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Old 29th June 2017 | 03:39
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tdracer
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From: Everett, WA
Bowed rotor is an issue on most current generation turbofan engines - it's just the severity that varies.
All HP rotors bow a bit after shutdown due to the differential cooling of the engine parts (i.e. heat rises). The exposure window varies a bit with different engines, but if you perform a re-start somewhere in the 30 minutes to 6 hours window after shutdown, the HP rotor will have some bow in it. Best case, you'll get higher than normal vibes during the start - worst case you can rub the compressor - possibly even damaging blades - and cause permanent performance loss. No first hand knowledge, but scuttlebutt is that the starting issues Pratt's having with their geared fan on the NEO and C-series are related to bowed rotor...
The first engine I was aware of with significant bowed rotor start issues was the GE90 - IIRC if you start a GE90 in the bowed rotor window, it'll max motor for ~20 seconds to let the HP rotor thermally stabilize a bit before fuel-ON. The problem got worse on the GEnx - there were instances on the GEnx where a bowed rotor start took chunks off of some compressor blades . During the bowed rotor window, the GEnx will motor for ~40 seconds - but it's not "max motoring' - that's too fast and still can cause damage. So on the 787 the electric starter reduces the torque to motor ~20-25 % N2. The pneumatic starter on the 747-8 can't do that so it holds the variable vanes open to keep the motoring N2 down.
The LEAP engine on the MAX will pulse the starter air valve open/closed/open to keep the motoring N2 in the desired range during a bowed rotor start.
The GE9X on the 777X has the most interesting solution - there will be a small electric motor on the gearbox (aka "the rotisserie") that will use aircraft power to slow turn the HP rotor after a shutdown to prevent bowed rotor from occurring.
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