In a dual axial flow or twin-spool compressor system with a free power turbine, Nf would indicate the speed of which component?

• 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 or twin-spool setup that includes a free power turbine, the Nf value represents the rotational speed of the free power turbine. This turbine is the output-side turbine that drives the load (through a gearbox in turboprops or turboshafts) and operates on a separate shaft from the gas generator spools. Because it isn’t directly tied to the compressor spools, Nf specifically tracks the output turbine’s speed, which is why it’s the parameter you’d monitor or control for power delivery. The other items are not speeds of the output turbine: turbine thrust indication is a thrust measure, exhaust gas temperature is a temperature, and the first stage compressor speed refers to the compressor spool speed (not the power turbine).