15.15–15.17 Alternate turbine constructions, turboprop and turboshaft engines

Scope

• 15.15 Alternate turbine constructions — B1 only, level 1: geared turbofan; variable fan blades; open rotor/propfan; hybrid turbine-electric concepts and electric power augmentation; future trends.
• 15.16 Turboprop engines — A 1, B1 2: gas-coupled/free turbine and gear-coupled turbines; reduction gears; integrated engine and propeller controls; overspeed safety devices.
• 15.17 Turboshaft engines — A 1, B1 2: arrangements, drive systems, reduction gearing, couplings, control systems.

Alternate turbine constructions (15.15)

Concept	Key idea
Geared turbofan (GTF)	Planetary gearbox (ratio about 3:1) between fan and LP spool; fan turns slowly, LP turbine/compressor fast → fewer LP stages, higher bypass ratio, lower noise and fuel burn. Gearbox needs its own lubrication and cooling (high heat load)
Variable-pitch fan	Fan blade angle adjustable in operation — optimises fan efficiency across the flight envelope and can give reverse thrust without a cascade reverser (weight saving)
Open rotor / propfan	Unducted, highly swept fan blades, often contra-rotating; very high effective bypass ratio and low fuel burn, but noise and blade-off containment are challenges
Hybrid turbine-electric	Gas turbine coupled to generators and electric motors: turbo-electric (no battery), series or parallel hybrid with batteries
Electric power augmentation	Motor/generator on an engine spool adds power (boost at take-off/climb) or extracts power; allows smaller core or better transient response
Future trends	Ultra-high bypass ratio, higher overall pressure ratio and turbine temperatures (ceramic matrix composites), SAF and hydrogen combustion, more-electric engines (electric oil/fuel pumps, no bleed)

Turboprop engines (15.16)

Fixed-shaft (gear-coupled) vs free-turbine (gas-coupled)

	Fixed-shaft / single-shaft (gear-coupled)	Free-turbine (gas-coupled)
Propeller drive	Through the reduction gearbox from the same shaft as the compressor	From a separate power (free) turbine with no mechanical link to the gas generator
Starting	Starter must turn compressor and propeller → high starting torque; propeller on start locks (low pitch)	Starter turns only the gas generator → easier start; propeller may be feathered on the ground
Speed	Engine runs at nearly constant rpm; power changed by fuel flow with propeller pitch absorbing it	Gas-generator speed (Ng) varies with power; propeller speed (Np) controlled by the governor
Examples of features	Negative torque sensing (NTS) — drives propeller towards feather if the propeller starts driving the engine	Propeller brake possible (APU mode on ground)

Key fact: On a free-turbine turboprop the gas generator and the power turbine are linked only aerodynamically by the gas flow — hence "gas-coupled".

Reduction gears

Propellers must turn far slower than turbines. Reduction gearboxes use spur/helical stages (offset gearbox, e.g. propeller shaft above the engine axis) or planetary (epicyclic) stages (in-line). They house the torquemeter (helical gear thrust balanced by oil pressure), propeller brake (some types), and drives for propeller governor/overspeed governor and propeller oil pump. Typical overall ratios are about 10:1 to 15:1 or more (type-dependent).

Integrated engine and propeller controls

• Power lever (throttle): sets gas generator power in the alpha (flight) range; below flight idle it enters the beta range where the power lever directly controls blade angle (ground fine, reverse).
• Condition lever (propeller lever): selects propeller rpm (governor setting), feather and fuel shut-off.
• On modern engines an EEC/FADEC plus propeller electronic control (PEC) — sometimes a single "single-lever" integrated control — schedule power and propeller speed together.
• Beta light illuminates when blade angle is below the flight-fine stop.

Overspeed safety devices

Device	Function
Primary propeller governor	Normal constant-speed control
Propeller overspeed governor	Separate governor set a few % above max rpm — dumps oil to increase pitch if the primary governor fails
Fuel topping governor / Np governor in the fuel control	Reduces fuel if propeller speed exceeds a limit (on free-turbine engines)
Pitch lock	Mechanically locks the blade angle if oil pressure is lost, preventing a runaway to fine pitch
Flight fine-pitch stop / low-pitch stop	Prevents blades going below flight idle angle in flight
Power turbine overspeed protection	Electronic or mechanical shutdown if the free turbine overspeeds (e.g. shaft failure)
NTS / auto-feather	Detects negative torque (windmilling drive) or loss of torque after failure and feathers the propeller