M15.16 Turbo prop engines

Question 1

What speed range are turboprops most efficient?

Answer 1

300 to 450 mph

Question 2

What altitudes are turboprops most efficient?

Answer 2

from sea level too 20000 feet plus

Question 3

How much thrust is typically extracted from a turboprop?

Answer 3

95%

Question 4

What are the advantages of a free power turbine engine?

Answer 4

Advantages of a free power turbine engine are:
* 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 shut down is not desired, for example, the gas
generator can be utilised as an APU.

Question 5

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

Answer 5

The direct coupled engine has an extra set of turbine disks which are still connected tot he compressor and reduction gearbox.

Question 6

What has to be integrated to ensure propper engine / prop running on a direct coupled engine?

Answer 6

Because the propeller is driven directly by the engine, a complex integrated
propeller control system is necessary to adjust the propeller pitch for the
power requirements of the engine. At normal operating conditions, both
the propeller speed and engine speed are constant. The propeller pitch and
fuel flow must be coordinated to maintain a constant speed condition. This
means that if fuel flow changes then propeller pitch must change to maintain
a constant speed

Question 7

How must the inlet of a turbo prop be disgned?

Answer 7

With a large radius lip to prevent airflow separation

Question 8

How does a reduction gearbox differ from turbo-prop to piston engines?

Answer 8

Turbo props have much higher reduction ratios

Question 9

To cope with the high stresses, reduction gears are made from?

Answer 9

High quality alloy steel forgings.

Question 10

How are the large shafts of a turbo prop supported?

Answer 10

By ball bearings

Question 11

How might a reduction gearbox be lubricated?

Answer 11

By the engine oil system or by an independent system

Question 12

What may be said about the oil used in the reduction gear lubrication system?

Answer 12

It is typically of a high viscosity to cope witht he heavier loads

Question 13

What type of gears can be found in reduction gearboxes?

Answer 13

Spur gear
Spur epicyclic
Bevel epicyclic
compound spur epicyclic
Two stage spur reduction

Question 14

How can noise and vibration be reduced in a spur epicyclic reduction gear box?

Answer 14

By using helically cut gears, these types of gear can also be used to measure torque

Question 15

Explain single lever control

Answer 15

Engine power is dependent on the mass and the temperature of the gas flow
into the turbine. The mass is controlled by varying the engine speed through a
variable pitch propeller and the temperature by varying the fuel flow.
Changes in power by increasing or reducing the fuel flow must be
accompanied by a change in the mass of air flow to maintain the correct gas
temperature at the turbine.
The propeller speed control is interconnected with the throttle to maintain
fuel and air flows in the correct ratio. This arrangement gives single lever
control of engine power to establish the correct combination of rpm, fuel flow
and propeller blade angle to create sufficient thrust.

Question 16

Explain twin lever control

Answer 16

This consists of a power lever for engine power output in the propeller
constant speed mode and propeller blade pitch directly in the beta control
mode. Propeller rpm is controlled in the constant–speed mode by movement
of the conditioning lever.

Question 17

On a free power turbine engine how is overspeed prevented during reverse pitch operation?

Answer 17

Free Power Turbine (Nf) Governor

This governor is fitted to protect the free power turbine from over speeding
during beta operation in reverse pitch, when the pitch of the propeller is being
controlled by the power lever.
It consists of a centrifugal governor sensitive to Nf ,which is operated at
between 97 and 86 Nf (Pratt & Whitney Canada PT6A-Series engines). The
lower speed being the re-datum rpm at full reverse pitch.
The unit consists of a centrifugal governor sensitive to Nf, when in the beta
range, which will re-datum through an air bleed link (reset arm) sensitive to
reverser lever movement.
As Nf rpm increases, the centrifugal bob weights will overcome the speeder
spring and an air bleed from the engine fuel control unit (FCU) opens.
This bleeding away of air is sensed by the FCU which reduces fuel flow to
compensate and slow down Ng, which in turn slows down Nf to the datum
value.