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Turbo-prop, Turbo-shaft & APUs
Reduction gearing, free-turbine layouts, turboshaft drives and auxiliary power units.
Turbo-prop, turbo-shaft & APUs — summary notes
FreeMain ideas
- A turboprop uses the turbine to drive a propeller through a reduction gearbox (propellers must turn far slower than the turbine). Most cruise thrust comes from the propeller, a little from the residual jet; it is very efficient at low/medium speed and altitude.
- Free-turbine (free-power-turbine) engines have a separate power turbine, mechanically independent of the gas-generator spool, driving the prop/rotor — the gas generator can idle while the output shaft is stationary, and starting loads are low. Fixed/direct-drive turboprops couple the prop to the main shaft.
- A turboshaft delivers virtually all its power to an output shaft (helicopter rotors, ships, generators) rather than to thrust; it too commonly uses a free power turbine.
- An APU (auxiliary power unit) is a small gas turbine, usually in the tail, providing electrical power and bleed air for engine starting and air-conditioning on the ground and as a backup in flight; it runs at essentially constant speed with its own fuel, oil, start and fire systems.
- Propeller control (constant-speed unit) and the reduction gearbox are part of the turboprop powerplant; torque (not EPR) is the key power indication.
- ⚠ Exam trap: in a FREE-turbine engine the power turbine is not mechanically connected to the compressor spool, so the output shaft can be stationary while the gas generator runs; turboprop power is set/read by TORQUE, and a reduction gearbox is essential because the prop must turn much slower than the turbine.
Solved examples
Why does a helicopter turboshaft commonly use a free power turbine rather than a direct-drive layout?
A free power turbine is mechanically independent of the gas-generator spool, so the engine can be started and run up with the rotor (and its huge inertia) stationary or disengaged, keeping starter loads low. The gas generator sets the gas flow and the free turbine then drives the rotor, allowing rotor speed to be governed largely independently of gas-generator speed.
Prop / shaft / APU concept map
FreeTurbo-prop, Turbo-shaft & APU
Turbo-prop, turbo-shaft & APU quiz
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