Nf is an indication of the speed of which component in a dual axial flow system with a free power turbine?

• A. Turbine thrust indication
• B. First stage compressor speed
• C. Free power turbine speed
• D. Exhaust gas temperature

Answer: (C)

In a dual axial flow engine with a free power turbine, Nf represents the speed of the free power turbine—the turbine that drives the output shaft and is not mechanically linked to the compressor. The free turbine is driven by the energy of the exhaust gases, and its rotational speed determines how much power is being delivered to the load. The engine’s control system uses Nf to regulate fuel flow and maintain the desired power output, independent of the gas generator speed that drives the compressor.

This is different from the compressor speed, which is tied to the gas generator and often labeled Ng; the turbine thrust indication and exhaust gas temperature are separate measurements and do not indicate the speed of the power-delivering turbine.