Theory

Question 1

Describe the operation of a gas turbine engine

Answer 1

Compressor - Generates low air pressure as it turns which sucks ambient air into the inlet. Airflow is then squezed by the blades in the compressor raising density, pressure and temperature

Combustion Chamber - Fuel is added to form an air-fuel mixture which is burnt at constant pressure to generate high velocity flow

Turbine - Connected to the compressor and converts some kinetic energy into mechanical energy to the drive shaft

Nozzle - Turns remaining kinetic energy into thrust

Question 2

Gas turbine engines are split into turbojets, turbofans, turboprops, turboshafts and ramjets. Describe each and explain how they’re used

Answer 2

Turbojet and Ramjet - Thrust generated from a fluid jet leaving the aircraft

Turbojet - Rotating compressors compress the airflow, mostly used in supersonic aircraft with operating speeds of less than Mach 3

Ramjet - Uses ram effect for compression, used in supersonic planes and missiles, operates at mach speeds greater than Mach 3

Turbofan/prop/shaft - Adaptation of the turbojet which supplies thrust using fans, propellors or shafts

Turbofan - Used in civil aviation

Turboprop - Used in small Civil aviation and large cargoplanes

Turboshaft - Used in helicopters

Question 3

Draw a plot of altitude against Mach number for propulsion systems, indicating limitations of different engine types

Question 4

Accurately Draw plots of how pressure, temperature and velocity change through a gas turbine engine

Answer 4

Lecture 8, page 16

Question 5

What is bypass ratio and how is bypass air beneficial to the overall efficiency

Answer 5

Bypass ratio is the mass flow rate of air through fans, propellors, or helicopter blades divided by the mass flow rate of air through the gas generator

High Bypass Ratio indicates a larger mass of air accelerated to a lower velocity for a higher propulsive efficiency, which increases overall efficiency.

Question 6

Use the parametric cycle analysis to derive equations for the specific thrust of an ideal ramjet by using the following steps.
i) Apply the thrust equation to the ramjet
ii) Express the velocity ratio V9/a0 in terms of Mach numbers and
temperatures
iii) Find the exit Mach number
iv) Find the temperature ratio T9/T0
v) Apply the first law of thermodynamics to the combustor
vi) Evaluate the specific thrust
vii) Evaluate the thrust specific fuel consumption

Answer 6

Look at 2017 answers, Q2a

Question 7

Calculate the specific thrust, thrust specific fuel consumption at the following Mach numbers M0 = 1;3;5
T0 = 217k
gamma = 1.4
Cp = 1.004
hpr = 42800J/kg
Tt4 = 2000k
tR = 1.8 (tau R)

Answer 7

Look at 2017, Q2b

Question 8

Describe and explain the operational envelope of an ideal ramjet

Answer 8

With higher specific thrust, less fuel is used. Max flight mach number will appear where thrust specific fuel consumption is infinitely close to zero

Question 9

What is the difference between static and stagnation enthalpy

Answer 9

Static Enthalpy: Represents potential energy of fluid stream per unit mass which doesn’t include kinetic energy. If kinetic energy = 0. The stagnation enthalpy = static enthalpy

Stagnation Enthalpy: Represents total energy of a flowing fluid per unit mass which remains constant in an adiabatic condidtion

Question 10

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

Answer 10

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

Question 11

Explain the principle of a converging-diverging nozzle

Answer 11

When air enters a converging nozzle, Pressure, Temperature and Velocity decrease whilst Mach number increases and density decreases. Pressure reaches critical pressure at the throat.

Question 12

Assume that air enters the above nozzle at negligible velocity at a pressure, p, of
0.2MPa and temperature, T, of 350K. Assuming isentropic flow, determine the
temperature and pressure of the air at the exit plane before the shock wave that results
in a Mach number, Mx = 2, and also the temperature, pressure, Mach number and
stagnation pressure after the shock wave (Assume P0y/P0x = 0.7209 across a shock
wave)

If the air in part b) had entered with a velocity of 200m/s, what would the temperature
and pressure at the exit plane, before the shock wave have been that resulted in a
Mach number, Mx of 2 (assume Cp = 1005Jkg-1K-1)

Answer 12

Look at 2017 Q3d and e

Question 13

Discuss the reasons why the thrust power generated by a piston engine propeller combination will be significantly lower than the indicated power derived from the analysis of the otto cycle

Answer 13

TP = npnmncnbIP

These are propulsive, mechanical, cooling and combustion efficiency. These inefficiencies, in addition to the piston pump work being finite leads to thrust power being less than the indicated power

Question 14

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

Answer 14

Piston engine output can be increased by supercharging

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

increase engine speed - Power output is directly proportional to engine speed. The penalty is increased piston speed and therefore engine wear.

Increasing engine compression ratio - Power output raises in direct relation to bmep. Increased mechanical loading of the engine may degrade its service life.

improve volumetric efficiency - Obtained with multiple valves and manifolds, these designs breathe easier.

Increase number of cylinders - Increases power output and has gone as high as 28 cylinders, however, it becomes very complex and reduces reliability

Question 15

Describe how propellor pitch angle is determined in relation to angle of attack. What is the advantage to variable pitch propellors?

Answer 15

TA is a function of B, and Ta relies on alpha.

A variable pitch propellor can be continuously varied to maintain maximum efficiency at all flight velocities.

Question 16

Derive an equation for thrust based on the Rankline-Froude actuator disk model

Answer 16

a = (V2-V1)/V1 V1 = a(V2-V1) V2 = V1(1+a)
b = (V3-V1)/V1 V1 = b(V3-V1) V3 = V1(1+b)

m = rhoV2A = rhoAV1(1+a)

T = m(V3-V1)
T = m(V1(1+b)-V1)
T = rhoAV1(1+a)(V1(1+b)-V1)
T = rhoaV1^2(1+a)b

T = 0.5ArhoV1((1-b^2)-1)

(1+a)b = 0.5((1+b)^2-1)
(1+a)b = 0.5(1+2b+b^2-1)
(1+a)b - 0.5(b^2+2b)
(1+a) = 0.5(2+b)
a = 0.5b

T = rhoAV1^2(1+a)2a
or
T = rhoAV1^2(1+b/2)b

Question 17

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

Answer 17

A ramjet without a compressor is a logical evolution of the gas turbine for supersonic flight speeds as the compressor efficiency drops dramatically due to shock wave and boundary layer seperation lesses when the blade tips are near supersonic speeds.

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

Question 18

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

Answer 18

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 19

Sketch how the geometry of ramjets and scramjets may be used to control the combustion Mach Number

Answer 19

2016 Q1C.

Each have 4 sections.

Scramjet converges 1-2. diverges slightly 2-3, diverges greatly 3-4/

Question 20

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

Answer 20

Combustion Chamber - Stores and contains propellant during high pressure burning

igniter - Starts the propellant burning

Solid propellant - Burns and produces gases for propulsion

Nozzle - Expands the combustion product gases to high velocity, thus generating thrust.

Question 21

Explain the difference between a scramjet and a ramjet

Answer 21

The combustion process of a ramjet takes place at low subsonic velocities. The combustion process of a scramjet takes place at supersonic velocities.

Question 22

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

Answer 22

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 23

Derive the Tsilkovsky relation for rockets by considering the force balance equation for purely vertical ascent

T-W-D = M*dV/dt

Answer 23

T = mc W = mg

2016 Q2C

Question 24

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

Answer 24

Maximum engine thrust only occurs in a vacuum (Pa = 0) when the thrust equation becomes Tmax = MVe + Pe*Ae

Whilst generating maximum thrust in outer space, the rocket engine also operates at maximum thrust is pe = 0 however, this requires an infinitely large nozzle area expansion which is not realistic.

Question 25

Describe what is meant by progressive, neutral and regressive burning in a rocket motor. Sketch diagrams and geometries

Answer 25

Progressive burning - Burning area increases thus chamber pressure and thrust increase with time

Neutral burning - Burning area, chamber pressure and thrust remain constant with time

Regressive burning - Burning area decreases thus both chamber pressure and thrust decrease with time

2016 Q2e for diagrams

Question 26

Explain what is meant by mean effective pressure

Answer 26

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 27

Describe the operation of a 4 stroke engine

Answer 27

4 stroke engine completes all the events in a 4 stroke cycle, two up and two down revolutions.

At the intake stage, the inlet valve is fully open and the exhaust valve is closed. The piston is descending so it sucks in fresh air-fuel charge into the cylinder through the open valve. Towards 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 descends of the power stroke.

During the power stroke stage, both the inlet and exhaust valves are closed. The air-fuel charge has been ignited and the combustion products are rapidly expanding; pushing the piston down on its power stroke. Towards the bottom of the power stroke, the exhaust cam has rotated such that it begins to lift the exhaust value whilst 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 through the open exhaust port. Towards the top of the exhaust stroke the valve begins to close and the inlet valve begins to open as a result of cam action.

Question 28

Describe why the IP based on the Otto cycle may not be accurate in thrust based calculations

Answer 28

Combustion/Exhaust processes are not instantaneous
Valves take time to open and close
Heat losses to the sidewalls, valves ect
Finite pumping work
Incomplete displacement of exhaust gas with fresh charge due to engine capacity and ambient pressure conditions.

Question 29

A six-cylinder air-cooled normally aspirated four-stroke aircraft piston engine has a
cylinder bore, d, of 150mm and a piston stroke, l, of 80mm. The volumetric
compression ratio, r, is 7. The engine is flying at sea level and operating at 3000rpm
(n = 50rps). Assume Rproducts=0.99Rair, where Rair =0.287kJkg-1K-1.
Using Hookers charge-mass law

Mc = nV/2RTc*(Pc-Pe/r)

Find the charge-mass flow rate, assuming that:
(i) Pressures pe=pc=1atm abs. and no preheat, Tc = Tsea level std
(ii) Pressures pe=pc=1atm abs. and the additional preheat for Tc is 50°C
(iii) Charge pressure pc= 0.9 atm abs, exhaust pressure pe=1 atm abs. and no
preheat as in Tc = Tsea level std
(iv) Charge pressure pc = 0.9, exhaust pressure pe=1 atm abs. and preheat of
Tc = 50°C

Answer 29

2016 Q3d

DV = 8.48E-3m^3
V = 9.89E-3m^3
i) 0.264
ii) 0.223
iii) 0.232
iv) 0.198

Question 30

Describe the operation of a gas engine

Answer 30

Suck, squeeze, bang , blow

Compressor turns and compresses the air to generate low air pressure with sucks ambient air into the inlet. Squeezed airflow then increases the density, pressure and temperature

Combustion Chamber - Fuel is added to form an air-fuel mixture then burned at constant pressure to generate a high-velocity flow by converting the chemical energy of fuel into the kinetic energy of the airflow.

Turbine - Connected to the compressor by the same shaft, the turbine converts some of the kinetic energy into mechanical energy to drive the compressor whilst the remaining kinetic energy is expelled through the nozzle

Nozzle - Converts remaining kinetic energy into thrust

Question 31

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

Answer 31

2016 Q4b

Question 32

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

Answer 32

The 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 airflow and increases the 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 33

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

Answer 33

Relationship between thrust specific fuel consumption (S) and thrust specific fuel consumption given by

S = TSFC(1-dinlet-dnozzle)

Where dinlet and dnozzle are the inlet loss coefficient and the 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 lower than TSFC

Question 34

Describe the key differences between a 2 shaft engine and a 3 shaft engine in terms of turbine and compressor components

Answer 34

In three-shaft engines, the big fan, high-pressure and low-pressure compressors are driven by 3 different shafts so that they operate at different RPM’s.

In 2 shaft engines, the big fan and low-pressure compressors are driven by the same shaft.

Question 35

The thrust of a gas turbine engine when the exit pressure is equal to ambient air pressure is given by T = m(Ve-V0)

Describe the 2 methods of increasing engine thrust and the engine types for which these principles are applied

Answer 35

Increase the mass flow rate for a turbofan engine design

Increase the exit velocity for a turbojet engine desgin

Question 36

A gas turbine engine has a sea-level gross thrust, TG, of 120kN and a characteristic
exhaust velocity, c, of 500m/s, which can be assumed to be constant with altitude. If
the mass flow of the intake air, is significantly larger than the mass flow of fuel,
calculate, for cruise conditions (Temperature, T=240K, pressure, p=0.25psls (sea-level
standard pressure), velocity, V=280m/s):-
i) Throughput, .
ii) Ram drag, RD.
iii) Net thrust, TN.
iv) Capture cross-section of free stream air, A.

Answer 36

i) m = TG/c = 240
ii)Rd = mV = 67.2KN
iii)Tn = Tg - Rd = 52.8KN
iv)RD = gammapressureM^2*A
A = 2.33m^2

Question 37

For a 1D steady flow, accounting for both external and internal forces on the control region. Derive an expression for thrust in terms of mass flow rate and the velocity increase (V4-V1)

Answer 37

2015 Q1a

Question 38

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 engine would expect to operate; Turbojet, High bypass ratio, turbofan, Lowbypass ratio turbofan, Turboprop

Answer 38

2015 Q1b

Question 39

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

Answer 39

2016 Q1c

Question 40

Describe the principles of operation of a propellor

Answer 40

Propellors 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 aircraft the through the air

Question 41

Draw a diagram of propellor efficiency as a function of J where J = V/nD

Describe why variable pitch propellors are used

Answer 41

2015 Q2b

3 lines, B1 to B3, B3 greatest nmax

Variable pitch propellors allow the propellor to be continuously pitched to maintain maximum efficiency at all flight velocities. And this can be visualised as riding along the peaks of the propellor efficiency curves as earlier drawn.

Question 42

Under what conditions is the propellor efficiency equal to zero and why does this occur?

Answer 42

For a propellor with a given n and D. J only depends on V. When V = 0 then J = 0. This propellor efficiency is zero at J = 0 as there is no motion of the aircraft, and hence no power is available.

At the other extreme, when V (and hence J) is made very large, the propellor loses lift owing to small angles of attack. Also, when the propeller tip speeds are near sonic, efficiency drops dramatically due to shock wave and boundary layer separation issues.

Question 43

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

Answer 43

Mechanical Losses - Delayy 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 44

What is the difference in Froude efficiency, nf, and the actual propulsive efficiency, np

Answer 44

Froude efficiency ignores all losses except that associated with the streamwise kinetic energy,

Actual propulsive efficiency takes every loss into account

Question 45

Describe the principles of operation of a gas turbine engine

Answer 45

Suck Squeeze Bang Blow

The compressor turns and generates a low air pressure which sucks ambient air into the inlet, the airflow is then squeezed by the blades which increase the air density, pressure and temperature.

Combustion Chamber - Fuel is added to form an air-fuel mixture which is burnt at constant pressure to generate a high-velocity flow by converting the chemical energy of fuel into kinetic energy of the airflow.

Turbine - Connected to the compressors by the same shaft, the turbine converts some of the kinetic energy in the flow into mechanical energy that drives the shaft, and therefor the compressor.

Nozzle - Converts remaining energy into thrust

Question 46

Why afterburning may be used in the gas turbine engine

Answer 46

The afterburner used unburnt oxygen in the jet exhaust to burn additional fuel

An extended heat pipe is placed between the turbine and the final nozzle, with flame on, the extra heat energy is added to the gas stream and thus raises the gas temperature and local sonic speed.

With the afterburner. The gas can be accelerated to a higher sonic choking speed in the nozzle and thereby generate higher thrust.

Question 47

Describe how the efficiency of the gas turbine engine has been increased. Focus of the compressor and turbine sections

Answer 47

For Compressor - Reducing tip losses due to leakage between moving rotors and stationary walls

Minimising surface/volume ratios

Developing low-drag laminar flow blade cascades

Careful study of 3D flow fields to minimise secondary flow loss

Using centrifugal compressors for small engines.

For Turbines - USe 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 48

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

Answer 48

Manufacturing blades and discs as a single structure (blisk) removes the need for leakage paths and blade fixings. Resulting in weight savings of upto 30%.

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 49

Describe with diagrams the principle of operation of 2 stroke and 4 stroke piston engines. You should include comments about the differences between 2 engine types. The stroke, compression ratio and swept volume.

Answer 49

2 stroke engines complete all events in a 2 stroke cycle. The crankshaft makes 1 revolution, so the 2-stroke engine is complete in 1 revolution. Since the incoming air-fuel mixture passes through the crankcase on its way to the cylinder, the crankcase is made gas-tight.

4-stroke engines complete all events in a 4-stroke cycle, 2 up and 2 down in 2 revolutions. Since the crankshaft makes 2 revolutions with on;y 1 power stroke and each valve opens once only during the cycle the cams must rotate once only in 2 revolutions of the crankshaft. Thus the camshaft is geared down 1:2 to the crankshaft.

2 stroke engines have a larger swept volume than 4 stroke engines, therefore with the same piston bore, 2 stroke engines have a higher compression ratio.

Question 50

Draw pressure-volume diagrams for the 2 stroke and 4 stroke piston engines indicating the main features of the cycle

Answer 50

2015 Q4b

Question 51

A 6 cylinder normally aspirated four-stroke piston engine develops a brake mean effective pressure of 10 atm, having a piston stroke, l, of 130 mm, a cylinder bore, d, of 140 mm and operates at n = 3000 rpm in sea-level-standard ambient conditions.The internal efficiency, ηi, is 73%. Find:-

i) The Brake Power
ii) The Indicated Power
iii) The Indicated Mean Effective Pressure

The engine in part c) has a volumetric compression ratio, r, of 7. Taking the charge mass gas constant Rc = 0.99 Rair, where Rair = 0.287 kJ kg-1 K-1 the inlet and exit pressures, pc and pe, to be 1atm, find the charge mass flow rate for the situation when the mixture is preheated by 60 ºC at sea-level-standard conditions.

Answer 51

i) BP = 101013250.13A3000/606/2 = 304.2
ii) BP = niIP -> IP = BP/0.73
iii) imep = IP/lANk = 1387.7kpa

2015 Q4c

Tc = 60+273 = 348k

V = dV/(1-1/r) = 0.014

m = nV/2RTc*(pc-pe/r)

Question 52

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

Answer 52

By the 1950’s the power-to-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 lift and greater safety in the operation of civil transport aircraft.

The specific weight of civil transport engines had been reduced from 9kg/kw to the of the original Wright Brother’s engine to 0.6kg/kw and the power of operational engines had peaked at 4300hp. It was during this period that piston engines got replaced by gas turbines.