GE90 Engine Noise During Start
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I would tend to go along with what 3holeover and lomapaseo have posted. The vibration sound and feel may also be influenced by harmonics as 777AV8R pointed out, depending on where you are seated.
There is no influence, one way or the other, caused by the composite fan blades.
The 20 minute stabilization after landing before shutting the engines down certainly helps, but it probably doesn't occur on a regular basis. With a bowed rotor on startup, the LPT area is most affected until things settle in. Additionally, as the rotors start to turn and pick up rpm's, there are a lot of frequencies being crossed which are designed to be outside normal engine operating range (idle through TO thrust). These sounds and vibrations are often transmitted from the engine, through the pylon to the wing and fuselage giving some strange sounds when seated near the engines or wing area. These all go away as the engine approaches idle speed and everything is in sync.
There is no influence, one way or the other, caused by the composite fan blades.
The 20 minute stabilization after landing before shutting the engines down certainly helps, but it probably doesn't occur on a regular basis. With a bowed rotor on startup, the LPT area is most affected until things settle in. Additionally, as the rotors start to turn and pick up rpm's, there are a lot of frequencies being crossed which are designed to be outside normal engine operating range (idle through TO thrust). These sounds and vibrations are often transmitted from the engine, through the pylon to the wing and fuselage giving some strange sounds when seated near the engines or wing area. These all go away as the engine approaches idle speed and everything is in sync.
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I have to support Papa2Charlie's contribution here, however he had the HP bleed valve confused with engine handling bleeds/start bleeds!
It is very obvious, that the deep 'awesome' vibration sounding resonance, experienced during engine start/spool-up, distinctly shuts off, disappears, as the engine reaches steady state idle, which can only result from some sudden change, which is the appropriate engine handling bleeds closing, hence silencing the aweful rumble, eminating from the mass compressor air mismatch of N1 to N2 airflow, during sub idle/operating RPM's.
An engine of these proportions, will probably never reach max motoring spec's without all the handling bleeds wide open, dumping the excessive air and unloading the starter turbine, let alone accelerate, once fuel burn is induced.
Once the GE90 reaches steadystate idle, it rapidly becomes incredibly quiet. Smooth, matched, controlled and sealed compressor airflow.
I agree with 3holelover about his 'bowed rotor effect' on GE engines, however this pertained specifically to the CF6-50/80's, which had very skinny & long LP shafts.
I doubt this phenomenon is creating the majority of the loud audible & physical rumble that every GE90 delivers on almost every engine start.
That kind of vibration/resonance being the result of a long shaft whipping within its bearings will no doubt have caused GE to issue an immediate AD to force 20 min cool-downs, as it would seriously shorten engine reliability and life.
It is very obvious, that the deep 'awesome' vibration sounding resonance, experienced during engine start/spool-up, distinctly shuts off, disappears, as the engine reaches steady state idle, which can only result from some sudden change, which is the appropriate engine handling bleeds closing, hence silencing the aweful rumble, eminating from the mass compressor air mismatch of N1 to N2 airflow, during sub idle/operating RPM's.
An engine of these proportions, will probably never reach max motoring spec's without all the handling bleeds wide open, dumping the excessive air and unloading the starter turbine, let alone accelerate, once fuel burn is induced.
Once the GE90 reaches steadystate idle, it rapidly becomes incredibly quiet. Smooth, matched, controlled and sealed compressor airflow.
I agree with 3holelover about his 'bowed rotor effect' on GE engines, however this pertained specifically to the CF6-50/80's, which had very skinny & long LP shafts.
I doubt this phenomenon is creating the majority of the loud audible & physical rumble that every GE90 delivers on almost every engine start.
That kind of vibration/resonance being the result of a long shaft whipping within its bearings will no doubt have caused GE to issue an immediate AD to force 20 min cool-downs, as it would seriously shorten engine reliability and life.
Usual disclaimers apply!
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Right just to clear up a few details.! The vibration and noise is caused by compressor airflow mismatch through the gas path. 
In normal operation the engine would never been operated at above say 70% N1 immediately prior to being shut down. However heavy use of the reversers should invoke the requirement of the 20 min cool down cycle. The bowed rotor effect is a GE thing, very noticeable on any subsequent start, you definately don't need loose fillings.. or kidney stones
The STB (Start Transient Bleed [valve]) was removed very early on in the engines service life, as it was found to be unnessessary! The Bypass Valves (10 off) are fully open during engine start and dump excess air from the back of the LP compressor (booster) into the fan stream they only start to modulate closed with advancing throttle, the VSV (Variable Stators) are fully closed at idle and only start to modulate open with increasing throttle. 
In normal operation the engine would never been operated at above say 70% N1 immediately prior to being shut down. However heavy use of the reversers should invoke the requirement of the 20 min cool down cycle. The bowed rotor effect is a GE thing, very noticeable on any subsequent start, you definately don't need loose fillings.. or kidney stones The STB (Start Transient Bleed [valve]) was removed very early on in the engines service life, as it was found to be unnessessary! The Bypass Valves (10 off) are fully open during engine start and dump excess air from the back of the LP compressor (booster) into the fan stream they only start to modulate closed with advancing throttle, the VSV (Variable Stators) are fully closed at idle and only start to modulate open with increasing throttle.
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Gaspath is absolutely correct. The VBV's modulate and are not closed at idle. (The VSV's when "closed" are obviously not closed shut like the VBV's can be, but they're as "closed" as they can get.)
I've probably run as many CF6-50's and -80's as many who fly with them have, and I never once experienced -nor even heard of - the bowed rotor effect with them... many other troubles, but not that!
As for your doubts Easyduzzit. What can I say.... GE didn't issue an AD, but they did issue the recommendation ( probably had a fancier name -SB? EI maybe? I forget) to cool the GE90 for 20 minutes after the throttles were off the stop. (never saw any such recommendation for any other GE motor)... and as you've noted, the noise/vibration on a GE90 start is not an every time occurrence, it ONLY occurs when there's been insufficient cooling time prior to shut down... and as it takes 20 full minutes, it is very often the case.
If you're ever running at idle for that long and you have the opportunity -waiting for deice, or great lines of congestion or whatever, try shutting one down and restarting.... you'll see. Smooth as a babies bum! Guaranteed! (but don't cheat! 20 minutes at idle! Not 18!, and not 20 minutes which included just a little breakaway throttle use ...it IS that fussy.) --and again, that's the GE90-115 (&-110) I'm referring to.... Don't know about the lesser GE90's.
CF6-50's were always smooth starters, especially when we adopted our PP engineers' recommended method of putting the fuel on at 10%.... Seemed odd at first, but made for a very smooth start, as the thing would light off at about 15% and, they claimed, the thermal shock was was much less that way compared to motoring to max motor before lifting the fuel cock. Haven't run a -50 for about 10 years now... I miss those beasts! Great engines to work on!
CF6-80's (been fiddling with them since about 1988 or 89, and still am) are also quite smooth during start.... Unless something's wrong of course.
I've probably run as many CF6-50's and -80's as many who fly with them have, and I never once experienced -nor even heard of - the bowed rotor effect with them... many other troubles, but not that!
As for your doubts Easyduzzit. What can I say.... GE didn't issue an AD, but they did issue the recommendation ( probably had a fancier name -SB? EI maybe? I forget) to cool the GE90 for 20 minutes after the throttles were off the stop. (never saw any such recommendation for any other GE motor)... and as you've noted, the noise/vibration on a GE90 start is not an every time occurrence, it ONLY occurs when there's been insufficient cooling time prior to shut down... and as it takes 20 full minutes, it is very often the case.
If you're ever running at idle for that long and you have the opportunity -waiting for deice, or great lines of congestion or whatever, try shutting one down and restarting.... you'll see. Smooth as a babies bum! Guaranteed! (but don't cheat! 20 minutes at idle! Not 18!, and not 20 minutes which included just a little breakaway throttle use ...it IS that fussy.) --and again, that's the GE90-115 (&-110) I'm referring to.... Don't know about the lesser GE90's.
CF6-50's were always smooth starters, especially when we adopted our PP engineers' recommended method of putting the fuel on at 10%.... Seemed odd at first, but made for a very smooth start, as the thing would light off at about 15% and, they claimed, the thermal shock was was much less that way compared to motoring to max motor before lifting the fuel cock. Haven't run a -50 for about 10 years now... I miss those beasts! Great engines to work on!
CF6-80's (been fiddling with them since about 1988 or 89, and still am) are also quite smooth during start.... Unless something's wrong of course.
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CF6-50's were always smooth starters, especially when we adopted our PP engineers' recommended method of putting the fuel on at 10%.... Seemed odd at first, but made for a very smooth start, as the thing would light off at about 15% and, they claimed, the thermal shock was was much less that way compared to motoring to max motor before lifting the fuel cock. Haven't run a -50 for about 10 years now... I miss those beasts! Great engines to work on!
CF6-80's (been fiddling with them since about 1988 or 89, and still am) are also quite smooth during start.... Unless something's wrong of course.
CF6-80's (been fiddling with them since about 1988 or 89, and still am) are also quite smooth during start.... Unless something's wrong of course.
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Originally Posted by grounded27
I have experienced with CF6-10 through [...]
) I'll bet they were good engines for their time.
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Hey, 3hole lover
We operate the largest fleet of them as converted to MD-10's on the airframe. They still run well. Operated domestically, the company is looking for a more efficient aircraft to fit the bulk of cargo this aircraft provides on domestic routes.
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They still run well.
That's cool to hear the -10's are still running! Say hello to a Dear old Diesel Dixie for me, will you? ....sigh....
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Originally Posted by Landroger
....could that be one of the odd features of those RR (or should that be VW?) makes?
Roger.
Last edited by Shanewhite; 6th Jul 2011 at 10:20.
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Let me just clarify something here....
When I said a HP Bleed Valve, I wasn't speaking about cabin bleed. I mean handling bleed valve(s) in the HPC. At low N2, the HPC BV's will be wide open to help stabilise the compressor airflow. As the engine nears idle then the offending BV will close. The remaining valves on the HPC will open / close as usual during accels / decels. Without knowing the details of the booster arrangement on the GE90, it's possible the noise could come from an LPC BV also but my intuition would say the HPC is more marginal on stability during engine start.
I don't know the in's and out's of the GE90-115B but from my own experience of hearing a couple of them start, the noise is consistent with a handling BV being open at start then closing once the engine nears ground idle.
When I said a HP Bleed Valve, I wasn't speaking about cabin bleed. I mean handling bleed valve(s) in the HPC. At low N2, the HPC BV's will be wide open to help stabilise the compressor airflow. As the engine nears idle then the offending BV will close. The remaining valves on the HPC will open / close as usual during accels / decels. Without knowing the details of the booster arrangement on the GE90, it's possible the noise could come from an LPC BV also but my intuition would say the HPC is more marginal on stability during engine start.
I don't know the in's and out's of the GE90-115B but from my own experience of hearing a couple of them start, the noise is consistent with a handling BV being open at start then closing once the engine nears ground idle.
