Turbine Engine Performance

Question 1

Define the following terms and describe the relationship between them, and their application to engine operation—
power

Answer 1

Rate of doing work. P = w/t

Work = Fd

Power = force x (d/t)

Power is used to describe how well an engine does the work within a timeframe. Higher power = more work.

Power = thrust x TAS since speed = distance /time

Question 2

Define the following terms and describe the relationship between them, and their application to engine operation—
Thrust

Answer 2

F=ma thrust = F.

Question 3

Define the following terms and describe the relationship between them, and their application to engine operation—
Torque

Answer 3

A force that can cause an object to rotate around an axis. Refers to the amount of work an engine can produce, while power is how quickly that work can be produced.

Dependent on the force exerted and the distance to the turning point.

Question 4

Define the following terms and describe the relationship between them, and their application to engine operation—
Gross thrust

Answer 4

The total thrust produced by the engine, when not taking into account losses from things like drag and other absorbing forces. It’s measured when engine isn’t static. Meaning its measured at mean sea level in ISA.

Question 5

Define the following terms and describe the relationship between them, and their application to engine operation—
Net Thrust

Answer 5

Thrust produced by engine in motion.

Engine produces drag, so net thrust is gross thrust - ram drag.

Question 6

Define the following terms and describe the relationship between them, and their application to engine operation—

Thrust horsepower

Answer 6

The actual thrust force generated by the accelerated airflow from a turboprop, a measure of the net thrust applied as a power rating.

Question 7

Define the following terms and describe the relationship between them, and their application to engine operation—

Shaft horsepower

Answer 7

The measure of engine power delivered to the propellor via the shaft.

A measure of the direct power output of an engine.

Question 8

Define the following terms and describe the relationship between them, and their application to engine operation—

Equivalent shaft horsepower (ESHP)

Answer 8

The shaft horsepower + any useful propulsion through the jet effluent exiting the exhaust. A measure of gross thrust in a turboprop.

Question 9

Define the following terms and describe the relationship between them, and their application to engine operation—

Specific fuel consumption (SFC)

Answer 9

SFC is the Fuel Flow divided by the thrust.

It is a measure of the efficiency of the engine in extracting the useful energy from the fuel and how well the engine converts this energy to useful thrust.

The lower the SFC the higher the efficiency of the engine.

Question 10

Describe the effect of the following factors on turbine engine performance—
(a) airspeed

Answer 10

increase in thrust causing an increase in drag (known as ram drag) and thus an overall reduction in thrust. This isn’t so hard to understand as the faster that one goes, the higher that parasite drag (and all of its components) becomes.

Question 11

Describe the effect of the following factors on turbine engine performance—
Ram effect

Answer 11

The engine produces drag when it is moving forwards, so net thrust is gross thrust minus what can be called ram drag.

This loss of thrust is theoretical and is is negated by ram effect. It’s because air is compressible and is apparent near 300kts.

F=ma so if we increase mass of air passing through engine by compression through the ram effect we increase thrust force.

Question 12

Describe the effect of the following factors on turbine engine performance—
Altitude, temp and pressure.

Answer 12

Altitude affects the performance of a turbine engine due to decreasing temp and pressure. Going back to F=ma

This is as increase with altitude air density falls and so pressure reduces and with it, the mass of air entering the engine so thrust decreases to this reduced mass of air.

Question 13

Describe the effect of the following factors on turbine engine performance—
Humidity

Answer 13

Humidity reduces the density of air, so thrust production is reduced. (Less dense air = lower mass of air entering the engine).

Water vapour can also reduce the temp of hot air that spins the turbine reducing thrust, and the compressor blades can be less efficient with a layer of moisture on them.

Effect of humidity is low tho.

Question 14

Describe the effect of the following factors on turbine engine performance—
Bleed air

Answer 14

air that is bled from the compressor of the engines - is used for a number of functions, including pressurisation, anti and de-icing, heating of fuel, sealing and cooling.

In removing some of this compressed air, the amount available for the production of thrust is lowered and consequently the amount of thrust produced (at that power setting) is lowered.

Question 15

Describe methods of thrust augmentation (afterburner) (3)
What its used on

Installation location

Build

Process

Answer 15

Afterburners - used on low-bypass turbofan engines.

Installed after the last stage turbine before the exhaust nozzle.

Near the entrance to the tailpipe (and after the turbines) is the fuel manifold which consists of fuel nozzles, which injects fuel into the tailpipe. To make sure that the fuel and air mix properly, a flame holder is used. It causes eddying of the air and fuel, ensuring that they swirl and mix together.

The fuel and the unburnt Oxygen mix, ignite and burn in the afterburner section.

Question 16

Describe the ignition system on afterburners

Answer 16

The fuel and unburnt Oxygen in the tailpipe will generally ignite thanks to the high temperatures that exist.

Accomplished via additional igniters placed near the fuel manifold.

Question 17

Describe the exhaust nozzle on afterburners

Answer 17

The exhaust nozzle on an engine with an afterburner is generally equipped with a variable exhaust nozzle to provide for operation under all conditions.

The nozzle will open or close to give an exit area suitable for the volume of the gas stream, which prevents any pressure increase from occurring in the jet pipe.

Question 18

Describe thrust augmentation (Water/methanol mix)

Answer 18

Water methanol mix cools the air and increases the density of the air being combusted.

Water is the reason that this method works and methanol is added as it acts as an anti-freeze agent. While it doesn’t evaporate (as the water does -that’s how the water raises the density/mass), methanol burns in the engine.

The water or water/methanol is injected either into the compressor inlet, or into the combustion chamber.

Question 19

Describe the propulsive efficiency of the following types of turbine engine—
(a) turboprop/turboshaft

Answer 19

More efficient at lower airspeeds. This is as faster airspeeds require faster propellor RPM but this comes at a cost of compressibility at the propellor tips (spins the fastest).

Also Propellor loses efficiency as it gains in altitude (less dense air means theres less mass of air that the prop can move rearwards)

Question 20

Describe the propulsive efficiency of the following types of turbine engine—
turbofans

Answer 20

They reach peak efficiency at higher airspeeds than a turboprop. Their efficiency grows as speed increases.

The fuel required to get to and maintain those higher speeds though means that this may not be feasible.

High bypass engines are also slightly greater in efficacy over airspeed then low bypass.

Question 21

Describe the propulsive efficiency of the following types of turbine engine—
Turbojet

Answer 21

They are extremely inefficient.

Question 22

Describe the propulsive efficiency of the following types of turbine engine—
Geared turbofans

Answer 22

a geared turbofan is a type of turbofan that has a planetary reduction gearbox between the fan and the low pressure shaft. This allows each to spin at the most efficient speed (which will be different for each).

Geared turbofans allow less stages of the low pressure compressor and turbine to be used (as that spool runs at a higher RPM to the fan), while still developing the same amount of thrust.

The fan speed can be slower, which allows higher bypass ratios, which in turn leads to reduced fuel consumption and much reduced noise.

Question 23

State the causes of the reduction in SFC with increasing airspeed in turboprop engines.

Answer 23

SFC = Fuel flow ÷ Thrust

So as equation says, increase in thrust/airspeed SFC decreases.