Doesn't work that way....a different core must go with a gearbox. The different RPM's of the low-spool/gearbox and high-spool would not match the current LEAP core. Close integration of the fan, low-compressor flow and and high-pressure compressor, among other things, would have to be redesigned also.
As Pratt found out with its GTF...the gearbox worked out great -- but the consequences of the gearbox and high shaft RPM contributed to other problems with the bearings, rotor bowing, oil seals, etc.
I'm no expert, but it's hard to characterize the GTF's issues based on what I've read in Tech Ops.
On GTF the N2 shaft rotation speed is not ground breaking, it is similar to what was on the IAE V2500 series.
From four years ago in tech ops:
Quoting Revelation (Reply 56):Being able to turn the core faster gives the GTF
The core as in the "N2 spool" does not run faster.
The LP turbine/power extraction for the fan runs faster.
( you can find the N1 N2 values in the EASA cert docs for both engines.
N2.fan is slightly lower on the GTF while N2.LP is 3 times higher.
For the Leap N2.fan is hardlinked 1.1 to the N2.LP rpm.)
And imho it is all in the gearbox as everything else gained hinges on it.
With the N2 spool at the rear end running at speeds ~= to the N1 spool power takeoff
per stage is much better and the smaller diameter reduces the number of blades
The N1 spool rotates 3x faster, which is the whole idea of the GTF, to get the low power spool rotating at a faster optimal speed whilst letting the fan spin at its slower optimal speed.
Therefore the main issue for bearings was N1 was spinning faster than before, but one would have hoped that would not have been much of a challenge, right?
It seems the PW bearing issue was an optimization (the "flying" bearing) introduced late in the design cycle with insufficient testing/maturing time.
It seems that was a bad idea that would have led to bad consequences on any other engine program.
Pratt itself stated the shaft bowing issue also happened on V2500; it's a mystery why it wasn't anticipated and/or detected earlier on the GTF.
If GE/CFM wants to do a gearbox -- a whole new engine is required. That would take much more than five years.
All you have to do is look at the GTF and you see fewer stages on the LPC because it spins at a higher rate.
Fewer stages mean redistribution of loads.
Higher rotational rates means different airfoils.
This means you need a different engine core.
The gear means you need a new case, thus a new engine.
Such a thing is within CFM's capabilities, but the time to execute such a program cannot be avoided.
Some interesting quotes on GTF vs LEAP from an older tech-ops thread: viewtopic.php?t=776091&start=50#p11195739
Last edited by Revelation
on Thu Feb 07, 2019 1:37 pm, edited 1 time in total.