N1 & N2

[Pilotech]

We all know and love these terms, nut does anyone know where the "N" comes from?

[Check Airman]

Probably "number"?

[spannersatcx]

it has been discussed before, a search might help, it maybe on one of the epr n1 debates.

[carholme]

N is the symbol for speed

N1 compressor speeds also Ng
N2 free turbine speeds also Nf


Nr Rotor Speed as in helicopters

etc., etc.

Regards

carholme

[tommoutrie]

Lundberg-Palmgren theory expresses the number of stress cycles to failure as N. When applied to gas turbines, N has come to mean the rotational speed that can be applied to a component that will give statistically infinite life. N1, usually the compressor stage, is the speed that you can whizz the compressor stage round at forever, N2, the power turbine etc. You get percentages higher than 100% N1 or N2 used because you accept the statistical reduction in disk life in order to obtain the transient increased power. Other factors limit engine life more than how fast you spin the turbine round so it is an acceptable trade off.


I am attempting to dredge up knowlege from many years ago and it might be because N was Frank Whittles favourite letter. Or it might stand for noisy. Or something else. And if it is something different I'd really like to know.

[8846]

Well that was something interesting I didn't expect to find as I trawled through pprune for one last time before bed!

I'm indebted to you for the information about percentages of N in excess of 100% and the trade off with engine life.

Just a quickie (ooh err missus!) - what, briefly, are the other factors that have a bearing on turbine engine life? I imagine cycles are important but any other major factors?

Thanks in anticipation....(I really MUST get out more...)

:ok:

[barit1]

It has simply been a convention for a few decades or more to represent rotor or shaft speeds by N. When there are multiple shafts, then N1, N2 etc are used. Sometimes Ng (gas generator) and Nf (free turbine) are seen, or Nl (low pressure) and Nh (high pressure) - different customers you know. :confused:

The 100% reference speeds are purely arbitrary, sometimes based on poor guesses before the machine is first fired up. Thus some engines run at 90% at TO, others run 115-120% at TO. Sorry 'bout that. :O

[tommoutrie]

where did you get that it is arbitrary?
and where did you do your degree?
and should you give it back?...

life is limited by lots of things - combustion chamber life, bearing failure, material degredation, heat cycling, crystalline structure degredation, fuel quality, corrosion, the balancing regime used, me firewalling it for fun on the way home sometimes...

who knows, its all witchcraft

[ATSU Misc]

Mnemonic from a turbo jet instructor.:ok:

Time to brush off the dust from the ol' Tedric Harris.;);)

[barit1]

where did you get that it is arbitrary?
and where did you do your degree?
and should you give it back?...

I get "arbitrary" from 30+ years in the business. As I said, some engines run at (certified) 118% indicated, meaning that 100% is a low-end cruise or holding pattern value. There's NO GOOD REASON to calibrate the tachometer that way, is there? I think some marketing director sat bolt upright in bed @ 0300 with that brilliant idea. Believe me, there's little sound engineering in THAT one. :}

But in the end, does it make any difference? Instead of % speed, or (in the case of piston engines) RPM, they could have picked radians per millisecond.
The numbers merely have to reflect the units chosen.

Of course, whatever speed the engine likes to run, the mfr. has to demonstrate overspeed tests, blade-out, bird ingestion etc. using the takeoff speed as reference, and conduct low-cycle fatigue tests and analysis on the same basis.

I beg you to put up hard evidence to the contrary!

[FE Hoppy]

For the first time in my 20 year career Ive found an engine with both N1 and N2 limit of 100%.
Sadly after further investigation it turns out that the real limit is somewhat different and the indicating system is biased to show a limit of 100%.

Oh well!

[XPMorten]

barit1 got it right :ok:

M