspan classification code is no longer a problem already even that we do not know moreover how far the wing can fold.

          Otherwise that’s no problem for a brand new popular standard…

          • …except that “a certain OEM” doesn’t have the funding to bring a TBW product to market…

          • If that OEM had funding they wouldn’t do a TBW albatross. 🙂

        • An open fan accelerates some air outside the disk area increasing bypass ratio even more.

        • for the same core power source you can cover a much bigger area.
          increasing a shrouded fan has the penalty of the fan shroud / engine cowl growing strongly in mass.

          The TP400 engine is a 5.5m Diameter 8MW driven fan.
          The P&W GTF for the NEO is 2m Diameter 24MW driven fan.
          efficiency is inversely proportional to the speed increment you need to achieve the same impulse ( eqv. to thrust )

          • Those big props do wonders for the A400M short takeoff run.

            It can do things from small runways that ‘Fat Antoine’ the 100 tonne A321 XLR can only dream of even with its engines set at max TO thrust

        • I am not an engineer but the way i explain it is through this mental image. With a ducted engine the air flow starts from the tip of the duct and extend outwards which is your “draw” area. While for un-ducted engine that conus of air starts from the bottom of the blades which results in larger area where air is being drawn from, thus it increases the bypass.
          Essentially air is being sucked in from the center of the engine instead of the tip of the duct.
          Draw it on something and you can calculate the area of the triangles that are formed which represent the air being actively drawn.

  5. A Truss Braced Wing, would be better combined with a brand new engine technology, than with the only engine technology around 2035 on “conventional wing”…

  6. A TBW easily allows integration of RISE engines of Safran where another conventional wing would not do as well
    to be integrated

    What is a 10% efficient conventional Wing aircraft with 150 seats worth in 2035 VS a 30% TTBW/m or BWB efficient aircraft of 120/130 seats in 2035?

    Nothing, the new standards established, the others become obsolete and disappear for a repositioned heart of the market.

    The folding wing is no longer a problem, the
    ICAO wingspan classification code is no longer a problem already even that we do not know moreover how far the wing can fold.

    Otherwise that’s no problem for a brand new popular standard…

    • “What is a 10% efficient conventional Wing aircraft with 150 seats worth in 2035 VS a 30% TTBW/m or BWB efficient aircraft of 120/130 seats in 2035? ”

      Amusing assumptions.

  7. A 250 pax 737MAX/A321neo successor designed as an oval widebody might require an engine like the ones on the 767-200. They were just over 50k thrust, still today you might design it lower than the 767-200 142.9 ton MTOW, maybe 20 tons less, but you end up with 757 type engines in 37-42k thrust that does not exist today. They are maybe a $10-$15bn project per engine manufacturer to make a great one and you want both Boeing and Airbus to use the same engine (like the CF6-50 found on both Boeing, Airbus and Douglas aircrafts). So if only Boeing moves the engine manufacturer will not until Airbus specifies a similar engine. Airbus has the A322 in its computers that will be very cost competetive equipped with production A321neo engines at 35k.

    • I think composites have so far not led to significant lower weight aircraft. Ref. A330-900 vs 787-9 OEW’s or the more composites 777-9 compared to the 77W.

      • 787( and especially the 777X) is/are a misleading sample(s). the thin 787 wing brings a weight penalty.
        Scaling that to 777X sizes brings more penalty.

        look at the A350XWB. ( vs A330 and 787 )
        keep in mind that cabin stuffings have over proportionally grown in mass.

        ( compare wing box dimensions at the wing interface )

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