columba
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Unducted Fan

Tue Aug 08, 2006 1:08 am

What were the disadvantages of this interesting technology and is there a chance that it will be further developed ?

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It will forever be a McDonnell Douglas MD 80 , Boeing MD 80 sounds so wrong
 
teixeim
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RE: Unducted Fan

Tue Aug 08, 2006 1:15 am

Strange - the unducted fan concept was all the rage in the late 80s/early 90s. I wonder what happened? I guess winglets are more practical than re-engineing aircraft. Also, I only saw the unducted engines on T-tailed aircraft.
 
Glareskin
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RE: Unducted Fan

Tue Aug 08, 2006 1:26 am

http://www.aviation-history.com/garber/images/udf-2.jpg
Maybe the sky-high fuel prices will cause the industry to reconsider further development. Economics are significantly better than with a jet engine.
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seanp11
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RE: Unducted Fan

Tue Aug 08, 2006 1:48 am

Quoting Columba (Thread starter):
What were the disadvantages of this interesting technology and is there a chance that it will be further developed ?

Three things-- first, its pretty loud. Even if regulation would allow it, it would still bother a majority of pax. Second, unlike a normal turbofan, there is no shroud to catch a fanblade that comes off. That means if it looses a blade, it would be much more dangerous than a regular turbofan. Finally, it looks like a turboprop, and the majority of pax wouldn't know the difference. Most of them still think that a prop plane is old. The noise issue could possibly be fixed, (how, i have no idea) but the other two are inherent to the design. I see the GTF (geared turbofan) as a much more possible technology.

Its a novel technology, but I really don't see it catching on.
 
dw747400
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RE: Unducted Fan

Tue Aug 08, 2006 1:53 am

You can do a search to find out some more information; however some key problems were noise and vibration. There is no way that these engines could meet modern noise standards--though further research could help in this regard, the engineers have a LOT of work ahead of them. Vibration was also an issue, as it made the aircraft less comfortable for passengers and may have had potential affects on long term structure and system integrity (someone more versed in the appropriate area of engineering could explain how big of an issue that actually was).

The fact that it looks like a prop turns passengers away, though most airlines seem willing to risk a few ill-informed pax for the efficiency of a turboprop these days. Safety concerns were also significant, as any blade failure could do major damage to the airframe. Finally, though it is faster than a turboprop, the Unducted Fan (UDF) is slower than most conventional jets. As with the issue of passenger appeal, I think more airlines will be willing to look the other way on the speed loss in order to save some gas.

If UDFs can be made quieter, smoother-running, and blade-out issues conclusively resolved, I think we may be seeing them in the future.
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ksupilot
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RE: Unducted Fan

Tue Aug 08, 2006 1:55 am

There were three main reasons the UDFs failed to take off the first time around:

1. New Concept- Anytime somethingis new, it is already at a disadvantage. People, and especially airlines like to stay with what has worked, and are afraid of drastic changes.

2. Noise- The UDFs were loud, cabin noise increased (jet were loud to begin with back then, now imagine even louder with UDFs) The forces that the tips of the blades produced could cause stressing of the fuselage in the areas around the engines. The sound wave sof the UDFs are at a low frequency and it even effected people on the ground who were in the area of the tests.

3. Even if 1 and 2 were overcome, the fuel prices eventually fell, therefore the airlines and MD and Boeing found it unncessary to further develop the project.

Boeing was working on the 7J7:

Larger image here: http://i.timeinc.net/popsci/flat_fil...ace/space0604boeing/images/7J7.jpg

MD was working on the MD-94X:


And doing tests on an MD-80:
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The Russian AN-70 uses UDF technology. The latest appearance of the UDF in a Boeing concept came in early June with the "Muppet Studies" that Boeing did on super efficient "green planes".


Article here:
http://seattletimes.nwsource.com/htm...y/2002973147_boeingconcepts05.html
 
ksupilot
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RE: Unducted Fan

Tue Aug 08, 2006 2:26 am

It has been a popular subject...My Boeing Green Planes thread, and I believe it came up in another disucssion as well. It is an interesting subject however.

I read somewhere that the UDF fan blades were to be made out of composite material. With the 787 it has come to the surface that composites do not handle heat very well. I wonder what effect this would have on the UDF blades, as engines do get rather hot.

And with the shrapnel problem...isn't this still a serious problem even your normal turbofan. If it sheds shrapnel it can rip the engine apart, and I don't see why it can go through the engine duct and still hit the side of the plane. Isn't that what happened to that plane that crash-landed in Sioux City? Shapnel went through the engine and cut the hydrolics. It seems that no matter what type of engine you have on the plane, if it sheds amterial, you are in deep trouble. Though I can see why this is more of a concern with the UDFs.

I guess I can see how it could be mistaken for a tubroprop...but those engines look a lot like a normal jet with Surfboard fins glued on. And strangely it reminds me of the "needler" from the game Halo.
 
DfwRevolution
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RE: Unducted Fan

Tue Aug 08, 2006 2:41 am

Quoting KSUpilot (Reply 7):
I read somewhere that the UDF fan blades were to be made out of composite material. With the 787 it has come to the surface that composites do not handle heat very well. I wonder what effect this would have on the UDF blades, as engines do get rather hot.

The fan blades would not have been "hot" portions of the engine, and carbon fiber would be a suitable material.

The Ge90 has used carbon fiber fan blades for over a decade with great success. The Genx will build on the Ge90 with a carbon fiber fan casing as well.
 
duke
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RE: Unducted Fan

Tue Aug 08, 2006 4:30 am

In 1992 when I was 12, I was interested in some of the new stuff that had come out of or was supposed to come out of Boeing and Douglas. I called someone at the Boeing plant (IIRC) and asked about the 7J7/about propfans. I was told the project had been shelved because they were afraid of what passengers would think when they saw them, maybe that passengers would feel uneasy about flying with such a plane.
 
seanp11
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RE: Unducted Fan

Tue Aug 08, 2006 4:41 am

Quoting KSUpilot (Reply 7):
And with the shrapnel problem...isn't this still a serious problem even your normal turbofan. If it sheds shrapnel it can rip the engine apart, and I don't see why it can go through the engine duct and still hit the side of the plane. Isn't that what happened to that plane that crash-landed in Sioux City? Shapnel went through the engine and cut the hydrolics. It seems that no matter what type of engine you have on the plane, if it sheds amterial, you are in deep trouble.

That's why they make engine shrouds out of kevlar and other strong polymers.  Smile
 
Rheinbote
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RE: Unducted Fan

Tue Aug 08, 2006 5:13 am

Quoting Dw747400 (Reply 4):
Vibration was also an issue, as it made the aircraft less comfortable for passengers and may have had potential affects on long term structure and system integrity

Structures adjacent to the GE36 open rotors suffered from a phenomenon called acoustic fatigue. Basically, the rotating shockwaves generated by the prop blades constantly pummeled the aircraft's skin. The immediate effect was visible at F'boro Airshow 1988, when the paint came flaking off the MD-80 demonstrator during a brief aerial display. In the long run, metal strucutures subjected to this treatment would become brittle and crack. Very similar to bending a paperclip until it snaps. I'm not sure how composites would behave in this environment, but I'd suspect delamination to occur very quickly.
Peak noise levels measured on the surface of the MD-80's vertical tail were in the region of 125-135dB. While this could be mitigated nowadays by modern 3D flow analysis and reverse airfoil engineering that was't available back then, I doubt that the noise level could be taken down to meet Stage 4 requirements.
 
ksupilot
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RE: Unducted Fan

Tue Aug 08, 2006 5:56 am

Quoting Rheinbote (Reply 11):
Peak noise levels measured on the surface of the MD-80's vertical tail were in the region of 125-135dB. While this could be mitigated nowadays by modern 3D flow analysis and reverse airfoil engineering that was't available back then, I doubt that the noise level could be taken down to meet Stage 4 requirements.

Then you have to question whether it is all truly worth it. You will notice in the Green Planes picutre above the "Beaker" concept. This uses your normal turbofan engines, yet they are super efficient, low emissions. The only downside is that the aircraft travels at the same speed as a plane with UDF engines.

On future aircraft we will see a new kind of engine, whether it will be a UDF is tough to say, but it is highly unlikely.

[Edited 2006-08-07 23:01:56]

[Edited 2006-08-07 23:03:14]
 
liedetectors
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RE: Unducted Fan

Thu Aug 10, 2006 11:57 am

Quoting Seanp11 (Reply 3):
Second, unlike a normal turbofan, there is no shroud to catch a fanblade that comes off. That means if it looses a blade, it would be much more dangerous than a regular turbofan.

This is correct. Don't forget though with out a fan case to catch the blade, you would need to heavily beef up the tail structure (in the case of the MD80 UDF). The added weight of this structure, made it uneconomical.
If it was said by us, then it must be true.
 
F14D4ever
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RE: Unducted Fan

Sat Aug 12, 2006 2:57 am

Quoting KSUpilot (Reply 5):
The Russian AN-70 uses UDF technology.

Not entirely. UDF is a GE trademarked name for a specific technology demonstrator engine, the GE36 Unducted Fan. The GE UDF featured composite blades attached directly to large-diameter, low-speed propulsor turbine rotors. The propulsor (lp) turbine was a two-rotor contra-rotating turbine with no gearbox, aerodynamically coupled to the two-spool gas generator.

The An-70's Progress D-27 engine, like the UDF, uses composite blades on contra-rotating rotors. The D-27, unlike the GE UDF, has a gearbox. A nice isometric drawing of the D-27 is visible at
http://www.aeronautics.ru/img002/an70-progres-d-27-drawing.jpg
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N231YE
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RE: Unducted Fan

Sat Aug 12, 2006 10:43 am

Quoting F14D4ever (Reply 13):
The GE UDF featured composite blades attached directly to large-diameter, low-speed propulsor turbine rotors. The propulsor (lp) turbine was a two-rotor contra-rotating turbine with no gearbox, aerodynamically coupled to the two-spool gas generator.

Sorry for asking this late in the thread,

But what exactly differentiates an Unducted Fan to a turboprop in a pusher configuration with many blades (like a Beech Starship)?
 
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Starlionblue
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RE: Unducted Fan

Sat Aug 12, 2006 10:56 am

Quoting N231YE (Reply 14):

But what exactly differentiates an Unducted Fan to a turboprop in a pusher configuration with many blades (like a Beech Starship)?

As I recall, it's speed. A turboprop has a relatively low speed prop. An unducted fan moves at speeds similar to the ducted fans found in ordinary turbofans. Thus the problems with vibrations, noise and containment compared to a turboprop.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
TEAtheB
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RE: Unducted Fan

Sat Aug 12, 2006 6:35 pm

Quoting F14D4ever (Reply 13):
UDF is a GE trademarked name for a specific technology demonstrator engine

So should we be calling this "open rotor"?
 
N231YE
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RE: Unducted Fan

Sun Aug 13, 2006 5:52 am

Quoting Starlionblue (Reply 15):

Thanks
 
F14D4ever
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RE: Unducted Fan

Sun Aug 13, 2006 11:52 am

Quoting TEAtheB (Reply 16):
So should we be calling this "open rotor"?

"Open rotor" seems to be in vogue now. Back when the UDF was in development, and Pratt was looking at a similar project, they were generically referred to as Ultra-High Bypass (UHB).
"He is risen, as He said."
 
F14D4ever
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RE: Unducted Fan

Sun Aug 13, 2006 11:57 am

Quoting N231YE (Reply 14):
what exactly differentiates an Unducted Fan to a turboprop in a pusher configuration with many blades (like a Beech Starship)?

The absence of a reduction gearbox between power ('propulsor') turbine and fan blades. You might call the UDF a 'direct drive' open rotor machine, as opposed to a turboprop, which employs a gearbox so that the fan rotor can turn at a lower (more optimal) rpm and the turbine powering it can turn at a higher (more optimal) rpm.
"He is risen, as He said."
 
Dakar
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RE: Unducted Fan

Sat Aug 19, 2006 2:19 pm

Does anyone know if these unducted fans have the same problem as the earlier rear-mounted high-bypass fans where the turbine and fan are tied directly together and causing dissimilar temps between hot turbine inner sections and fan blades in the cold bypass air?

Nick
 
Fly2HMO
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RE: Unducted Fan

Sat Aug 19, 2006 2:59 pm

I'm probably asking for the impossible, but, if anybody has heard them, what did they sound like? Turboprop-ish I suppose? Or like the an extremely loud version of the buzzaw as heard in current engines?

Any videos with sound on them?

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