I got PMDG NGX for 2 weeks and I still don't know what is the diffrence between N1 and N2. Can somebody help?

N1 is low pressure shaft rpm (in simple terms, how fast the big fan at the front is spinning)

N2 is high pressure shaft rpm (how fast the core of the engine is spinning)

 

N1 is generally used as a power reference (much like RPM in a car or manifold pressure in a piston-engine aircraft) because it roughly describes how much of the available thrust/power you're using. N2 is used for some specific things, such as when to introduce fuel during engine start.

 

The PMDG NGX is a very complicated addon, I would strongly suggest you complete the included tutorials (the Boeing supplied documentation can be rather impenetrable to a lay person) or search on YouTube for a video tutorial.

Thanks for help. I've already known how to use the aircraft, but didn't understand the diffrence. Anyways - thank you.

Here's a discussion on N1 for output power indication in the context of a comparison against EPR (engine pressure ratio):

http://theflyingengineer.com/flightdeck/cockpit-design-epr-vs-n1-indication/

 

The basics:

- N1 responds more quickly to commanded changes in power output, but only shows the performance for a single component (low pressure compressor).

- EPR is a better indication for the performance of the entire engine, but reacts more slowly than N1.

 

It's more of a "Pepsi vs Coke" thing than anything serious. Pratt and Rolls like EPR, GE and SNECMA like N1. And even in EPR equipped engines, your secondary engine performance indicator is N1.

The issue i have with EPR is that it's just a number (e.g. 1.543) rather an easily understood percentage. "% EPR" would be good but I've never seen or heard of this being used.

 

Also I forgot to mention previously that N2 does not always refer to the high pressure spool; on three shaft turbofans (like most Rolls-Royces) N2 is the intermediate spool and N3 is the high pressure spool.