GE and NASA to Revive Open Rotor Jet Engines

[Chris]

Back in the mid 1980s, GE and NASA began testing on a new propulsion system that used counter rotating fan assemblies in an attempt to boost efficiency. The testing ultimately led to the design of the GE36 experimental aircraft engine, but was then set aside to explore other options. Now, GE and NASA are planning on revisiting the design and adding some new technology to the mix.



The design is ultimately a hybrid between a Turbofan and Turboprop, which was aptly named a Propfan or Unducted Fan. The process begins by using a turbofan engine with the fans relocated to the outside of the engine housing. As the exhaust from the turbofan engine is forced out, they pass over a turbine which is connected to a prop on the exterior, while the other prop is driven by the turbofan.

Both sets of props have variable pitch blades that featured a dramatic curve, similar to a scimitar, which would allow the engine to reach speeds close to Mach .75. Along with the high flight speeds, the engine also showed a 30% to 35% decrease in fuel consumption. The only real drawback to using a Propfan engine was the noise that was present inside the cabin.  This, when combined with a shifting market, ultimately led to the design being shelved.

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The last time I flew in a turboprop, it was a Saab commuter jet -- I got stuck sitting behind the wing and the engine.  It was so flippin' loud, my head was numb by the end of the flight.  Probably the first time I've ever been scared on a plane... it was nerve-wracking flying on that little thing.  I've flown on a couple small commuter flights (buses with wings) since then, and Cessna-type float planes that weren't nearly as bad.

I suppose guilty liberals will gladly deafen themselves to save the environment.  Me, I'd have to fly one to form an opinion, but it seems like an interesting concept if it saves money.

[Chris_]

The last time I flew in a turboprop, it was a Saab commuter jet -- I got stuck sitting behind the wing and the engine.  It was so flippin' loud, my head was numb by the end of the flight.  Probably the first time I've ever been scared on a plane... it was nerve-wracking flying on that little thing.  I've flown on a couple small commuter flights (buses with wings) since then, and Cessna-type float planes that weren't nearly as bad.

I suppose guilty liberals will gladly deafen themselves to save the environment.  Me, I'd have to fly one to form an opinion, but it seems like an interesting concept if it saves money.

NeoCommies will scream about the "carbon emissions", and they will scream about the "noise pollution"; any way you look at it they're going to scream at you.  Unless you're willing to forego everything human inginuity has created over the past 6,000 years and become a Luddite like them (well, not like them, because the evils of technology are only evil in your hands, not theirs) they're going to scream at you no matter what you do.  So my philosophy is that if they're going to scream, give 'em a damned good reason to do so.

[thundley4]

It might be an option on big cargo jets and the cockpit would only need to be soundproofed.

[docstew]

I wouldn't expect it to be much louder than a C-130 (ok, not the quietest bird ever), until it got close to top speed and the fan tips started pushing against the sound barrier.

[NHSparky]

So my philosophy is that if they're going to scream, give 'em a damned good reason to do so.

Throw them into plastic shredders?

[Wineslob]

By the looks of that thing, geese need to be worried.....................

[DumbAss Tanker]

Noise was a huge factor with the previous experimental engine, the high-subsonic tip speeds generate quite a roar and as the Tu95 more than amply demonstrates, the clashing soundwaves from contrarotation seem to really kick it in high gear...though you would think that, at least from some angles and at certain speed/pitch combinations, it should be possible to engineer it so the the sound waves cancel out.

[Chris_]

Throw them into plastic shredders?

Usually, my thoughts turn to wood chippers, but I think you have the right idea.

[rich_t]

That is one strange looking engine.  I'm not sure I'd want to ride in a jet with an engine that looks that way.

[Chris_]

Noise was a huge factor with the previous experimental engine, the high-subsonic tip speeds generate quite a roar and as the Tu95 more than amply demonstrates, the clashing soundwaves from contrarotation seem to really kick it in high gear...though you would think that, at least from some angles and at certain speed/pitch combinations, it should be possible to engineer it so the the sound waves cancel out.

If I had the time I could give you about a dozen good sound engineering reasons why this is a "really bad" idea from a commercial aircraft point of view.........efficiency notwithstanding, it would never actually be practical without a "shroud" on the exposed blades to prevent low-speed "blade stalls", and I agree, the noise would be astronomical......the additional weight of the extra sound deadening material to make parts of the cabin livable would cancel out the efficiency gains......

Back in the '50s Lockheed experimented with a VTOL aircraft with a turbine mounted in the nose and counter-rotating props.......the noise level in the cockpit approached 180 dB, and other than a bunch of other failings, it was unbearable to attempt to fly.

The blade shape is similiar to a Russian design for one class of their Nuke subs, also with counter rotation, and they could never solve the noise problem either, even at very low RPM....

doc

[Chris_]

That is one strange looking engine.  I'm not sure I'd want to ride in a jet with an engine that looks that way.

That engine was the "missing link" that connects the modern, high bypass turbofan with the earlier generations of turbojet engines that were powering the jetliners rolling off the line in the '60s and '70s.  

Along with the high flight speeds, the engine also showed a 30% to 35% decrease in fuel consumption.

Problem with this statement is that it isn't comparing apples to apples: the 30%-35% decrease in fuel consumption was as compared to the turbojet that the experimental engine was based on, not as compared to the modern turbofan of equivilent power output.

Back in the late '70s/early '80s - with the effects of the oil embargo still fresh in their minds - Boeing actually had a prototype constructed using that experimental engine as the power source (designated the 7J7) based upon the old 707/727 airframe.  You don't see many of those in the skies, because the high bypass turbofan  came along as a better solution: equivilent fuel efficiency, and less noisy besides.

[DumbAss Tanker]

If I had the time I could give you about a dozen good sound engineering reasons why this is a "really bad" idea from a commercial aircraft point of view.........efficiency notwithstanding, it would never actually be practical without a "shroud" on the exposed blades to prevent low-speed "blade stalls", and I agree, the noise would be astronomical......the additional weight of the extra sound deadening material to make parts of the cabin livable would cancel out the efficiency gains......

Back in the '50s Lockheed experimented with a VTOL aircraft with a turbine mounted in the nose and counter-rotating props.......the noise level in the cockpit approached 180 dB, and other than a bunch of other failings, it was unbearable to attempt to fly.

The blade shape is similiar to a Russian design for one class of their Nuke subs, also with counter rotation, and they could never solve the noise problem either, even at very low RPM....

doc

As far as noisemakers, probably none of those get to the same level of the XF-84H, a Thunderstreak fitted with a 5,000 horsepower turboprop and a supersonic(!) prop.

[Chris_]

That engine was the "missing link" that connects the modern, high bypass turbofan with the earlier generations of turbojet engines that were powering the jetliners rolling off the line in the '60s and '70s.  

Problem with this statement is that it isn't comparing apples to apples: the 30%-35% decrease in fuel consumption was as compared to the turbojet that the experimental engine was based on, not as compared to the modern turbofan of equivilent power output.

Back in the late '70s/early '80s - with the effects of the oil embargo still fresh in their minds - Boeing actually had a prototype constructed using that experimental engine as the power source (designated the 7J7) based upon the old 707/727 airframe.  You don't see many of those in the skies, because the high bypass turbofan  came along as a better solution: equivilent fuel efficiency, and less noisy besides.

Correct, I suspect that is why you see NASA involved in the project.......any competent aircraft designer working for a private firm has already discounted this as not feasable considering state of the art.  

A government agency however is perfectly capable of "reinventing the wheel".....

doc

[AllosaursRus]

A government agency however is perfectly capable of "reinventing the wheel".....

Boy, ain't that the frikkin' truth! If the guvmnt can't find a way to waste money, nobody can!