Bjorn’s Corner: New engine development. Part 4. Propulsive efficiency

By Bjorn Fehrm

April 19, 2024, ©. Leeham News: We have started an article series about engine development. The aim is to understand why engine development now dominates new airliner development when it comes to the needed calendar time for development and the risks involved.

To understand why engine development has become challenging, we need to understand engine fundamentals and the technologies used to achieve them. Last week, we discussed propulsive efficiency and learned that it depends on the Overspeed the engine gives its exhaust air-gas mix.

We then used two direct-drive engines from CFM (CFM56 for the 737 ng and LEAP for the 737 MAX) to give us examples of Overspeeds and their corresponding Propulsive efficiency. Now, we look at geared turbofans.

Figure 1. One of the first geared turbofans (if not the first), the Turbomeca Aubisque used on the SAAB 105 jet trainer. Source: Swedish Military Airplane History.

Geared Turbofans

We learned last week that lowering the air Overspeed in the thrust equation increases Propulsive efficiency:

         Thrust = air massflow through the engine times the air Overspeed

The method to lower the air Overspeed in a turbofan is to increase the ByPass Ratio, BPR. You do this by increasing the fan size and reducing the core size.

To drive a larger fan, you need more power, which in a turbofan comes from the core’s low-pressure turbine (Figure 2). The Gasturb graphic shows the generic buildup of a geared turbofan.

The fan is driven through a planetary gearbox (red box) by the low-pressure turbine (yellow box). A geared fan is a complication compared to a direct drive fan (which has no gear between the low-pressure turbine and fan, just the straight shaft). But, you want the turbine to run much faster than the fan because turbines gain power efficiency with blade speed. So, for a reasonable-diameter power turbine with few stages, you want to spin the turbine shaft fast.

However, the large-diameter fan then spins the blades at a high speed, causing them to have a supersonic tip flow, which reduces fan efficiency. The power from the low-pressure turbine can be obtained in a slow-running turbine shaft by increasing the turbine diameter and number of stages (which the LEAP does).

A large-diameter low-pressure turbine with many stages gets heavy, so at some point, it’s better to de-couple the fan and turbine RPM with a gearbox. In the case of the Pratt & Whitney GTF, it’s a 3:1 planetary gearbox.

An added benefit is that the low-pressure compressor (called the booster, blue box) can be made with a smaller diameter and with fewer stages in a geared design, once again, as its blades turn faster.

Data for direct drive versus a geared design

The LEAP and Pratt & Whitney GTF are two engines that show that both paths are viable. The engines are made for the same power range and have the same fuel consumption at cruise speed, but they have very different architectures.

The PW1130G (Figure 3) for the Airbus A321neo has a Bypass Ratio (BPR) of 11.8 at TakeOff power for the 30klbf version. At cruise at 35,000ft and Mach 0.78, the BPR is 13.2. To generate 4,800lbf of cruise thrust, the engine passes an air massflow of 514lb/233kg per second, giving it an Overspeed (Specific thrust) of 301ft/s or 92m/s. The true cruise speed is 450kts, and the air Overspeed is 178kts.

Figure 3. The PW1100 in cut-through with key parts. Source: Pratt & Whitney.

We shall compare this to the LEAP-1A30 used on the A321neo. It has a Bypass Ratio (BPR) of 10.8 at TakeOff. At cruise at 35,000ft and Mach 0.78, the BPR is 11.2. To generate 4,800lbf of cruise thrust, the engine passes an air massflow of 449lb/204kg per second, giving it an Overspeed (Specific thrust) of 345ft/s or 105m/s. The true cruise speed is 450kts, and the air Overspeed is 204kts.

The PW1130G then has a Propulsive efficiency of 83.5% versus 81.6% for the LEAP-1A30. As both engines have the same cruise fuel consumption at 35kft and Mach 0.78 we can deduce that the LEAP core efficiency is 1.9% higher than the PW1130G.

Geared Turbofans are nothing new

Pratt & Whitney likes to give the impression that the GTF (PW1100G for the A320 series, PW1500G for the A220, and PW1900G for the E-Jet E2s) are new and a first for commercial airliners. They’re not.

The possibly first geared turbofan was the engine I had in my SAAB 105 military jet trainer, the Turbomeca Aubisque (Figure 1), which ran on the stand in 1961 and entered into service in 1967. Then we had the Bizjet engine Garreth TFE731, now Honeywell TFE731 (Figure 4), with its first run in 1970, and the Lycoming ALF 502 airliner engine for the BAe 146 with its first run in 1971.

Figure 4. The Honeywell TFE731-5, a late development of the original TFE731. The planetary gearbox is between the fan hub/bearing and the axial compressor. Source: Honeywell.

All these engines used the gearbox to slow down the fan from high-revving cores (turboprop cores for the Aubisque and ALF 502 and, in the case of the TFE731, from a DC10 APU). As they had low specific thrust, they all had good fuel economy from rather low-tech and thus thirsty cores.

26 Comments on “Bjorn’s Corner: New engine development. Part 4. Propulsive efficiency

  1. Excellent piece, Bjorn, thank you.

    You are correct that the PW geared fan is not new, I would argue that compared to the other older engines mentioned that the PW is the first HIGH bypass geared fan developed.

    I liked your analysis on SFC efficiency comparison with both PW and LEAP…. Very similar. So, airline customers really choose based on preference and what contractual deals the OEM will provide. Leaving out both of the OEM reliability issues.

    In the airline world we had a name for the BAe 146:
    Bring Another engine….
    That Lycoming reliability was horrible.

    • High Bypass engine for a sought after thrust class.
      Thrust class is quite a step up from previous incarnations.
      ( add in vast amounts of PR drumming up the “first ever …” meme )
      .. and wrapping this is a predictive maintenance programme.

      GTF works on leveraging rpm transformation using a conservative core. ( the gearbox allows a “less parts higher rpm and smaller” LPT section.)
      How much of the more risk solutions from the CFM can be transferred/leveraged on the GTF side?

    • The ‘bring another engine’ phrase eventuated because it being a 4 engines when all similar planes had two- the benefit was the large reduction in noise for an RJ.
      It was based on the long running T55 turboshaft core around since the 50s so wasnt using new technology.
      The new CFM-56 engine in 1971 wasnt the reliable one it became decades later either
      The little Aubisque GTF- based on existing Bastan turboprop- in the Saab 105 was soon replaced by GE J85 turbojet for the export trainers and the Swedish ground attack version.

      • Not quite. The very special Aubisque (to fly with, as it had variable guide vanes for the fan that made thrust come in with a “delay-delay-oomph”, was active on the Swedish Sk60 until the late 1990s, when it was replaced by the Williams FJ 44 turbofans. The GE J85 was for the Austrian 105 export order only. It had serious Ooomph, I would have loved to fly one (but also serious fuel consumption 🙂 )

  2. Bjorn:

    None of those previous engines were in the class that P&W GTF is. P&W does not claim its new, but they do say its new in that thrust application and it is by a lot.

    Now I don’t want to put P&W on a Pedestal, they should not be.

    But they did spend a lot of years developing that engine as well as a seriously large investment that might not be returned. That alone makes them a mark well above Boeing.

    No disagreement P&W has had issues on the GTF and in surprising areas that are not GTF related. It may be they focused so hard on the part that would trash the program if it failed and not enough to the details around it.

    I look on it as, you can fix those surrounding issues but RR showed us what happens when you don’t even understand your own core and repeat it. I am not seeing that with the PW GTF.

    What I am also seeing is P&W has an easy upgrade path. They were conservative in areas of high head unlike LEAP that had to go there.

    I believe the first upgrade is focused on reliability which is good and the reports I have read indicated the PW engine is a bit ahead of LEAP SFC wise (and worse with the nit noid problems the PIP should not only solve but also0 the longevity issues they both have)

    But as I understand it, P&W were pretty conservative with the whole package. Reports are that they can increase the SFC by 1% a year. I think it was variable nozzles on the table that they determined were not needed but there in the package if they were.

    They may get a bit of that with the PIP but it would be the 2nd PIP that starts to grab the full ability.

    So while LEAP is going to be small incremental, P&W GTF looks to have a lot of upside SFC wise and there is nothing in the architecture pushed so reliability should also be there as they sort out that new beast.

    I also believe they were conservative with the fan setup, so not they can play with that as well as the core in using more exotic higher heat capable materials and increase there as well.

    I am impressed P&W stuck with it. All the NASA studies I have seen baseline a GTF to get the improvements they want in any of the combination of air-frames they are looking at.

  3. Is the GTF a little quieter too? It seems to me that they are as a passenger…

    • A big bypass helps reduce noice and improe T-O and climb performance. P&W did have problems even before certification “rotor bow” and it just continued with burner life problems before the powder metal issue. Statistically they should also have seen and solved a sleeve of additional problems, hopefully they get everything right with the Advantage upgrade.

      • 3200rpm ( PW1100 ) vs 3800rpm ( Leap-1A ) for the fan
        should make quite a bit of difference too.
        Even though the GTF fan is 4% larger lifting tip speed slightly.

        • However for the Leap-1B engine the low pressure turbine speed- which drives the fan is 4500rpm [A engine is 3800 as mentioned]
          For the PW1100G the max LP speed is 10,000 rpm and thats geared down for the fan to be around 3500rpm

          The leap engines use highly twisted carbon fibre fan blades, and less of them.

          • The LEAP engines borrows alot from the GEnX design. The GEnX is a hot rod for long range flying and can live with expensive almost non repairable parts. The LEAP engine must be able to take much more cycling and be repairable to keep operating cost down. A bit like RR’s Trent overhaul cost problem. I do not know the LEAP-1A fleet leader cycles and if it has been thru a full 20 000 cycles OVH yet with a cost summary.

  4. The move to develop the GTF for narrow body applications in the 1990s to take on the CFM56, after the JT8D/JT8D-200 was at the fag end of its lifespan and after the PW6000 disaster, was a bold and brilliant move indeed from Pratt & Whitney leadership given the $10 billion investment it entailed and CFM56’s absolute hegemony of the narrow body market.

    • Pratt and Rolls and others had the CFM-56 equivalent V2500 engine ( the V stood for the 5 member consortium) long after the JT8D was history. Certification was 1988 and 7500 engines were made, like all new engines it had reliability issues.

      • While I get how jet engines work and can follow the tech involved to a degree, I am not able to compare two engines of the same thrust class.

        Or more accurately, I get the SFC and some of the aspects involved like maint, time on wing and overhaul costs. On the bigger engines as well as the current LEAP and GTF you can see that playing out as well.

        So yes I can see why the GenX beat out RR and that was before the debacle of the various failures on the Trent 1000.

        V2500 I have not seen similar comparisons. So what was different enough about it that it did not stand on equal footing with CFM-56?

        I thought it was a strange decision for RR to get out of that program and a smart none for PW to be in it and then take up RR portion.

        No questions that PW failed in some basic areas on the GTF, but they did keep their foot in the Single Aisle Market with the V2500 to not be absolutely out of touch.

        And it kept the partners PW still has on the GTF in the business as well.

        The V2500 does not seem to be a bust but it was not on par with the CFM-56 so that is puzzling to me.

        • The V2500 had initial durability problems with the V2500-A1. The -A5 was a big improvement still not trouble free, P&W fooled RR into some stupid design mistakes like only one VSV actuator. Still the CFM56-5B architecture with only one HPT stage reduce cost. The CFM56-7 on the 737NG got the improved parts first then they came onto the CFM56-5B. The CFM56-7B got amazing time on wing for the 737NG and a cost effective design. The LEAP-1A is of another scale of exotic materials and complexity (just the modulated HPT blade cooling flow is one example). It will take decades before its maintenance cost per hour on wing to reach CFM56-7 levels.

    • I remember PW pushing for PW8000 GTF testing on the A320 25 years ago. Not mature enough at that stage.. PW management sure had a lot of faith in the technology, keeping investing billions when ROI was very uncertain. After 9-11, GFC, a decade of cheap oil.

      • Afair Airbus was outspoken to want a GTF engine under the IAE umbrella. The SuperFan debacle seems to have burned them soundly.
        On the other hand: no SuperFan engine lead to the “overwinged” A340/A330 layout that later leveraged the major success of the A330.

      • “The PW8000 was supposed to be a geared fan section on a PW6000 power core[A318 engine] , and the article in the SAE journal archives is most interesting. The engine was scheduled for certification two years on, which would have put the release date about 2001. There is also a very nice article in Flug Review which described the history and construction of the PW8000.
        But the PW8000 never went anywhere despite ten years of research and $350 million in development money.”
        Not billions
        There was also the IAE Superfan a geared fan development of the V2500 core , which was supposed to be the engine for the then new A340. It got the CFM-56 modified for long range cruise instead. The Boeing 7J7 150 seater was also a target plane.
        What all could have been !

      • Not being in in depth engine guy, I was wondering if there was any reason why the V2500 was not a better engine than the CFM-56.

        Newer by some years, somewhat successful but not a match for the popularity of the CFM-56.

        I can see the maturity aspect but a newer engine should be a fair amount better SFC and latter maint costs once its mature (and match for reliability)

        • @Transworld

          There was relative sales parity on the A320…of course the B737 wing is owned and funded by CFM. A lot of people forget thats why any derivative of the current B737 wing was going to be CFM exclusive.

          By the time PW offered the GTF commercially around 2007 there was not much industry confidence in PW being an engine OEM. RR had the inside track on the MRJ and PW needed a moonshot to get back into the business. RR was not interested in an equitable partnership on a next-gen IAE. PW was offered an IHI or MTU level share.

          • “the B737 wing is owned and funded by CFM.”
            Really? Could you tell us more? Thanks.

          • @thesupermoop

            When the time came to launch the 737NG, a commercial deal was reached where CFM would fund a new wing in exchange for exclusivity and other commercial terms. In 1992, PW elected not to offer an engine on the B737…a decision I am sure they regretted immediately. GE/CFM has been sole source ever since.

            When the Max was launched, Boeing looked into competing engines, but the decision not to re-wing the aircraft again meant that the exclusivity agreement remained in effect.

            Finding web articles to go into detail for something this old is hard to find. Attaching a link here that hints around the issue when you read the comments. Maybe someone else has a better archived story.


          • @Casey

            Actually, as I wrote in my book Air Wars, Boeing had the option to place the GTF on the MAX; it was a matter of the engine being better than the LEAP. Boeing could also have offered a choice of engines. In the end, commercial terms between GE/Safran and Boeing kept exclusivity for CFM.

            Air Wars:


  5. A geared turbofan should weigh significantly less than a non-geared equivalent. Not only does it need fewer turbine stages and a shorter case & shaft at the back, but the fan can rotate more slowly than it otherwise would. The centrifugal forces are lower and the fan disc, a great forging of titanium, can be trimmed down. This has a knock on effect that runs through the entire structure of the engine – fan blade off forces are lower too, so the fan support bearings and their structural supports can be trimmed down.
    Against that is the weight of the gearbox itself.

    • There are some fear of geared turbofans since the Lycoming LF502. The PW1100G did not help the geared fan reputation for durability yet, still the Garrett TFE731 was good for its time. You have benefits in mass if you can design a much lighter and smaller LPT/LPC. Avoiding a huge fan case and matching nacelle reduce cost/mass and wing/nacelle drag interference.

      • Ancillary issues.
        I haven’t seen any mention of a broken gearbox ( yet ).

        significantly raising LP turbine rpm …
        and afair on the A320 the selection of mounting points had issues.

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