Optimizing LEAP for 737 MAX

There has been a great deal of debate over whether Boeing can meet the SFC reduction targets for its 737 MAX. This debate revolves in part because neither Boeing or CFM have been forthcoming about details how the CFM LEAP engine is being optimized for MAX.

We’ve obtained some details to explain how CFM is proceeding.

Details are still sketchy and hard to come by. But our source has direct knowledge of the program.

Our source compared the requirement to reduce the fan size of the 737 LEAP from 78 inches on the Airbus A320 neo to 68 1/2 inches on the MAX to the fan reduction on the GEnx from the 787 to the 747-8. The 747-8 engines are optimized for this aircraft despite the smaller fan size.

Reducing the fan enables CFM to eliminate some LPT stages, our source explains, which also cuts other parts.

This eliminations allow the LEAP to be shorter, which also allows the engine mounting to be shorter.

CFM is also using ceramics to the MAX LEAP.

These are some of the key ways CFM is optimizing LEAP for MAX.

40 Comments on “Optimizing LEAP for 737 MAX

  1. With less energy scavenged from the core airmass will be ejected at higher temps and speeds.
    Cost is propulsive efficiency of the core and higher noise levels, right?
    With less energy for a smaller fan bypass airflow will not change much speedwise, though less mass and thus less thrust ( taken up by core thrust )

    • Reducing turbine stages would result in less energy captured from the core, only if the remaining stages are unchanged – It’s pretty obvious this is something CFM would not do. We have lots of circumstantial evidence CFM will be putting different airfoils in the LEAP-1B turbines to capture the energy with fewer stages. The most telling evidence is the fact they will be applying ceramics – an admission the airfoils in at least the first stage or two of will be seeing higher temps.

      • I would say that the ceramic blading is to offset a higher T4 (HPT inlet temp if you don’t speak gobbelidock…), which in turn is needed to get the efficiency up with the smaller BPR of the LEAP MAX.

        Now, there is absolutely nothing, from a technical pow, that prevents CFM adding the same ceramic HPT blading (and assciated higher T4) to the LEAP NEO.

        As for the LPT, they will not extract the same energy from the core flow in the LEAP MAX as in the LEAP NEO. The LPT drives the fan and the MAX fan is smaller, hence needs less power, thus the LPT will extract less power. Otoh, the power into the LPT will likely be less in the LEAP MAX since it makes little sense to burn the same amount of fuel in a lower thrust engine…

        The two cores are the same, or at least very similar, and since the NEO core was the starter I would venture that its size is larger than had they made a core from scratch for the LEAP MAX. But otoh, again, you can do all sorts of neat tricks, such as subracting a stage from the LPC in the MAX version (the opposite was e.g. done in the GE09 when moving from the -90B to the -115B).

    • Maybe they will simply inject less energy into the core airmass? (less fuel)
      Or, and this can be radical as well, reduce the core airmass to retain the optimal bypass ratio…

      • Assuming similar tsfc the amount of fuel required for similar thrust will be … similar 😉

  2. Thanks for the info.
    I think no one disputes, that a larger fan is probably better when optimizing for minimum fuel burn (if not, why did they propose it for the A320?).
    However, the additional effect if the last inches is very difficult to quantify.
    This problem is comparable to the ideal wing aspect ratio.
    By theory any increase in aspect ratio reduces the lift-induced drag. But: the wing structural weight increases with aspect ratio. At some point the drag reduction cannot compensate the added weight. Where exactly this point is, tough question. You see from the aircraft designed in the 1960ies, that they considered the optimum elsewhere (cost structure at that time was vastly different than today’s, so minimum fuel burn wasn’t necessarily the prime consideration).

  3. Take a look at this piece which reports that GE, Spirit, and B have agreed on just about everything related to the LEAP X to be used on the MAX. If this were just B talking, I would discount the claim substantially, but neither GE nor Spirit has anything near B’s reputation for routine mendacity, so I am inclined to take them at their respective words, at least provisionally. According to them, things look very good.


  4. Of course the Leap will be optimized for the 737 MAX. It has to.

    The difference between “optimizing”and “damage control” is semantics.

    It will also be optimized for the NEO, C919.. maybe even more aircraft in the future.

    And CFM will use any lessons from the different variants to improve the others too.

    Specially the version for the NEO version, because that one will be in direct competitions with the PW GTF. Losing market share if if sub optimizied..

  5. mneja :
    I would say that the ceramic blading is to offset a higher T4 (HPT inlet temp if you don’t speak gobbelidock…), which in turn is needed to get the efficiency up with the smaller BPR of the LEAP MAX.

    Hmm, the smaller fan allows slightly higher rpm for the lpt ( and probably a slight reduction in blades/surfaces there) .

    • Hmm, true, and higher LPT speed allows it to have fewer stages, although I would be surprised if it was a big difference. I guess 1 stage maxiumum reduction.

      But ceramics wouln’t be needed just for this. Must be a T4 hike as well or they just made themselves a more expensive HPT…

      The thrust should a little less for LEAP MAX than LEAP NEO, so a little less fuel/energy will be converted in the core.

  6. I suppose, then, that we could expect a PIP shortly after introduction for the NEO to incorporate gains from the MAX excperience to widen the gap between the two, unless that makes bad business for CFM to tweak Boeing like that

  7. I will sort of agree more with keesje here than with Uwe. keesje is right the LEAP-1B engine will be optimized for the B-737MAX, and the LEAP-1A engine will be optimized for the A-320NEO. CFMI has already demonstrated they can do this as the CFM-56-5B engine is optimized for the A-32X and the CFM-56-7B is optimized for the B-737NG. Both the -5B (68″ fan) and -7B (61″ fan) engines put out about the same amount of thrust and are also very close to one another in SFC, dispite the difference in fan size. They are also fairly close to each other in BPR (-5B BPR 5.4 to 6.0, the 7B BPR 5.1 to 5.5 ).

    There is no indication that CFMI will build an inferior engine for Boeing and a superior engine for Airbus.

    Yes, the LEAP-1B engine will run hotter, and faster, and will have ceramics, so what? My guess is Boeing already knows this and has accepted it, just as Airbus has accepted the make up and performance of the LEAP-1A engine.

    It seems to me that it is CFMI’s responsibility to make the LEAP engine (both versions) as competitive to the GTF as possible. They seem to be doing that.

    • Noise will be higher with a faster fan speed (everything else equal technologically, and there is no reason why it shouldn’t).

      They can probably make as efficient a gas generator (core) for the MAX by upping T4 a little but and use ceramic blades. But this would likely mean shorter maintenance intervals or higher cost blades (or a little of both), depending of course on how much life improvement the ceramic blading gives. And no reason why the NEO engine could not do this too (both LEAP and GTF), this is not an exclusive to MAX (if it is on the LEAP they risk falling behind NEO, so I doubt it).

      So: more noise and higher main’t cost (whether it comes out in the intervals or part cost remains to be seen).

      • After CFM upped the LEAP specs for the NEO there was some speculation around that they would initially use materials
        with shorter service life and than replace with CMC based
        parts ( time horizont for availability after NEO EIS). No idea if that would be before or after MAX EIS.

        • Uwe, there has also been numerous speculations on when the world will end. That shows there is no relationship between speculation and reality, or fact.

      • KC, baring the introduction of some pretty advanced planet moving technology, life on Earth will have ended by the time the Sun has entered its red giant phase some 1 billion years from.

        • Yes, I know. I was referring to religious beliefs, alien invasion, and other possibilities. BTW, the estimates I have heard is our sun will become a red giant in about 8-10 billion years. Either way, I will miss it.

      • Yes, you’re right of course. Red giant phase starts some 5.5 billion years from now. What I meant to say was that due to the ever increasing luminosity of the sun, 1 billion years from now, life on earth is pretty much over by then.

        Now, silly specuation such as doomsday prophecies by nutcases is not really equal to the speculation on whether or not CFM International would use silicon carbide-based ceramic matrix composites on the first iterations of the Leap-1B engine, is it? 😉

      • I’m not sure the noise level for the LEAP-1A/-1B/-1C will be any higher than that of the current CFM-56-5B/-7B, or GEnx 1B/2B engines. Boeing is planning on adding chevrons to the engine nacells, which, I believe Airbus is going to copy, too. The MAX and NEO will meet the noise standards in both the US and EU.

        The LEAP-1B engine should have a slightly faster fan speed, since it is about 15% smaller than the -1A/-1C engines. But that does not translate to a 15% faster fan speed, it is not a linier line on the graph. The LEAP engines will spin slower than both the CFM-56-5B/-7B engines. Thus they should be slightly quiter (by a few dbs) than the engines now on the A-32X or B-737NG.

      • According to Heidegger, we probably don’t need to wait 1 billion years : of the original four moons circling Earth, three have already crashed, so we’re only waiting for n° 4 and last one : our present Moon … when does celestial mechanics predict its final narrowing in on Earth and conclusive contact, OV-099 ?

  8. I guess it will be undesirable for CFM to make to many differences between the different LEAP versions. And the customers will demand the best. E.g. Norwegian and AA will demand optimized, similar engines for their 737 and A320 fleets.

    So assuming both will have the same level of technology, CFM will at one point tell us what will be the performance differences betweem a LEAP with a 68 inch fan and flattened inlet and that LEAP with a 78 inch and circular inlet.

    If the 78 incht is better Airbus probably will shrink the fan and flatten the inlet. If the 78 inch is better, Boeing will install the 68 inch flattende variant and say it is optimized for the 737 and it is much to simple to look at just the fan.


  9. KC135TopBoom :
    Uwe, there has also been numerous speculations on when the world will end. That shows there is no relationship between speculation and reality, or fact.

    From you that is high praise, isn’t it ;-?

    Actually a question of scope: “overscoped” questions will lead to overly speculative answers.
    This is not the case here: availability of required CMC parts does not match up with given EIS. There is not much room for solutions.
    A: move EIS to the right
    B: offer a shorter initial livespan solution with an upgrade path.

  10. The CFMI LEAP engine will come out in the next few years in 3 different versions. The LEAP-1A for Airbus, the LEAP-1B for Boeing, and the LEAP-1C for COMAC. Funny how that worked out. But the LEAP-1C will be the first version to come out, followed by the LEAP-1A, and finally the LEAP-1B.

    The LEAP engines have always been planned to use composite materials, a blisk fan for the compressor, and a second generation TAPS II combustor. It will have two different fan sizes, a 1.70m for Boeing, and a 2m for Airbus and COMAC.

    The LEAP engines should have a BPR around 10:1-11:1.

  11. There is a lot of clamoring about the GTF, but right now it is unknown what the reliability of the GTF will actually be compared to the LEAP and other engines. Historically, turboprop engines have had shorter service intervals compared to the same turbine without the gearbox. Yes, things look pretty good right now but what happens if the GTF service interval is only 90% of the LEAP. In particular, even a slightly higher time between events could play havoc with the ETOPS rating.

    Often, with new technology it is better to sign up for the 2nd generation rather than be early in the 1st gen. Besides for the effort to include a 2nd engine on the MAX, Boeing may be wanting the GTF to mature a bit more before including it.

    • For tne NEO buyers seem to balance the GTF advantages against the “good pedigree” low risk position of CFM ( and GE ). But the initial CFM offering seems to not have balanced the scales. With the resultant first performance upgrade risk rose for the CFM side.
      For the MAX LEAP that requires another significant infusion of bleeding edge tech this risk
      would increase further. Just like Boeing the engine manufacturers have lost a bit of shinyness
      during the gestation of the Dreamliner ( and with a cause ).

      IMHO the “don’t buy a Mk1” advisory probably holds for both offerings.

      • I think BPR is often held to be the motherlode of engine efficiency. While it is a significant factor, it’s not the entire answer. Just like RR is speeding up the fan of the XWB -1000 engine 6% to allow the fan of the smallest engine maintain the BPR on the 15% more thrust -1000 engine, CFM will no doubt be speeding up the smaller fan of the -1B to get their required BPR.

        While increasing fan size is the easiest way to increase BPR, it’s not the only way.

        One disadvantage of faster bypass flow, is more noise but if the current CFM’s meet all the noise regs, the Leap engines shouldn’t have much problem with it.

      • They both are using bleeding edge tech on their engines…just in different places. Pratt’s is in the gearbox and CFM’s is in the core…and neither has been proven in service.

        Both have been working on their solutions for a very long time and the technologies required to make a reliable gearbox are probably not much less exotic that what is required to make an efficient and reliable core.

    • I don’t think Boeing can care about the GTF right now. I believe they have an exclusive agreement with CFMI.

    • What the general public isn’t privy to (and will never be) is whether there exists (??) some hidden incentive to single-sourcing the 737 powerplants, of the kind “retrocessions of GE/Safran sales proceeds back to Boeing”. If something of the kind does exist the routing back to the airframer would be made more impenetrable than the Ways of our Lord : CFMI strategists weren’t born out of the last shower of rain.

      • More than electrons, Uwe : I note you also know how to herd flows of US dollars ! Let Gecas purchase aircraft from Boeing at artificially “inflated” prices, then let Gecas lease aircraft at market prices. Or let Gecas purchase engines from GE-Safran, then let Gecas lease these engines cheap to Boeing customers : both constructions theoretically would actually allow for veiled $-flow re-routings …

  12. Frequent Traveller :

    You need a method and an instrument to bind!

    The same change we have observed in the banking sector happens in leasing.
    From mediator / “impedance match” for a fee these institutions have moved to
    being major profit takers. Profits the airlines ( and the airframers? *) lack.
    And the hamstrung position a significant number of airlines show aggravates this: Higher perceived risk, higher leasing rates. My guess is that aircraft appraisers like the rating agencies in the main financial arena help the money side in this situation ( i.e partisanship )

    * GECAS and other manufacturer attached financing arrangements try to fix this imho.

  13. OK, Uwe : in business (as in love-makin’), everything is acceptable between consenting (legally capacitated) partners … so let’s leave the Boeing/CFMI/Gecas threesome (?) and Back to Essentials, ie, back to H2XQR Series and H3XQR Series : why does Scott Hamilton label (1+3+1) for the NEO and/or for the MAX “goofy” ?? How can we make LeeHam/AirInsight professionals come around to giving H2XQR Series a “fair review” ?

    • As Mort knows well, from many emails back and forth, AirInsight has reviewed the economics and our conclusion is that that numbers don’t work. He doesn’t like the answer, he won’t accept that we simply do not believe in his concept as economically feasible or our conclusion that airlines won’t cut capacity. His constant bleating to us long ago crossed over into the insulting and annoying phase. I won’t answer his emails as a result and his whining here won’t get any traction, either. I’m allowing him to debate his concept but lobbying isn’t going to get him anywhere. Nor can he seem to get “Leeham” correct. He also wants AirInsight to undertake a full study for free but only if we tell him in advance if we will agree with him.

      Drop it, Mort.

      Scott Hamilton

  14. Pingback: Boeing MAX v Airbus NEO; Fan size and optimizing the LEAP for MAX, Part 2 « Leeham News and Comment

  15. leehamnet : … we simply do not believe in his concept as economically feasible …

    Here’s where the shoe hurts, Scott : you reduce (or elevate ?) your appraisal to a matter of “belief”, as if SMR Feeder air transport economics is some Religion. Yet, you are fully aware that I’ve developped a proper economic THESIS to sustain the resilience of my concepts. About AirInsight debating my concepts on the web, I’ve told you that I’d readily accept the challenge, all I’ve asked for is a “droit de réponse”. I’ve also intimated that obviously, a public debate concerning the H2XQR Series concept with AirInsight offering the soapbox (forum) wouldn’t exactly please Boeing or Airbus, so I understand why you’d rather not. This proposition is received at your end as an insult. Never mind, I don’t blame you, no hard feelings, Scott. I am a patient man. Where you yourself see a “goofy” idea, others (closer to yourself than you’d imagine !) will one day ignite to the rationale of (1+3+1) in Y-class and (1+2+1) in B-class, in lieu of (3+3) in Y-class and (2+2) in B-class. Even RYR or SWA or Tony Fernandes could awaken to a sweet “199-er” H21QR : it makes sense, pocket calculator in hand. Let all the Nostalgics jubilate into pension, Scott : Bon Vent ! Forget the Hindenburg chute !

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