SteelChair
Topic Author
Posts: 498
Joined: Fri Aug 25, 2017 11:37 am

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
Posts: 908
Joined: Tue Sep 04, 2007 9:50 am

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
Posts: 187
Joined: Fri Dec 10, 2004 8:19 pm

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.

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