GE90 Engine Noise During Start
 

Any ideas as to why GE90s are so noisy during start?

I second the question.

I sat between a pair of them for the first time a couple of weeks ago on a new New Zealand 773ER, and noticed they were remarkably noisy when starting compared with the PW4000s or Trents that usually push me places, or what I recall from earlier model GE90s.

As I remember, it was an almost-constant-tone whine-grind noise, rather than a rising tone from something whose speed was increasing. I assumed it had to be from an air turbine, which didn't turn again for the rest of the flight after the engine started. I also see something perhaps similar has been discussed before at
http://www.pprune.org/fragrant-harbo...ml#post6008093

I asked the same question to someone a while ago and if I recall the answer I was given was:

the airflow through the engine is no yet stable and the awful noise is various compressor blades at a stall relative to the airflow in the engine.

or something along those lines this may not be correct however.

My own experience with GE90's (-110's and -115's) is that they only make ugly noises during start if they weren't cooled properly prior to their last shut-down. They call it "bowed rotor effect" and if one doesn't wait the recommended 20 minutes since the throttle was last off the idle stop prior to shut down, the next start will be noisy.

So, obviously, if it takes less than 20 minutes from landing roll-out to gate, (and surely, it often does!?), it's going to happen.

Harmonics...
 

Natural harmonics......

The "cool down period" is detrimental to this motor, shaft warping can cause major problems. The 115/derated110 have huge fan blades, suspect this is the source.

:ooh: Could you expand on that statement? Is this a language thing, or did you really mean to say one shouldn't cool the GE90 before shut down?

In any case, I'm not just spittin' wind here... I'm tellin' ya, if you cool them down nicely, the next start will be no more noisy than a cf6-80 or PW4000 or even one of those funny backwards turning, three spooled, weird things the Brits call engines. ;) .....and if they're not cooled really well, I can guarantee your next start will sound awful!

I never heard a GE90 that I didn't like

and I have no idea what this subjective noise sounds like or what causes it

When talking about cool down in jet engines (large or small) some issues that come to mind are:

Depending on mass and shape different parts cool at different rates. and even the same part will cool faster at some of it geomentry areas than others.

Exampes are:

Case structures surrounding the rotor blades cool much faster than the rotor disks themselves burried deep in the center of the engine. Beacuse of this rate difference there could be a critical time when the blade tip clearenace is not optimum (too loose or too tight). The too tight part of it is at one time and the too loose part of it at yet another time.

too loose creates air distortions at the blade tips leading to rumbles and sometimes even a surge.

Too tight leads to binding at very low RPM and inability for a starter to overcome and/or excessive rub out which will show up as too loose when the rotors get to speed and equalize.

Then there is the effect of depth between the top of the engine and the bottom of the engine in the gas cavities where heat rises creating a bow in the rotor. This acts like a tire that has been allowed to sit too long leaving a flat spot and sets up harmonic vibration (noise) of some parts createing a cacophony of sounds until the spinning itself evens out the temperatures around the rotor.

All engines are subject to all the above, but to different degrees and rarely does once change a design just to change a subjective noise that only acts for a brief time.

Engine surge is of course to be avoided

The problem I'm having is an inability to cut and paste from employer provided info... They'd get upset about that. But I've seen text from GE that says what I'm saying... I'm pretty certain they called it "Bowed Rotor effect" (the flippin' things shake and shiver while they find their happy place) and it's cause is said to be inadequate cool down.
I've heard/felt it many a time during many starts... And, I've also seen, many a time, when I give the poor dears their twenty minutes at idle after a hard run... the next start is as smooth as a baby's bum.

We ran into the same thing with naval steam turbines. It always made me wonder why the OEMs don't fit small electric jacking motors to get them turning at a relatively slow RPM to cool off properly.

The most likely cause are HP bleed valve(s) without silencers being open during the start. As the engine reaches idle, the valve will close and the noise levels will drop significantly.

:hmm: The HP valve is definitely not closed at idle. ....Given that you're evidently not too familiar with the beasts, I'm a little curious as to how it is you feel you can say what "the most likely cause" is? HP bleed valve "silencers"?? Never run across one on a GE or a Pratt? ....could that be one of the odd features of those RR (or should that be VW?) makes?

In any case, the noise I've been talking about, and which I took the question to be about, is not an air related noise at all.... it's an ugly sounding vibration. Maybe I'm in left field though.... wouldn't be the first time.

Its to do with fan blade material. GE90 fan blades composite so make diffrent noise to metal ones.

I agree.... These GE90 (115 & 110) are damn big engines & their design needs to be respected just as any engine does. As a side note, it is a personal pet dislike seeing pilots disregard the need to confirm stabilized engines prior to pressing the TO/GA switch. Laziness and complacency.

3holelover
 

Trolling under the wrong bridge there, I think, bro'. Rolls-Royce, the makers of cars - automobiles? - are owned by Volkswagen, but Rolls-Royce (1971) Ltd., - or whatever they are called nowadays - make some of the world's finest aeroplane engines. And you can see from the name, how far back it was that they were the same company. Keep with the programme. :)

Roger.

LandRoger
 

;) Thanks Mate... I thought so, but can't seem to resist poking some fun at me Limey Cousins for those funny three spooled backwards turners. :E

and the winner is.....
 

Skip all the rest, go straight to 3holelover's first post - "Bowed Rotor effect" for all of the reasons given.

Case closed your honour:D

Hmm, how about a sub-idle debow run to increase service life? Or would that be too Concorde-esque? ;)

http://fromtheflightdeck.com/MEL/PPRune/Whoa.jpg

Hahahaha, had to look for a while, but here it is:
Yet another Concorde question [Archive] - PPRuNe Forums
Apparently one had to run the Olympus for a while at sub-idle to debow the shaft prior engine start if the engine had been run recently...