Originally Posted by
Bus Driver Man
Out of interest, why wasn’t this an issue on the previous CFM/IAE generation? Bigger NEO engine squeezed in a tighter space?
What vilas said. The trend for decades has been for bigger fans and small, faster spinning cores (faster spinning cores tend to be more efficient) operating at higher temperatures, with ever tighter clearances to minimize losses. The bowed rotor phenomena is not new, but wasn't a major concern until about 20 years ago. I believe the GE90-115B was the first engine to really suffer from bowed rotor problems - and the autostart incorporated a bowed rotor mitigation (IIRC and extra 17 seconds of motoring before fuel ON). In addition to bowed rotor mitigation, engines with smaller cores and bigger fans just naturally take longer to start so start times keep going up.
GE9X on the 777X takes bowed rotor mitigation one step further - there is a small electric motor on the gearbox that uses aircraft power to slowly rotate the core after shutdown at roughly one rotation per minute - that will even out the cooling of the core and prevent the rotor from bowing. It's commonly referred to as the 'rotisserie' for what should be obvious reasons.