B737MAX - the next diameter!

UPDATE: Jon Ostrower reports that the exact diameter will be 69.4" and that the engine will also get smaller core size. I have not seen the article myself as I do not have a WSJ subscription, but Scott Hamilton has an update on the story, too.
Scott writes that a smaller core fits better under the wing. Well, yes and no - the core, meaning the HPC and HPT are physically smaller, but as the worksplit between the high and low spool changes, the LPT has to do more work now - and at the same time needs to run slower because of the larger fan. So the diameter of the LPT has to move out as described earlier. And there could really be a need for an additional LPT stage with the higher loading of the LPT. Only if the efficiency of the LPT can be kept constant the smaller core size and higher bypass ratio will do any good for SFC and  fuelburn. Temperatures in the core will rise anyway - what this means for maintenance costs customers will see only a few years after putting the engine in service.

It is becoming a real saga: the fan diameter of the B737MAX LEAP-1B engine. Now we are at 70", compared to the 61" of the CFM56-7BE and the last announced 68.4". Read the story at Leeham News here:
Sorry - but, but for me Boeing is loosing credit with every change that is announced (or, as in this case, reported by a credible source). For months Boeing now said "Bigger is not better", defending the B737MAX and the LEAP-1B against the A320neo and the LEAP-1A/PW1100G with their considerably larger fan sizes.
So, is the larger 70" fan just for optical reasons? Just a joke, of course...
A larger fan needs to run slower to keep the efficiency high - or in other words: the tip speed needs to be the same as it was before. But this means that the LPT also needs a larger diameter to keep it's efficiency - or the LPT needs an additional stage. If this is beneficial in the end? It looks like Boeing looks only at fuel burn now, not at overall operating costs in the first place. I eagerly wait for the next press release from Boeing regarding MAX.


  1. The "sweet spot" wasn't so sweet after all, was it?

  2. Well, I think someone said that the actual sweet spot of a given engine fan diameter range was 4-5 inches. So, depending from where they started, it could still be withing that range. Also, I read someone mentioned that the fan diameter increased did not add much (apparently, that person saw a slide presented by Boeing)to the Boeing Max efficiency (weight and drag penalty). Does it have more to do with engine maintenance (core temperature less hot)and also less engine noise?

  3. With the smaller core associated with the larger fan the temperatures in the core will rise, so maintenance cost should go up. Noise will go down with the larger fan though.

  4. Will the use of ceramic parts and or coatings compensate for this factor? Also, does ceramic add to the costs but make up for it through maintenance reductions?

  5. Ceramics would help primarily to better engine SFC, as you need less cooling air taken out from the compressor. They are costly - no doubts. But the most important question is: are they ready? Until now they are not, according to my information.

  6. Besides ceramics, are there any other materials which could be incorporated into the engine that would alleviate this issue. I read from time to time of conbinations that promise superior qualities but do not know if they are or will be available for this particular endeavor.

    1. I am not an expert on the material sector, but I am not aware of any that would be superior to the ceramics. There are some "powder" materials under consideration or in lab testing that could probably be as good as ceramics.

  7. The engine makers are always looking into the material sciences to see what improvements can bear fruit. As usual, however, the only way to really understand the efficiency and effectiveness is in the long run. But the materials used presently are an improvement over prior years and hopefully the curve towards even better ones will continue.

    My understanding is that ceramics have been widely used in many forms including body armor and they may be more advanced in engine parts as they have been used in racing cars engines