Turbo-prop, Turbo-shaft & APUs
Reduction gearing, free-turbine layouts, turboshaft drives and auxiliary power units.
Turbo-prop, turbo-shaft & APUs, summary notes
- 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.
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
Turbo-prop, Turbo-shaft & APU
Turbo-prop, turbo-shaft & APU quiz
Turbo-prop, Turbo-shaft & APUs, quiz
1. A turboprop needs a reduction gearbox because:
The propeller must turn much slower than the turbineThe prop must turn faster than the turbineIt has no turbine2. In a free-turbine engine the power turbine is:
Mechanically independent of the gas-generator spoolBolted to the compressorNot fitted3. A turboshaft engine's output is:
Shaft power (e.g. to a helicopter rotor)Pure jet thrustPropeller thrust4. A turboprop's power is set and read by:
TorqueEPRBypass ratio5. An APU is:
A small gas turbine for ground/backup electrical and bleed airThe main propulsion engine, run at reduced power when the aircraft is on the groundA hydraulic pump only, driven from the accessory gearbox to pressurise the ground systems6. A benefit of the free-turbine layout for helicopters is:
Low starter load, the rotor can be stationary at startHigher fuel burnNo need for a gearbox7. Most of a turboprop's cruise thrust comes from the:
PropellerJet exhaustAPU8. An APU typically runs at:
Essentially constant speedContinuously varying speed with throttleOnly maximum RPM