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DanielBednar
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Posts: 16
Joined: Fri Jul 05, 2019 5:40 pm

N1/N2 RPM question

Fri May 01, 2020 2:29 am

Hello all,

N1 and N2 are driven by separate shafts and are mechanically independent of each other, allowing them to have different rotational speeds because they are on different shafts. But why is N1 significantly slower (5,000 RPM) compared to the N2 RPM (15,000 RPM)? Does N2 need to rotate faster in order to achieve the necessary amount of compression or something like that? Thanks a lot everyone.
 
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jetmech
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Joined: Wed Mar 29, 2006 2:14 am

Re: N1/N2 RPM question

Fri May 01, 2020 4:02 am

DanielBednar wrote:
. But why is N1 significantly slower (5,000 RPM) compared to the N2 RPM (15,000 RPM)? Does N2 need to rotate faster in order to achieve the necessary amount of compression or something like that?

The issue at hand is the tip speed of the various turbine and compressor blades.

IIRC, there is an optimum tip speed for these components. Too fast and losses due to compressibility become excessive, too slow and as you say, one does not realise the full capacity of the turbo machinery.

Fans (N1 spool) are usually sized as large as possible to get the largest mass flow through the engine with the compressor (N2 / N3 spool) being a smaller diameter partly due to the smaller mass flow through the core and partly to physically facilitate the provision of bypass.

Assuming that the ideal tip speeds are similar, the differing diameters results in different RPM's for the spools.

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair :shock: .
 
426Shadow
Posts: 236
Joined: Tue Sep 11, 2007 8:13 am

Re: N1/N2 RPM question

Sat May 02, 2020 1:44 am

TL:DR The N1 turbine speed is limited by the fan that is connected to it. This is why Geared Turbofans are technically more efficient (this way you can let the turbine spin closer to the speed that it actually wants to and gear the speed down for the fan). Also you need a lot of torque to turn the fan so a bigger set of larger diameter turbine blades are needed to turn it.
We are all just fanboys, our opinions don't make or break businesses.
 
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jetlife2
Posts: 203
Joined: Sun Jul 09, 2006 11:32 am

Re: N1/N2 RPM question

Mon May 04, 2020 12:12 pm

jetmech wrote:
tip speeds are similar, the differing diameters results in different RPM's


Correct. Also the reason that small gas turbines (like helicpter engines) have rotor speeds of 30,000 rpm or more.
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