15.16 Turboprop Engines

Question 1

What are some advantages of a turboprop engine?

Answer 1

Provide the best specific fuel consumption of any gas turbine engine and perform efficiently from sea level to height over 6000 m

Question 2

How much power can a turboprop engine extract?

Answer 2

95%

Question 3

Where does this turboprop power get used?

Answer 3

to drive the propeller/reduction gearbox, compressor, and engine accessories, while the remainder of the energy is used as residual thrust

Question 4

What are the advantages of a free power turbine?

Answer 4

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.

Question 5

How are free power turbine engines connected?

Answer 5

With gas flow between the gas generator and free power turbine

Question 6

How are direct coupled turbine engines connected?

Answer 6

Mechanically to the compressor and reduction gear box

Question 7

How are forward motions produced?

Answer 7

By the exhaust gases passing through the additional turbine stages

Question 8

How does a turboprop engine maintain a constant speed?

Answer 8

Propeller pitch and fuel flow must be coordinated to maintain a constant speed

Question 9

At normal operating conditions what should the propeller and engine speed both be?

Answer 9

Constant

Question 10

What is the function of the reduction gear assembly?

Answer 10

To reduce high rpm from the engine to a propeller rpm that can be maintained without exceeding max propeller tip speed

Question 11

How is reduction achieved?

Answer 11

Through a series of spur and epicyclic gears

Question 12

What are spur gears used for?

Answer 12

Smaller reductions

Question 13

What are epicyclic gears used for?

Answer 13

Target high torque reduction

Question 14

What are the gears made of?

Answer 14

Quality alloy steel forging

Question 15

What are the two main types of reduction gears?

Answer 15

Parallel spur gear type and epicyclic type

Question 16

What are the features of straight cut spur gears?

Answer 16

Easier to cut and have the lowest friction losses and are very noisy

Question 17

What are the features of helical engagement?

Answer 17

Allows a more gradual onset of torque changes leading to quieter but less efficient transmission of power

Question 18

How is the reduction gear ratio figured out on a epicyclic reduction gears?

Answer 18

Calculated by dividing the number if driven teeth by the number of driving teeth

Question 19

What is a fixed annulus gear?

Answer 19

As the planet gear carrier rotates it drives the propeller shaft in the same direction

Question 20

What is a rotating annulus gear?

Answer 20

The ring gear is connected to the propeller shaft and spins in the opposite way to the power shaft

Question 21

What is the range called when in flight?

Answer 21

Alpha range

Question 22

What is the range called when on ground?

Answer 22

Beta range

Question 23

What are the major assemblies for a propeller?

Answer 23

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

Question 24

Why do modern turboprop engines use FADEC?

Answer 24

To control both the engine and the propeller

Question 25

Where is the throttle quadrant located?

Answer 25

Mounted in the centre pedestal

Question 26

What is single lever control?

Answer 26

Gear-coupled engines may be fitted with a single thrust lever where both the fuel and propeller control units are interconnected

Question 27

What are dual lever controls?

Answer 27

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

Question 28

What is a thrust lever?

Answer 28

The thrust lever will schedule fuel flow from idle to the take-off range

Question 29

What is beta control?

Answer 29

The degree of fining off into reverse is governed by the position of the thrust control lever in the Beta range.

Question 30

What is the propeller lever?

Answer 30

This lever is connected directly to the PCU and selects the required rpm by applying or releasing pressure on the governor control spring

Question 31

What is control lock?

Answer 31

The control lock ensures that the aircraft control surfaces do not flutter excessively on the ground with the engines running

Question 32

How does the turboprop balance power?

Answer 32

By presenting the blades at an angle of airflow

Question 33

What is the blade angle change controlled by?

Answer 33

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

Question 34

What should you do if the oil supply to the propeller fails?

Answer 34

Must be feathered quickly to avoid windmilling, if a negative torque is sensed a signal is sent to the auto feather system

Question 35

What happens if it goes into a negative torque?

Answer 35

A signal is sent to the auto-feather system. This energises the pitch increase solenoid in the governor and electrical feathering pump

Question 36

What are the two backup locking mechanisms?

Answer 36

Mechanical and hydraulically

Question 37

What happens if the propeller speeds exceed 105%?

Answer 37

A separate governor will dump oil pressure to the return

Question 38

What is a governor?

Answer 38

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

Question 39

What is turbine shaft failure?

Answer 39

If a power turbine detaches from its shaft, the potential turbine overspeed results in a catastrophic breakup of the engine