Pratt & Whitney´s geared turbofan

[Johansson]

There seems to be a "groundbreaking" new concept with a geared turbofan which both lowers noise and saves fuel compared to the traditional turbofan powered by its own turbine stage.



Why has it taken until now to come up with such an engine? I assume that the gearbox can be difficult to build, but still.

[ripcrow]

Damn I always thought they were geared down. I wouldn't think it would be that hard to fit or build a gearbox for it.

[enginewhisperer]

Turboprops were already geared


It makes sense though if you can run a smaller turbine engine at higher rpm and put the power through a larger, more efficient fan.

[finiteparts]

Pratt's marketing team should be commended on their ability to make something that has been done before seem new.

Honeywell's TFE-731 engine has been doing the "geared turbofan" thing since the early 1970's. Lycoming's ALF-502 has been around since the 1970's also.
A development engine was tested by Turbomeca in 1969 with a gear reduction and a variable fan...

www.flightglobal.com/pdfarchive/view/1972/1972%20-%200931.html


There are some great papers out there on the work that GE did with NASA on their Quiet Clean Shorthaul Experimental Engine (QCSEE) in the early 1970's...a few examples are given here"

ntrs.nasa.gov/search.jsp?R=19800006861&hterms=QCSEE&qs=Ntx%3Dmode%2Bmatchallany%26Ntk%3DAll%26N%3D0%26No%3D10%26Ntt%3DQCSEE
ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19800006847.pdf

This engine also ran a gear reduction to the fan stage with a variable pitch fan and even demonstrated the ability to produce reverse thrust with the variable pitch fan blades in a manner similar to what the turboprops do.

If you go through their reports, you will see that the gear work was subcontracted to Curtiss-Wright because they had some experience with high horsepower gearing systems that they used on their large turboprops. The uninstalled thrust of the Over-the-Wing mounted QCSEE was on the order of 21,000 lbf and at that thrust the fan power was around 17,200 hp delivered through a 198 lbm, 2:1 gear reduction. At take-off the gear efficiency was around 99.11% which was around 150 hp lost to heat in the 30 gpm oil flows. Now this might not seem like a lot, but when engine designers are trying to reduce every loss source and achieve the highest efficiency possible, a straight shaft vs gear reduction at take off is worth a .89% efficiency hit.

Now, if we look at the design of a gearbox for a flight engine, well, that is a huge challenge! Absorbing more than 17,000 hp in a gearbox that weighs on a few hundred pounds gear set is a huge challenge. Keeping the gears aligned under the engines deflections in maneuver loads for a pitchline speed over 24,000 fpm is a huge challenge. Often the quoted pitchline speed limit for spur gears is around 10,000 fpm...this is sometimes extended to 20,000 fpm if extreme care is taken to understand the vibration and loading over the operational envelope. So their certifying a gear reduction drive is a great accomplishment on their part.

I think the geared turbofan concept makes a ton of sense from the fan aerodynamics and propulsive efficiency point of view, but the fact that they make it out like it is a whole new concept that they came up with seems a bit silly!

Chris

[rcman50166]

Pratt and Whitney. Hmm I might know a thing or two about them. I also may know a thing or two about the NGTF or Pure Power as it's marketed. The revolution came as it's a zero service part. The gear box is designed to last the entire life of the engine without any maintenance required. Any previous effort couldnt do this. The engine also boasts an increase of up to 16% fuel efficiency and up to 75% noise reduction. For airliners, fuel efficiency is everything. Planes can go further, longer, and cost less to run. There has been a monumental effort to get the manufacturing line up to speed since Pratt can't make the engines fast enough. I know other details that make the engine insanely advanced, but I'm stuck to the fact cards due to EAR laws.

[Johansson]

Now that I think about it the Turbomeca Aubisque that powered the Swedish jet trainer Saab SK60 had a gear driven turbofan, it must be the production version of the Astafan prototype mentioned in the paper you posted Chris.



Still a very interesting concept that just makes perfect sense when you think about it. Why go through all of the trouble with a separate turbofan shaft when you can "simply" gear down the core shaft?

Thanks for explaining this to me guys!

[finiteparts]

Hi Anders,

Only the Turbomeca engine connects the gear drive to the core spool. The Pratt, Honeywell, Lycoming and GE engines that I mentioned all drive off a second LP spool.

The added complexity of having a gearbox, as compared to a straight shaft, is traded against the efficiency gains that can be realized by better matching the fan speed and the LP turbine to their ideal speeds for the flow sizing. If that trade is good enough, then the gearbox makes it on the engine.

Pratt has some really smart guys and one of them is Alan Epstein of previous MIT background. He gives a really great lecture to the University of Michigan's Engineering department here...

www.youtube.com/watch?v=i-6cRVGvFas

~ Chris