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speedracer1407
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Advantages Of Two-stage Fan

Thu Nov 29, 2007 5:17 am

So first, I'm fairly certain of the obvious DISadvantages of a two-stage on a modern turbofan, what with various advances in aerodynamics, computer modeling, and materials since, for example, the the JT8D was designed.

But as one of (or perhaps THE) most abundant commercial aviation engine ever produced, the JT8D's designers must have had a good reason for going with a two stage fan.

So, what were the advantages of the design in the era that the JT8D was designed? Is it simply that two fans could accelerate more air than one given the limited diameter that the designers were working with?

And besides the C-5's original engines (can't remember the military designation), what other designs have included a two-stage fan?

Thanks for your replies folks,

O
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Fly2HMO
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RE: Advantages Of Two-stage Fan

Thu Nov 29, 2007 5:56 am



Quoting Speedracer1407 (Thread starter):
Is it simply that two fans could accelerate more air than one given the limited diameter that the designers were working with?

That would be my guess. Two smaller fans doing the same (or close to the same) job as one big one. So the two stage solution is probably lighter and has less drag (due to smaller area).
 
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Jetlagged
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RE: Advantages Of Two-stage Fan

Thu Nov 29, 2007 9:57 pm

Rolls-Royce low bypass ratio turbofans like the Conway and Spey also had multi-stage fans. In fact the whole LP compressor acted as the fan. Bypass split occurred between the LP and HP compressors.

The JT3D, like the JT8D had a two stage fan.

So the single stage, large fan came into its own as bypass ratios became ever larger.
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F14D4ever
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RE: Advantages Of Two-stage Fan

Fri Nov 30, 2007 4:02 am

Thrust is, in very simple terms, mass flow rate out the back of the engine (m/t) multiplied by the velocity difference between inlet and exit flow (l/t). m l / t^2 = force (thrust).

The JT8D has a higher fan pressure ratio (FPR), hence higher exhaust gas velocity, than would a single stage fan. A modern high bypass turbofan of comparable thrust capability has a greater fan diameter, passing more mass at lower FPR hence lower exhaust gas velocity than that of the JT8D.

The JT8D was designed in an era when noise wasn't as big an issue as it is now. Since shear due to velocity difference between exhaust gas and ambient air is a major contributor to engine noise, it has become desireable to decrease exhaust velocities to meet ever more stringent noise constraints. To offset this, newer designs continually seek to increase mass flow. That's what brought us the GE90-115B et al. The idea is to pump as much mass as is feasible at a low FPR, which is constrained at the low end by, among other factors, fan stall margin.

Is this helpful?
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xv408
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RE: Advantages Of Two-stage Fan

Fri Nov 30, 2007 12:28 pm

The major factor that originally drove high bypass engines is efficiency (hence range/payload), a goal way before noise was an issue. As F14D4ever notes, thrust is related to the change in momentum of the air through the engine. This change in momentum can be served by either accelerating a little bit of air a lot, or a lot of air a little bit. Now the kinetic enery is related to the square of the velocity, so accelerating n ammount of air to x m/s uses 4 times as much fuel as accelerating 2n of air to x/2 m/s. (Sort of). So it is more efficient to accelerate a lot of air a little bit than the other way around.
However, of couse, installed thrust is related to jet velocity relative to aircraft velocity, so as speed rises, so the thrust falls off, eventually to nothing. Hence why short haul aircraft tend to be props (equivalent to c. 40:1 bypass ratio), long haul tends towards high bypass fans (say 6-8:1 BPR) and fast jets are more like 1:1 bypass ratio, and a lot noisier to boot.
The 2 stage fan was, I guess, an early step in the direction of higher bypass ratios, before the technology of the big fans was developed. Vibration, containment, etc are all difficult issues in the big fans. The TF39 in the C-5 was a bit of an oddball. Looking at WIkipedia (http://en.wikipedia.org/wiki/General_Electric_CF6), with a comment on the TF39, the first fan stage was 1/2 height and acted as a core boost stage, with a full-height stage 2 LP fan behind it. No idea why this unusual architecture was chosen. Big fans use LP core boosters behind the fan today.
 
ex52tech
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RE: Advantages Of Two-stage Fan

Tue Dec 04, 2007 9:28 pm



Quoting Xv408 (Reply 4):
No idea why this unusual architecture was chosen. Big fans use LP core boosters behind the fan today.

Patent time limits were the reason, the same reason that the aft fan engine was used on the Convair 990's CJ-805-23 G.E. engines. Pratt had the patents on the forward fan.

P&W finally gave in and shared technology with G.E. when it became apparent that they needed to use variable stator vanes in the JT-9. G.E. had the patents on V.S.V.
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MarkC
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RE: Advantages Of Two-stage Fan

Tue Dec 04, 2007 10:20 pm



Quoting Ex52tech (Reply 5):
Patent time limits were the reason, the same reason that the aft fan engine was used on the Convair 990's CJ-805-23 G.E. engines. Pratt had the patents on the forward fan.

P&W finally gave in and shared technology with G.E. when it became apparent that they needed to use variable stator vanes in the JT-9. G.E. had the patents on V.S.V.

Thats interesting about the patents. I did not know that.

The F100 and F119 have 3 fans. I think the F135 does also.
 
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Jetlagged
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RE: Advantages Of Two-stage Fan

Wed Dec 05, 2007 3:27 am



Quoting Xv408 (Reply 4):
Big fans use LP core boosters behind the fan today.

Not the RB211/Trent. They have an IP compressor instead. The LP compressor is a single stage, i.e. the fan.

Quoting Ex52tech (Reply 5):
Patent time limits were the reason, the same reason that the aft fan engine was used on the Convair 990's CJ-805-23 G.E. engines. Pratt had the patents on the forward fan.

Had Boeing patented their particular configuration (i.e. an LP compressor with front fan and core booster stages)? Because RR were producing front fan turbofans, with the bypass split between the LP and HP compressors, at the same time as GE was producing aft fans rather than front fans. Presumably there would have been no reason why GE could not have followed RR's lead?
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
F14D4ever
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RE: Advantages Of Two-stage Fan

Wed Dec 05, 2007 4:33 am



Quoting MarkC (Reply 6):
The F100 and F119 have 3 fans. I think the F135 does also.

You mean 3 stage fans. As do the GE F101, F110, F136, F404, and F414. All intended for applications with roughly the same supersonic capability, hence requiring a high exhaust nozzle pressure ratio, in order to achieve the momentum exchange explained by Xv408 so well in Reply #4.
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747fan
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RE: Advantages Of Two-stage Fan

Wed Dec 05, 2007 10:52 pm



Quoting Speedracer1407 (Thread starter):
And besides the C-5's original engines (can't remember the military designation), what other designs have included a two-stage fan?

The big GE's that are on the C-5 are TF39's, the ancestor to the CF6. I don't know if any other engines had a two-stage fan, but the reason its rare is likely due to a lack of efficiency. Not to mention that the TF39 is LOUD, which may also be partly to blame for the two-stage fan. One of the reasons that Pratt & Whitney's JT9D beat out the TF39 (CF6) for the original 747-100 was due to noise concerns - though the JT9D is also loud, its definitely quieter than the TF39. The original civilian CF6 on the DC10-10, on the other hand, was not a two-stager and was quieter than both the original TF39 and JT9D.
I'll look up two-stage fans and see what advantages they have and if I find anything, I'll post about it.
 
ex52tech
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RE: Advantages Of Two-stage Fan

Fri Dec 07, 2007 10:22 am



Quoting 747fan (Reply 9):
Not to mention that the TF39 is LOUD, which may also be partly to blame for the two-stage fan.

Some sources say that the TF39 has a one and a half stage fan, while other sources call it a two stage fan. I personally have always thought of the forward/smaller fan as the core compressor booster, and thus a fan and a half. I still say that it was G.E.'s way of tap dancing around Pratt and Whitney's forward fan patents. They had to know that the core booster in the immediate gas path of the core compressor was more efficient, than in front of the fan.

I was floored when they hung the TF39 off of the C-5B. Talk about throwing the baby out with the bath water. I know that decision was driven by cost, but how much money did it cost to eventually re-engine both the A&B models. Not to mention the initial cost of TF39 spares, pylon mods for the CF6-80C2L1F, the -80C2s and it's spares. The TF39 belonged in a museum by the time the C-5B came along.
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