In a free turbine configuration, although the compressor (N1 or Ng) turbine is not directly linked to the power turbine (N2 or Nf) the turbine rotors are still directly adjacent to each other in their respective engine chamber.
After the fuel air mixture is ignited in the combustion section, the expanding gas is routed over first over the N1 (compressor) turbines which share a common shaft with the engine's compressor section. After passing over the N1 turbines, the exhaust gas then passes over the N2
(power) turbine which then is linked to some sort of gearbox (generally being backshafted through the middle of the existing N1 shaft) before being used to drive a propeller/rotor/transmission, etc.
Klaus is correct....the engine starter drives the compressor section until its RPM is high enough to keep the engine self sustaining. With a free-turbine set-up, it's also possible to start the engine without having the N2
turbines turn. This is what allows a number of helicopters to do a "locked start" utilizing the rotor brake. In this case, the N1 is section is up and running (albeit at a low RPM), while the Nf turbine remains stationary.
The short answer to the thread starter question is that in the "suck-squeeze-bang-blow" model of jet engine operation, both the N1 and N2
turbines are in the "blow" section. So even though the starter has dropped out, once sufficient combustion is occurring the N1 turbines are still turning.
My experience has been mostly the PT6, T53, and T64....so if I'm off-base, I'm all ears.
Hope that helps.
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