SteelChair
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Core engine speeds

Mon Oct 29, 2018 11:16 pm

We generally see aircraft gas turbine engine rotor speeds expressed as a percentage.

I have read repeatedly that the GTF allows the small parts (the core) to turn at their more optimum speed, and the big fan to run at its more optimum speed (much slower) versus a traditional turbofan.

Does anyone know how fast the core engine on a GTF is turning (how many rpms, not %) versus a CFM56 at a typical cruise power setting?

Furthermore, does anyone know how rpm affects engine life? I generally think of engine life being more affected by temperature than rpm, but I could well be wrong.

I'm thinking that tiny core on the GTF swinging that huge fan is going to have durability problems.
 
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akiss20
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Re: Core engine speeds

Mon Oct 29, 2018 11:34 pm

The GTF lets the low pressure compressor and turbine spin faster while letting the fan spin slower, the high pressure compressor and turbine are on a different shaft and can spin as fast as they need to. If I recall the GTF is 3 to 1 gear ratio. The CFM-56 has a 100% N1 of about 5200 RPM [1] and the GTF is not that much slower at 4000-5000 RPM, so the LPC and LPT are spinning at 12000-15000 RPM [2]

The biggest issues introduced by the increasingly smaller cores has been rotordynamic. What used to be a big, stiff shaft is now much smaller and less stiff. GTF specifically has experienced a lot of thermal problems with rotor bow.

1 http://www.b737.org.uk/cfm56_soi.htm

2
https://en.m.wikipedia.org/wiki/Pratt_% ... ey_PW1000G
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tepidhalibut
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Re: Core engine speeds

Thu Nov 01, 2018 9:10 am

The other easy source of information is the Type Certificate Data Sheet, which includes the operating limits for the shafts. For the PW1100G, and gives the Maximum LP speed as 10,047rpm and maximum HP speed as 22,300rpm. It doesn't detail typical speeds at cruise, but I'd bet that they'd be around >90% of those speed limits.

Furthermore, does anyone know how rpm affects engine life? I generally think of engine life being more affected by temperature than rpm, but I could well be wrong.

Stresses rise with the square of speed, but cyclic life counting for critical parts is a very complex process, I'm not a company expert, but I could still spend hours describing the difficulties in real life. As regards temperatures, that has an effect, but it depends on which part of the engine. Turbines - very hot, very significant effect. Fans - not much above ambient air temp - not a huge effect.
 
Yikes!
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Re: Core engine speeds

Mon Dec 03, 2018 4:05 am

Hi SteelChair: Typically, first stage HP turbine will be turning ~38,000 RPM at takeoff thrust. The geared turbofan at a substantially less RPM. During takeoff with a modern GTF installation, approximately 75% of the takeoff thrust comes from the fan. During cruise, only about 60% from the fan and 40% from the core thrust. I made a post here about 20 years ago (can't find it) giving the mathematics to those numbers. Feel free to search!

Engine life is more dependent on temperatures, not so much rpm's.
 
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Francoflier
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Re: Core engine speeds

Mon Dec 03, 2018 6:11 am

Yikes! wrote:
Hi SteelChair: Typically, first stage HP turbine will be turning ~38,000 RPM at takeoff thrust. The geared turbofan at a substantially less RPM. During takeoff with a modern GTF installation, approximately 75% of the takeoff thrust comes from the fan. During cruise, only about 60% from the fan and 40% from the core thrust. I made a post here about 20 years ago (can't find it) giving the mathematics to those numbers. Feel free to search!

Engine life is more dependent on temperatures, not so much rpm's.


I don't know of an engine of the class of the CFM 56 or larger in which the HP spool would turn at anywhere near 38,000 rpm...
The -56 HP assembly spins at around 15k and the Leap's at around 20k.

You'd have to go to much smaller gas turbines, mostly turboprop engines, to see stuff spinning above 30k.
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DocLightning
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Re: Core engine speeds

Mon Dec 03, 2018 6:51 am

SteelChair wrote:
Furthermore, does anyone know how rpm affects engine life? I generally think of engine life being more affected by temperature than rpm, but I could well be wrong.


I don't know about materials and such, but I do know that in my former life as a molecular biologist, we had ultracentrifuges that were capable of 100,000 RPM with centrifugal force accelerations at the radius approaching one million xg (https://www.thermofisher.com/us/en/home ... fuges.html). The radius of these rotors was roughly comparable to the radius of the HP spools I've seen on some turbofans, such as the PW2000.

Now, admittedly, those rotors were not operating at thousands of degrees under dynamic aerodynamic stresses (in fact, ultracentrifuges are evacuated so as to improve efficiency and prevent heating of the rotor), but the bottom line is that properly selected materials can handle those kinds of stresses.
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gloom
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Re: Core engine speeds

Mon Dec 03, 2018 8:41 am

tepidhalibut wrote:
The other easy source of information is the Type Certificate Data Sheet, which includes the operating limits for the shafts. For the PW1100G, and gives the Maximum LP speed as 10,047rpm and maximum HP speed as 22,300rpm. It doesn't detail typical speeds at cruise, but I'd bet that they'd be around >90% of those speed limits.


Not really. Max is usually a couple of percent above nominal speeds (I'd say up to 10% for fan and up to 5% for high pressure). Typical cruise would be over 90% in the old days, but nowadays more and more planes cruise at 80s. The difference between max and cruise would suggest cruise at below 8000rpm for fan, and probably something like 19000rpm on HP.

Cheers,
Adam

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