Turbojets & Turbofans: Geared Shafts & Fans

[Zipper730]

After reading about the development of the L-1000/XJ37 engine, which evolved from a mixed-flow compressor to a 2-1/2 speed compressor (the LP section was driven off the main-shaft via a reduction gear, and a hydraulic torque-converter was fitted to the first 4-stages in order to assist with engine starting and higher altitude performance), I'm curious why nobody just drove part of a compressor off a gearing system instead of using the twin-spool arrangement commonly seen?

I have also heard of ideas for geared-turbofans in which the LP/HP compressor shafts were pretty normal, but a gear-box would drive the fan instead of having the fan either part of the LP section (as on most turbofans), or on it's own shaft (as on RR's engine designs), yet only a very small number of designs were produced: Off the bat, I can only think of the Garrett TFE731, the Lycoming ALF-502R/507L, Pratt & Whitney PW1000G.

FLYBOYJ fubar57 M m13katyusa2020 T ThomasP T thunderbird

 

[FLYBOYJ]

After reading about the development of the L-1000/XJ37 engine, which evolved from a mixed-flow compressor to a 2-1/2 speed compressor (the LP section was driven off the main-shaft via a reduction gear, and a hydraulic torque-converter was fitted to the first 4-stages in order to assist with engine starting and higher altitude performance), I'm curious why nobody just drove part of a compressor off a gearing system instead of using the twin-spool arrangement commonly seen?

I have also heard of ideas for geared-turbofans in which the LP/HP compressor shafts were pretty normal, but a gear-box would drive the fan instead of having the fan either part of the LP section (as on most turbofans), or on it's own shaft (as on RR's engine designs), yet only a very small number of designs were produced: Off the bat, I can only think of the Garrett TFE731, the Lycoming ALF-502R/507L, Pratt & Whitney PW1000G.

FLYBOYJ fubar57 M m13katyusa2020 T ThomasP T thunderbird

My guess the twin spool was the easier route to manufacture and it would also save weight. Remember the L-1000 was the first of it's kind and we can only speculate how it would have performed when fitted to an airframe and flown at altitude.

Another issue I see is if you're going to have a gearbox driving the turbofan, you have to have a way to decouple it should the fan start driving the engine and should there be a sudden a failure in the gearbox.

 

[Zipper730]

My guess the twin spool was the easier route to manufacture and it would also save weight.

I guess the J37's 1235-1600 lb. weight figures were optimistic then?

Another issue I see is if you're going to have a gearbox driving the turbofan, you have to have a way to decouple it should the fan start driving the engine and should there be a sudden a failure in the gearbox.

That doesn't sound good

 

[FLYBOYJ]

I guess the J37's 1235-1600 lb. weight figures were optimistic then?

Perhaps - I can only speak on the maintenance side. Remember, the goal was to sell the engine to the government

That doesn't sound good

No, not a good thing. This is a huge consideration when designing turboprops.

 

[Zipper730]

Remember, the goal was to sell the engine to the government

Generally, most weight figures are often optimistic (sometimes verging on terminological inexactitude).

No, not a good thing.

What typically causes a failure in the gear-box?

 

[FLYBOYJ]

What typically causes a failure in the gear-box?

You can have over torque or side loads placed on it, depending on configuration, also if you lose any oil from the gear box so you're always checking for leaks

 

[spiralcopter]

In your average gas turbine engine, about 60-70% of the energy harvested by the turbines goes straight back into the compressors. So, in the case of, say, an LM1500 industrial gas turbine (basically a J79 but not in an airplane) that 15,000 horsepower is actually a fairly small percentage leftover after the compressors have taken their cut.

So, while there definitely would be advantages to running the compressor off of a gearbox or hydraulic unit that kept it at the ideal speed as opposed to spinning at turbine shaft speed, this reduction gearbox or hydrodynamic device would need to be able to handle roughly maximum engine output x3. Designing a gearbox that can handle that kind of power while still being light enough to not completely offset any efficiency gains would be hard.

 

[kitplane01]

After reading about the development of the L-1000/XJ37 engine, which evolved from a mixed-flow compressor to a 2-1/2 speed compressor (the LP section was driven off the main-shaft via a reduction gear, and a hydraulic torque-converter was fitted to the first 4-stages in order to assist with engine starting and higher altitude performance), I'm curious why nobody just drove part of a compressor off a gearing system instead of using the twin-spool arrangement commonly seen?

I have also heard of ideas for geared-turbofans in which the LP/HP compressor shafts were pretty normal, but a gear-box would drive the fan instead of having the fan either part of the LP section (as on most turbofans), or on it's own shaft (as on RR's engine designs), yet only a very small number of designs were produced: Off the bat, I can only think of the Garrett TFE731, the Lycoming ALF-502R/507L, Pratt & Whitney PW1000G.

FLYBOYJ fubar57 M m13katyusa2020 T ThomasP T thunderbird

The gas stream coming off the combustor is very high velocity, pressure, and tempature. Because of that, it's energy is most effeciently harvested by a very high speed turbine. Further down, the velocity, pressssure, and tempature are lower, and the energy is best harvested by a slower turbine. Using two spools allows one to have turbines running at different speeds.