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Old 14th Feb 2006, 09:54   #1 (permalink)
 
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N1 and N2

Could someone clarify what N1 and N2 mean for me? My understanding is that N1 is the fan speed and N2 is the turbine speed, but they seem to be expressed as a percentage. Is it as a percentage of the max RPM? If so, how come it is possible to get over 100% for an N readout in an aircraft?
Also, I thought that the fan and turbine were linked and therefore would run at the same speed - are N1 and N2 measured on different stages, or do I just have an over-simplified view of how the engine works?

Any help would be greatly appreciated.

Cheers, Ginger

(PS, if anyone has any RPM data for a Rolls BR715, i'd love you to share it!)
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Old 14th Feb 2006, 10:07   #2 (permalink)

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You are right, Ginger; N1 is compressor speed, and N2 turbine speed. That is with a free turbine, of course, where the two are not connected, and the turbine is driven by the hot gases from the combustion chamber.

100% is a speed decided upon by the engine manufacturer, and is not necessarily the maximum speed, merely a reference speed. Instead of a percentage, the gauge could be calibrated in RPM, but the former is more convenient to use. You should realise that all the limitations on engine speed are not limitations on the engine, but possibly on the gearbox. So, on a twin-engined helicopter, if an engine fails the other engine can then increase its power output to some extent.
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Old 14th Feb 2006, 10:16   #3 (permalink)
 
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Actually, this is wrong.
Compressor and turbine have to be linked, otherwise the engine couldn't run What would drive the compressor to provide the combustors with "new" air and start another cycle?
N1 and N2 represent the speed of the two stages of the engine. N1 is the low pressure compressor/turbine and fan, and N2 is the high pressure compressor and turbine. N1 and N2 run independently on most engines.
If you have an N3 like on some RR engines than this is the high pressure stage, and N2 represents the RPM of the intermediate stage.

A free running turbine (= a turbine stage not linked to a compressor) is used to drive the rotor or propeller via a gearbox in turboshaft engines but is not present on normal jet engines.
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Old 14th Feb 2006, 10:24   #4 (permalink)
 
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That's a lot clearer, thanks guys. The problem I was having related to calculating the tip speed of fan blades. Using the LP turbine speed (which I assumed would be the same as the fan RPM), I got the tip speed as 496 m/s .
Clearly i'm not taking something crucial into account here.
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Old 14th Feb 2006, 16:50   #5 (permalink)
 
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We have to be careful about what kind of engines we are talking about here - is it a helicopter installation, or a turbo-fan? The nomenclature may be different for the different engines.
And for A-Floor, it is possible to have the two items separated physically - happens all the time in helicopter power turbines. They are only joined by a gas path.
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Old 14th Feb 2006, 19:44   #6 (permalink)
 
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By all the rules I have seen over a couple decades:

N1 is the forward-most rotor. In the case of a modern front-fan engine this is the fan, which is directly coupled to the low-pressure turbine (or occasionally through a gearbox). In an aft-fan engine (CF700 or CJ805-23) it's the core rotor - compressor and HP turbine. In a helo it's the core again.

N2 is the second independent rotor the airflow sees. In a front-fan 2-spool engine, it's the core or HP rotor. In a helo or aft-fan it's the free turbine.

And for the Rollers, there's this N3 thingie. It's the core HP rotor, and the intermediate spool logically becomes N2.

So - simply follow the inlet air though the engine and count the rotors, and they will be in N1, N2, (N3) sequence.

There are other conventions too: Ng & Nf for Gas Generator (core) & Free turbine, or Nh & Nl for High-pressure & Low-pressure. These may be specified by a military user.
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Old 14th Feb 2006, 23:15   #7 (permalink)
 
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By all the rules for power turbines (for helicopters and some turbo-props), N1 is compressor speed, and N2 is power turbine speed. So, be careful when asking what is what - which engine type are is being referred to makes a big difference.
There is supposedly a convention that talks about which station is which number - N1 is at the inlet to the compressor in this convention. Is the convention more honoured in the breach than the observance???
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Old 17th Feb 2006, 10:24   #8 (permalink)
 
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It's a two-stage turbofan, Rolls Royce BR715. We're basically trying to work out if we can theoretically increase the fan diameter slightly.

Another really useful piece of information (if anyone knows) is how much an engine liek this would cost No matter how many emails I send, or research I do it seems impossible to find out a cost for an engine such as this.

Cheers, Ginge
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Old 17th Feb 2006, 13:16   #9 (permalink)
 
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The engine COST (the builder's cost of production) is a closely-guarded secret, and varies from year to year as production improvements & simplifications are made.

The PRICE it sells for depends on how much money the maker thinks he can make in spare parts sales (that's where the real profit is) over the life of the engine. If it's a big customer, the engines may go for less than cost!

So basically it's the same business as Schick or Gillette; give the razors away, make money on the blades...
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Old 19th Feb 2006, 14:17   #10 (permalink)
 
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Damn, I had a suspicion it wasn't simple. We've been told about the "after-sales is where the profit comes from" thing before, so I thought putting a unit cost on the engine would be difficult.

Cheers though,

Ginger
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Old 19th Feb 2006, 20:13   #11 (permalink)
 
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hi
if you talk about helicopter engines, n2 is the power turbine. 100% will be maintained to have 100% NR. N1 is the compressor, is driven by a starter(during start up) and after that by the comressor turbine(behind combustion chamber, extracts around 60% of the energie). power turbine and compressor turbine a seperated. N2(power turbine) is the"engine output".
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Old 19th Mar 2006, 21:16   #12 (permalink)
 
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diagrams

anyone have some diagrams of airflow and N1 and N2? might help clarify. I'm looking for diagrams and photos of the GE CF-34-3B1 engines.

vasi075@yahoo.com
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