15.17 - Turbo-Shaft Engines

Question 1

What Aviation machines use turbo-shaft technology?

Answer 1

Helicopters and APU’s

Question 2

Why do Auxiliary Power units need at run at a constant RPM?

Answer 2

They need to run at a constant RPM as they do not contain a CSD and so must maintain a constant RPM themselves to run the AC generator they power at a constant frequency.

Question 3

What control devices are found on helicopter engines.

Answer 3

A side ‘’collective’’ stick can be raised up and down, it alters the angle of attack of all rotor blades at the same time to increase or decrease RPM and thus altitude. If the collective stick is raised, the angle of attack of the rotor blades is increased which increases drag, slowing down RPM. As a constant rotor RPM is desired, this change in rotor blade pitch must be matched by a change in fuel flow. This fuel flow is accomplished by twisting the side ‘’collective’’ stick.

Question 4

How is the APU accelerated to 100% RPM?

Answer 4

Starter motor de-energises at around 35%. It requires ignition up to 95% and fuel metering valve via the ECB up to 100%

Question 5

What Non-Aviation related machines utilise turbo-shaft technology?

Answer 5

Boats, ships, tanks and stationary equipment requiring shaft power.

Question 6

List the 2 types of turbo-shaft engines?

Answer 6

1. APU
2. Helicopter

Question 7

What reasons are there for automatic shutdown of an APU

Answer 7

Fire

Low oil pressure

High oil temp

overspeed

High EGT

Battery switch off on ground

Slow start

Load compressor stall

Low start EGT

Loss of sensor signals

Question 8

Do turbo-shaft engines produce thrust?

Answer 8

Occasionally, a turboshaft engine is designed to produce some hot exhaust thrust (up to 10%), while some are not.

One consideration in this design is whether or not the rotor alone will produce the desired airspeed while another is whether or not the helicopter can satisfactorily hover with constant forward thrust