15.16 - Turboprop Engines

Question 1

How fast do turboprop engines go

Answer 1

300-450mph

Question 2

How much power can a turboprop engine extract from total power to drive the shaft

Answer 2

As much as 95% with the rest used as residual thrust.

Question 3

Free power turbine engines

Answer 3

No mechanical connection between the gas generator and the free power turbine.

Question 4

Advantages of a free power turbine engine.

Answer 4

Engine is easier to start especially in colder climates.
Vibration is not directly transmitted to the gas generator from the propeller and gear box.
The prop 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 shut down is not desired, allowing the gas generator to be utilised as an APU.

Question 5

Gear/ direct coupled turbines.

Answer 5

Extra turbine discs
A complex propeller control system is needed for pitch control
Propeller speed and engine speed is constant
Prop pitch and fuel flow must be co-ordinated for constant speed conditions.

Question 6

Reduction gears.

Answer 6

Reduce rpm to a propeller rpm that can be maintained without exceeding maximum tip speed ( speed of sound).
Spur gears and epicyclic gears used.
Large and high torque engines use epicyclic.
Has own oil with higher viscosity than turbojet and turbofan oil due to heavier loads.

Parallel spur gears are simple
Straight cut have lower frictional losses but are noisier.
Helical engagement given a more gradual onset of torque.

Question 7

Epicyclic gear

Answer 7

Used as reduction gears for turboprop.
Fixed annulus=prop shaft turns in same direction
Rotating annulus = prop shaft spins in opposite direction.

Question 8

What is alpha and beta range

Answer 8

In reference to turboprops.
Alpha = flight
Beta = ground ops.

Question 9

Why do turboprops have feathering assemblies

Answer 9

For when engine is shut down in flight.

Question 10

Single lever turboprop controls

Answer 10

Gear coupled engines.
Prop control unit (PCU) governs the max rpm of the engine.

Question 11

What is dual lever turboprop controls

Answer 11

For free power turbine engines.
One lever is for power the other is for prop control.

Question 12

Automatic drag limiting system

Answer 12

If oil supply to prop fails it is feathered to stop windmilling and reduce drag.
A pitch increase solenoid force it fully up.

Question 13

Prop over speed govenor

Answer 13

If prop speed exceeds 105% a separate govenorwill dump oil pressure to the return - blade angle to increase.