15.16 Turboprop Engines Flashcards
What are some advantages of a turboprop engine?
Provide the best specific fuel consumption of any gas turbine engine and perform efficiently from sea level to height over 6000 m
How much power can a turboprop engine extract?
95%
Where does this turboprop power get used?
to drive the propeller/reduction gearbox, compressor, and engine accessories, while the remainder of the energy is used as residual thrust
What are the advantages of a free power turbine?
Vibration is not directly transmitted to the gas generator from the propeller/reduction gearbox.
Engine is easier to start, especially in cold weather.
The propeller can be held at low rpm during taxiing, giving low noise and low blade erosion.
A propeller brake can be used to prevent propeller movement during aircraft loading when engine shutdown is not desired, for example, the gas generator can be utilised as an APU.
How are free power turbine engines connected?
With gas flow between the gas generator and free power turbine
How are direct coupled turbine engines connected?
Mechanically to the compressor and reduction gear box
How are forward motions produced?
By the exhaust gases passing through the additional turbine stages
How does a turboprop engine maintain a constant speed?
Propeller pitch and fuel flow must be coordinated to maintain a constant speed
At normal operating conditions what should the propeller and engine speed both be?
Constant
What is the function of the reduction gear assembly?
To reduce high rpm from the engine to a propeller rpm that can be maintained without exceeding max propeller tip speed
How is reduction achieved?
Through a series of spur and epicyclic gears
What are spur gears used for?
Smaller reductions
What are epicyclic gears used for?
Target high torque reduction
What are the gears made of?
Quality alloy steel forging
What are the two main types of reduction gears?
Parallel spur gear type and epicyclic type
What are the features of straight cut spur gears?
Easier to cut and have the lowest friction losses and are very noisy
What are the features of helical engagement?
Allows a more gradual onset of torque changes leading to quieter but less efficient transmission of power
How is the reduction gear ratio figured out on a epicyclic reduction gears?
Calculated by dividing the number if driven teeth by the number of driving teeth
What is a fixed annulus gear?
As the planet gear carrier rotates it drives the propeller shaft in the same direction
What is a rotating annulus gear?
The ring gear is connected to the propeller shaft and spins in the opposite way to the power shaft
What is the range called when in flight?
Alpha range
What is the range called when on ground?
Beta range
What are the major assemblies for a propeller?
Barrel
Dome
Low-pitch stop assembly
Overspeed governor
Pitch control unit
Auxiliary pump
Feather and unfeather valves
Torque motor
Spinner
Deice timer
Beta feedback assembly
Propeller electronic control
Why do modern turboprop engines use FADEC?
To control both the engine and the propeller
Where is the throttle quadrant located?
Mounted in the centre pedestal
What is single lever control?
Gear-coupled engines may be fitted with a single thrust lever where both the fuel and propeller control units are interconnected
What are dual lever controls?
Dual-lever controls are normally used on turbopropeller engines with a free turbine. One lever is a power lever that has two distinct ranges of operation, and the other is a propeller control lever
What is a thrust lever?
The thrust lever will schedule fuel flow from idle to the take-off range
What is beta control?
The degree of fining off into reverse is governed by the position of the thrust control lever in the Beta range.
What is the propeller lever?
This lever is connected directly to the PCU and selects the required rpm by applying or releasing pressure on the governor control spring
What is control lock?
The control lock ensures that the aircraft control surfaces do not flutter excessively on the ground with the engines running
How does the turboprop balance power?
By presenting the blades at an angle of airflow
What is the blade angle change controlled by?
The blade angle change mechanism is hydraulically operated by the engine oil. If the engine were to fail in flight, the controlling oil pressure can be lost, and the blades will try to return to a zero angle under the influence of the centrifugal twisting moment
What should you do if the oil supply to the propeller fails?
Must be feathered quickly to avoid windmilling, if a negative torque is sensed a signal is sent to the auto feather system
What happens if it goes into a negative torque?
A signal is sent to the auto-feather system. This energises the pitch increase solenoid in the governor and electrical feathering pump
What are the two backup locking mechanisms?
Mechanical and hydraulically
What happens if the propeller speeds exceed 105%?
A separate governor will dump oil pressure to the return
What is a governor?
The governor is a safety device designed to operate if the normal propeller governor and the fuel control overspeed governor fail to contain the engine speed
What is turbine shaft failure?
If a power turbine detaches from its shaft, the potential turbine overspeed results in a catastrophic breakup of the engine
