15.16 Turboprop Engines Flashcards
What is a gas-coupled turbine?
An engine designed with no mechanical connection between the gas generator and the free-power turbine, relying solely on gas flow.
This design allows for flexibility in engine configuration.
What are the advantages of a free-power turbine engine?
- Vibration is not transmitted to the gas generator from the propeller/reduction gearbox
- Easier to start, especially in cold weather
- Low RPM during taxiing for low noise and low blade erosion
- Use of a propeller brake to prevent movement during loading
The gas generator can function as an APU during engine shutdown.
What is the core engine of a turboprop engine commonly known as?
Gas generator or gas producer.
It produces high-velocity gases to drive the power turbine.
What is the primary function of a turboprop engine?
To provide efficient power for aircraft operating in the 483 to 725 km/h speed range.
It is more efficient than turbojets in this speed range.
How much total power can a turboprop engine extract to drive the propeller and other components?
Up to 95% of the total power produced.
The remaining energy is used as residual thrust.
What differentiates gear-coupled turbines from free-power turbines?
Gear-coupled turbines have extra turbine discs mechanically connected to the compressor and reduction gearbox.
This allows for increased power transmission generated by the exhaust gases.
What must be coordinated to maintain constant speed conditions in gear-coupled turbines?
Propeller pitch and fuel flow.
Changes in fuel flow necessitate adjustments in propeller pitch.
True or False: The propulsive efficiency of a turboprop remains constant in the normal cruising range.
True.
In contrast, the efficiency of a turbojet increases rapidly with airspeed.
What is the relationship between propeller speed and engine speed in gear-coupled turbines?
Both the propeller speed and engine speed are constant at normal operating conditions.
This requires a complex integrated propeller control system.
Fill in the blank: The turboprop engine provides the best specific fuel consumption of any _______ engine.
gas turbine
This efficiency is maintained from sea level to altitudes over 6000 metres.
What is the primary function of a reduction gearbox?
To reduce the high rpm from the engine to a propeller rpm that can be maintained without exceeding the maximum propeller tip speed.
What reduction ratio does the Pratt & Whitney PT6A-34 engine use?
A two-stage 15:1 reduction ratio.
At what rpm does the propeller rotate when the power turbine rotates at 33,000 rpm?
2200 rpm.
What types of gears are used in reduction gearboxes?
Both spur gears and epicyclic gears.
What is the advantage of using spur gears for smaller reductions?
They are mechanically simple and have lower frictional losses.
What is a disadvantage of straight-cut spur gears?
They are very noisy.
What type of gear arrangement allows for quieter operation but is less efficient?
Helical engagement of the gear teeth.
What is the function of ball bearings in reduction gearboxes?
To absorb and transmit all loads imposed on the gears to the casing.
How are gears in engine reduction gearboxes typically manufactured?
Machined from high-quality alloy-steel forgings.
What lubrication systems can be used for reduction gearboxes?
Lubricated from the engine’s lubrication system or a self-contained system.
What are the two main types of reduction gears?
- Parallel spur gear type
- Epicyclical type
What is the basic operational principle of epicyclic reduction gears?
A sun gear drives several planet gears engaged with a fixed or rotating ring gear.
How is the reduction gear ratio calculated in an epicyclic arrangement?
By dividing the number of driven teeth by the number of driving teeth.
In a fixed annulus arrangement, what is stationary?
The outer fixed gear ring or annulus.
