open ultra bypass

ge testing with boeing


it has some pretty lofty ideas/sales pitch being shown off and explained at the test site

a reply to: penroc3

So how come we don’t see these being used? The video is old. And it states the engine would be released in 1992.

a reply to: KKLOCO

It didn't perform quite up to expectations, and didn't meet noise requirements IIRC.

There's still a lot of debate over noise. Under the latest noise regulations, you'd save about 9% over a conventional turbofan to meet the requirements. Snecma claims they can make an engine that has the same noise levels as their CFM56 engine.

So there are a couple basic ways to create thrust for conventional jet air planes.

Thrust is Newtonian. You throw mass out the back going faster than the air is already moving, and an equal opposite reaction occurs. The mass in a jet engine is air (primarily. Occasionally they inject water, there's also fuel in there, but mostly it's just a lot of air). So really at it's base we're looking at F=m*a. There are a lot of variables affecting the mass to account for, and airspeed at intake to account for, but we're accelerating a mass of air out the back.

So there's a couple ways to produce X-amount of thrust.

One way is to take a given volume of air, run it through the compressor and combustion chamber, and throw it out the back going really, really fast. That is generally inefficient. That's the root of a simple turbojet engine. Speed up the air really fast, kick it out the back.

Another way to get the same amount of thrust is to use a bigger fan or propellers to move a lot more air. This takes energy away from the core, and sends it down a shaft moving the fan/propellers, so we are moving more airmass, but accelerating it more slowly to make M*A still equal the first method. This is generally more efficient, but has losses to the shaft, reducing gears, etc. Also, as airspeed increases in your inlet, thrust goes down quickly, because the air is not being accelerated as much when it leaves. Since the acceleration is smaller, the thrust drops off at speed more quickly than the first method.

A propfan is basically a turbofan like the second method. But it moves even more air with larger unducted fans at a lower acceleration to produce X-ampunts of thrust. At higher air speeds, it has to work harder to accelerate it more.


Tangential speed is quite high for the fan tips, so there is a practical limit to fan/blade diameter that is coupled with rpms. If I double my propfan's diameter, I double the tangential speed of the tip at the same engine RPM. Since the blades are losing efficiency at higher speeds because of drag and compressibility, this rapidly becomes a game of diminishing returns. Also, as the vehicles speed through the air increases, it needs to accelerate the air faster than that flow. If the exit speed of my exhaust efflux is 700mph, but I am traveling at 650mph, I am only accelerating the air 50mph. And my prop tips are meeting the air even faster than they would on the ground at zero air speed and the same RPM.

Related to tip speed is the noise level. Quite high in the cabin for a propfan.

The biggest issue facing propfans right now is the efficiency is tied to a generally lower speed than a turbofan and that it is more mechanically complex. Also the fact that everytime someone seemingly makes a break through on propfan efficiency, they realize turbofans are increasing efficiency at the same or higher rate.
The airlines wouldn't care about the speed as much if the efficiency was high enough. If my engine is 25% more efficient in flight, but my flight is ten percent longer because of lower flight speeds to get that efficiency, I'm not really getting 25% efficiency over the first aircraft. And if my engines need more frequent service times, that is more time my plane is sitting on the ground doing nothing, and if my airplane is uncomfortably noisy, passengers aren't super happy. You can shield noise from the ground, but it's hard to hide it from the cabin.

Forgive my ignorance but isn't noise wasted energy so if something generates more noise it is less efficient. See the latest iteration of F1 cars as an example.

Not necessarily.

Most of the noise here is generated by fast moving air meeting slower moving air. To reduce the noise, we would have to reduce the efflux speed by accelerating it less. That makes less noise, but lowers thrust.

A conventional turbofan moves some air really fast through the core, and more air a little less fast through the bypass. This helps "sheath" the very high velocity efflux, letting it mix and slow before getting to the outside air so it makes less noise. And at low speeds is typically more efficient.

If you have a muffler on an F1 car (or any other) you can reduce the high-velocity exhaust from your engine and reduce noise, but you are creating back-pressure on the engine which reduces your airflow through the entire system starting at the efflux all the way up to the intake. That makes your engine less efficient, but less noisy.

a reply to: RadioRobert

I think it sounds cool with the doppler shift.

At the end of the video they referred to it's 'special acoustic properties' and i take that to mean that it is loud.

It would be interesting to see a modern remake of these with the better modeling we can do now, the shape and other aspects of the blades might be able to be modified to lessen the noise. That being said and prop aircraft is going to be loud.

I forgot what helicopter's blades move almost at the speed of sound at the outermost tip of the blade, but modern helicopters apply some sort of voodoo to make them quieter.




with better computers they can make the blades very very efficient

It would be interesting to see a modern remake of these with the better modeling we can do now,

Someone somewhere is always working on this. It just can't catch up to turbofan development which is also moving forward. The same advances in modelling and material sciences which help blade design for propfan are applicable to the less mechanically complex turbofans. We may see a break through one day, but the fact noone is using them despite it being 30 some year old or more technology is not inexplicable. The advantages don't outweigh the drawbacks.

Propfans/UDF are effectively a dead end for commercial airliner use. They may have a place as an alternative to turboprops but even there I'm not convinced. The problems with unshrouded fan systems is you no longer have a method of directing or masking noise that would normally be attenuated by the cold stream duct in a turbofan. Because of the design of the propfan rotors you need to keep rotation speed down to avoid reaching transonic/supersonic tip speeds which create both noise and reduce airfoil efficiency. So by default you need to have a lower cruise speed of at least 10% over current designs. Then you have to be able to build incredibly strong blades to avoid the obvious effects of blade failure. A bad bird strike could result in a blade or blades detaching and penetrating the fuselage. In a conventional turbofan you have the nacelle to build in blade failure containment rings a UDF has none of that. Lastly and partly to do with the previous point, propfans limit your placement options. You either need a lot of ground clearance so a high wing design works better, or you need to mount them on the empennage a la DC-9/MD90/717 style. Neither of which is ideal for the kind of efficiency and operating characteristics that would make a compelling case to build one. In the end whilst researches are no doubt occasionally revisiting this idea there simply hasn't been an efficiency gain or operating characteristic compelling enough to build a business case on.

a reply to: thebozeian

They may have a place as an alternative to turboprops

They basically are a turboprop --one optimized for higher RPM's and air speed, so shorter blade diameter. They invented a new word to avoid using the word "propellers" and "turboprop". It's basically marketing. They are still making up new words.

Blade failure is essentially a non-factor. How many total times has anyone thrown a propeller in the last forty years? If it happens it doesn't happen in a well-maintained commercial service. Much greater of conventional fan failure. Compressor blade failure is a shared concern across the board by all of them.

The rest is spot on. Higher noise, greater complexity with gearboxes driving shafts, speed limitations, noise and vibration limits. Carrier haven't see enough benefit in economy to outweigh the negatives.

originally posted by: RadioRobertThe rest is spot on. Higher noise, greater complexity with gearboxes driving shafts, speed limitations, noise and vibration limits. Carrier haven't see enough benefit in economy to outweigh the negatives.

Modern turbofans are having gearboxes too and have to deal with vibrations. So the only benefits of turbofans become speed and noise.

And then there is the A400M with 4 turboprops and a cruise speed of 422 kn, which is quite impressive.

a reply to: moebius

Yes, some newer fans have a reduction of about 3:1. Most simply accept the losses that come from the fan running too high a RPM and the fan and turbines share the same shaft/RPM, and some use a larger, slower turbine section which is less efficient than otherwise as a compromise.

But even for the fans using a gearbox, they aren't bringing the RPM down as much as a turboprop or propfan, so the gearbox is not taking the same type of strain.The gearbox in a turboprop is more mechanically complex, has to be more robust, is bigger, heavier, and fails at a higher rate because of the torque applied to the gears and shaft.

Turboprops typically run ~15:1 reduction (compared to the 3:1 in the fans which use them) because a prop is more susceptible to losses at higher tangential speeds, and almost all thrust is provided by the prop, very little from the turbine core flow. The prop blades have a much higher diameter, and so higher tip speeds for the same RPM.

So the propfan represents a middle ground. They have the same options as the turbofan, but because of larger prop diameter, need slower RPM than most fans, because they are taking energy from the turbine flow to power the larger heavier props, so are getting less thrust from the core and more from the prop(fan). They can run an intermediate reduction lower than a typical turboprop because the blades are shorter and optimized for higher RPM. Or they can run a much slower, less efficient core. Or both. Both represent compromises in efficiency.

ETA: You mentioned the Atlas; look up A400 reduction box and read about all the problems they are having, and that's why there isn't a great desire for bigger, larger turboprops with carriers. A propfan with 3:1-6:1 reduction might get traction, but the bigger your engine, the bigger you need the propellers to take advantage. That means slower RPM needed for the propellers, and more reduction. They'll need to find a sweetspot.