Bjorn’s Corner: New engine development. Part 5. Turbofan design problems

By Bjorn Fehrm

April 26, 2024, ©. Leeham News: We do 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 and risks.

To understand why engine development has become a challenging task, we need to understand engine fundamentals and the technologies used for these fundamentals.

We discussed geared versus direct-drive turbofans last week. Now, we’ll examine some design problems for these engines.

Figure 1. The Pratt & Whitney PW1100G geared turbofan, with its unique aluminum fan. Source: Pratt & Whitney.

Turbofan design problems

We learned that a High Bypass Ratio (BPR) can increase Propulsive efficiency by lowering the air Overspeed that passes through the engine.

To keep the fan RPM low while the compressor and turbine RPMs stay high, we use a planetary gearbox between the fan and the core low-pressure shaft, Figure 1.

The geared design allows a high RPM design for the booster compressor and the low-pressure turbine, thus keeping power efficiency high and stage count low.

The gearbox also enables a fan design that avoids blade tips that pass into supersonic flow during high power (and thus high RPM) settings.

The Pratt & Whitney Geared Turbofans (PW GTFs) have a fan that stays subsonic during takeoff and climb power settings. This has enabled a fan design using hollow aluminum blades, something unique in the turbofan market.

It lowers the weight of the fan and also of the nacelle, as the fan shroud, which shall contain a broken blade, can be made less strong.

I know of no other turbofan fan made of aluminum. Typical materials for fan blades are Titanium (solid for smaller fans, hollow for larger) and, recently, Carbon Fiber Reinforced Polymer, CFRP, blades.

The LEAP fan is CFRP, made with a resin infusion process (Figure 2), which is more adapted to high volumes than the pioneering CFRP fan of the GE90. These fan blades (and those for the GEnx and GE9X) are made of hundreds of hand-layup prepreg sheets.

Figure 2. The fan blade form for the LEAP resin infusion CFRP fan blade. Source: CFM.

It has created a high-quality blade (no blade has ever broken off in service), but it’s not suitable for high-rate production.

The LEAP has a fan that goes supersonic on the outer part during takeoff and climbs. It thus must have thin outer blade parts, something that can be a challenge with CFRP.

That’s why Rolls-Royce and Pratt & Whitney opted for hollow titanium fan blades on their Trent and PW4000 engines.

Rolls-Royce has now developed a resin-infusion CFRP blade (Figure 3) for the next generation of turbofans, called Ultrafans.

Figure 3. Next-generation Rolls-Royce fan using CFRP fan blades. Source: Rolls-Royce.

Heavy powerplants

As fan sizes increase for a thrust class, the mass of the engine increases as well. Modern high bypass engine installations are heavy.

Comparing the PW1130G and the LEAP-1A30 engines’ mass, the geared principle has made the GTF 290 kg lighter (2870kg vs. 3160kg) despite the GTF having a three-inch larger fan (81 inches versus 78 inches) and a fan gearbox.

A modern high BPR engine also increases the whole powerplant mass. The CFM56-5 for the Airbus A320 weighs about 3.6 metric tonnes with nacelle and pylon. A LEAP-1A with nacelle and pylon weighs about 4.7t, an increase of 30%.

The reason is that as the BPR increases, the fan, fan shroud with fan housing, nacelle with thrust reverser, and pylon increase in mass.

If the search for propulsive efficiency drives the desired BPR above 20, it’s time to look at fan designs that skip the shrouding nacelle.

These used to be called Open Rotors. Open Fan is also a good name. We look at this alternative technology in the next Corner.

36 Comments on “Bjorn’s Corner: New engine development. Part 5. Turbofan design problems

  1. One of engine experts mentioned in his speech that reducing number of LPT stages on GTF has negative impact on blades and vanes.

    The high thrust produced by core simply can not be handled (more correct technical term converted i assume) by only 3-4 stages on GTF’s, opposite to direct-drive turbofans where number of LPT increased to 6 stages. Of course the main reason is decreasing Fan RPM with making LPT Rotor heavier.

    Can you share you though if there is impact on LPT durability and reliability because of less stages on GTFs, that simply was neglected because main focus was gearbox and front section of GTF.
    Thank you

    • The design principle in a GTF LPT is that the LPT can be optimized around a higher RPM module and thus extracting more power per stage and needing fewer stages. The trick like every other engine part is making sure the stages you have left are durable enough for full engine intervals.

      • The LPT on a geared fan is a bit unoptimal until it reaches design rpm’s. Hence it can run hotter in the rear stages until rpm gets over a threshold were the LPT sucks most energy out of the gas stream.

    • To my knowledge, there is no general rule that says this. It’s about the detailed design and material choice in both cases.

      An example; The direct drive LPT has a large diameter and many stages by necessity. Thus, you try to limit the mass by using light materials, especially for the last cooler stages. One such material is titanium aluminide. I know of engines where the TiAl was designed in from the beginning and where it later was replaced with more classical Nickel based alloys as the long term durability was not there. I’m not pointing to any specific engine or case; it’s all about the detailed design and use of materials.

      • Fewer stages would also be less costly for mfg as well as the overhaul when it is due.

        Jet engines are truly the ultimate in trade offs to get a desired end result.

  2. P&W PW4000 got hollow fan blades for the 112″ version only and then onto the GP7200, both have seen fan blade problems with UAL, JAL and Air France.

    • Indeed on the UAL fan blade failure…some of the fan blades are now pushing 30 years old. UAL was the launch customer of the B777 and is still operating the majority of their aircraft.

        • Fundamentally it was a maintenance scheme that did not contemplate inspections for micro-cracks.
          Agreed Aloha was the poor appreciation of high cycle aircraft fatigue.

          The GP7000 Fan Hub was found to be cold dwell…still not particularly well understood.

          Therein lies one of the greatest challenges to engine performance. Operating at RPMs and/or temperatures that have never been seen requires new materials. Materials science is at the bleeding edge of technology and the trade secrets. Sometimes the materials do not perform as expected. Predictive models without historical industry performance have their limits.

          • “Agreed Aloha was the poor appreciation of high cycle aircraft fatigue.”

            Casey: I recently re-read the NTSB report.

            It was illuminating. They did understand it.

            Boeing fully understood the high cycle issue as well as the environment Aloha operated in.

            They not only knew about where the failure would start, they had inspection for specifically for that as well as what to do about it once detected.

            Aloha did not want to take aircraft off line to do the inspections right let alone the work. Their paperwork said they did the inspection and found nothing when in fact they were not doing the inspections.

            Aloha inspectors pencil whipped it to keep aircraft flying and after the blow off, the other aircraft revealed exactly what Boeing said they needed to be finding and the rupture took place exactly where Boeing said needed to be inspected (and fixed when not if found).

            Like the Exit Plug blow off, its mind boggling that the Aloha 737 did not crash.

            Its an area I truly get angry in. When they know there is an issue and fail to deal with it for whatever reasons, its criminal. And we see it over and over again.

            In Aloha case it was the management and inspectors caving to management.

            In the fan blade case it was PW using the system not to implement a process and training to ensure that the blade losses did not happen and the FAA with letting them slide.

            Its beyond appalling.

            And in the case of Boeing, two MAX crashes get nothing done because the FAA is not doing its job and only an extraordinarily lucky escape finally gets the attention needed.

    • Claes:

      The GP7000 was a not understood mechanical phenomena so that is one that truly was not able to be foreseen and I believe it was a single incident.

      The 777 issue were both a regulatory failure (FAA) and PW failure for a inspection procedure that was well known to be required but failed to implemented.

      There is a lot of managing things like that in the Aviation world. If done right its not an issue, not done or done wrong it is.

      Aloha Air blowout was one of those. The Issue was well understood by Boeing and inspection and fix was in place to ensure it was nipped in the bud.

      Aloha was pencil whipping their inspections and the FAA inspector was severely over tasked.

      • The hollow fan blades was new to PWA and produced a bit differently than RR hollow fan blades (that has had its problems and inspections). It is not that easy to define life limits and NDT inspections that catch all relevant defects on a totally new design. You always have defects and you want to control/check its growth rate by design and NDT. PWA had a different problem on the A220 engines fan module.

      • @TW

        “Aloha Air blowout was one of those. The Issue was well understood by Boeing and inspection and fix was in place to ensure it was nipped in the bud.”
        What a nice picture you painted! Why on earth, more than two decades later, WN 2294 and (less than two years later) WN 812 happened? BA had to rush out a SB for additional inspections after WN 2294 and FAA had to mandate additional periodic inspections for as few as 500 cycles after WN 812.
        Clearly FAA was extremely worried for a third incident.

  3. Do you have any idea what the P&W GTF fan disc is manufactured from? Surely not aluminium?

    • Clearly says so
      ‘The Pratt & Whitney Geared Turbofans (PW GTFs) have a fan that stays subsonic during takeoff and climb power settings. This has enabled a fan design using hollow aluminum blades, something unique in the turbofan market”
      The leading edges are titanium

  4. I really enjoy you articles on new jet engine technology. I was in the USAF for 10 years and worked on TF33p7a (C -141 A & B models) and TF-39 (C-5A) TURBOFAN ENGINES. Like I stated I really enjoy reading about new turbofan engines. I think some of the new technologies are really fascinating. Please keep up the great work.

  5. Good article. True that the engines are as important for a new airframe as the airframe.
    That leads to the question, what is the Ultrafan made for? That would be a massive 2 engine airframe, possibly a re-winged A380 or a double-decker 350?

    • @Halken
      RR is pitching the UltraFan up to 110K of thrust. I would venture that the A380 is gone for good. It was too much plane as would a double decker A350.

      The GE9X has a 134″ fan…Ultrafan at 140″ which is not out of range with what is made today. The efficiency gains on fthe UltraFan really come from being able to shrink the core driving this monster fan.

      • I think RR analysis point to a streched A350-1000 with a new optimal wing for it that would suit the Ultrafan. That would be ideal for many Asia to Europe/US routes. Another application is the 787-10 stretch with a new wing design. That aircraft with +400 seats in airline 3-class config and 787-9 range. GE would not be happy if that aircraft with RR engines took over many of the 777-9 routes. But BA, SQ, Cathay, Qantas, ANA might vote for it.

        • The first ultrafan plane will be the Boeing Midsize.

          The 787-10 isnt going to be stretched and undercut the 777X range and the A350K is more likely to have the GE9X than a different RR engine. The ‘stretch’ was done by upping the gross weight and increasing the interior width within the same outside frame

          • @Duke
            Any ultrafan installation is likely to need an entirely new centerline variants. It would be one thing if there was one already in service…but there isnt. A one variant installation is an economic disaster. And i really do not see any aircraft over 400 seats getting greenlit anytime soon.
            The ball would seem to be in Boeings court in whatever they do next (737 replacement / NMA / or B787 re-engine)

          • I think RR got exclusivity on the A350 series as Boeing got on the 777. The next step for RR Ultrafan would be altitude testing at The Aero-propulsion Systems Test Facility at Arnold Engineering Development Complex “designed to test aircraft propulsion systems in true mission environments without leaving the ground”.

          • Cassy:

            RR did a re engine (Trent 10) on the 787 and did them no good.

            Boeing will hold pat there for another 10 years.

            P&W and RR would then offer their versions of the GTF.

      • The A380 is also 100t heavier than the 747. So this engine could be put on a 2 engine 747 replacement. Tim Clark what A380 replacements, and in many fleets the 747 is also reaching its age limit. What is really needed is a 747 2-engine replacement, and Boeing does not have the funds to do it. That leaves Airbus, if they want something that both satisfy Tim Clarks needs and can replace the 747 with something modern that does not require extra adaptatio as the big and heavy A380 do and only have two engines.
        If there are 250 A380 and 400-500 747 to replace, that could make for a better business case and steal some sale from 777x.
        This is what I would if I was Airbus.

        • You would have to re-desing a A380 or a 747 wing and no longer in production, not happening.

          And that assumes you could stuff such a monstrous big enough engine under the wings.

          777X has pushed the limits of a twin.,

    • The A380 is not going to be resurrected let alone putting two engines on it.

      A double deck A350 is a pure fantasy project. It would cost 3-4 times what an all new double deck would (which is also not going to happen).

      RR picked the upper thrust range to match anything foreseeable. In this case it would the the talked about A350-2000 (and various other monikers)

      The design is the top limit, RR says it can be scaled down to single aisle size if that opens up.

      GTF is the future, while RISE in theory has its promises, its not a reality and and prop aspect is a game stopper.

      The EV in cars was a thing and its proving to be niche. I know of one long run between two cities that has a generator firing a charging system.

      So you pull off, spend an hour or so getting charged up while that generators just keeps on chugging away.

      • Most likely RR Ultrafan would be just over 100k thrust. In theory you can shrink it but cost does not shrink as fast. You could tolerate a very expensive big engine on a power by the hour agreement if it flies extreme long range and thus see very few cycles during its life but a huge amount of chargeable hours like SYD-LHR.

        • Simply not happening.

          RR would have a shot if Airbus went with a A350-2000. P&W would be in that mix as well. But one or the other would want an exclusive as not enough business for two.

          GE will not try.

          • @TW

            I dont even think there enough business for one. Airbus would have to subsidize this variant to get anyone interested.
            RR could justify a new ultrfan of that magnitue “only” if there was some other derivative installation to come.

            Reading the tea leaves…there seems to be the most fertile ground in the mid-size market between Boeing and Airbus. Boeing needs to launch something.

      • “A double deck A350 is a pure fantasy project. It would cost 3-4 times what an all new double deck would”

        Please explain your calculation. How do you come up with this??

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