voltage
Topic Author
Posts: 35
Joined: Fri May 04, 2007 6:50 am

Three-spool GTF Hybrid?

Wed Jul 16, 2008 5:55 pm

Ok, so I was just thinking, if RR is already building three spool engines that drive 3 different compressor stages from 3 different turbine stages, why can't an engine be built that has 2 compressor stages and 3 turbine stages, with the 3rd turbine stage driving the fan?

Two spool:
HPC driven by HPT
Fan/LPC driven by LPT

Three spool:
HPC driven by HPT
IPC driven by IPT
Fan/LPC driven by LPT

GTF:
HPC driven by HPT
LPC driven by LPT
Fan driven by LPT (gear-reduced)

My proposed design:
HPC driven by HPT
LPC driven by IPT
Fan driven by LPT

Is it possible? Is it preferable?
 
Nicoeddf
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RE: Three-spool GTF Hybrid?

Wed Jul 16, 2008 6:51 pm

I guess (but as always tdscanuck will correct me Big grin ) that you get difficulties in providing the power you generate at the places you need them.

Meaning it might be easier to dedicate the LP/IP/HP-compressor systems via their own shaft to their related turbine components to gain max efficiency.

Just a guess, though.
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MarkC
Posts: 238
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RE: Three-spool GTF Hybrid?

Wed Jul 16, 2008 11:21 pm



Quoting Voltage (Thread starter):
My proposed design:
HPC driven by HPT
LPC driven by IPT
Fan driven by LPT

But this is a 3 spool. There is no LPC. Correct me if I'm wrong, I'm no RR expert.

But even with this, the LPT is moving so slow. The slower it goes, the more stages. A GTF LPT moves much faster than it would without a gear, and can be smaller diameter, less blades and vanes, less stages, and this means a lot lighter.
 
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jetmech
Posts: 2322
Joined: Wed Mar 29, 2006 2:14 am

RE: Three-spool GTF Hybrid?

Thu Jul 17, 2008 12:30 am



Quoting Voltage (Thread starter):
Is it possible?

I don't see why not. The gearbox appears to be the main issue with a GTF, but I believe P&W has one ready for flight testing, so the major hurdles with the gearbox may prove to have been overcome in the near future. To me, the main issue with a three spool GTF, assuming mature gearbox technology would be size and packaging constraints.

Quoting Voltage (Thread starter):
Is it preferable?

I think this gets back to the old money vs. benefit analysis. Purely in thermodynamic terms, it may actually be desirable for each compressor stage to be driven by its own turbine, such that it could turn at its own, desired RPM.

In reality, such an engine, especially for an aircraft application, would most likely be hideously complex, large and heavy, so quite apart from the cost of manufacturing it in the first place, any thermal efficiency you gain from such an engine, would most probably be readily negated by the higher drag and weight of the engine.

Quoting MarkC (Reply 2):
But this is a 3 spool. There is no LPC. Correct me if I'm wrong, I'm no RR expert.

RR terminology denotes the fan as the LP compressor. IIRC, because the fan and booster on a GE / P&W are tied together, the whole unit is referred to as the LP compressor. Either that or fan + booster. The remaining compressor stages are then denoted as HP.

Thus, with respect to Voltage's proposal, and keeping in mind RR terminology, I would hazard a guess that a three spool GTF may be denoted as;

HPC driven by HPT
IPC driven by IPT
Fan (LPC) driven by LPT (via gearbox)

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair :shock: .
 
tdscanuck
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RE: Three-spool GTF Hybrid?

Thu Jul 17, 2008 2:39 am



Quoting Voltage (Thread starter):
Ok, so I was just thinking, if RR is already building three spool engines that drive 3 different compressor stages from 3 different turbine stages, why can't an engine be built that has 2 compressor stages and 3 turbine stages, with the 3rd turbine stage driving the fan?

As far as I know, this is how RR engines work. The OEM's are a bit schitzophrenic about how they label the fan/booster/LPC.

Tom.
 
voltage
Topic Author
Posts: 35
Joined: Fri May 04, 2007 6:50 am

RE: Three-spool GTF Hybrid?

Thu Jul 17, 2008 1:36 pm

So the design I'm proposing is actually the design RR has been using since the RB211? The RR Fan and compressors are already all running at optimum speed? In that case, what makes the GTF so much better than the design RR has been using for ages?
 
nomadd22
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RE: Three-spool GTF Hybrid?

Thu Jul 17, 2008 1:51 pm

Would Pratt ever consider something like supplying gear boxes to other engine makers to use with their cores? Or would that be too much a case of competing with themselves?
Anon
 
rwessel
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Joined: Tue Jan 16, 2007 3:47 pm

RE: Three-spool GTF Hybrid?

Fri Jul 18, 2008 1:37 am



Quoting Voltage (Reply 5):
So the design I'm proposing is actually the design RR has been using since the RB211? The RR Fan and compressors are already all running at optimum speed? In that case, what makes the GTF so much better than the design RR has been using for ages?

No, while the three spools all run at different speeds, you still really want to fan stage to be running much slower than that for best efficiency, and the last turbine stage to be spinning faster. The fan is disproportionately large compared to the compressor stages, so that while the intermediate and high pressure sections may be running at reasonably near optimal speeds for both the compressors and turbines, even with the three stage design, the fan and last turbine stages are still respectively running too fast and too slow.
 
tdscanuck
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Joined: Wed Jan 11, 2006 7:25 am

RE: Three-spool GTF Hybrid?

Fri Jul 18, 2008 1:38 am



Quoting Voltage (Reply 5):
So the design I'm proposing is actually the design RR has been using since the RB211? The RR Fan and compressors are already all running at optimum speed?

Yes to the first part, as far as I know. No to the second. In an RR engine the fan must turn at the same speed as the LP turbine. Given that the turbine is smaller diameter (and in a hotter flow) there's no way that both are at their optimum speed. That match on the HP and IP spools should be pretty good.

Quoting Voltage (Reply 5):
In that case, what makes the GTF so much better than the design RR has been using for ages?

The GTF decouples the fan speed from the LP turbine speed. That lets both go at their optimum speed. Since the huge majority of the thrust is coming from the LP shaft, there's a lot of bang for the buck in optimizing this part of the engine.

Quoting Nomadd22 (Reply 6):
Would Pratt ever consider something like supplying gear boxes to other engine makers to use with their cores?

I'm sure they'd think about it, but it would only make business sense if the gearbox sales were worth more than the engine sales P&W would lose as a result. That means they'd have to make the gearboxes really expensive. At that point, it may become better for each maker to develop their own rather than buy from P&W.

Tom.
 
baroque
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Joined: Thu Apr 27, 2006 2:15 pm

RE: Three-spool GTF Hybrid?

Fri Jul 18, 2008 5:15 am



Quoting Voltage (Reply 5):
So the design I'm proposing is actually the design RR has been using since the RB211?

Yes.

Quoting Voltage (Reply 5):
The RR Fan and compressors are already all running at optimum speed?

As Voltage and Tdscanuck write, not so. The advantage is that they are close to optimum than with a twin spool design, but the turbine is still spinning too slowly and the fan too quickly which is where the gearbox come in.

The tip speeds tend to give the clue. Even with transonic speeds at the tips of a large fan, the much smaller radius turbine blades are waffling along at quite slow speeds. Even a four spool would not assist the radius problem, you need a reduction in revs to conquer that.  spin   spin   spin  No smilies with different revs unfortunately.
 
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jetmech
Posts: 2322
Joined: Wed Mar 29, 2006 2:14 am

RE: Three-spool GTF Hybrid?

Fri Jul 18, 2008 9:10 am

Quoting Voltage (Reply 5):

G'day Voltage,

I think I misread your orignal post slightly, namely;

Quoting Voltage (Thread starter):
Three spool:
HPC driven by HPT
IPC driven by IPT
Fan/LPC driven by LPT

As far as I know, with current RR's, there are no additional compressor stages on the same shaft as the fan itself. Thus, as pointed out by others, current RR's are close to the proposed configuration you also mentioned in your original post.

The only changes are that the LPC in your proposed configuration is denoted by RR as the IPC, and the fan in your proposed configuration is denoted by RR to be the LPC.

Quoting Voltage (Thread starter):
My proposed design:
HPC driven by HPT
LPC driven by IPT
Fan driven by LPT

In other words, your proposed configuration differs to actual RR RB-211 / Trent configurations in terminology only.

Regards, JetMech

[Edited 2008-07-18 02:13:25]
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