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.
What does the planet gear carrier do in a fixed annulus arrangement?
Drives the propeller shaft in the same direction of rotation as the power turbine shaft.
In a rotating annulus arrangement, how does the direction of the propeller shaft compare to the power turbine shaft?
It drives in the opposite direction.
What are the two types of bearing arrangements involved in helical gears?
- Ball bearings
- Tapered roller bearings
True or False: The reduction ratio depends on the relationship between the size of the driving and fixed gear.
True
Fill in the blank: Reduction gearboxes may have their own _______ system.
self-contained
What is the primary purpose of a turbopropeller?
To provide an efficient and flexible means of using the power of the engine at any condition in flight.
What are the two operational ranges of a turbopropeller?
- Alpha range
- Beta range
What is the function of the low-pitch stop assembly in a turbopropeller?
To prevent the propeller blades from moving below a specified pitch angle.
What does FADEC stand for?
Full Authority Digital Engine Control
What is the role of the spinner assembly in a turbopropeller?
To reduce drag by enclosing the dome and barrel.
What does the synchro-phasing system do?
Maintains a pre-set angular relationship between the master and slave propellers.
What type of control system do larger turboprop engines typically use?
Dual acting hydraulic control systems.
What is the purpose of the feathering system in turboprop control assemblies?
To feather the propeller when the engine is shut down in flight.
Where is the throttle quadrant located in the flight deck?
On the centre pedestal.
In a single-lever control system, what happens when the thrust lever is moved forward?
Increases fuel flow and selects a higher rpm.
What is the function of the High-Pressure Fuel Shut-Off Valve during feathering?
Shuts off fuel flow to prevent engine overheating.
What are the two distinct ranges of operation for the power lever in dual-lever controls?
- Power range
- Propeller control range
What does the thrust lever do when retarded from idle to reverse?
Schedules fuel flow and blade angle from +20° to -20°.
What is the Beta range in turboprop operation?
Ground operating range where the thrust lever influences blade angle.
How does the propeller lever function in relation to the PCU?
Selects the required rpm by applying or releasing pressure on the governor control spring.
What is the purpose of a control lock in turboprop engines?
To prevent excessive flutter of control surfaces on the ground.
Fill in the blank: The degree of fining off into reverse in Beta range is governed by the position of the _______.
thrust control lever
What is the primary purpose of overspeed protection systems for propeller aircraft?
To prevent extreme stress to the propeller and engine that can lead to rapid and total failure in flight.
What causes overspeed in propeller aircraft?
Overspeed can result from engine power output exceeding the propeller’s ability to absorb it.
How does centrifugal force affect propeller blades during overspeed?
Centrifugal force tries to reduce the blade angle to zero, potentially leading to a negative angle of attack.
What happens to the propeller blades if the engine fails in flight?
The blades try to return to a zero angle, causing windmilling and uncontrollable RPM.
What is the feathering angle of a propeller?
The angle where the blade chord is in line with the airflow, stopping the propeller from rotating.
What is the function of the Automatic Drag Limiting System (ADLS)?
To quickly feather the propeller blades to avoid windmilling if oil supply to the propeller fails.
What triggers the auto-feather system in the ADLS?
Negative torque indication sensed by the torque indication system.
What is the role of the pitch increase solenoid in the auto-feather system?
To pass oil pressure to the control valve and force the propeller blades to feather.
What are the two backup locking mechanisms for pitch locking?
- Mechanical
- Hydraulic
What happens if the fine-pitch oil line pressure fails?
The mechanical pitch lock engages and prevents movement towards fine pitch.
What does the propeller overspeed governor do?
It dumps oil pressure to increase blade angle if propeller speed exceeds 105%.
What are the two functions of the propeller overspeed governor?
- Control fuel flow to hold RPM in ground range
- Limit maximum RPM in flight range
How does the governor maintain RPM during flight?
A governor slide valve spills metered fuel to maintain the given RPM.
What happens in the event of a turbine shaft failure?
A mechanical system shuts off the engine-metered fuel supply to prevent catastrophic breakup.
What mechanism may be used to activate the closure of a fuel valve during turbine shaft failure?
A centrifugal mechanical switch.
Fill in the blank: The reduction gearing can be as low as ______.
1:22
True or False: The propeller acts as a brake when overspeed occurs.
True
What is the result of the propeller driving the engine during overspeed?
The aircraft becomes uncontrollable.
What is the function of the shuttle valve in the mechanical pitch lock?
To drain the oil holding the locks out of engagement if oil pressure fails.
