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DocLightning
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The GTF Gearbox

Sat Feb 13, 2010 5:05 am

I was reading an article (that I can't find) that claimed that P&W will have to prove that the GTF continues to run "even if the gearbox should seize."

Now...the fan and core counter-rotate in any planetary gear arrangement. So what material exists that is made of electrons, neutrons, and protons that can absorb such a force without turning to dust instantly that could fit in the available space for a reasonable weight? I doubt even diamond is that strong.
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
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rwessel
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RE: The GTF Gearbox

Sat Feb 13, 2010 5:28 am

Actually you can use any of the three parts of a planetary gear system as input, and either of the remaining two as output. Depending on which combination you select, you either get a reversal of the direction of rotation, or not. For example, if you drive the sun gear, and use the annulus (the outer ring) as output, you do get a reversal. If you instead hold the annulus fixed, drive the sun gear, and then use the rotation of the planetary carrier as output, the direction of rotation will be the same. IOW, you'd attach the output shaft to the frame (missing from the animation) holding the yellow planetary gears:

http://www.youtube.com/watch?v=QDJLYp-l8XY

edit: I watched this animation a few more times, and the planetary gear carrier is not actually omitted, it's just the featureless gray structure behind the planets.

You can also use a double set of planetary gears to generate a non-reversed rotation of the annulus relative to a driven sun gear.

I don't know what P&W is doing, but if the fan counter-rotates, the requirement is impossible - if the gearbox froze (and things didn't just go to pieces), the fan would start to rotate backwards. If P&W uses one of the non-reversing arrangements, then the fan hypothetically would still turn (at the wrong speed), but would still be subject to a huge shock as is speed suddenly changed. I'm also not sure if any of the non-reversing configurations of planetary gears could freeze in a way that didn't just prevent rotation at all.

Still, there are plenty of aircraft with critical gearboxes (all multi-engine helicopters, any turboprop), that I can't see that surviving a total failure like that is likely to be a requirement.

[Edited 2010-02-12 21:40:13]
 
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DocLightning
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RE: The GTF Gearbox

Sat Feb 13, 2010 5:49 am

Quoting rwessel (Reply 1):
if the gearbox froze (and things didn't just go to pieces), the fan would start to rotate backwards. If P&W uses one of the non-reversing arrangements, then the fan hypothetically would still turn (at the wrong speed), but would still be subject to a huge shock as is speed suddenly changed. I'm also not sure if any of the non-reversing configurations of planetary gears could freeze in a way that didn't just prevent rotation at all.

I hadn't thought about using the planetary gears as the output, but it makes sense. If the outer gear is held truly fixed, then you are correct, there is no way that the system could seize up and yet continue to turn. But if the outer gear is attached to bearings that can give way in such an event, then it would become the output.

The problem is that this would lead to a fan spinning faster than its intended speed and that would cause a thrust imbalance.

I don't see how this requirement is possible.
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
-Carl Sagan
 
rwessel
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RE: The GTF Gearbox

Sat Feb 13, 2010 6:36 am

Quoting DocLightning (Reply 2):
The problem is that this would lead to a fan spinning faster than its intended speed and that would cause a thrust imbalance.

I think that suddenly eliminating the reduction gearing would probably cause the fan to spin *slower*. The problem is that you cannot make the spin faster without more power, and the sudden coupling of the fan and turbine to the same speed would slow the turbine down much more than it would speed up the fan (the fan being far more massive), and with the turbine now spinning far too slowly, it would be significantly limited in terms of the amount of power it could extract from the hot gas flow, thus requiring the fan to slow down.

So there would be a thrust imbalance, but in the other direction - the engine with the failed gearbox would be producing less thrust. Of course that's something the aircraft has to handle anyway (a conventional engine failure would eliminate all the thrust on one side), and a partial reduction in thrust on one side would certainly be less traumatic than a complete loss of power on that side.

I'm still not sure about the practicality of the supposed requirement, however.
 
kl671
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RE: The GTF Gearbox

Wed Feb 17, 2010 1:41 am

Quoting rwessel (Reply 1):
if the gearbox froze (and things didn't just go to pieces), the fan would start to rotate backwards.


Since the fan is directly attached to the gearbox output shaft, the fan must come to a stop if the gearbox seizes. It cannot do anything else unless the gearbox output shaft shears.

Quoting DocLightning (Thread starter):
I was reading an article (that I can't find) that claimed that P&W will have to prove that the GTF continues to run "even if the gearbox
should seize."

It is a pity you don't have the article to help us understand what "run" means. From Pratts web site it appears that the GTF is a three shaft design. If the gearbox seizes the turbine driving the gearbox will also stop.

Perhaps the requirement to "run" after a seized gearbox event means that the LP and HP shafts have to contine running and provide bleed air, electrical power and hydraulics via the AGB, even though the PT shaft has stopped.
 
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DocLightning
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RE: The GTF Gearbox

Thu Feb 18, 2010 1:51 am

Quoting KL671 (Reply 4):

Since the fan is directly attached to the gearbox output shaft, the fan must come to a stop if the gearbox seizes. It cannot do anything else unless the gearbox output shaft shears.

Not necessarily. If the output is the annulus, then the output would become 1:1 coupled with the input.
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
-Carl Sagan
 
kl671
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RE: The GTF Gearbox

Fri Feb 19, 2010 11:39 pm

Quoting DocLightning (Reply 5):
Not necessarily. If the output is the annulus, then the output would become 1:1 coupled with the input.

We seem to disagree on the defination of "Seize".

My defination of a seized gearbox is that all shafts simply will not turn. Otherwise the gearbox is not seized. A gearbox that continues to turn after a component failure is not seized, even if it goes round at an off design speed.

Semantics perhaps, but with some ambiguity in the failure mode that the engine has to tolerate, it makes it difficult to respond to your original post.

Any luck in finding the original article?
 
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DocLightning
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RE: The GTF Gearbox

Sat Feb 20, 2010 2:11 am

Quoting KL671 (Reply 6):

My defination of a seized gearbox is that all shafts simply will not turn. Otherwise the gearbox is not seized. A gearbox that continues to turn after a component failure is not seized, even if it goes round at an off design speed.

In this case, the whole gearbox would rotate while no gears inside would turn, right?
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
-Carl Sagan
 
kl671
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RE: The GTF Gearbox

Sat Feb 20, 2010 6:06 am

Any luck in finding the original article?

Quoting DocLightning (Reply 7):
In this case, the whole gearbox would rotate while no gears inside would turn, right?

Correct, unless the mounting system remains intact to prevent the gearbox from rotating. Newtons third law.
 
LimaNiner
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RE: The GTF Gearbox

Sat Feb 20, 2010 7:58 pm

Quoting rwessel (Reply 1):
http://www.youtube.com/watch?v=QDJLYp-l8XY

Wow, thanks guys! This is the kind of thread that keeps me coming back to Tech Ops. I just spent half an hour clicking through the linked YouTube video, and Related Videos -- learned a lot!

  

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