A photo of Safran's contra rotating fan prototype appears in a Flightglobal article about ZeroE initiatives.....


https://d3lcr32v2pp4l1.cloudfront.net/P ... 3_crop.jpg

https://www.flightglobal.com/flight-int ... 96.article


Is this finally that design's vindication and would it mean its eventual widespread application? What about the noise issue before?

Can somebody explain the difference between turboprop and UDF please? They appear to have different configuration, but I have no idea the advantages of UDF over turboprop.

LCDFlight wrote:

Can somebody explain the difference between turboprop and UDF please? They appear to have different configuration, but I have no idea the advantages of UDF over turboprop.

The first article is fairly lengthy, but should help.

https://www.britannica.com/technology/j ... an-engines

https://www.grc.nasa.gov/WWW/k-12/airplane/aturbp.html

IAHFLYR wrote:

LCDFlight wrote:

Can somebody explain the difference between turboprop and UDF please? They appear to have different configuration, but I have no idea the advantages of UDF over turboprop.

The first article is fairly lengthy, but should help.

https://www.britannica.com/technology/j ... an-engines

https://www.grc.nasa.gov/WWW/k-12/airplane/aturbp.html

Thanks, not too lengthy, helped a lot.

It still leaves me wondering, why did it take >50 more years to perfect UDF technology (assuming we are near that today), after turboprop was essentially perfected 50 yrs ago? Is there a materials science issue, or was the aerodynamics of the scimitar-shaped UDF fan blades that difficult?

LCDFlight wrote:

IAHFLYR wrote:

LCDFlight wrote:

Can somebody explain the difference between turboprop and UDF please? They appear to have different configuration, but I have no idea the advantages of UDF over turboprop.

The first article is fairly lengthy, but should help.

https://www.britannica.com/technology/j ... an-engines

https://www.grc.nasa.gov/WWW/k-12/airplane/aturbp.html

Thanks, not too lengthy, helped a lot.

It still leaves me wondering, why did it take >50 more years to perfect UDF technology (assuming we are near that today), after turboprop was essentially perfected 50 yrs ago? Is there a materials science issue, or was the aerodynamics of the scimitar-shaped UDF fan blades that difficult?

AFAIK the big difference is supersonic tip speeds on UDFs. Not sure if this is in all flight regimes or only at high thrust.

The aerodynamics are indeed difficult. However, I don't think that is the big hurdle. UDFs have been unacceptably loud, for starters. The other issue is the energy level of a blade if it detaches. This would be much higher than the propeller equivalent.

Starlionblue wrote:

LCDFlight wrote:

IAHFLYR wrote:

The first article is fairly lengthy, but should help.

https://www.britannica.com/technology/j ... an-engines

https://www.grc.nasa.gov/WWW/k-12/airplane/aturbp.html

Thanks, not too lengthy, helped a lot.

It still leaves me wondering, why did it take >50 more years to perfect UDF technology (assuming we are near that today), after turboprop was essentially perfected 50 yrs ago? Is there a materials science issue, or was the aerodynamics of the scimitar-shaped UDF fan blades that difficult?

AFAIK the big difference is supersonic tip speeds on UDFs. Not sure if this is in all flight regimes or only at high thrust.

The aerodynamics are indeed difficult. However, I don't think that is the big hurdle. UDFs have been unacceptably loud, for starters. The other issue is the energy level of a blade if it detaches. This would be much higher than the propeller equivalent.

I’m interested in this. Why does a UDF fan blade have more energy than a full-size propeller blade? I understand the UDF fan spins faster, but how much faster? Because a propeller blade is a much larger piece of kit.

kitplane01 wrote:

Starlionblue wrote:

LCDFlight wrote:

Thanks, not too lengthy, helped a lot.

It still leaves me wondering, why did it take >50 more years to perfect UDF technology (assuming we are near that today), after turboprop was essentially perfected 50 yrs ago? Is there a materials science issue, or was the aerodynamics of the scimitar-shaped UDF fan blades that difficult?

AFAIK the big difference is supersonic tip speeds on UDFs. Not sure if this is in all flight regimes or only at high thrust.

The aerodynamics are indeed difficult. However, I don't think that is the big hurdle. UDFs have been unacceptably loud, for starters. The other issue is the energy level of a blade if it detaches. This would be much higher than the propeller equivalent.

I’m interested in this. Why does a UDF fan blade have more energy than a full-size propeller blade? I understand the UDF fan spins faster, but how much faster? Because a propeller blade is a much larger piece of kit.

Higher RPM. That being said, it isn't that much higher. A turboprop prop maxes out under 2000rpm. A turbofan fan spins at around 3000rpm.

Certification requires a fan blade failure to be contained. That is not the case for a propeller blade. At a guess, due to the relative damage potential.

One difference is that even if all the blades on a propeller come off, it is unlikely that more than one will hit the fuselage. In the case of a UDF, more than one could hit something important. If it is wing mounted blades can easily hit the wing. If it is tail mounted, the empennage.

Would be interesting to see some actual energy figures on this.

What MD found out many years ago that while the UDF is great on gas, the noise and vibration were just horrendous.

^^^Untrue. The UDF was quieter than existing turbofans at the time, which reporters noted when they were able to view or ride test flights. Vibration was experienced but was described as minor.

McDonnell Unveils Plane With Revolutionary Engine - Los Angeles Times, 2/4/1988

A major concern early in the UDF program was noise levels both inside and outside the plane. But the prototype engine demonstrated Wednesday--called a “proof of concept” engine--has apparently cleared up such concerns.

Vibration Still a Problem

Bruce Gordon, GE’s UDF program manager, said the flight test program at Douglas has shown that the UDF is quieter by 5 decibels, a measurement of sound, than conventional fan jets. Orlowski, the Douglas engineer, said the company was able to eliminate more than 1,000 pounds of insulation and airframe structure because the sound levels were lower than expected.

Still, GE and Douglas have a job ahead in reducing vibration inside the passenger compartment. On the demonstration flight Wednesday, Douglas officials readily conceded that vibration levels are higher in some areas of the aircraft than airlines are willing to accept. But they believe that they are already on the way to a solution.

Devilfish wrote:

Is this finally that design's vindication and would it mean its eventual widespread application?

I hope so, but the timing still appears far away:

Manufacturers see no single ‘silver bullet’ to reach aviation’s CO2 reduction target - Flight International, 11/26/2020

The engine maker is preparing demonstrator flight tests with a modified, hybrid-powered turboprop in 2023-24 and is studying the possibility of distributed electrical propulsion systems on new aircraft designs beyond 2030. But for large aircraft, GE Aviation is looking at the open rotor to increase efficiency of gas turbine engines.

A next-generation open-rotor demonstrator is to be fight-tested in 2025-26, says Hegeman. Open-rotor designs – featuring counter-rotating fan blades outside a nacelle containing a gas turbine core – have been studied for decades, and GE tested such demonstrator engines during the late 1980s. But excessive noise and lower cruise speed versus conventional turbofans have been major obstacles for these designs in the past.

Hegeman says that new blade geometrics have facilitated a noise reduction below required standards with “sufficient margins”, and that the new engine designs would achieve speeds “around Mach 0.8”, comparable to current-generation narrowbody aircraft. If realised, open rotor engines could deliver 25% fuel burn savings, he says.

Also, for composite blades, wouldn't a bird strike or some other impact result in delamination instead of detachment?

Starlionblue wrote:

AFAIK the big difference is supersonic tip speeds on UDFs. Not sure if this is in all flight regimes or only at high thrust.

So, it's the blade tip speed that was causing the excessive noise level and not the counter rotation?


https://leehamnews.com/wp-content/uploa ... scaled.jpg

Interesting that the engine is hanging from the tailplane in this configuration.

LMP737 wrote:

What MD found out many years ago that while the UDF is great on gas, the noise and vibration were just horrendous.

I think Boeing came to pretty much the same conclusion.....

CowAnon wrote:

^^^Untrue. The UDF was quieter than existing turbofans at the time, which reporters noted when they were able to view or ride test flights. Vibration was experienced but was described as minor.

Invited media could be relied upon to provide a positive 'spin'...a more recent report was guardedly optimistic about it.....

https://www.imeche.org/news/news-articl ... ome-viable

Quote:

"Contra-rotating open rotor engines that are quieter than today’s turbofan engines could be developed in the near future, one of the top engineers at Rolls-Royce has predicted.

Speaking at a conference in London on disruptive aerospace technologies, organised by the IMechE, Parker said: 'We believe we can make an open rotor significantly quieter than today’s aircraft and significantly more efficient. We are talking about an open rotor that could be 13dB quieter than today’s A320, which I think should be quiet enough for most people.'"


I believe RR is part of the industry group working on the EU Clean Sky project.

CowAnon wrote:

I hope so, but the timing still appears far away:

Indeed...just imagine if the next generation ATR would look like this.....


https://leehamnews.com/wp-content/uploa ... roject.jpg

Assuming this technology matures to the level to which Safran aspires...

In the US at least: The flying public seems averse to seeing moving parts outside of the aircraft (hence one of the reasons 121 turboprop operations are all but dead for short hops, despite their economic advantages over RJs).

And back to Starlion's point: I would hazard a guess that there would be a massive amount of centripetal force on an unducted N1 propulsor, with a heck of a lot more blades shedding potential energy than a prop, should the worst case scenario happen.

You know, front-loading fans have those "ducts" that have to demonstrate containment should a blade or two fly off. Back-loading fans without such a mandated safety structure?

In my humble opinion: it's a solid idea, that is unlikely to go into ever go into mass production.

It's not the first time engines on tailplanes have been proposed - Antonov was going to build the An-180 in the 1990s with a similar configuration (but on the tailplane tips).


Antonov An-180 model by Kevin BIETRY `ღ´ ✈, on Flickr

That Avio Aero concept is based on GE's Unducted Single Fan (USF). I'm not a big supporter of it, since the aft fan doesn't rotate. It only varies its pitch to straighten out the airflow, so it does gain back the 10% or so of energy that was lost to swirl. But the USF still has to be a lot larger in diameter than a contra-rotating setup, for the same amount of thrust. Maybe the USF setup should be the standard configuration for large turboprops, though. Then Airbus/Europrop would've been able to avoid the handed versions of the TP400 on the A400M, and the Lockheed C-130J Super Hercules wouldn't have had the stall issues in the 1990s that were caused by the AE2100's large swept propellers.

The contra rotation was causing a problem in that the aft fan's blade tips were having to chop through the wake of the fore fan's blade tips. Reducing slightly the diameter of the aft fan solved that issue. Noise was also higher on engines like the Kuznetsov NK-12 because of the high blade loading. This was fixed by increasing the blade chord, which lowered the aspect ratio of the blades but added more blade surface area to spread the thrust around to.

Boeing/GE would've tackled noise/vibration and other issues on the UDF successfully, it's just that the additional fuel efficiency wasn't needed in a period of dropping fuel prices. It had trouble finding many buyers, but SAS really wanted the 7J7 (which Boeing practically let SAS design the fuselage cross-section for) and other larger or smaller propfan aircraft that it wanted Boeing to produce. If Boeing had been brave enough to launch with just SAS as its launch customer, history would've been different. Also, there were many other ultra-high bypass engines proposed during the 1980s that were ducted (like the PW ADP, MTU CRISP, IAE SuperFan, and RR ContraFan). If the main showstoppers were noise and vibration, some of those other offerings would've succeeded, but they didn't.

R-R unveils the future - Flight International, September 6, 1986
Denver to Seoul, nonstop - Forbes, May 29, 1989

I believe Rolls-Royce dropped out of the open rotor part of Clean Sky. It's just Safran involved now, I think.

Devilfish wrote:

Starlionblue wrote:

AFAIK the big difference is supersonic tip speeds on UDFs. Not sure if this is in all flight regimes or only at high thrust.

So, it's the blade tip speed that was causing the excessive noise level and not the counter rotation?


https://leehamnews.com/wp-content/uploa ... scaled.jpg

Interesting that the engine is hanging from the tailplane in this configuration.

LMP737 wrote:

What MD found out many years ago that while the UDF is great on gas, the noise and vibration were just horrendous.

I think Boeing came to pretty much the same conclusion.....

CowAnon wrote:

^^^Untrue. The UDF was quieter than existing turbofans at the time, which reporters noted when they were able to view or ride test flights. Vibration was experienced but was described as minor.

Invited media could be relied upon to provide a positive 'spin'...a more recent report was guardedly optimistic about it.....

https://www.imeche.org/news/news-articl ... ome-viable

Quote:

"Contra-rotating open rotor engines that are quieter than today’s turbofan engines could be developed in the near future, one of the top engineers at Rolls-Royce has predicted.

Speaking at a conference in London on disruptive aerospace technologies, organised by the IMechE, Parker said: 'We believe we can make an open rotor significantly quieter than today’s aircraft and significantly more efficient. We are talking about an open rotor that could be 13dB quieter than today’s A320, which I think should be quiet enough for most people.'"


I believe RR is part of the industry group working on the EU Clean Sky project.

CowAnon wrote:

I hope so, but the timing still appears far away:

Indeed...just imagine if the next generation ATR would look like this.....


https://leehamnews.com/wp-content/uploa ... roject.jpg

Motors hanging off the horizontal stab make great desktop models. Real planes not so much.

mikeinatlanta wrote:

Motors hanging off the horizontal stab make great desktop models. Real planes not so much.

Well the An-180 and Clean Sky setup doesn't look as strange as engines embedded in the vertical stabilizer, but ugliness didn't stop their presence on trijet airplanes.

This apparently was one of McDonnell Douglas's earlier concepts for a DC-10 derivative:


From “Props” for McDonnell Douglas: The Unducted Fan Projects That Never Took Off (avgeekery.com).

Higher RPM. That being said, it isn't that much higher. A turboprop prop maxes out under 2000rpm. A turbofan fan spins at around 3000rpm.

Not quite that much. The GE36 UDF had a fan blade tip speed of 780 feet per second (238 meters per second) at cruise and 850 fps (259 mps) at takeoff (UDF design report, p. 141).

• Since the length the blade tip travels in one revolution is the fan circumference, you have (blade tip distance traveled per one revolution) = 2 * pi * (fan radius) = pi * (fan diameter).
• Multiplying by rotational speed, you get (blade tip speed) = pi * (fan diameter) * (revolutions per minute, or rpm).
• Converting to seconds results in (blade tip speed in fps or mps) = pi * (fan diameter in ft or m) * (rpm) / (60 seconds per minute).
• Solving for rotational speed yields the equation (rpm) = (60 / pi) * (blade tip speed in fps or mps) / (fan diameter in ft or m).

The source for the blade tip speeds also lists a diameter of 11.67 feet for the GE36, so you get a calculated rotational speed of 1,277 rpm at cruise and 1,391 rpm at takeoff.

You need the cruise speed to calculate the helical (3-dimensional) speed, though. Page 17 of the UDF engine test report lists the maximum cruise speed at Mach 0.80 at an altitude of 35,000 feet. The Aviation Calculator website converts that into 778 fps, so the cruise speed is about the same as the rotational speed. Applying vector geometry means the helical speed is the square root of (780 fps ^ 2 + 778 fps ^ 2), or 1,102 fps (which is Mach 1.13 if you run that number through the website).

Also, page 163 of the design report has the heaviest blades weighing 22.5 pounds, if you want to do some kinetic energy calculations.

===
What set GE's UDF apart from the other open rotor proposals is that it was a gearless direct-drive setup. The contra-rotating propulsors were driven by free-power turbines (unattached to either high-pressure or low-pressure compressor), but one was attached to a regular shaft, while the other was attached to a rotating drum. The regular shaft had turbine blades representing one-half of each power-extraction stage attached to it, as expected. The rotating drum had a propulsor attached to its exterior wall, but its other half of turbine blades for each stage were attached to the interior wall of the drum. Since both halves of each turbine stage were rotating parts going in opposite directions, there wasn't a traditional non-rotational stator element as in normal turbines, and the effective rotational speed of the turbine was twice of the speed of the contra-rotating propulsors (so 2 * 1,277 = 2,554 rpm). Since the effective turbine rpm is fairly close to a comparable turbofan's fan rpm, the free-power turbine on the UDF can extract power using as few as (or almost as few as) the number of stages in the turbofan's power turbine.

Also, since it had a contra-rotating fan rotor setup as free-power turbines, the GE36 UDF was a three-shaft architecture (plus a rotating drum). When the contra-rotating propulsors are behind the engine core, like the GE36 was, the two compressor shafts are coaxial with each other, but then the propulsor shaft+drum don't have to be coaxial with the compressor shafts. (The drum propulsor might even be coplanar with one of the turbine stages.) GE claimed that the UDF could be designed with the propulsors in front of the engine core, like how turbofans and most turboprops are normally configured, but I think I read that they had trouble with that setup. The propulsor shaft would then have to be coaxial with the compressor shafts, and the drum would have to extend forward of the engine core to attach to its propulsor, but still extend aft of the core to attach to the turbine blades.

The more recent open rotor proposals have all included gearboxes instead of the 1980s UDF design. I don't think GE will need to reprise the direct-drive design, either. It has the 7,500-horsepower GE38/T408 turboshaft engine in service now. With a contra-rotating open rotor, the gearing requirement is supposedly two gearboxes supporting half of what the total horsepower requirement is. 15,000 horsepower might be enough to power the open rotor engines of a narrowbody plane family.

IMO, the more promising application of the contra-rotating direct-drive design might be for ducted engines. Rolls Royce (and probably Pratt & Whitney) have gearboxes powerful enough to handle widebody engines, but GE is a question mark. The GE9x produces 105,000 pounds of thrust using a 134-inch fan. If GE wanted to do an equivalent contra-rotating ducted fan with the same fan tip speed and the same number of turbine stages (ignoring the larger beneficial increase of bypass ratio), it would double the fan diameter, which would halve the rpm. But the effective turbine rpm would be the same. Somehow I doubt the airframers would be willing to fit a 268-inch fan diameter engine onto their planes, so the fan diameter increase would be smaller, which would still lower the rpm from the original GE9x, but the effective turbine rpm would now beat the GE9x. So the UDF design -- essentially a 2:1 ratio invisible gearbox -- could still potentially reduce the number of turbine stages in ducted engines, just like real gearboxes do.

CowAnon wrote:

Well the An-180 and Clean Sky setup doesn't look as strange as engines embedded in the vertical stabilizer, but ugliness didn't stop their presence on trijet airplanes.

Purely from an enthusiast's POV - whatever structural, electro-mechanical and maintenance compromises were done to put that number two engine on the vert stab, I find MDD's solution more elegant and sexier than LM's massive S-duct in its L-1011's tailend. Maybe if the noise and vibration problems could be overcome (geared design may increase weight) a slimmer, lower thrust engine like Safran's would fit in the tail better given that the counter-rotating fans are behind outside

CowAnon wrote:

This apparently was one of McDonnell Douglas's earlier concepts for a DC-10 derivative:

That is one amazing concept. ZeroE goals might make a configuration like that with much more advanced materials, wings and tips viable again. Tri-motors just have an enduring, endearing appeal.

CowAnon wrote:

mikeinatlanta wrote:

Motors hanging off the horizontal stab make great desktop models. Real planes not so much.

Well the An-180 and Clean Sky setup doesn't look as strange as engines embedded in the vertical stabilizer, but ugliness didn't stop their presence on trijet airplanes.

This apparently was one of McDonnell Douglas's earlier concepts for a DC-10 derivative:


From “Props” for McDonnell Douglas: The Unducted Fan Projects That Never Took Off (avgeekery.com).

That's actually pretty cool.

CowAnon wrote:

Higher RPM. That being said, it isn't that much higher. A turboprop prop maxes out under 2000rpm. A turbofan fan spins at around 3000rpm.

Not quite that much. The GE36 UDF had a fan blade tip speed of 780 feet per second (238 meters per second) at cruise and 850 fps (259 mps) at takeoff (UDF design report, p. 141).

[list][*]Since the length the blade tip travels in one revolution is the fan circumference, you have (blade tip distance traveled per one revolution) = 2 * pi * (fan radius) = pi * (fan diameter).

The blade tip path is not a circle but a spiral. ( vector addition: (c)ircumfenretial path + (p)rogress in the axis of rotation. = root( c² + p² )

WIederling wrote:

CowAnon wrote:

Higher RPM. That being said, it isn't that much higher. A turboprop prop maxes out under 2000rpm. A turbofan fan spins at around 3000rpm.

Not quite that much. The GE36 UDF had a fan blade tip speed of 780 feet per second (238 meters per second) at cruise and 850 fps (259 mps) at takeoff (UDF design report, p. 141).

• Since the length the blade tip travels in one revolution is the fan circumference, you have (blade tip distance traveled per one revolution) = 2 * pi * (fan radius) = pi * (fan diameter).

The blade tip path is not a circle but a spiral. ( vector addition: (c)ircumfenretial path + (p)rogress in the axis of rotation. = root( c² + p² )

Right, but for better or worse, it seems that the convention is just to call it the tip speed without having to qualify that it refers only to the rotational direction (circumferential path). The fan diameter, "tip speed", and rpm in the following article are slightly different but are close enough to confirm that my 1,277-rpm cruise calculation is about right:

GE developing unducted fan for 1990s transports (Aviation Week, 4/9/1984 -- rotate image clockwise to read)

The General Electric unducted fan demonstrator features two counterrotating propfans with eight swept blades each. Planned tip speed of the 12-ft.-dia. blades is 800 fps. Propfan rotational speed will be slightly less than 1,300 rpm. Reverse thrust is obtained by revesing the propfan blades.

Also, if you run the 780 fps tip speed through the aviation calculator at 35,000 cruise altitude, you get ... Mach 0.8017. If that were the total (spiral) speed, and the engine is designed for an airplane cruise speed of Mach 0.80, that leaves just a tiny contribution from rotational speed. So the propfans would turn too slowly to generate the thrust needed to counter the resulting drag.

So...the target is mid-2030s EIS...that is 45 years at least after GE announced the UDF for 1990.....

https://www.flightglobal.com/engines/cf ... 41.article

Wonder what new airplane launches on the horizon would benefit from this 20% better fuel burn?.....

Devilfish wrote:

So...the target is mid-2030s EIS...that is 45 years at least after GE announced the UDF for 1990.....

https://www.flightglobal.com/engines/cf ... 41.article

Wonder what new airplane launches on the horizon would benefit from this 20% better fuel burn?.....

Nearly 2 generations after the UDF was supposed to enter service ... aviation moves so quickly!

A few tidbits from the FlightGlobal article:

Fan diameter has been significantly reduced, to 144-156in (365-396cm) – in line with the external diameter of a current-generation single-aisle engine – enabling installation on a narrowbody-size aircraft.
...
The RISE demonstrator engine will be sized to deliver 30,000lb of thrust (133kN), says Dijoul, but “we have flexibility on that”. Cruise speeds equal to current levels “and even a little bit further” will be achievable with the open rotor design.

Although CFM is initially targeting the short- and medium-haul segment, the design “can be adapted to customer need”, even potentially scaling to widebody applications.

The UDF demonstrator in the late 1980s used 140-inch diameter contra-rotating fans to produce 25,000 pounds of thrust. I'm not sure how the RISE demonstrator will be able to produce more thrust with just the stator vanes and a slightly higher diameter without increasing noise, since the noise emission increases with blade loading. Maybe they increased the number of fan blades and widened the chords? But that was already done in the 1980s, so I don't know how much surface area can be added without choking off the airflow. Or maybe the stator vanes muffle the noise?

It's interesting that CFM is talking placing RISE on widebodies, since it's been widely assumed that open rotors were strictly for narrowbodies. The only times I've seen reference to widebodies were in that avgeekery article/DC-10 derivative photo (which I'm not completely convinced isn't a Photoshop hoax) and the 1984 Aviation Week article I linked to earlier.

Here's more reporting about the RISE:

• Aviation Week: CFM Unveils ‘Open Fan’ Demonstrator Plan For Next-Gen Engine
• Seattle Times's Dominic Gates, via Bakersfield.com: GE and Safran tout new ‘open rotor’ engine future for sustainable aviation
• Leeham News: CFM announces the RISE engine program
• Reuters: GE, Safran venture to develop radical new jet engine
• AINonline: GE, Safran Launch Green Open-rotor Engine Development

And the Civil Aviation forum thread on this topic: viewtopic.php?f=3&t=1461795

LCDFlight wrote:

Can somebody explain the difference between turboprop and UDF please? They appear to have different configuration, but I have no idea the advantages of UDF over turboprop.

It's a fairly hazy line, but a fan involves a "greater" number of blades, while a prop involves "lesser" number blades. I can't really define these terms, but let's say props have 3-6 blades, and fans have more than 6 blades. The UDF name came from taking the shroud off a ducted fan, or "unducting" it, so the fan retains multiple small curved blades, spaced close together. So remove the duct (or shroud) of a ducted fan engine, and you now have the unducted fan.
Interestingly, the newer props are also beginning to resemble fan blades, as for example the A400M turboprop has multiple blades that are curved, which are beginning to resemble the "unducted fan" but placed at the front of the engine.
The newest proposed unducted fan has the fans in the front. This design allows the engines to be mounted more conveniently on wings rather than the original UDF's being mounted on the rear of the fuselage. In my opinion, it is not a good idea to attach a UDF close to the fuselage, as the air turbulence from the fan blades cause vibrations on the fuselage, which will cause the aluminum skin to fatigue.
I'm very curious to see how the new incarnation of the UDF, with the fans forward, will work out in testing.

delta-flyer wrote:

LCDFlight wrote:

Can somebody explain the difference between turboprop and UDF please? They appear to have different configuration, but I have no idea the advantages of UDF over turboprop.

It's a fairly hazy line, but a fan involves a "greater" number of blades, while a prop involves "lesser" number blades. I can't really define these terms, but let's say props have 3-6 blades, and fans have more than 6 blades. The UDF name came from taking the shroud off a ducted fan, or "unducting" it, so the fan retains multiple small curved blades, spaced close together. So remove the duct (or shroud) of a ducted fan engine, and you now have the unducted fan.
Interestingly, the newer props are also beginning to resemble fan blades, as for example the A400M turboprop has multiple blades that are curved, which are beginning to resemble the "unducted fan" but placed at the front of the engine.
The newest proposed unducted fan has the fans in the front. This design allows the engines to be mounted more conveniently on wings rather than the original UDF's being mounted on the rear of the fuselage. In my opinion, it is not a good idea to attach a UDF close to the fuselage, as the air turbulence from the fan blades cause vibrations on the fuselage, which will cause the aluminum skin to fatigue.
I'm very curious to see how the new incarnation of the UDF, with the fans forward, will work out in testing.

Actually, fan blades are beginning to resemble turboprops. NASA's ATP program had swept blades -much more radically swept than even any current turbofan- in the 1970's. Turbofans only started to incorporate swept blades in the early 90's.

CFM's new open rotor is technically a turboprop as it has a gearbox, just like P&W's 578DX.

I've always thought using the number of blades as the differentiator between open rotor and turboprop made no sense.

As for the difference between propfan/UDF/open rotor nomenclature, I would just call them all turboprops as they are all evolutions the Advanced Turboprop Program. To put it another way, the open rotor concept came from the turboprop, not the turbofan. That being said one could fairly argue that to be a turboprop you have to have a gearbox, at least that is the definition used since the late 80's and GE's gearbox-less UDF.

^^^I think most people these days consider the presence of a contra-rotating propeller as what separates an open rotor from a turboprop. So the CFM RISE makes things even more confusing, since there's a 2nd propeller, but it doesn't rotate. I agree with your conclusion.

===
To double back to the 1980s UDF effort for a moment, here's what Jim Worsham (president of Douglas Aircraft at the time) said at a Financial Times conference on the eve of the 1988 Farnborough Air Show (where the MD-80 UDF demonstrator flew on daily flights):

I heard yesterday that it is noisy. It is not noisy, it sounds like a benevolent bumblebee. With our MD91 airplane we can be exempt from all noise requirements, take off and land in Washington National after curfew, fly anywhere we want to fly out of John Wayne exempt of noise requirements. I have heard from some other speakers that it is not here yet. It is here: go out to Farnborough and watch it and listen to it. I have heard it is not practical: it is practical. I have heard that it is slower than other airplanes: not true. We have flown our demonstrator airplane several times at .865 mach and in commercial service we will fly at exactly the same cruise speed as the MD-80. We have had more than 118 flights on our demonstrator airplane with a GE engine. I have personally flown over 20 times including back at the beginning when I had to wear a parachute. We have flown more than 214 hours, and at Farnborough we do not expect to demonstrate any big deal. We just want to demonstrate three things: it is quiet, it is just like another commercial transport...

That's a lot of bravado about something that supposedly didn't work. I can envision how maybe the cabin noise and vibration effects can be (deceptively) hidden through unrealistically excessive shielding, but I'm not sure how the demonstrator would be able to fake low community noise for the large and savvy audience at the air show. My opinion is that because of the dropping price of oil in the late 1980s, the new technology in the engine cost too much to recoup through fuel savings.

Good find with that quote!

744SPX wrote:

delta-flyer wrote:

LCDFlight wrote:

Can somebody explain the difference between turboprop and UDF please? They appear to have different configuration, but I have no idea the advantages of UDF over turboprop.

It's a fairly hazy line, but a fan involves a "greater" number of blades, while a prop involves "lesser" number blades. I can't really define these terms, but let's say props have 3-6 blades, and fans have more than 6 blades. The UDF name came from taking the shroud off a ducted fan, or "unducting" it, so the fan retains multiple small curved blades, spaced close together. So remove the duct (or shroud) of a ducted fan engine, and you now have the unducted fan.
Interestingly, the newer props are also beginning to resemble fan blades, as for example the A400M turboprop has multiple blades that are curved, which are beginning to resemble the "unducted fan" but placed at the front of the engine.
The newest proposed unducted fan has the fans in the front. This design allows the engines to be mounted more conveniently on wings rather than the original UDF's being mounted on the rear of the fuselage. In my opinion, it is not a good idea to attach a UDF close to the fuselage, as the air turbulence from the fan blades cause vibrations on the fuselage, which will cause the aluminum skin to fatigue.
I'm very curious to see how the new incarnation of the UDF, with the fans forward, will work out in testing.

Actually, fan blades are beginning to resemble turboprops. NASA's ATP program had swept blades -much more radically swept than even any current turbofan- in the 1970's. Turbofans only started to incorporate swept blades in the early 90's.

CFM's new open rotor is technically a turboprop as it has a gearbox, just like P&W's 578DX.

I've always thought using the number of blades as the differentiator between open rotor and turboprop made no sense.

As for the difference between propfan/UDF/open rotor nomenclature, I would just call them all turboprops as they are all evolutions the Advanced Turboprop Program. To put it another way, the open rotor concept came from the turboprop, not the turbofan. That being said one could fairly argue that to be a turboprop you have to have a gearbox, at least that is the definition used since the late 80's and GE's gearbox-less UDF.

Wouldn't it be more appropriate to differentiate the propeller engines (both turboprop and piston engine) from the turbofans / UDF by the fact that in a propeller engine, the power is entirely transferred to the propeller while in turbofans and UDF, there is still a fraction of the thrust generated by the core of the engine?

In turboprops there is always a fraction of the overall thrust generated by the core. Sometimes that fraction can be quite large, as in the 15,000 hp Pratt and Whitney T-57 turboprop which produced a whopping 5000 lbs core thrust.

A tweet from the author of the FlightGlobal article says CFM downplayed the role of the gearbox but noted that the details were proprietary. I interpret that as saying CFM doesn't have a good gearbox solution and has a lot of work ahead of it. If they really have improved the noise performance of the rotating part of the fan as much as CFM says, I would still consider trying to go with contra-rotating direct-drive turbines. Then you could reduce the fan diameter to maybe 110 to 120 inches. If you assume that roughly half of that diameter would go above the wing and half underneath, then it would fit on an existing 737 without any clearance issues. (The single-fan version of the RISE would already be mountable on an A320 under that assumption.)

The Leeham article talks about how the diameter reduction affected other aspects of engine performance:

The second advantage is the engine is less sensitive to higher speeds. The reduced diameter reduces the engine’s bypass ratio (in technical speak it increases the specific thrust). It puts the RISE engine’s high-speed characteristics closer to high bypass turbofans, which in turn makes the engine suitable for airliners that fly longer routes, up to our typical single-aisle routes.

CowAnon wrote:

A tweet from the author of the FlightGlobal article says CFM downplayed the role of the gearbox but noted that the details were proprietary. I interpret that as saying CFM doesn't have a good gearbox solution and has a lot of work ahead of it. If they really have improved the noise performance of the rotating part of the fan as much as CFM says, I would still consider trying to go with contra-rotating direct-drive turbines. Then you could reduce the fan diameter to maybe 110 to 120 inches. If you assume that roughly half of that diameter would go above the wing and half underneath, then it would fit on an existing 737 without any clearance issues. (The single-fan version of the RISE would already be mountable on an A320 under that assumption.)

The Leeham article talks about how the diameter reduction affected other aspects of engine performance:

The second advantage is the engine is less sensitive to higher speeds. The reduced diameter reduces the engine’s bypass ratio (in technical speak it increases the specific thrust). It puts the RISE engine’s high-speed characteristics closer to high bypass turbofans, which in turn makes the engine suitable for airliners that fly longer routes, up to our typical single-aisle routes.

Agreed.

AW published a follow-up with a few more details:

CFM Details Open-Fan Plan For Next-gen Engine (Aviation Week, 6/25/2021)

The RISE open fan will include a new compact high-pressure core to boost thermodynamic efficiency, as well as a recuperating system to preheat combustion air with waste heat from the exhaust. The demonstrator will also incorporate the use of advanced materials such as ceramic matrix composites in the hot section and resin-transfer-molded composite fan blades.

AFAIK, Western engine OEMs haven't attempted designing recuperators into their aircraft engines, so this should be interesting. The CFM LEAP-1A has a cruise SFC of 0.51 lb/lbf/hr (source: AIN 8/19/2019), so a 20% reduction from that gets you to 0.408. A NASA study from 2013 estimates a geared contrarotating propfan would have an SFC of 0.415, which is almost identical to the RISE's target, but without needing a recuperator. The lowered bypass ratio of the RISE (compared to "normal" open rotor efforts) must have a big negative effect if CFM feels it needs to also incorporate recuperation.

CowAnon wrote:

A few tidbits from the FlightGlobal article:

Fan diameter has been significantly reduced, to 144-156in (365-396cm) – in line with the external diameter of a current-generation single-aisle engine – enabling installation on a narrowbody-size aircraft.
...
The RISE demonstrator engine will be sized to deliver 30,000lb of thrust (133kN), says Dijoul, but “we have flexibility on that”. Cruise speeds equal to current levels “and even a little bit further” will be achievable with the open rotor design.
...

The UDF demonstrator in the late 1980s used 140-inch diameter contra-rotating fans to produce 25,000 pounds of thrust. I'm not sure how the RISE demonstrator will be able to produce more thrust with just the stator vanes and a slightly higher diameter without increasing noise, since the noise emission increases with blade loading. Maybe they increased the number of fan blades and widened the chords? But that was already done in the 1980s, so I don't know how much surface area can be added without choking off the airflow. Or maybe the stator vanes muffle the noise?

This old article might explain how to go from contra-rotation to single-rotation without a large increase in diameter or noise:

The Return of the Propeller: The demand for shorter, cheaper flights is driving new research into turboprops (Scientific American, November 2014)

“The computational power that’s avail- able now has really made the difference,” says Dowty’s Jonathan Chestney, noting that researchers can analyze data on an individual-blade basis. “It’s an exciting time for us,” he remarks. “We’re able to see much more detail, like a scientist who just got a microscope for the first time.”

Dowty engineers are currently exploring two novel spacing ideas for eight- blade propellers. One positions the blades unequally around the circumference of the propeller hub; the other staggers the blades axially, with four blades mounted farther forward on the hub than the others. These spacing schemes break up and change the audible frequencies created in flight. Dowty is in the midst of testing the corresponding cabin sounds on volunteers to see which ones they prefer.
...
Novel designs are not far from the tarmac. Says Dowty’s Chestney, “We expect to see some key players going public with new aircraft designs in the next couple of years.”

More pictures and info on the Dowty website.

The Aviation Week archive has a side view of the staggered blade propeller. It looks like two rows of rotors, just like with the old contra-rotating open rotor concept. But those rows co-rotate instead of contra-rotate because they're on the same fan hub, so there's no sound issue caused by fan blades passing by each other in opposite directions. Also, the back row of rotors appear shorter than in the front row, just like how the aft rotor on contra-rotating GE36 engine was shorter than the fore rotor to avoid cutting (loudly) through the tip vortexes of the fore rotor's blade tips. With these two noise-reduction fixes in place, maybe the designers had room to double the blade loading without causing an overall increase in decibels.

Devilfish wrote:

So...the target is mid-2030s EIS...that is 45 years at least after GE announced the UDF for 1990.....

https://www.flightglobal.com/engines/cf ... 41.article

Wonder what new airplane launches on the horizon would benefit from this 20% better fuel burn?.....

The Boeing 737MAX-Mark ii Series 4.

In March of 2021, Boeing purchased two former DL MD-90s from the boneyard in BYH and flew them to VCV (N962DN & N908DA).

It’s plausible these aircraft were acquired to be used as testbeds. Might an updated UDF concept be a conceivable purpose for these aircraft?

https://www.planespotters.net/airframe/ ... ing/el05ve

https://www.planespotters.net/airframe/ ... bcc/epdwke

DeltaMD95 wrote:

In March of 2021, Boeing purchased two former DL MD-90s from the boneyard in BYH and flew them to VCV (N962DN & N908DA).

It’s plausible these aircraft were acquired to be used as testbeds. Might an updated UDF concept be a conceivable purpose for these aircraft?

If that's what Boeing is going for, then perhaps they're going for a quicker EIS. The CFM RISE is designed for wing mounting, and won't be coming out before 2035 at the earliest. I'd also guess that Pratt & Whitney would be providing the engine, although they seem to have been mostly against open rotor for most of the concept's existence. Maybe PW could just slap a "conventional" contra-rotating fan unit with updated gearbox onto the core of its PW1000G family of engines.

Scott Hamilton is calling for regulation to get the industry to start using open rotors:

https://leehamnews.com/2021/10/11/pontifications-biofuels-hydrogen-batteries-are-nice-but-far-in-the-future-solution-exists-now/

He's mostly focusing on short-haul flights up to 2,500 miles, but there's an old ICAS-published study from 1982 that shows the fuel reductions increasing for flights beyond 2,000 nautical miles. It pegs the fuel savings for a long-haul 6,000-nmi flight at 20 percent for Mach 0.8, 30% for M0.7, and over 40% for M0.6.

http://www.icas.org/ICAS_ARCHIVE/ICAS1982/ICAS-82-4.5.2.pdf#page=4 (zoom in on figure 5)

CowAnon wrote:

Scott Hamilton is calling for regulation to get the industry to start using open rotors:

https://leehamnews.com/2021/10/11/pontifications-biofuels-hydrogen-batteries-are-nice-but-far-in-the-future-solution-exists-now/

He's mostly focusing on short-haul flights up to 2,500 miles, but there's an old ICAS-published study from 1982 that shows the fuel reductions increasing for flights beyond 2,000 nautical miles. It pegs the fuel savings for a long-haul 6,000-nmi flight at 20 percent for Mach 0.8, 30% for M0.7, and over 40% for M0.6.

http://www.icas.org/ICAS_ARCHIVE/ICAS1982/ICAS-82-4.5.2.pdf#page=4 (zoom in on figure 5)

I don't see it happening any other way. Today's airline executives are controlled by the low cost competition. They simply cannot afford to invest in new technology. They can barely pay for rehashed old designs. And post Covid, their balance sheets are a wreck.

WIederling wrote:

CowAnon wrote:

Higher RPM. That being said, it isn't that much higher. A turboprop prop maxes out under 2000rpm. A turbofan fan spins at around 3000rpm.

Not quite that much. The GE36 UDF had a fan blade tip speed of 780 feet per second (238 meters per second) at cruise and 850 fps (259 mps) at takeoff (UDF design report, p. 141).

[list][*]Since the length the blade tip travels in one revolution is the fan circumference, you have (blade tip distance traveled per one revolution) = 2 * pi * (fan radius) = pi * (fan diameter).

The blade tip path is not a circle but a spiral. ( vector addition: (c)ircumfenretial path + (p)rogress in the axis of rotation. = root( c² + p² )

For those in the plane the sound issue (supersonic tips) only needs to take account of the speed of the rotation as they have the same forward speed.

I'd imagine on the ground the relative motion of the tip to the observer would give a smaller and smaller zone of "Booms" the closer to M1 that the tips became. Oh the joys of rotating sound sources!

Fred

On the topic of tip noise, has there even been experiments with "propeller sharklets"?

The idea being that you could get the same thrust from a smaller diameter and thereby lowering the supersonic tip problem. I could also imagine the turbulence from the tip becoming less strong.

Problems I could imagine is the asymetric mass and profile causing vibrations.

CowAnon wrote:

This apparently was one of McDonnell Douglas's earlier concepts for a DC-10 derivative:

Boeing also evidently looked in a UDF version of the 747-500:




From https://www.youtube.com/watch?v=KybOWKxkObw

Boeing also evidently looked in a UDF version of the 747-500:



From https://www.youtube.com/watch?v=KybOWKxkObw

That's a misleading video -- there wasn't an unshrouded fan proposed for the 747-500. (See fredocarroll's comment on the YouTube video page.) There's confusion between the generic term "unducted fan" and the "GE Unducted Fan (TM)". The latter was a General Electric trademarked term that covered not just the unshrouded nature of the GE36 engine, but the contra-rotating direct-drive technology behind it, which could be applied to both shrouded and unshrouded fans. Engines for higher-speed widebody airliners using contra-rotating gearless fans might have looked like this:

https://archive.org/details/sim_airline ... ew=theater (rendering of an airliner with four RB529 Contrafan engines, which were proposed by Rolls-Royce in 1986)

Very cool, but wouldn't moving the engines aft of the wing require moving the wing further aft to accommodate the new CG?

Probably, though I wonder if having the engines attached to the trailing edges of the wings would've been a bigger issue.

For those in the plane the sound issue (supersonic tips) only needs to take account of the speed of the rotation as they have the same forward speed.

I'd imagine on the ground the relative motion of the tip to the observer would give a smaller and smaller zone of "Booms" the closer to M1 that the tips became. Oh the joys of rotating sound sources!

I'd like to understand why people think noise is a problem due to supersonic flow around the exposed propellers (despite testimonials of the GE36's quietness on 727/MD80 flying testbeds), but also say it isn't a problem when there's still supersonic flow over the wings (supercritical or not) of typical passenger airliners?

On the topic of tip noise, has there even been experiments with "propeller sharklets"?

The idea being that you could get the same thrust from a smaller diameter and thereby lowering the supersonic tip problem. I could also imagine the turbulence from the tip becoming less strong.

Problems I could imagine is the asymetric mass and profile causing vibrations.

My guess is if you do that with variable pitch propellers, the sharklets would be highly suboptimal at certain pitch angles.

SteelChair wrote:

I don't see it happening any other way. Today's airline executives are controlled by the low cost competition. They simply cannot afford to invest in new technology. They can barely pay for rehashed old designs. And post Covid, their balance sheets are a wreck.

I'm assuming any proposed regulation wouldn't apply to airplanes that currently are in service.

The CFM RISE will have twelve "3-D" CFRP fan blades and a bypass ratio of 75, thanks to a smaller core.

Commercial-Engine OEMs Detail Emissions Work (AIN, 11/10/2021)

Travis Harper, GE Aviation’s program manager for RISE, said that—unlike SAE’s 2017 two-rotor-stage design—CFM’s Open Fan design contemplates only a single fan stage. Today, CFM envisions 12 fan blades for the Open Fan stage, aerodynamically three-dimensional in design and made of woven carbon-fiber composite material—a technology developed and perfected by SAE.

The blades will cover a much greater diameter than those in CFM’s latest Leap production turbofan engines. But because no fan case will contain the blades, the overall fan diameter could match the overall outer diameter of the Leap nacelle, according to Harper. With such large blades—and also making use of a smaller core than typically found in today’s engines—CFM expects its Open Fan R&D design to produce an eye-opening bypass ratio of 75:1, nearly an order of magnitude greater than the 11:1 achieved by the Leap-1A and other latest-generation turbofans.

Engine makers GE and P&W race to boost efficiency as net-zero carbon goal looms (FlightGlobal, 11/18/2021)

At the same time, the RISE team aims to shrink the size of its next narrowbody engine’s core – to be the same size as cores in today’s business jet engines, says Ali. The smaller core will run hotter and at greater air pressures, yielding improved thermal efficiency. The design can also allow for increased bypass ratios.

The smaller core increases the open fan BPR, but doesn't it also increase the optimal BPR of a turbofan? I've heard a turbofan could get to a BPR of 15-18 before drawbacks start to outweigh the benefits, but maybe that can go higher if the core airflow can be decreased without losing thrust.