This kind of testing is new to me, I never heard that these kind of
tests were done. In the end it is very expensive to build a test engine solely
to measure the performance with so called “end of life ”clearances”. Normally
the clearances in the engine are constantly measured so you get a relation
between the change of the clearances due to wear and rub ins and the change of
the performance (both fuel burn and compressor stability).
To build in intentionally larger tip clearances makes sense only for the
compressors to test compressor stability. And if these tests showed a SFC shortfall
of 4-5% (I now heard 5%) then either the performance retention of the LEAP
engine is very bad or there indeed is a performance shortfall as the effect of
larger tip clearance should not lead to a loss in SFC larger than 2%.
Now CFM always mad big noise
about the good performance retention the LEAP engine family would have, in particular
compared to the P&W GTF Family. One could ask: how do they want to know
that? And I think, neither CFM nor P&W nor anybody else can know today and
will know until real life data is available. With “real life data” I mean data
that will come out of the field once airlines really fly the aircraft and engines
for some time.
So we don’t know for sure how much of the (obviously measured) 4-5% are attributable
to a new and with production tip clearances build engine. But CFM’s argumentation
does not add up to the point where they would meet not only their own expectations.
To meet the own pretest prediction within 0.5% does not mean a whole lot to the
outside world until you know where the prediction was relative to specification,
i.e. relative to the target SFC where the B737MAX would meet 14% fuel burn
reduction vs. the B737NG.
Hello AeroT.
ReplyDeleteI will mention again about a comment left on your previous post about the CFM LEAP-1B. If right now CFM is only able to get about 9-10% (14 - 5%) improvement on its new LEAP engine for the B737, they be better off using the core of the legendary CFM56-7B tweaking it a bit to get about 3% better SFC and then put the new composite fan and fan case to it along with the new 69.4" composite fan. With its better air flow technology, they would have about 10% improvement right there. That will be so much better for CFM who will not have to spend so much money developing and building something exotic and expensive. The customer will receive the standardize and superb reliability of the CFM56 engines, at a outstanding price as well. Sure the engine will be about 5% less efficient than the hoped for LEAP, but they will be getting something really good very cheaply and easier to maintain. I just don't get how they can miss performance that bad on an engine they're been working on for so long and supposedly understood its capability well before building the first metal piece for it.
Unfortunately you can't mix and low and high spools that easy. The LEAP engines have a higher overall pressure ratio and thus need a smaller core to produce the same thrust. Scacling down the CFM56 would not help as the pressure comuing out of the HPT would not be high enough to drive the LPT and hence the fan.
DeleteThat should read "mix and match" in the first sentence....
DeleteYes, that is true about the mix and match issue, but judging that we now have the CFM56-1A with fan at 69"
ReplyDeleteputting the new more efficient, better flow, lighter composite fan at 69.4" to the same engine with the new composite fan case to boot would not bring the fuel burn down about 7% or so for the CFM56-7B which has an smaller fan but the same core as the CFM56-1A?
There is a lot more flow going through the core of the CFM56 than through the core of the LEAP engine. Thus you would not get the benefit of the higher bypass ratio. The fan of the LEAP and the core of the CFM56 are not matched to work together. The whole engine would not work.
Delete