In reference to a rotorcraft turbine engine (turboshaft):

"How can a power turbine rotor speed N2 (or Npt) and rotorcraft rotor speed Nr both be at 100 % while the gas generator speed N1 (or Ngg) is at 85 % and similarly both the N2 and Nr can be at 100 % while the N1 is at 99 %?"

I know the two turbines are not physically linked but just joined by a gas coupling. How ever if fuel flow changes to speed up the N1 how could it not directly affect the N2?

  • $\begingroup$ Remember the “power” in power turbine. $\endgroup$
    – Jim
    Jul 13, 2022 at 1:36
  • 1
    $\begingroup$ Are you comparing 2 different power settings or flight conditions? It looks like the question is a little incomplete. And yes, gas turbines for helicopters (turboshafts) deliver various powers (hence called the power turbines) but maintain the power turbine speed (basically fixed speed) and in effect the the helicopter rotor speed (gearbox is fixed ratio). $\endgroup$
    – 0scar
    Jul 13, 2022 at 11:07
  • $\begingroup$ So i presume what your saying Oscar is that in order for the N2 to maintain a constant rotor speed with different torques applied to it the N1 needs to be able to turn faster or slower depending on the needs of the N2? $\endgroup$
    – Wes
    Jul 13, 2022 at 15:24
  • $\begingroup$ Done via a fuel controller and governor? $\endgroup$
    – Wes
    Jul 13, 2022 at 15:33

1 Answer 1


Most rotorcraft use a fixed rotor speed (note that slowing the rotor speed could bring benefits in some flight situations, but that is not applicable here) flying different operating/flight conditions. The rotor is driven by the power turbine of the turboshaft through a (main) gearbox (reducing the turbine spool speed of several thousands of revolutions per minute to several hundreds). This fixed coupling makes that the power turbine and rotor speed are directly linked, so, maintaining a constant rotor speed implies a constant power turbine speed. Do note that the different flight phases of a helicopter require different power settings, e.g. the hoover requires more power (lift the whole mass of the rotorcraft) than a forward flight motion where the body generated lift. This mean that the power to the rotor is not constant, but depending on the flight condition. This power is generated by the power turbine and knowing that torque is related to the power by the speed:

$ Power = Torque \times Speed $

with a constant speed, the torque on the power turbine needs to be varied to get the appropriate power for the rotor.

A turboshaft engine consists of a gas generator and power turbine, these are coupled by the gas path, so, these can spin independently from each other.

To provide the gas power to the power turbine the gas generator creates a hot, pressurized stream of combustion products that drive the power turbine.

For that reason you see that the gas generator spool speed is low when the rotor requires less power (low fuel flow), or high (high fuel flow) if the rotorcraft is in a high power demanding flight condition like the hoover (there even is a difference in hoover in free air or just above the ground in terms of needed power!).

  • $\begingroup$ That makes perfect sense! Thank you so much Oscar!!! $\endgroup$
    – Wes
    Jul 14, 2022 at 17:14

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