15.16 Turbo-Prop Engines

Question 1

At what speed range is a turboprop engine the most efficient?

Answer 1

Between 300 to 450 mph (483-725km/h)

Question 2

How much energy can a turboprop engine extract from the total power produced to drive the propeller?

Answer 2

95% of the power

Question 3

In a turboprop engine, what is the core part commonly known as?

Answer 3

The gas generator or the gas producer

Question 4

What is a gas coupled or free power turbine?

Answer 4

A turbine which has no mechanical connection between the gas generator and the free power turbine

Question 5

What are the advantages of a free power turbine?

Answer 5

• vibrations are not directly transmitted
• easier to start
• propeller can be held at low RPM for taxiing etc (reducing wear and lower noise)
• a propeller brake can be used when engine shut down is not required but propeller movement is required to be stopped

Question 6

What is the problem with a direct coupled turbine?

Answer 6

There must be a complex control system that adjusts propeller blade angle (pitch)

Question 7

How does a direct coupled turbine differ from a free power turbine?

Answer 7

It is mechanically connected via a reduction gear to the propeller

Question 8

For turbo-prop engines, how is the intake lip designed?

Answer 8

Large lip radius preventing flow separation

Question 9

What is the disadvantage of a turboprop engine?

Answer 9

The intake lip can ingest FOD

Question 10

How is FOD prevented from entering a turboprop engine?

Answer 10

There is a movable vane which creates a severe directional change to the air, but not the high momentum particles which means they can be ejected

Question 11

What is important about the lubrication for reduction gears?

Answer 11

The viscosity of the oil is higher than comparable turbo-jet engines due to the heavier loads imposed on the gears

Question 12

What is different about reduction gears in turbo-prop engines compared to ones fitted to piston engines?

Answer 12

They have much higher reduction ratios than ones fitted to piston engines

Question 13

What type of material are reduction gears made from?

Answer 13

They are machined from high quality alloy-steel forgings

Question 14

What is a disadvantage of spur gears on turboprop engines?

Answer 14

They would be very bulky and noisy for the amount of reduction necessary

Question 15

To overcome the problems with spur gears, what type of gears are used?

Answer 15

Spur Epicyclic reduction gears which output on the same axis as the input shaft

Question 16

Where are Bevel Epicyclic gears used?

Answer 16

On piston engines - due to weight and dimension required

Question 17

What is the advantage of using helically cut gears?

Answer 17

They reduce noise and vibrations whilst providing a means of measuring torque

Question 18

What is the advantage of Compound Spur Epicyclic gears?

Answer 18

It consists of high speed and low speed gear train which gives greater speed reduction without larger components

Question 19

What is a rotating annulus gear?

Answer 19

The propeller rotates in the opposite direction to the engine

Question 20

What is a fixed annulus reduction gear?

Answer 20

The rotation of the propeller and engine are the same

Question 21

What is the advantage of using a single lever control (throttle and propeller speed control) for a turboprop engine?

Answer 21

The engine can establish the correct RPM, fuel flow and blade pitch angle.

Question 22

What does a twin lever control consist of, for a turboprop engine?

Answer 22

It consists of a power lever and a separate propeller conditioning lever

Question 23

What overspeed devices are used in a free power turbine?

Answer 23

An engine governor and an overspeed governor which can be re-datumed for ground test purposes