What are N1 and N2?

  • When I'm reading something about jet engines, it's common to see references to N1 and N2.

    I've never fully understood what N1 and N2 are on engine instruments and how they relate to thrust and the position of the throttle.

    why is it called "N"?

    @erich - It's been a year, but in case anybody else wanders by... "N" = "Nominal", see the first sentence of Airman01's answer.

    N3 for RR and other 3 shaft engines.

  • Ralph J

    Ralph J Correct answer

    5 years ago

    Conceptually, N1 is the fan (or, fan speed) and is most related to your thrust (since the fan produces more thrust than the core on today's big motors). The N2 is the engine core, and the N2 gauge is used mainly during engine start: on initial starter engagement, the N2 starts turning first, and things like adding fuel+ignition and then later disengaging the starter are based on specified N2 speeds. As long as you have N1 rotation before lighting the engine off, N1 isn't all that important during the start. But once it is running, power is generally set with reference to the N1 (or fuel flow or EGT or EPR) rather than the N2.

    Picture of 727 engine instruments

    This picture of 727 instruments shows, from top to bottom, the N1, EPR, EGT, N2, and Fuel Flow indicators. You can tell (hopefully) how the N1 and N2 are both tachometers, showing percentages instead of values, because it's a lot easier to talk about 52% N2 than 6350 N2 RPM. The picture below shows an example of the engine stack displayed on a modern EICAS panel, with the N1 and the EGT displayed prominently, and everything else just shown as digital values (with no EPR indication in this aircraft). This is a typical in-flight presentation, showing you the two things you most care about with large displays, and everything else available if you really need it.

    EICAS engine display

    Nice answer, thank you. Why would the landing gear have an "off" position? Is that for emergency use, such as gravity deployment?

    You should probably start a new question for this, however... with the landing gear control lever in the "OFF" position, hydraulic pressure is blocked at the selector Valve. Both the down line and the up line are connected to return. OFF is the normal position in cruise. Boeing has 'OFF', Lockheed has 'Neutral'. Douglas has an 'Uplatch Check' position.

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