In the meantime there were news in different but reliable media (LeehamNews, aspire aviation) that the diameter went up to 68 3/4" or 68.5". So now it is 68.4" - what does that tell us?
- Size matters - although Boeing denies that. Well, of course there is a sweet spot for every aircraft/engine combination, as Scott Hamilton writes in a recent column, so you can't go everywhere with fan size. But apparently CFM still tries to maximize the fan diameter of the LEAP-1B so it just fits under the wing and to get the best possible fuel burn for the aircraft. That means that 68" is not the sweet spot, but that is is somewhere higher.
- It means that the definition of the engine is probably still not finished yet. If the fan size changes, this potentially changes everything else - at least if you want to maximize efficiency of the whole engine. In turn that means that there is still a lot work for Boeing to define to pylon and the pylon/wing interface.
Although an not an engineer and I don't know much about the sweet spot, common sense would tell me that there must be an engine size for any airplane where it must not be bigger, or it will start impacting the fuel efficiency down. Weight is probably the first one to come to mind, as well as the force of the engine on the wing (trust). So, is bigger always better for any airplane?
ReplyDeleteBigger is not always better, that's right: there is a sweet spot for aircraft/engine combination. The question is: is the sweet spot for the MAX where Boeing and CFM tell us it is, coincidentally being the maximum diameter the LEAP can have under the wing under the MAX?
ReplyDeleteWell, doing a rough estimation of fan are increase on both NEW and Max. The NEO fan goes from 68 inches to 78 inches, so is that about an increase of about .1471 (78/68)?
ReplyDeleteNow, if we do the same for the Max we have an increase of about .1213 (68.4/61). Although, the increase is smaller than that of the NEO, is it as big a difference for the perceive increase in efficiency for the NEO over the Max? What is the difference in fan area increase for each?
NEO fan area increases by roughly 31%, MAX fan area by 24%, meaninig that the fan pressure ratio can be set lower for the NEO engine, resulting in higher propulsive efficiency.
DeleteOk, thanks for the response. So the difference is bigger than one might think at first. I did not think that the area for either fans was increasing that much.
ReplyDeleteIs it possible to figure out how much a pressure differential would it be between the two fans? Thanks again and keep up the great work you'd been doing.
If you compare two fan area's (more exactly cou would have to compare the fan nozzle areas), you get that v2/v1=(A1^2/A^2). Per Bernoulli you get then that p2-p1= (density/2)* (v2^2-v1^2).
DeleteI think that if 68-69 inch would be the optimum fan size for the LEAP engine (taking into consideration weight, drag, fan pressure ratio), Airbus would put that one on the A320 series.
ReplyDeleteIf 78 inch would be better, Boeing would do the business case and conclude 68.4 is the maximum they can get on the 737 at reasonable costs.
Saying it's the sweetspot & we are the best anyway is just PR. What else can they say? Sorry, but this is the max we can do?
Please remember that the B737 has a lower thrust setting than the A320, so the sweet spot for the B737 should be somewhat lower than the sweetspot for the A320. And the exact sweetspot might be different for different engines. It might be higher for a GTF than for a conventional trubofan like LEAP-1B.
DeleteWell, yes, people forget that the engine on the A320 classic is bigger today than the one on the B737NG, still they are about equal in efficiency as a whole. So, I do not see why it would not be the same for the NEO and the MAX planes.
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