15.16- Turbo-prop Engines

Question 1

What is the purpose of the free-power turbine?

Answer 1

It is a separate turbine which drives the main shaft (propeller)
Only around 20% of the engines power is used by the free-power turbine

Question 2

What are the two types of turbo-prop?

Answer 2

Single-shaft (also gear coupled and direct coupled)

Free power turbine

Question 3

What is a single shaft engine? (Gear coupled/direct coupled)

Answer 3

The shaft that the propeller reduction gears are connected to are driven by the same shaft which drives the compressor for the gas generator.

Question 4

What is the purpose of idler gears in turbo-prop accessory gear boxes?

Answer 4

They reverse the rotation and can alter the final ratio of several drives

Question 5

What is a spur and pinion gear train?

Answer 5

The smaller gear (pinion) and larger gear (spur) are used to reduce the ratio of the shaft speed

Question 6

What is the advantage of using a pinion gear inside the spur gear?

Answer 6

Both gears rotate in the same direction & considerable space can be saved by using this arrangement.

Question 7

What are planetary gear systems also referred to as?

Answer 7

Epicyclic gears

Question 8

Where would you find a planetary gear system?

Answer 8

In a propeller reduction gearboxes

Question 9

What is the main advantage of a planetary gear system?

Answer 9

High torque output with low backlash.

Question 10

What is the formula for gear ratio?

Answer 10

Teeth on drive gear + teeth on fixed
Gear ratio = ————————————
Teeth on drive gear

Question 11

What is the alpha range of a turbo prop?

Answer 11

The propeller blade angle and fuel flow for any given power setting is governed automatically according to a predetermined schedule called the alpha range.

Question 12

What is the beta range of a turbo-prop?

Answer 12

A ground handling range (lower than flight idle) propeller blade angle is controlled by the lever position

Question 13

Why are turbo-prop called constant speed engines?

Answer 13

They operate at near 100% throughout the operational cycle. The propeller pitch changes to produce and reduce the thrust depending on the requirement.

Question 14

What is the CLA?

Answer 14

The condition lever angle

Question 15

What is the propeller blade angle at full feather?

Answer 15

86° +/- 5°

Question 16

What is the purpose of a propeller latch/stop?

Answer 16

They confine the angular movement of the blades within limits appropriate to the phase of flight/ground handling

Question 17

What are the overspeed and safety devices used for on turbo-prop engines?

Answer 17

They prevent an overspeed condition of the propellers, which could cause blade separation from centrifugal forces

Question 18

What is the purpose of the overspeed governor?

Answer 18

It is a backup for the propeller governor. It’s mounted on the reduction gearbox, having its own fly-weights and pivot valves, which in turn increases the propeller pitch, reducing RPM

Question 19

What is the pitch control unit on a FADEC turbo-prop engine?

Answer 19

The PCU controls the pitch and the propeller/power turbine rotor speed. It will prevent overspeed also so no damage to the engine is done.

Question 20

Where is torque measurement taken?

Answer 20

At the reduction gearbox

Question 21

How is fuel trimming controlled on a turbo-prop engine?

Answer 21

Controlled by the governor

Question 22

During takeoff, what happens to the blade angle and RPM?

Answer 22

It has a low blade angle and high RPM