Sponsor Message:
Aviation Technical / Operations Forum
My Starred Topics | Profile | New Topic | Forum Index | Help | Search 
Gearboxes And Generators  
User currently offlineDocLightning From United States of America, joined Nov 2005, 20358 posts, RR: 59
Posted (5 years 1 month 1 week 2 days 17 hours ago) and read 21361 times:

So one thing I've been wondering about is the talk of gearboxes and generators in jet engines.

How does one extract the rotational energy of the shafts without interrupting the airflow?

Do both N1 and N2 have attached gears? What's the arrangement that allows for that?

And how is the generator handled? It needs to produce a constant supply of juice, but with the engine speed going up and down all the time how is this regulated?

31 replies: All unread, showing first 25:
 
User currently offlineWoodreau From United States of America, joined Sep 2001, 1053 posts, RR: 7
Reply 1, posted (5 years 1 month 1 week 2 days 16 hours ago) and read 21358 times:

In general, the gearbox is usually attached to the N2 compressor and all the accessories are attached to the gearbox (the starter, generator, oil pumps, fuel pump, hydraulic pump, generator for the FADEC - if it has a FADEC, etc.)

The generator itself is attached to a constant speed drive. The constant speed drive is attached to the gearbox. The generator requires a constant RPM, so that it can produce a steady 400Hz. It's the variation in the frequency that messes up electronics.

There are some generators out there that don't produce a constant frequency (they aren't attached to a constant speed drive, so they experience RPM changes as the engine accelerates and decelerates) - they're called "wild" generators, but I've only seen them in turboprops.

Again that's a generic explanation.



Bonus animus sit, ab experientia. Quod salvatum fuerit de malis usu venit judicium.
User currently offlineDocLightning From United States of America, joined Nov 2005, 20358 posts, RR: 59
Reply 2, posted (5 years 1 month 1 week 2 days 16 hours ago) and read 21341 times:



Quoting Woodreau (Reply 1):
In general, the gearbox is usually attached to the N2 compressor and all the accessories are attached to the gearbox (the starter, generator, oil pumps, fuel pump, hydraulic pump, generator for the FADEC - if it has a FADEC, etc.)

The generator itself is attached to a constant speed drive. The constant speed drive is attached to the gearbox. The generator requires a constant RPM, so that it can produce a steady 400Hz. It's the variation in the frequency that messes up electronics.

So these are the parts I'm specifically asking about.

How does one attach a gearbox to the N2 shaft, which lies in the center of a bunch of whirling blades? Where does the mechanical linkage sit so that it doesn't interrupt the airflow?

And how does a "constant speed drive" work? Do jet engines have some sort of continuously-variable transmission?


User currently offlineWoodreau From United States of America, joined Sep 2001, 1053 posts, RR: 7
Reply 3, posted (5 years 1 month 1 week 2 days 15 hours ago) and read 21334 times:

It's probably easier to see than explain:

There is a power takeup unit which connects the two. It doesn't get in the way of the airflow if that's what you're wondering.

This is what I could find - googled accessory gearbox under images.

http://media.photobucket.com/image/a...ssory%20gearbox/evo65/accessor.jpg

The constant speed drive is a "box/device" which provides the constant speed that the generator requires, it's not part of the engine. How does it work - don't need to know how it works in order to fly the plane.  Smile It either works or it doesn't. If it doesn't work, you get to do some real life abnormals. disconnect the generator and start up the APU.

So - each thing is it's own "box/device" in between the arrows:

N2 compressor -> Power Takeup Unit -> Accessory Gearbox -> Constant Speed Drive -> Generator

again not talking about any specific jet engine



Bonus animus sit, ab experientia. Quod salvatum fuerit de malis usu venit judicium.
User currently offlineNoWorries From United States of America, joined Oct 2006, 539 posts, RR: 1
Reply 4, posted (5 years 1 month 1 week 2 days 15 hours ago) and read 21328 times:

Quoting DocLightning (Reply 2):
And how does a "constant speed drive" work? Do jet engines have some sort of continuously-variable transmission?

There's folk a lot more knowledgeable than me that will probably jump in here, but essentially a gearbox not unlike a differential allows the shaft connected to the generator to rotate at constant speed. Look up CSD (constant speed drive) and IDG (integrated drive generator)

Newer systems power alternators that are rectified to DC and then inverted to whatever frequency is needed -- more efficient that using gear regulated velocity,

[Edited 2009-11-16 14:40:03]

User currently offlineRwessel From United States of America, joined Jan 2007, 2416 posts, RR: 2
Reply 5, posted (5 years 1 month 1 week 2 days 14 hours ago) and read 21303 times:
Support Airliners.net - become a First Class Member!

The usual approach is to put a bevel gear onto the turbine shaft, and then run a shaft out the side of the engine. Usually it's the high pressure shaft, and often that shaft doubles as the drive shaft for the starter.

There are some good drawings here:

http://www.globalsecurity.org/milita...brary/policy/army/fm/1-506/Ch8.htm

The shaft does disrupt the airflow inside the engine, but so do the variety of structural members that hold the rotating parts in place. In at least some case, the shaft is run, wholly or partially, inside one of those structural members.

Usually the constant speed drive on a jet is not a CVT in the sense of varying-diameter pulleys, but rather a combination of a hydraulic pump and a hydraulic motor, and a bit of regulator to maintain the speed. In principal this is similar to the torque converter on an automatic transmission on a car.

For electricity generation, the more modern approach is to run the generator at variable speed, and then electronically convert that (varying output) to the required (fixed) output levels.


User currently offlinePGNCS From United States of America, joined Apr 2007, 2858 posts, RR: 48
Reply 6, posted (5 years 1 month 1 week 2 days 14 hours ago) and read 21291 times:



Quoting Woodreau (Reply 1):
There are some generators out there that don't produce a constant frequency (they aren't attached to a constant speed drive, so they experience RPM changes as the engine accelerates and decelerates) - they're called "wild" generators, but I've only seen them in turboprops.

I have never heard that term, but have no turboprop time to speak of. I can tell you the MD-90 changed the rather bulletproof DC-9 CSD system that worked great to a Variable Speed Constant Frequency (VSCF) similar to what you describe. It is my least favorite feature of the MD-90, and one that was certainly not a step forward in my opinion.


User currently offlineDocLightning From United States of America, joined Nov 2005, 20358 posts, RR: 59
Reply 7, posted (5 years 1 month 1 week 2 days 14 hours ago) and read 21271 times:



Quoting Rwessel (Reply 5):

http://www.globalsecurity.org/milita...8.htm

Ah, that answered a lot. So it seems that the gearbox sits within the diameter of the compressor disks so as to not impede airflow and a thin shaft (presumably in an aerodynamic fairing) passes out of the core to the gearbox.


User currently offlineRwessel From United States of America, joined Jan 2007, 2416 posts, RR: 2
Reply 8, posted (5 years 1 month 1 week 2 days 13 hours ago) and read 21256 times:
Support Airliners.net - become a First Class Member!



Quoting DocLightning (Reply 7):
Ah, that answered a lot. So it seems that the gearbox sits within the diameter of the compressor disks so as to not impede airflow and a thin shaft (presumably in an aerodynamic fairing) passes out of the core to the gearbox.

Terminology quibble:

It's the bevel gears and shafts that are in the engine core, what's usually called the gearbox is outside the engine, on the other end of that shaft. The accessories then attach to the gearbox in various ways.


User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 9, posted (5 years 1 month 1 week 2 days 12 hours ago) and read 21247 times:



Quoting DocLightning (Thread starter):

How does one extract the rotational energy of the shafts without interrupting the airflow?

You need to support the engine core, since that's where all the loads come in via the bearings. That means you need fixed structures between the outer case and the inner non-rotating structure...these take the forms of aerodynamic struts that go in between the rotating disks. The shaft is usually run inside one of these, so the shaft doesn't disrupt the airflow any more than it already was.

Quoting DocLightning (Thread starter):
And how is the generator handled? It needs to produce a constant supply of juice, but with the engine speed going up and down all the time how is this regulated?

787 just skips the whole problem and uses variable frequency generators.

Tom.


User currently offlineDocLightning From United States of America, joined Nov 2005, 20358 posts, RR: 59
Reply 10, posted (5 years 1 month 1 week 2 days 12 hours ago) and read 21235 times:



Quoting Rwessel (Reply 8):

Terminology quibble:

Thanks for the correction.  Smile


User currently offlineFLY2HMO From , joined Dec 1969, posts, RR:
Reply 11, posted (5 years 1 month 1 week 2 days 11 hours ago) and read 21218 times:

Here's a cutaway of the CF6 as seen from the bottom-rear:

http://www.flightglobal.com/airspace/photos/aeroenginesjetcutaways/images/5605/general-electric-ge-cf6-50-cutaway.jpg

On almost all (civilian a tleast) engines the gearbox is somewhere between the 4 and 8 o'clock position. It's hard to make out but you can see the gearbox's drive shaft just aft of the bypass duct guide vanes in wide vertical fairing at the 6 o'clock position. This engine is a GE CF6-50.

In this pic of a CFM 56, you'll note the gearbox and accessories are offset more to the side, due to the 737's short height and thus producing the distinctive bulge on the 737 cowlings.



[Edited 2009-11-16 19:19:20]

User currently offlineJetMech From Australia, joined Mar 2006, 2699 posts, RR: 53
Reply 12, posted (5 years 1 month 1 week 2 days 7 hours ago) and read 21171 times:



Quoting DocLightning (Thread starter):
How does one extract the rotational energy of the shafts without interrupting the airflow?

You can see the set-up in the following picture of an RB211. Basically, there is a bevel gear set (circled in blue) that takes rotational energy off the N3 spool. This is transmitted via a set of drive shafts (red lines) to the external gearbox (boxed in green). The gearbox then distributes this to the various accessories such as the IDG. The only part of the set-up intruding into the gas path is the housing for the vertically orientated drive shaft.

http://www.flightglobal.com/airspace/photos/aeroenginesjetcutaways/images/5645/rolls-royce-rb211-535-cutaway.jpg
http://i94.photobucket.com/albums/l118/Jet-Mech/Jet.jpg

http://www.flightglobal.com/airspace.../rolls-royce-rb211-535-cutaway.jpg

Quoting DocLightning (Reply 2):
Do jet engines have some sort of continuously-variable transmission?



Quoting Rwessel (Reply 5):
but rather a combination of a hydraulic pump and a hydraulic motor

Pretty much. A CSD consists of a variable displacement hydraulic pump supplying fluid (usually engine oil) to a fixed displacement hydraulic pump. The variable displacement hydraulic pump is usually of the multi-piston type with a tilting swash-plate. The input from the gearbox rotates the shaft, pistons and the cylinder block containing the pistons. The swash-plate (yellow) does not rotate, but pivots on a fixed axis as shown below. The small slipper feet (dark green ) on the bottom of the pistons slide upon the surface of the swash-plate.



http://home.planet.nl/~brink494/axpp_v.htg/axpp_v.gif

http://www.emeraldinsight.com/fig/0180560404021.png

http://www.emeraldinsight.com/fig/0180560404021.png

You can see how the tilt of the swash-plate varies the strokes of the pistons, and hence the volumetric output from the pump. When the engine speed is high, the swash-plate tends to move to a neutral position to minimised the stroke of the pistons, and hence output of the pump.

When this fluid is fed to the constant displacement motor – which drives the generator - it will produce a gearing down effect, that is, the constant displacement hydraulic motor will spin at a reduced RPM compared to the variable displacement pump

When the engine speed is low, the swash-plate tends to move to a tilted position to maximise the stroke of the pistons, and hence output of the pump. When this fluid is fed to the constant displacement motor – which drives the generator - it will produce a gearing up effect, that is, the constant displacement hydraulic motor will spin at an increased RPM compared to the variable displacement pump.

The swash-plate constantly modulates between these two extremes to ensure the generator spins at a constant RPM whatever the engine RPM.

Regards, JetMech



JetMech split the back of his pants. He can feel the wind in his hair.
User currently offlineCelticMech From Ireland, joined Oct 2008, 216 posts, RR: 2
Reply 13, posted (5 years 1 month 1 week 1 day 21 hours ago) and read 21091 times:



Quoting JetMech (Reply 12):
Pretty much. A CSD consists of a variable displacement hydraulic pump supplying fluid (usually engine oil)........................

Ah yes...the days of learning about all this stuff from the basic licences now comes flooding back to me! I knew it was somewhere in my head!!!  Big grin


User currently offlineDocLightning From United States of America, joined Nov 2005, 20358 posts, RR: 59
Reply 14, posted (5 years 1 month 1 week 1 day 7 hours ago) and read 20941 times:



Quoting JetMech (Reply 12):

The swash-plate constantly modulates between these two extremes to ensure the generator spins at a constant RPM whatever the engine RPM.

Wow. Fascinating! Hardly an obvious solution to a problem. Now, what regulates the swashplate? Is it an act of physics, or is there a motor in the shaft that adjusts the angle to keep the output constant?


User currently offlineRwessel From United States of America, joined Jan 2007, 2416 posts, RR: 2
Reply 15, posted (5 years 1 month 1 week 1 day 6 hours ago) and read 20931 times:
Support Airliners.net - become a First Class Member!



Quoting DocLightning (Reply 14):
Now, what regulates the swashplate? Is it an act of physics, or is there a motor in the shaft that adjusts the angle to keep the output constant?

The swashplate is invariably moved hydraulically (after all, there's plenty of hydraulic pressure around). The regulation for that is more variable. In simple designs that are more concerned with constant output pressure, it can be simple feedback from the pressure on the output side pushing against a spring - if the output pressure drops, the swashplate angle is increased by the spring - if the output pressure rises, the hydraulics will push the swashplate towards flat. For better output flow control, you can attach a small constant displacement pump to the motor being driven, and use the pressure from that to move the swashplate (if the motor slows down, the pressure will be reduced). Or you can go highly electronic.


User currently offlineJetMech From Australia, joined Mar 2006, 2699 posts, RR: 53
Reply 16, posted (5 years 1 month 1 week 1 day 5 hours ago) and read 20919 times:

Quoting CelticMech (Reply 13):

I think one of the most interesting parts is how some of the pressurised fluid is bled off to lubricate the slipper foot. I think there is also a small amount of leakage between the piston and cylinder for lubrication purposes also. I presume both of these fluid streams make up the case drain fluid.

Quoting DocLightning (Reply 14):



Quoting Rwessel (Reply 15):

As mentioned by Rwessel, the multi-piston axial pump with tilting swash plate is also the same configuration of device used for aircraft hydraulic pumps, in this case, the swash plate tilts to modulate pressure.

Quoting Rwessel (Reply 15):
if the output pressure drops, the swashplate angle is increased by the spring - if the output pressure rises, the hydraulics will push the swashplate towards flat.

http://i94.photobucket.com/albums/l118/Jet-Mech/CSDpic.jpg

http://home.planet.nl/~brink494/axpp_v.htg/ppmpreg.jpg

With respect to the CSD, GE quotes that the N2 spool of the GE90 spins at 6610 RPM during ground idle, and 10.850 RPM at max power. It thus seems that the maximum speed range that needs to be accommodated by a CSD is a somewhat modest 2:1.

http://www.geae.com/education/engines101/

Regards, JetMech

[Edited 2009-11-18 01:38:31]


JetMech split the back of his pants. He can feel the wind in his hair.
User currently offlineMD11Engineer From Germany, joined Oct 2003, 14139 posts, RR: 62
Reply 17, posted (5 years 1 month 1 week 1 day 2 hours ago) and read 20891 times:



Quoting CelticMech (Reply 13):
Quoting JetMech (Reply 12):
Pretty much. A CSD consists of a variable displacement hydraulic pump supplying fluid (usually engine oil)........................

Ah yes...the days of learning about all this stuff from the basic licences now comes flooding back to me! I knew it was somewhere in my head!!! Big grin

When you BAEC part 6 exams, did you also have to describe the internal workings of a CSD in detail?  Silly

Jan


User currently offlineLMP737 From , joined Dec 1969, posts, RR:
Reply 18, posted (5 years 1 month 1 week 21 hours ago) and read 20851 times:



Quoting PGNCS (Reply 6):
I can tell you the MD-90 changed the rather bulletproof DC-9 CSD system that worked great to a Variable Speed Constant Frequency (VSCF) similar to what you describe. It is my least favorite feature of the MD-90, and one that was certainly not a step forward in my opinion.

The problem with the VSCF system on the MD-90 is that it was way too sensitive. The system would latch onto faults way to easily. When AA had the ex-Reno MD-90's it was common practice during the day to keep the APU running and not hook up ground power. Gate power had a tendency to fault the system, something you don't have to worry about with an MD-80.

The concept of a VSCF system is a good on. Unfortunately Douglas made a big mess. If they had done it right they would have sold a lot more MD-90's. Or if they had stuck an IDG on the aircraft instead. And put a new wing on it.

I hear Delta is getting some second hand MD-90's for their fleet. I'm not sure whether to pity or envy the A&P's over at Delta. Pity them because the 90 can be a handful, envy because it's almost guaranteed overtime.  Wink


User currently offlineMD11Engineer From Germany, joined Oct 2003, 14139 posts, RR: 62
Reply 19, posted (5 years 1 month 1 week 21 hours ago) and read 20847 times:



Quoting LMP737 (Reply 18):
The concept of a VSCF system is a good on. Unfortunately Douglas made a big mess. If they had done it right they would have sold a lot more MD-90's. Or if they had stuck an IDG on the aircraft instead. And put a new wing on it.

Back in the 1990s LH tried VSCF systems with their 737 classics. It was a big fail. The systems constantly went U/S. in the end they went back to conventional CSD / generator combinations.

Jan


User currently offlineNoWorries From United States of America, joined Oct 2006, 539 posts, RR: 1
Reply 20, posted (5 years 1 month 1 week 19 hours ago) and read 20812 times:

Do any new-design commercial jets use these types of systems. The 787 doesn't, I don't thin

Quoting Tdscanuck (Reply 9):
787 just skips the whole problem and uses variable frequency generators.

Will any new designs still use these types of constant speed systems? I didn't think the A380 used them either.


User currently offlineEx52tech From United States of America, joined Dec 2006, 559 posts, RR: 1
Reply 21, posted (5 years 1 month 1 week 13 hours ago) and read 20745 times:



Quoting Rwessel (Reply 8):
It's the bevel gears and shafts that are in the engine core, what's usually called the gearbox is outside the engine, on the other end of that shaft.



Quoting DocLightning (Reply 7):
Ah, that answered a lot. So it seems that the gearbox sits within the diameter of the compressor disks so as to not impede airflow and a thin shaft (presumably in an aerodynamic fairing) passes out of the core to the gearbox.

GE calls that gearbox inside the compressor case (frame) an inlet gearbox, it is the power takeoff for the main gearbox shaft. They were touchy to build, quite a few measurements and lash checks to accomplish. On a CF6 that shaft passageway through the case strut is also used to scavenge the oil from the N1 compressor rear bearing, and the N2 compressor front bearing, or the "B" sump.



"Saddest thing I ever witnessed....an airplane being scrapped"
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 22, posted (5 years 1 month 1 week 10 hours ago) and read 20725 times:

In one of those lovely twists of fate, I found a lovely cross-section of a Trent 1000 that I was able to liberate from it's home this morning. It is an excellent overview of the major engine components and layout, and has a great view of the point inside where the power is extracted from the spools.

Here's the full view:


And here's a close-up of the bevel gears:


The right gear is the N3 spool, the left gear is the N2 spool. As you can see, there are actually separate output shafts from each spool that go into the gearbox just under the core, which sends it out to the main gearbox on the bottom of the fan case.

I have the full resolution gory detail scan (~200 MB as a TIFF) if anyone wants it. Send me a msg and we'll figure something out.

Tom.


User currently offlineEx52tech From United States of America, joined Dec 2006, 559 posts, RR: 1
Reply 23, posted (5 years 1 month 1 week 10 hours ago) and read 20720 times:



Quoting Tdscanuck (Reply 22):

The right gear is the N3 spool, the left gear is the N2 spool. As you can see, there are actually separate output shafts from each spool that go into the gearbox just under the core, which sends it out to the main gearbox on the bottom of the fan case.

They must be operating different portions of the main gearbox my guess would be the more loaded components would be powered by the N3, but it is just a guess. Does anyone know which spool is powering what components, and why they are operating from two different inputs?  scratchchin 



"Saddest thing I ever witnessed....an airplane being scrapped"
User currently offlineLMP737 From , joined Dec 1969, posts, RR:
Reply 24, posted (5 years 1 month 6 days 21 hours ago) and read 20683 times:



Quoting MD11Engineer (Reply 19):
Back in the 1990s LH tried VSCF systems with their 737 classics. It was a big fail. The systems constantly went U/S. in the end they went back to conventional CSD / generator combinations.

I remember reading about the VSCF system being used on some 737 classics at one point. I did not know LH had it on theirs.

The 777 uses a VSCF system for backup power. Each engine has a Back Up Generator mounted on the lower r/h fan case, at least on the Trent. The two BUG's supply power to a backup power converter which converts the variable frequency to 115/400. From what I've seen it's a pretty reliable system. There was a brief period of time when the BUG's had a higher than normal fail rate. That problem seems to be rectified. Which is good because I had to change one in freezing drizzle about four years ago.

Quoting NoWorries (Reply 20):
Will any new designs still use these types of constant speed systems? I didn't think the A380 used them either.

I believe both the A380 and 787 use the VSCF system.


25 Tdscanuck : A380 and 787 are not VSCF, they're VSVF. Variable speed-variable frequency. Tom.
26 NoWorries : As I undestand it then, this means that the AC generators are geared directly to the engine and their speed varies with engine speed. It's strictly s
27 Bri2k1 : There are some electrical components that don't care about the frequency, and IIRC there are both variable frequency and constant frequency buses. Is
28 Tdscanuck : Yes. For systems that need it, yes. As Bri2k1 noted, many systems don't care so you don't have to bother feeding them constant frequency power. Tom.
29 NoWorries : It looks like there could be quite a few trade-offs when considering CSD vs VSVF systems -- systems cost, installation, space, weight, maintenance cos
30 Tdscanuck : The big enabler for VF systems was good, reliable, high power-density solid-state electronics. The CSD is a mechanical nightmare and took a long time
31 TristarSteve : The British Airways B734 fleet was also delivered with VSCF units. But due to unreliability they eventually gave up and converted them to conventiona
Top Of Page
Forum Index

Reply To This Topic Gearboxes And Generators
Username:
No username? Sign up now!
Password: 


Forgot Password? Be reminded.
Remember me on this computer (uses cookies)
  • Tech/Ops related posts only!
  • Not Tech/Ops related? Use the other forums
  • No adverts of any kind. This includes web pages.
  • No hostile language or criticizing of others.
  • Do not post copyright protected material.
  • Use relevant and describing topics.
  • Check if your post already been discussed.
  • Check your spelling!
  • DETAILED RULES
Add Images Add SmiliesPosting Help

Please check your spelling (press "Check Spelling" above)


Similar topics:More similar topics...
747 Upper Deck And Balance posted Fri Nov 13 2009 14:30:43 by DocLightning
Taxiing And Airport Signage posted Tue Nov 3 2009 09:42:57 by Meristem
Landing Gears On L1011 And DC10... posted Fri Oct 30 2009 22:12:50 by Aircanada014
Mobile Phone Apps And Flying posted Thu Oct 29 2009 14:30:58 by GolfOscarDelta
Private Jets And Traffic Control posted Wed Oct 28 2009 16:12:41 by WROORD
A319 Trim And Tailplane posted Tue Oct 13 2009 19:55:10 by FX772LRF
AA Logo Size On 733 And 738: Why Is 738 Bigger? posted Tue Oct 13 2009 11:13:48 by September11
Landing Lights On And Off In Cruise posted Thu Oct 1 2009 09:21:53 by DocLightning
Piston Power And Turbines posted Mon Sep 28 2009 20:55:37 by AFGMEL
BAe-146 - How Was It From The A/P And Pilot POV? posted Mon Sep 28 2009 19:29:30 by Pmk

Sponsor Message:
Printer friendly format