A turboprop engine's design incorporates a 'free turbine' (power turbine) arrangement. Which statement best describes this configuration?
- AThe power turbine is aerodynamically driven by the gas generator's exhaust gas but is mechanically independent, driving the propeller through a reduction gearbox
- BThe compressor and propeller share a single common shaft with no gearbox required
- CThe free turbine drives only the accessory gearbox, not the propeller
- DThe propeller is bolted directly to the compressor shaft
Show answer & explanationAnswer & explanation
Correct answer: A. The power turbine is aerodynamically driven by the gas generator's exhaust gas but is mechanically independent, driving the propeller through a reduction gearbox
In a free-turbine turboprop, the gas generator (compressor plus its turbine) is mechanically separate from the power turbine. The power turbine spins only because of the energy in the exhaust gas stream and drives the propeller through a reduction gearbox, allowing the propeller speed to vary somewhat independently of gas generator speed.
Why the other options are wrong
- B. A common shaft describes a fixed-shaft (single-spool) turboprop, not a free-turbine design.
- C. The free turbine's primary job is to drive the propeller through the reduction gearbox, not just accessories.
- D. Direct drive without a gearbox is impractical due to turbine speed being far too high for a propeller.
Free Turbine (Turboprop)
A power turbine stage mechanically independent of the gas generator, driven purely by exhaust gas energy and connected to the propeller through a reduction gearbox.
- Also called the power turbine
- Mechanically separate shaft from gas generator
- Allows propeller speed to be controlled independently of N1
Memory trick: 'Free' means it floats on gas flow, not bolted to the gas generator.