N1, N2 and engine start.

[Vic]

I have two quick questions about turbofan engines.
As I understand it, the N1 is the fan, the N2 is the compressor (am I correct?)

What is the start up sequence for turbofan engines? (No autostart cheating ) The way I understand it, you have to spin either the N1 or the N2 to a certain % after which you turn the fuel on and after that you ignite it...am I correct?

Vic

[beefhole]

I'll answer the second.  Normally you start it 2,1 (right, left) in a two-engine aircraft.  Hit E+2, and crack the throttle.  Hold ctrl+shift, and keep you finger hovering over the F4 button.  When ready, use your other hand to press and HOLD the Right engine starter switch on START.  Watch N1 and N2.  Make sure that N1 is increasing as N2 is increasing.  In general, you want to hit F4 (which will open the fuel valve) when N2 is at 21%.  Repeat for engine 1.  Hope that wasn't confusing.

[Nexus]

The N1 (fan)is the Low pressure compressor.
The fan is normally mounted on the front of a number of core compression stages (twin spool engine) or on a separate shaft, diven by its own turbine (triple spool)

The N2 is the High pressure compressor and this is the one you must start first (often by pneumtatic power)
When the N2 is at around 25%, then you have adequate airflow thru the engine so you can add the fuel. Not all aircrafts can start both engines at the same time (the APU delivers the pneumatic) and some aircraft's must have the air conditioning turned off, because of the same reason

[beefhole]

When the N2 is at around 25%, then you have adequate airflow thru the engine so you can add the fuel.

In FS, N2 doesn't reach 25% before spooling down. You'll need to charge it at 21%.

[Nexus]

In FS, N2 doesn't reach 25% before spooling down. You'll need to charge it at 21%.

Must be an FS program issue, since neither the PMDG 737 nor Level D 767 spools up the N2 according to their realistic counter parts.

[Saratoga]

Ya in reality, just hold the start switch, watch the N2 begin to rise, then the N1, wait 'til 20-25 or so (in modern turbofans, you have a wide range that will work), flip the fuel on, and she'll spool right up.

[Nexus]

How'd you get those N2; N1 to look like that?
Been trying to do that for ages  :)

[ChrisM]

You use the subscript and supscript buttons.  4 from the right bottom row on the 'add yabbc tags'

[Nexus]

Thanks you Chris!  :)

[Vic]

Thanks for that info guys!

beefhole: the 3 spool engine you are refering to is some Rolls Royce, right? (Someone told me that a rolls royce has N1, N2 and N3!)
Vic

[beefhole]

beefhole: the 3 spool engine you are refering to is some Rolls Royce, right? (Someone told me that a rolls royce has N1, N2 and N3!)
Vic

I just reread my stuff, I didn't mention any N3, but I've also heard of this on some engines.

[Nexus]

Correct Vic.

The tripple spool engine is has an N3 stage, and that stage is the High pressure compressor, whereas the N2 is the intermediate stage compressor. All spools ar each driven by their own turbine and connection shafts.

Advantage compared to twin-spool engines are that the N1 fan can have a wider chord because it is not restricted to any connection to booster conpressors-
This allows for a very large frontal area and the N1 can be driven closer to its optimal speed

The actual fan - which works more like a shrouded prop - is responsible for approx 75% of the engine's thrust in the form of bypass airflow to the atmosphere via the engines bypass ducts behind the fan.

The air that flows thru the N2 and N3 compressors becomes highly compressed , of which 1/3 is used for combustion and 2/3 for internal engine cooling.

And yes I have a poster of the RB211 on my wall  ;D

[Saratoga]

Can't think of any aircraft which has an N3 gauge. I believe though if the N1 and N2 were within operating limits, the N3 would be forced into that same position, correct?

[Nexus]

Look no further than the L1011.  :)
I believe the gauges are arranged as EPR, N1, TGT, N3 and FF

[Vic]

Thanks Nexus! So the L1011 engines used the triple spool? Well, that would be a great advantage, so whats the disadvantage (cheese is only free in a mousetrap) so I take it that the fuel burn is higher and a two spool? Thats just my guess to why newer engines are usually two-spool, right?
Vic