P&W Geared Turbofan?

I'm interested to hear what our resident experts might have to say about the efficiency claims.

Also.... What's to stop me from putting a cowling around the prop on a C-130 and calling it a gear driven turbofan? Seems like it's mostly a matter of the number of blades in the fan.

 

Not exactly reliable yet Bombardier had to ground there CS100 prototype a few months as well as Airbus has there A320neo with the GTF grounded due to issues with it.

 

Everything is a turbofan nowadays. They're either shrouded or not shrouded.

 

Fans have high solidity and generate positive pressure (pressure ratios of 1.2 to 1.7 are common) in the duct behind the fan, propellers are wing sections and create forward lift, but the overall pressure isn't increased in the volume behind the prop. Fan blades run at higher speeds and the tips are supersonic, props aren't.

The airfoil sections are fundamentally different between the two.

The idea of gearing a fan has been around for a long time. With turbo-machinery there is a proper tip speed and area relationship in the turbine that you would like to hold for the best efficiency. If you bring the tip out the blade gets shorter (for the same flow area), and the tip losses as a percentage of flow get higher.

If you pull the tip diameter in, for the same shaft speed, the blade runs slower and you can't do as much work with that stage. In short, you would like a higher tip speed with a longer blade in the turbine than you can get driving a big fan on the same shaft.

To make up for that in conventional turbofans you run the fan at its correct speed and the turbine runs at that same speed and you design blades that have a good efficiency in the early stages. That is you bring down the diameter of the turbine and put in efficient blades, but that reduces the tip speed and limits the amount of work you can do in each stage. Since that tip speed is much lower than is optimum, you have to put in a lot of stages to get the work out of it. The aerodynamic speed/load relationship isn't good, and therefore the efficiency isn't as good as you would get running it at the correct speed. That said the efficiencies of the turbine stages in a conventional turbofan are pretty high, on the order of 90% typically, so there aren't a lot of losses in the system.

What a geared fan lets you do is run the turbine at the right speed and then you can design a more efficient turbine to drive your fan.

The downside is that there are gearbox efficiencies that you have to take into consideration and the weight and reliability of the gear train. The amount of power in the gearbox is huge, in the PW engine it's about 25 mW, or about 33,500 hp... That's a pretty big and robust set of gears no matter how you cut it. They claim a 1% loss in the gearing, which sounds pretty good, but that's still about 350 hp worth of oil cooling that they need to do to keep it running.

Given that there isn't a big loss in the turbine of a conventional turbofan where does the higher fuel efficiency (like 15% better) coming from? The short answer is that it's coming from a bigger fan that you can run with the gearing. As we've seen a constant march of bigger and bigger fans and higher bypass ratios since the first high bypass ratio engines were developed in the early 70's, that's where the big gains are made.

The geared fan is just the next logical step in that progression and as the fans got bigger and slower the turbines would have had to have added more and more stages to do the work. At some point (and we are there), the gearbox weight starts to pay off against the bigger turbine with more and more stages . Now you have a smaller higher speed turbine that is optimized and you gear the fan and let each end of the engine run at the speed that it wants to run.

 

Which is why Rolls uses a 3-shaft design.

 

Yes, pretty much the same thing, keeping the IP compressor speed and the speed of the turbines driving it higher as opposed to putting them on the fan spool and needing more stages to drive it and fewer stages in the compressor.

Not magic and all of the engine companies have to follow the same rules that are basically just physics...

 

I thought it was all ball bearings nowadays...???

 

1989 Geared turbofan. These guys have worked on this for a long time!

. . . a 25% to 45% improvement in fuel efficiency . . . " (!) I hope so!

https://youtu.be/zxVAaIsfPIY

 

The propfans were a NASA sponsored effort. Pratt and Allison collaborated on their UnDuctedFan (UDF). Allison actually did the gearbox on that program and had the lead in high power gearbox technology at the time. The UDF doesn't run as fast as a conventional fan, it was called the "propfan" and was about half way between a fan and a prop and the tips were supersonic.

The GE engine didn't have gearing, but by "rotating the cases" they basically doubled the relative velocities in the turbines and that let them get by with fewer stages than if they had a conventional system.

The propfans were both loud, as I said the tips were supersonic and that was noisy to say the least. Better fuel efficiency than the huge fans or even the geared fans, but noise was always going to be the big issue.

 

These windows are filthy.

 

Very informative post, thanks for taking the time to type it. Just curious what is a typical gear ratio in these engines and what is the gear box like. I would guess it would be some kind of planetary system.

 

I don't know what the new PW geared fan gear ratio is.

The gear ratio on the 578DX was about 13:1 IIRC and that left you with a prop speed of just over 1,000 rpm, which is pretty fast for a prop that big.

 

There is all kinds of wild stuff coming on turbine engines, some driven by new requirements for a 6th generation fighter engine. Even though low bypass engines, some of the technology will be applicable to high bypass engines.

AFRL Achieves First in Advanced Engine Technology