The gearbox in the GTF is one thing that, so far, has been pretty much bulletproof. The problems have been coming from other new tech Pratt used in the engines.
It seems to me that to get similar efficiencies to the GTF, the LEAP has to push the core a lot harder. They can because they are really good at that but it seems that they are getting close to running out of headroom on with the direct drive low pressure spool. Being able to spin the low pressure spool faster means fewer compressor and turbine stages, which means less weight and increased efficiency.
Coupling a GTF with LEAP tech seems a logical way to go.
GE has tons of experience with gears on their helicopter engines and they have been going to court to overturn Pratt and RR GTF patents so they are definitely looking at the possibility.
I think GTF engines and folding wings will grace the 797.
I suspect that might have something to do with physical engine size and the size of the planes the engines go on. Bigger engines have more size and weight margin for more compressor stages.
First, what the engine manufacturers have done with computational fluid dynamics is nothing short of incredible. They are doing more with less when it comes to the compressors and the turbines. And they aren't finished yet.
Second, once upon a time, size was a limitation. But now the engine manufacturers get more work out of a given engine stage, so size is less. But on the size front, the Pratt GTF for the A320NEO is over 6000 lb and has an 81" fan. The CF6 for the 767, 747, DC10 has an 86" fan. So the aircraft manufacturers are accepting much bigger engines for narrowbodies.
Between the two factors, it appears that the "small" engines can get quite close to the big engines with regards to fuel efficiency.
That's true...kind of. You're comparing old tech to new tech. A new small engine is more efficient than an old big engine, but a new big engine is more efficient than a new small engine. It seems to me that efficiency still scales with size, if technology remains pretty much the same across the sizes. Today's Pratt NEO engine, has an 81" fan. The CF6 had an 86 inch fan, the similarly powered GEnx-2b, has an 104" fan, with significantly higher efficiency, which is not just due to fan size, but improved technology throughout the design.
The newer, more efficient engines are heavier than the older engines of similar power. CFM56-5's weight is 5200lbs and the PW1100 weight 6300 lbs.
Efficiency definitively scales with size. Think of the blades in an engine:
1. At the tip air leaks (backwards in a compressor, forwards in the turbine). The tip clearance is a technology (constantly being reduced). For a larger engine there is less leak area to flow area, do the compressor efficiency is notably higher and slightly more efficient turbines.
2. Larger engines have less surface area drag. The easiest way to see this is the inner and outer flow path cylinder areas are less than the flow path area for a larger engine, this makes the engine more efficient, lower ratio of cooled area to flow path area in the hot sections (less cooling per unit of thrust improves efficiency).
3. Weight of components per unit of thrust could go down. In a larger engine, the pressure vessel just weights less per unit of thrust. Because of this, the optimal pressure ratio goes up bringing back up the weight.
4. Value of adding technology to an engine goes up with size. The more fuel burned but engine, the earlier the business case. For example, another low turbine stage price goes up less than a linear function of thrust. So for a Pratt GTF, it is not worth putting on a 4th turbine stage of a design optimized for the 2 hour mission (Airbus NEO design point). For a higher thrust engine optimized for a longer mission (4 or 5 hours in my opinion, the 787 engines were optimized for 8 hours for reference). This pays for a 4th turbine stage to drive a higher bypass fan.
The optimal mission length is critical. Certain technology increases climb fuel burn to reduce cruise fuel burn (e.g., bypass ratio beyond the climb optimal). Variable cycle technology breaks the trade at a maintenance and purchase cost. For example the GTFs were to have variable fan nozzles, a technology that breaks even at the 1.5 hour mission. Pratt wisely decided to remove the tech to cut risk. GE has a valve to vary turbine cooling. This was expensive to develop tech (tough packaging), but pays off at 1 hour,but this won't fit in less than 25,000lbf thrust engines.
For longer missions, turbine clearance cooling should be more precise and granular. This requires heavier and more expensive flow control. Stuff that isn't worth it for the 2 hour mission but is for the 4 hour optimization. The GE9x is taking this to the extreme. No just turbine clearance controlled by cooling the casing. , But new cooling that takes the engine to an insane level of precision. Precision decreases cruise fuel burn, but cannot be used in climb without high risk, so the added weight hurts climb fuel burn as the systems opperate very open in climb to prevent frying the turbine. Cruise is much easier to predict..
So larger engines are more efficient and engines optimized for longer missions are more efficient (at range). Wait for the GE9x, that engine is GE's big technology risk.
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