Pure Theory

Question 1

Draw graphs of thrust/airflow as a function of Mach number and Specific fuel consumption as a function of Mach number. Indicate the regions of the graphs where the following engines would be expected to operate; Turbojet, High bypass ratio turbofan, Low bypass ratio turbofan, Turboprop.

Answer 1

2015 Q1b

Question 2

On a graph of fuel consumption versus specific thrust, indicate the desired trend for future technology.

Answer 2

2015 Q1c

Question 3

Describe the principles of operation of a propeller

Answer 3

Propellers are made up of aerofoil sections designed to generate an aerodynamic force.
During operation, the pressure difference of the aerofoil provides thrust to push the
airplane through the air

Question 4

Draw a diagram of propeller efficiency as a function of J and hence describe why variable pitch propellers are commonly utilised.

Answer 4

With variable pitch propeller, the propeller pitch can be continuously varied to maintain
maximum efficiency at all flight velocities. This can be visualised as riding along the
peaks of the propeller efficiency curves shown earlier, as shown by the dotted line.

Question 5

Under what conditions is propeller efficiency equal to zero? Why does this occur?

Answer 5

Consider a propeller with given n and D, J depends only on 𝑉∞. When 𝑉∞= 0 then J = 0.
Thus, propeller efficiency is zero at J = 0 because there is no motion of the airplane, and
hence no power available.
At the other extreme, when 𝑉∞ (and hence J) is made large, the propeller loses lift owing
to small angles of attack. Also, when the propeller tip speeds are near sonic, η drops
dramatically due to shock wave and boundary-layer separation losses.

Question 6

Based on the Otto cycle an indicated power can be calculated for a piston engine.However, a significantly lower value of power will be determined as thrust power.Explain the contributing factors that lead to this decreased value.

Answer 6

Three major factors contributed to power loss:
Mechanical loss: Delay in valve and piston timing, mechanical wear of seals
Heat loss: heat loss to the sidewalls and other engine parts
Combustion loss: incomplete combustion, incorrect mixture ratio, poor flame propagation
and poor combustion chamber design

Question 7

What is the difference between the Froude efficiency,nF, and the actual propulsive efficiency,np.

Answer 7

Froude efficiency ignores all losses except that associated with the streamwise kinetic
energy; Actual propulsive efficiency takes every loss into account.

Question 8

Describe the principles of operation of the gas turbine engine.

Answer 8

The gas turbine engine has four stages of work: suck, squeeze, bang and blow.
When the compressor turns, it generates a low air pressure which sucks ambient air into
the inlet. The airflow is then squeezed by rotating blades in the compressor to increase the
air density, pressure and temperature before entering the combustion chamber.
Once it reached the combustion chamber, fuel is added to form a fuel air mixture, then
burned at constant pressure to generate a high velocity flow by converting the chemical
energy of fuel into kinetic energy of the airflow.
The high energy exhaust is then blasted through the turbines, which is connected to the
compressors by the same shaft. The turbines convert some of the kinetic energy in the
flow into mechanical energy that drives the shaft, therefore drives the compressor and
produces power.
The flow is then passing through the exhaust nozzle to convert the remaining kinetic
energy into thrust.

Question 9

Explain why afterburning may be used in the gas turbine engine.

Answer 9

The afterburner uses unburned oxygen in the jet exhaust to burn additional fuel.
An extended exhaust pipe is placed between the turbine and the final nozzle. With flame-
on, the extra heat energy is added to the gas stream, thus raises the gas temperature and
local sonic speed.
With afterburner, the gas can be accelerated to a higher sonic choking speed in the nozzle
and thereby generate higher thrust.

Question 10

Describe processes by which the efficiency of the gas turbine engine has been increased. Your answer should focus mainly on developments in the compressor and turbine sections of the engine.

Answer 10

For compressors:
* Reducing tip losses due to leakage between moving rotors and stationary walls (tighter
clearance)
* Minimizing surface/volume ratios
* Developing low-drag laminar-flow blade cascades
* Careful study of 3-D flow fields to minimize secondary flow loss
* Using centrifugal compressors for small engines

For turbines:
* Using single-crystal structure blades, especially nozzle guide vanes
* Implementing multi-pass cooling in turbine blades
* Applying thermal barrier coating to the blades
* Casting bleed air holes into the blades’ trailing edge
* Manufacturing the blades and discs as a single structure

Question 11

Describe how the introduction of blisks and blings in the gas turbine leads to improved efficiency.

Answer 11

Manufacturing blades and discs as a single structure (blisk) removes the need for leakage
paths and blade fixings, resulting in weight savings of up to 30 per cent.
Bladed rings (blings) can save even more weight by removing the disc inner ring with the
remaining ring carrying all the centrifugal loads.
Manufacture and repair techniques also contribute to making blisks the best life-cycle
value solution.

Question 12

Describe, with the aid of diagrams, the principle of operation of the two stroke andfour stroke piston engines. You should include comments about the differencesbetween the two engine types, the stroke, compression ratio, bore and swept volume.

Answer 12

As the name implies, two stroke engines complete all the events in a two-stroke cycle. In
two strokes the crankshaft makes one revolution, so the two-stroke cycle is complete in
one revolution. Since the incoming air-fuel mixture passes through the crankcase on its
way to the cylinder, the crankcase is made gas-tight.

Four stroke engines complete all the events in a four-stroke cycle, two up and two down
in two revolution. Since the crankshaft has made two revolutions with only one power
stroke and each valve open once only during the cycle, the cams must rotate once only in
two revolutions of the crankshaft. Thus, the camshaft is geared down 1:2 to the
crankshaft.

Two stroke engines have a larger swept volume than four stroke engines, therefore, with
the same piston bore, two stroke engines should have a higher compression ratio.

Question 13

Draw pressure-volume diagrams for the two stroke and four stroke piston engines indicating the main features of the cycle.

Answer 13

2015Q4b

Question 14

Describe briefly the evolution of the piston engine in the first half of the 20th century,including the reasons why larger engines were gradually replaced by gas turbine engines.

Answer 14

By the 1950s, the power/weight ratio and power output of piston engines significantly
increased due to the higher speed and performance demand for military aircraft; and the
need for increased speed, reliability, longer life and greater safety in the operation of civil
transport aircraft.

The specific weight of civil transport engines had been reduced from 9kg/kW of the
original Wright brother’s engine to 0.6kg/kW and the power of operational engines had
peaked at 4300hp (3200kW). It was during this period however that piston engines began
being supplanted by gas turbines.

Question 15

What is the difference between a ramjet and a scramjet?

Answer 15

The combustion process in an ordinary ramjet takes place at low subsonic
velocities; the combustion process of scramjet takes place at supersonic velocities.

Question 16

Describe, with the aid of diagrams or otherwise, how the geometry of ramjets and
scramjets may be utilised to control combustion mach number.

Answer 16

Lighter fixed-geometry engines are then possible by the simpler method of
adapting the fuel flow input rate to the varying airflow as flight speed is changed.
Heat added in supersonic combustion to the constant-area (Rayleigh) combustor
design would generate an increasing static pressure pb and falling supersonic Mach
number Mb between an inlet and exit of the combustor. To prevent choking (Mb = 1
at the combustor exit to combustor) or to sustain constant pb, or constant Mb
combustion, the flow cross section area will have to increase in the combustion
zone.

2014 Q1b

Question 17

By considering the relative wind speed observed by a rotating propeller, explain why
propeller blades are generally twisted, i.e. the pitch angle, , is a function of the
radius, r.

Answer 17

Swept velocity is determined by radius (r) multiply by rotational velocity (ω). In
this case, airstream velocity (V∞) and ω are constant from root to tip. Since radius
is smaller at root, its swept velocity is smaller, where swept velocity is larger at tip.
Pitch angle (β) must be larger at root [see (a)] and smaller at tip [see (b)] to
maintain the same angle of attack (α).

Question 18

For a typical gas turbine engine, plot a diagram illustrating the change in gas velocity,
temperature and pressure for each stage of the engine.

Answer 18

2014 Q3a

Question 19

Describe the operation of the compressor in a gas turbine engine. Include in your
answer comments about temperature and pressure changes across an axial
compressor.

Answer 19

Compressor in a gas turbine converts the kinetic energy of the drive shaft into the
potential energy of the airflow. Axial compressor has multiple steps of rotors and
stators, their blades form divergent ducts, which slow down the incoming air flow
and increase the air pressure and temperature. The annulus height is decreasing
throughout the length of the axial compressor to increase air density and maintain
constant axial flow speed.

Question 20

Describe the role of rotors and stators in a gas turbine compressor.

Answer 20

The blades of the rotors and stators are installed with an angle to ensure effective
capture of the incoming airflow. when the rotors turn, they suck in air and
straighten the airflow by the blade geometry. Each step of rotors is followed by a
step of stators to further pressurize and straighten the airflow.

Question 21

Describe the key differences between a two shaft and a three shaft engine in terms of
turbine and compressor components.

Answer 21

In three shaft engines, the big fan, high-pressure and the low-pressure compressors
are driven by three different shafts so that they operate at different rpm; in two
shaft engines, the big fan and low-pressure compressors are driven by the same
shaft.

Question 22

The thrust, T, of a gas turbine engine for the condition when exit pressure is equal to
ambient air pressure is given by

wherem
 is the mass flow rate, Ve is the exit velocity and V0 is the air speed. Describe
two methods of increasing thrust and the engine types for which these principles are
applied.

Answer 22

Increase the mass flow rate (turbofan) or increase the exit velocity (turbojet).

Question 23

What is the difference between thrust, T, at the design condition and maximum thrust
of a rocket?

Answer 23

Maximum engine thrust only occurs in a vacuum (pa = 0) when our thrust equation
becomes: 𝑇𝑚𝑎𝑥 = 𝑚̇ 𝑉𝑒 + 𝑝𝑒 𝐴𝑒

While generating maximum thrust in outer space (pa = 0), rocket engines also
operate at optimum thrust only if pe = 0 as well. However, this requires an infinitely
large nozzle area expansion (Ae/A* = ∞), which is not realistic.

Question 24

Describe why a ramjet travelling at supersonic velocities may use a diffuser prior to
the combustion process

Answer 24

Ramjet engine without compressors has seemed as a logical evolution of the gas turbine appropriate to
supersonic flight speeds, it is because the compressor efficiency drops dramatically due to shock wave
and boundary-layer separation losses when the blade tip speeds are near sonic.

The removal of the gas turbine rotors simplifies the engine mechanical configuration, which eliminates
mechanical loss that appears in in gas turbine engines. It also prevents swirling, as in purely axial flow,
there are no radial components of airstream velocity, the tangential velocity would be larger, which
directly contributes to larger thrust generation.

Question 25

Either graphically or by description, indicate how the following quantities vary
through all points from the diffuser to the entry to the combustion chamber; pressure,
temperature, velocity, stagnation pressure, stagnation temperature and density

Answer 25

The inlet or diffuser slows the air velocity relative to the engine from the flight velocity V0 to a smaller
value V3. This decrease in velocity increases both the static pressure P3 and static temperature T3.
However, the stagnation pressure and temperature remain unchanged for an ideal ramjet engine as the
process is assumed isentropic

Question 26

Explain the difference between a ramjet and a scramjet

Answer 26

The combustion process in an ordinary ramjet takes place at low subsonic velocities; the combustion
process of scramjet takes place at supersonic velocities

Question 27

State, and describe the function of, the four main components of a solid rocket motor.

Answer 27

A simple solid rocket motor has four main components:
* A combustion chamber, which stores and contains propellant during high-pressure burning
* An igniter, which starts the propellant burning
* The solid propellant: burns and produces gases for propulsion
* A nozzle: expands the combustion product gases to high velocity, thus generating thrust

Question 28

What loads would the walls of the combustion chamber be required to withstand

Answer 28

The walls of the chamber must be sufficiently thick to withstand stresses from various sources:
* Pressure load during combustion
* Thermal stresses from high temperature combustion and possible aerodynamic heating
* Dynamic loads during launch and flight manoeuvres
* Clustering and staging loads

Question 29

What is the difference between thrust, T, at the design condition and maximum thrust
of a rocket?

Answer 29

Maximum engine thrust only occurs in a vacuum (pa = 0) when our thrust equation becomes:
𝑇𝑚𝑎𝑥 = 𝑚̇ 𝑉𝑒 + 𝑝𝑒 𝐴𝑒
While generating maximum thrust in outer space (pa = 0), rocket engines also operate at
optimum thrust only if pe = 0 as well. However, this requires an infinitely large nozzle area
expansion (Ae/A* = ∞), which is not realistic.

Question 30

Describe, using diagrams if required, what is meant by progressive, neutral and
regressive burning in a rocket motor. Sketch potential propellent geometries for
progressive, neutral and regressive burning

Answer 30

Progressive burning: Burning area increases thus chamber pressure and thrust increase with
time.
Neutral burning: Burning area, chamber pressure, and thrust remain practically constant with
time.
Regressive burning: Burning area decreases thus both chamber pressure and thrust decrease
with time.

Question 31

Explain what is meant by mean effective pressure

Answer 31

It is a fictitious pressure that, if it acted on the piston during the entire power stroke, would
produce the same amount of net work as that produced during the actual cycle

Question 32

Describe the operation of the four stroke piston engine.

Answer 32

Four stroke engines complete all the events in a four-stroke cycle, two up and two down in two
revolution.

At the intake stage, the inlet valve is fully open and the exhaust valve closed. The piston is
descending so it is sucking a fresh air-fuel charge into the cylinder through the open inlet valve.
Toward the bottom of the suction stroke the inlet valve begins to close.
Moving on to the combustion stage, both the inlet and exhaust valves are closed. The piston is
ascending and is compressing the fresh air-fuel charge into the combustion space. Ignition of
the charge occurs towards the top of this stroke; once the stroke is completed, the piston again
begins to descend on the power stroke.

During the power stroke stage, both the inlet and exhaust valves are again closed. The air-fuel
charge has been ignited and the combustion products are rapidly expanding, pushing the piston
down on its power stroke. Toward the bottom of the power stroke the exhaust cam has rotated
such that it begins to lift the exhaust valve while the inlet valve remains closed.

Finally, at the exhaust stage, the exhaust valve is fully open and the piston is ascending. As the
piston ascends, it pushes the combustion products out through the open exhaust port. Towards
the top of the exhaust stroke the exhaust valve begins to close and the inlet valve begins to open
as a result of cam action.

Question 33

Describe why the indicated power based on the Otto cycle may not be accurate in
thrust based calculations.

Answer 33

Three major factors contributed to power loss:
Mechanical loss: Delay in valve and piston timing, mechanical wear of seals
Heat loss: heat loss to the sidewalls and other engine parts
Combustion loss: incomplete combustion, incorrect mixture ratio, poor flame propagation
and poor combustion chamber design

Question 34

Describe the operation of a gas turbine engine

Answer 34

The gas turbine engine has four stages of work: suck, squeeze, bang and blow.

When the compressor turns, it generates a low air pressure which sucks ambient air into the
inlet. The airflow is then squeezed by rotating blades in the compressor to increase the air
density, pressure and temperature before entering the combustion chamber.
Once it reached the combustion chamber, fuel is added to form a fuel air mixture, then burned
at constant pressure to generate a high velocity flow by converting the chemical energy of fuel
into kinetic energy of the airflow.

The high energy exhaust is then blasted through the turbines, which is connected to the
compressors by the same shaft. The turbines convert some of the kinetic energy in the flow
into mechanical energy that drives the shaft, therefore drives the compressor and produces
power.

The flow is then passing through the exhaust nozzle to convert the remaining kinetic energy
into thrust.

Question 35

For a typical gas turbine engine, plot a diagram illustrating the change in gas velocity,
temperature and pressure for each stage of the engine.

Question 36

Describe the operation of the compressor in a gas turbine engine. Include in your
answer comments about temperature and pressure changes across an axial
compressor

Answer 36

Compressor in a gas turbine converts the kinetic energy of the drive shaft into the potential
energy of the airflow. Axial compressor has multiple steps of rotors and stators, their blades
form divergent ducts, which slow down the incoming air flow and increase the air pressure and
temperature. The annulus height is decreasing throughout the length of the axial compressor to
increase air density and maintain constant axial flow speed.

Question 37

Explain the difference between installed and uninstalled thrust specific fuel
consumption and why the differences arise

Answer 37

The relationship between installed thrust specific fuel consumption (S) and uninstalled thrust
specific fuel consumption (TSFC) is given by:

𝑆 = 𝑇𝑆𝐹𝐶(1 − 𝛷𝑖𝑛𝑙𝑒𝑡 − 𝛷𝑛𝑜𝑧𝑧𝑙𝑒)

Where 𝛷𝑖𝑛𝑙𝑒𝑡 and 𝛷𝑛𝑜𝑧𝑧𝑙𝑒 represents inlet loss coefficient and nozzle loss coefficient
respectively. These two coefficients are directly related to the friction drag from the inlet and
nozzle, which decrease the installed engine thrust value and thus decrease the value of S,
making S always smaller than TSFC.

Question 38

Describe the key differences between a two shaft and a three shaft engine in terms of
turbine and compressor components.

Answer 38

In three shaft engines, the big fan, high-pressure and the low-pressure compressors are driven
by three different shafts so that they operate at different rpm; in two shaft engines, the big fan
and low-pressure compressors are driven by the same shaft.

Question 39

The thrust, T, of a gas turbine engine for the condition when exit pressure is equal to
ambient air pressure is given by

where is the mass flow rate, Ve is the exit velocity and V0 is the air speed. Describe
two methods of increasing thrust and the engine types for which these principles are
applied.

Answer 39

Increase the mass flow rate (as in turbofan engine design) or increase the exit velocity (as in
turbojet engine design).

Question 40

Describe the operation of a gas turbine engine

Answer 40

The gas turbine engine has four stages of work: suck, squeeze, bang and blow.

When the compressor turns, it generates a low air pressure which sucks ambient
air into the inlet. The airflow is then squeezed by rotating blades in the compressor
to increase the air density, pressure and temperature before entering the
combustion chamber.

Once it reached the combustion chamber, fuel is added to form a fuel air mixture,
then burned at constant pressure to generate a high velocity flow by converting the
chemical energy of fuel into kinetic energy of the airflow.
The high energy exhaust is then blasted through the turbines, which is connected
to the compressors by the same shaft. The turbines convert some of the kinetic
energy in the flow into mechanical energy that drives the shaft, therefore drives
the compressor and produces power.

The flow is then passing through the exhaust nozzle to convert the remaining
kinetic energy into thrust.

Question 41

Gas turbine engines can be classified into the following; turbojets, turbofans,
turboprops, turboshafts, ramjets. Describe what is meant by each of these and how
they may be used.

Answer 41

The thrust of the turbojet and ramjet result from the action of a fluid jet leaving the
aircraft (Hence the name jet engine).

Turbojet uses rotatory compressors for compression, and it is mostly used in
supersonic aircrafts with operating speed lower than Mach 3; ramjet uses ram
effect for compression, and it is used in supersonic missiles and aircrafts with
operating speed higher than Mach 3.

Turbofan, turboprop and turboshaft engines are adaptations of the turbojet to
supply thrust or power using fans, propellers or shafts.
Turbofan engines are mostly used in civil passenger aircrafts; turboprop engines
are used in small civil aircrafts and large cargo planes; turboshaft engines are used
in helicopters.

Question 42

What is meant by the bypass ratio of a gas turbine, and how is bypass air beneficial to
overall efficiency?

Answer 42

Bypass ratio is the mass flow rate of air through fans, propellers or helicopter blades over the
mass flow rate of air through the gas generator. High bypass ratio indicates a larger mass of air
is accelerated to a lower velocity for a higher propulsive efficiency, which increase the overall
efficiency.

Question 43

Based on these calculations, describe and explain the operational envelope of an ideal
ramjet.

Answer 43

Base on the calculation, we can see that with a higher specific thrust, less fuel
will be used. As fuel air ratio is constant in this calculation, the maximum flight
Mach number will appear at where Thrust specific fuel consumption is infinitely
close to zero, which sets a speed range of an ideal ramjet.

However, since the geometry is not included in parametric cycle analysis, the
plots of specific thrust and thrust specific fuel consumption, against Mach
number are not portraying the behaviour of a specific engine.

Question 44

What is the difference between static and stagnation enthalpy?

Answer 44

Stagnation enthalpy is also known as total enthalpy, it represents the total energy
of a flowing fluid stream per unit mass, which remain constant in an adiabatic
situation. Static enthalpy only represents the potential energy of fluid stream per
unit mass, which doesn’t include the kinetic energy part. However, if kinetic
energy is zero, stagnation enthalpy will equal to static enthalpy.

Question 45

On an enthalpy-entropy diagram, how does the actual stagnation state differ from the
isentropic stagnation state?

Answer 45

the stagnation enthalpy of the fluid is the same for both cases. However, the
actual stagnation pressure is lower than the isentropic stagnation pressure since
the entropy increases due to fluid friction.

Question 46

Explain the principle of a converging-diverging nozzle.

Answer 46

When air enters the converging nozzle, its pressure, temperature and velocity of
sound decrease while the fluid velocity and Mach number increase in the flow
direction. The density decreases slowly at first and rapidly later as the fluid
velocity increases. The Mach number is unity and pressure reach critical
condition at the location of the smallest flow area, called the throat. After that,
the air reached the divergent nozzle where the velocity of the fluid keeps
increasing with further reductions in pressure, although the flow area increases
rapidly in that region. This increase in velocity past the throat is due to the rapid
decrease in the fluid density.

Question 47

Discuss the reasons why the thurst power generated by a piston engine-propeller
combination will be significantly lower than the indicated power derived from
analysis of the Otto cycle.

Answer 47

Thrust Power (TP) = 𝜂𝑝 × 𝜂𝑚 × 𝜂𝑐 × 𝜂𝑏 × 𝐼𝑛𝑑𝑖𝑐𝑎𝑡𝑒𝑑 𝑝𝑜𝑤𝑒𝑟(𝐼𝑃)

Where ηp, ηm, ηc and ηb are propulsive, mechanical, cooling and combustion
efficiencies respectively.
Propulsive efficiency is directly related to the propeller design;
Mechanical efficiency accounts for the power loss by mechanical means, such as
friction loss within the joints, seals, bearings and gears;
Cooling efficiency accounts for the heat loss by cooling, which can be
substantial;
Combustion efficiency accounts for incomplete displacement of exhaust gas with
fresh charge due to engine capacity and ambient pressure conditions.
Combustion/exhaust processes are not instantaneous, and valves take time to
open and close, this rounds off the corners of the cycle.
The piston pumping work is also finite, which is shown as a negative work loop
on the p-V diagram.
Due to the reasons and factors stated above, thrust power will always be smaller
than indicated power.

Question 48

What methods have traditionally been applied to increase piston engine output, and
what are the associated problems?

Answer 48

Piston engine input can be increased by supercharging.

Evaporative cooling: Evaporative cooling of the intake charge by excess fuel
and/or water injection raises the density, the charge-mass flow, and hence the
power output.

Increasing engine speed: Other things being unchanged, power output is directly
proportional to engine speed n. However, the penalty is increased piston speed
and therefore engine wear.

Increasing engine compression ratio: Power output rises in direct proportion to
bmep. However, again, increased mechanical loading of the engine may degrade overtime

Improving volumetric efficiency: This can be obtained with multiple valves and
manifolds. Such designs “breathe easier”, increase volumetric efficiency, and
increase the charge-mass flow.

Increasing the number of cylinders: If the physical aspects of the cylinder are
optimised, power output can be increased only by adding more cylinders. The
number has gone as high as 28. however, it becomes more mechanically complex
and reduces its reliability. Further, significant improvement in specific power
output has only been achieved by supercharging.

Question 49

Describe how propeller pitch angle is determined in relation to the angle of attack, .
What is the advantage of variable pitch propellers?

Answer 49

TA is function of β, and TA is basically an aerodynamic phenomenon which is
dependent on angle of attack α.
With variable pitch propeller, the propeller pitch can be continuously varied to
maintain maximum efficiency at all flight velocities. This can be visualised as
riding along the peaks of the propeller efficiency curves shown earlier, as shown
by the dotted line.

Question 50

Describe the four forces on a fluid element which are considered when
deriving thrust equations

Answer 50

End Face Force
Sidewall Force
Friction Force
Body Force

Question 51

Describe the operation of the gas turbine engine.

Answer 51

The gas turbine engine has four stages of work: suck, squeeze, bang and blow.

When the compressor turns, it generates a low air pressure which sucks ambient
air into the inlet. The airflow is then squeezed by rotating blades in the compressor
to increase the air density, pressure and temperature before entering the
combustion chamber.

Once it reached the combustion chamber, fuel is added to form a fuel air mixture,
then burned at constant pressure to generate a high velocity flow by converting the
chemical energy of fuel into kinetic energy of the airflow.
The high energy exhaust is then blasted through the turbines, which is connected
to the compressors by the same shaft. The turbines convert some of the kinetic
energy in the flow into mechanical energy that drives the shaft, therefore drives
the compressor and produces power.

The flow is then passing through the exhaust nozzle to convert the remaining
kinetic energy into thrust.

Question 52

Describe the operation of an axial compressor in a gas turbine engine. How
does a centrifugal compressor differ from this and why do modern engine
designs show a preference for axial compressors?

Answer 52

Compressor in a gas turbine converts the kinetic energy of the drive shaft into the
potential energy of the airflow. Axial compressor has multiple steps of rotors and
stators, their blades form divergent ducts, which slow down the incoming air flow
and increase the air pressure and temperature. The annulus height is decreasing
throughout the length of the axial compressor to increase air density and maintain
constant axial flow speed.

Question 53

Describe the manufacturing process for a modern jet engine turbine blade,
which allows for operation in a gas stream that exceeds the melting
temperature of the turbine blade.

Answer 53

Interpass cooling - Passages are designed into the turbine blades to ensure long life whilst operating at high temperatures. Cool air (bleed air) is taken from the compressor and fed around the combustor into the blades to cool the aerofoils.

Thermal barrier coatings are also used to protect the turbine blade from oxidisation and hot corrosion.

Question 54

In order to save further weight in the gas turbine engine, designers are
considering the use of ‘blisks’ and ‘blings’. What are these components and
what are the manufacturing methods and materials which allow them to be
used in service?

Answer 54

A blisk is a blade and disk manufactured as a single piece, it removes the need for leakage paths and blade fixings; resulting in weight savings of upto 30%.

A bling is a bladed ring and saves even more weight than a blisk.

Blisks have better life cycle value due to better manufacture and repair techniques

Smaller Blisks are manufactured from a solid block whilst larger Blisks are made by friction welding a forged aerofoil onto the disk.

A titanium metal matrix composite with embedded silicon carbide fibres to carry stresses allow Blings to be 100% stiffer and 50% stronger than unreinforced titanium

Question 55

What materials would typically be used in a piston engine and what part of the
engine is likely to operate at the highest temperature?

Answer 55

The cylinder head is the hottest part of the piston engine.

Due to low cost, steels most often used, cylinder heads most likely to be hottest. Cr content reflects costs – higher Cr content, more temperature capability. Switch to titanium (similar temperature capability) in order to save weight if cost effective.

Question 56

Explain how upper temperature limits in a ramjet can be avoided if supersonic
combustion is utilised

Answer 56

High flight speeds of ramjets lead to high overall stagnation temperatures and pressures. If subsonic combustion is required (ramjet) then reduction in gas speed leads to high static pressure and temperature, which may exceed material capability or lead to energy loss in dissociation reactions. By allowing supersonic combustion (scramjet) static temperature and pressures can be reduced

Question 57

Why are composite materials likely to be used in future jet engine production?
Describe two potential areas of use and what some of the limitations may be.

Answer 57

Composite materials could be used in fan blades or the compressor disk. They are likely to be used in future jet engine production due to being stiffer, stronger and lighter than conventional materials.

However, the cost of producing the composite in addition to the complexities of manufacturing them could act as limitations.

Question 58

Why may liquid rocket motors be preferable to solid rocket motors

Answer 58

Liquid rocket motors can be throttled and turned on or off at the pilots discression. This is in contrast to solid rocket motors which, when activated, will only turn off once the propellant has run out.

However, liquid propellants are heavier and more complex than solid rocket propellants.

Question 59

Describe the three main categories of solid propellants used in rocket fuel

Answer 59

Double Base Propellant - Molecule contains both the fuel and oxidiser, is highly explosive

Composite Propellant - Made up of crystalline oxidiser and powdered fuel in a hydrocarbon binder

Composite double base propellant - Contains oxidiser and powdered aluminium in a matrix of double based propellant

Question 60

What are the four main components of a solid rocket motor?

Answer 60

Combustion Chamber - Stores and contains propellant during high pressure burning
Ignitor - Ignites the propellant

Propellant - Burnt to produce high velocity gas for propulsion

Nozzle - Expands combustion products to high velocity gases which generates thrust

Question 61

Draw and describe the Brayton Cycle upon which the gas turbine operation is BASED

Answer 61

Fresh, ambient air is drawn into the compressor where the temperature and pressure are raised.

High pressure air is then burnt at constant pressure in the combustion chamber.

The high temperature gases are then sent through the turbine where they are expanded to atmospheric pressure which produces power.

Exhaust gases are then thrown out and therefore the cycle is classified as an open cycle.

1-2: Isentropic Compression
2-3: Constant pressure heat addition
3-4: Isentropic expansion
4-1: Constant pressure head rejection

Question 62

Turbine entry temperatures provide a limiting consideration for the gas turbine
engine. Describe the technologies and materials that now allow for operation in gas
streams of over 1800K.

Answer 62

Modern turbine blades use thermal barrier coatings which allows for turbine temperatures of greater than 1550C. Complex high temperature alloys, including nickel, are used in the turbine.

The first row of turbine blades are critical. Metallurgical innovations allowed for solidified blades or even single crystal blades.

Forced air convection and film cooling surround the blades in a protective layer of cooler bleed air which is fed through internal passages in the turbine blades.

Question 63

What benefits are offered by a composite fan blade as opposed to more traditional
titanium fan blades?

Answer 63

Composite fan blades enable their blades to be lighter than conventional titanium blades in addition to being 100% stiffer and 50% stronger than titanium.

Question 64

What is the difference between static and stagnation enthalpy?

Answer 64

Stagnation enthalpy is also known as total enthalpy, it represents the total energy
of a flowing fluid stream per unit mass, which remain constant in an adiabatic
situation. Static enthalpy only represents the potential energy of fluid stream per
unit mass, which doesn’t include the kinetic energy part. However, if kinetic
energy is zero, stagnation enthalpy will equal to static enthalpy.

Question 65

Describe, for high speed aircraft, how a limiting cycle temperature leads to the
requirement for supersonic combustion as flight speeds increase.

Answer 65

At high speeds, stagnation pressure and pressures are experienced by an engine. To combust at subsonic velocities, the reduction in gas speed leads to high static pressures and temperatures which might exceed the material properties of the engine.

By allowing combustion at supersonic speeds, this reduces the static temperatures and pressures on the engine.

Question 66

Describe the criteria for fan blade material in the gas turbine engine.

Answer 66

Light Weight
High strength/stiffness
Good fatigue performance
Good impact resistance
Adequate fracture toughness
Temperatures usually -50C to 50C