Propulsion

Question 1

Describe the components and function of a gas generator

Answer 1

Acts as the “heart” of a gas turbine engine
Comprised of the compressor, the combustor and the turbine
The purpose is to supply high-temperature and high pressure gas

Question 2

How does a turboJET differ from the other types of turbine aircraft engines, and what is its main applications

Answer 2

A turbojet has all air travel through the gas generator
Used in high-speed, high altitude aircraft due to high thrust but low fuel effiency

Question 3

How does a turboFAN differ from the other types of turbine aircraft engines, and what is its main applications

Answer 3

Similar to a turboJET, but has a large fan at the front. Makes more thrust by bypassing air around core
Widely used in commerical aviation as its more fuel efficient, can operate from sub to transonic
Lower TSFC than turbojet

Question 4

How does a turboPROP differ from the other types of turbine aircraft engines, and what is its main application

Answer 4

Uses a gas generator’s turbine to drive a propeller
More fuel efficient at lower speeds and altitudes compared to other turbine engines, ideal for short to medium haul flights
Better for STOL

Question 5

How does a turboSHAFT differ from the other types of turbine aircraft engines, and what is its main application

Answer 5

Similar to a turboprop but delivers power to a shaft.
Used in helicopters and ships. Efficient power and high power-to-weight ratio

Question 6

EQ: Explain the principles of operation of the gas turbine engine

Answer 6

Suck, Squeeze, Bang Blow
Air is sucked into the low pressure inlet as aircraft flies, travels through the compressor. This ‘sqeezes’ the air to increase pressure, density and temp.
It then is mixed with fuel and ignited, causing combustion. This generates a high velocity flow by converting chemical energy to KE.
The reaction mixture then expands through high then low turbines, which converts some flow energy to drive the compressor.
This expansion causes the reactants to accelerate, after which they exit from the exhaust. The spare pressure at the exhaust gives the thrust

Question 7

EQ: Describe the four forces acting on a fluid element which are considered in deriving the thrust equation

Answer 7

Sidewall Force and End Face Force
Frictional Forces and Body Forces aren’t required for thrust eq and are usually neglected

Question 8

EQ: Describe ‘blisks’ and ‘blings’ in an aircraft engine and what manufacturing processes allow these to be used?

Answer 8

Short for ‘blade integrated disks’ and ‘blade integrated rings’
Compressor disks experience areas of stress concentration. With a blisk, blades are linear friction welded to the disk, and weight is saved, blades are more efficient and more reliable.
Using a material with a high enough hoop stress allows for material removal in the centre of the disk, again reducing weight.

Question 8

EQ: Describe the operation of an axial compressor and how does it differ from a centrifugal?

Answer 8

Used to prevent separation during diffusion and to and convert high KE to high pressure, ideally an isentropic process.
Axial Compressor usually multi-stage, made up of rotors and stators.
Centrifugal compressor takes air in at the eye of the impeller, the radial vanes compress the air then it exits out of the size. has larger frontal area and larger single stage pressure differential

Question 9

EQ: what materials are likely to be used in piston engines and what part of the engine is likely to operate at the highest temperature?

Answer 9

Piston engines experience rapid fluctuations in temperature and pressure. Hottest part is at top dead centre when compressed air fuel mix is ignited.
Common materials are irons or steels for engine block, with aluminium used to save weight where possible.
Pistons are usually high chrome content steels, sometimes titanium.

Question 10

EQ: How is upper temperature limit in ramjets avoided by using supersonic combustion?

Answer 10

When in supersonic flight the air enters the compressor at freestream velocity. When the gas is decelerated to increase pressure, the temperature subsequently increases as well. The larger the flight speed, the higher this temperature increase, to a point where the air enters the combustion chamber at the limting temperature of the brayton cycle. At this point, no useful work can be done, meaning no thrust is produced. Scramjets can perform supersonic combustion, negating the need to slow down the gas,

Question 11

EQ: Why are composite materials likely to be used in future jet engine production? List 3 potential materials

Answer 11

Polymer Matrix Materials are lightweight, good fatigue properties but brittle, can be used for fan blades
Metal Matrix Composites have a high strength, can be used for blisks/blings
Ceramic Composites for ultra high temperature applications, but also very brittle

Question 12

EQ: Describe the differences between a liquid and solid rocket motor in terms of their performance

Answer 12

SRM: Instantaneous ignition, simple and reliable design. Cannot be stopped, but burning can be progressive, regressive or neutral
Liquid: More flexible use, engines can be throttled or stopped and restarted. More complex deisgn and heavier

Question 13

EQ: Describe the 3 types of Solid Rocket Motor propellant

Answer 13

Homogenous Propellant, molecule contains both fuel and oxidiser. Highly explosive, such as nitroglycerine
Hetrogenous Propellent, made of crystalline oxidisers and powdered fuel dispersed in a hydrocarbon binder. Putty-like substance that is polymerised. Less hazardous
Composite double based propellent, combination of the two other types. Contains oxidiser and powdered fuel in matrix od double based propellent

Question 14

EQ: What are the four main components of a solid rocket motor

Answer 14

Combustion Chamber: Stores propellent and where high-pressure burning occurs
Igniter: Starts the burning process
Solid Propellent: Burns and produces exhaust gases
Nozzle: Expands combustion produced gases to high velocities, generating thrust

Question 15

EQ: Why does an axial compressor have both rotors and stators?

Answer 15

The rotors transfer the energy from compressor to air, the stators transfer that energy into increased static pressure and return flow to normal direction

Question 16

EQ: How does a pressure drop across a turbine compare to the rise across a compressor in the gas turbine engine

Answer 16

It is less than across a compressor as left over pressure through the turbine is used to produce thrust

Question 17

EQ: What is the mean effective pressure?

Answer 17

The pressure, that if kept all the way round an otto cycle, that would give the same amount of output work.
If losses are included, this becomes IMEP or BMEP

Question 18

EQ: Describe why the indicated power based on the otto cycle may not be accurate in thrust based caclulations

Answer 18

This is due to the loses causing inefficiencies
Mechanical, cooling, combustion, volumetric, propulsive efficiencies all reduce the indictated power. Inefficiencies can be combined into internal efficiency, maximum is around 85% efficient

Question 19

EQ: Describe how the plots of velocity, temperature and pressure vary through a typical gas turbine engine (In exam, may be asked to plot)

Answer 19

Through compressor: V remains freestream. T increases. P increases exponentially
CC, before combustion: V decreases. T is stagnant. P increases slightly
CC, after combustion: V increases. T rapidly increases. P decreases slightly
Through Turbine: V spikes, then fluctuates at high speeds. T decreases. P rapidly decreases
Through Exhaust: V decreases, stagnates, then spikes at outlet. T stays stagnant. P stays stagnant then drops back to freestream

Question 20

EQ: List major components of a gas turbine engine and the materials used for these parts:

Answer 20

Compressor Disc: steel or nickel based alloys, creep resistant
Compressor Blade: SC Titanium alloys
CC: ceramics and nickel alloys
Turbine Blades: SC nickel alloys with thermal barrier coating

Question 21

EQ: Describe key differences between two and three shaft gas turbine engines in terms of compressor and turbine components

Answer 21

Three shaft engines have an additional intermediate stage between low and high pressure compressors/turbines. This increases efficiency and performance but increases complexity and weight

Question 22

EQ: Describe the Brayton cycle upon which the gas turbine is based

Answer 22

Brayton cycle is thermodynamic model used to describe operation of gas turbine engine. Fresh air enters compressor, which raises its temperaturea and pressure. This high pressure air enters combustion chamber and burnt with fuel at a constant pressure. The now high temp gases enter the turbine where they expand to atmospheric pressure, producing power, driving the turbine which is connected to the compressor. Gases are not recycled, meaning it is an open Brayton cycle
Draw P-V and/or T-s diagrams. 1234 clockwise from bottom left
P-V: Flat top and bottom, constant s on curves. 3-4 has greater volume change than 1-2
T-s: Flat sides, 2-3 is greater temp change

Question 23

EQ: Describe methods that allow operation of turbines in gas streams greater than the materials melting temperature

Answer 23

Cooling channels: Turbine blades are cast with cooling channels built in to pass compressor bleed air through to extract excess heat
Single Crystal Cast: Blades are casting into a single crystal by only having 1 nucleation point, increasing creep and oxidisation resistance
Thermal Barrier Coating: Metal composite blades are coated in ceramic matrix thermal barriers that have higher thermal resistance but lower strength

Question 24

EQ: Describe the operation of a 2 stroke engine

Answer 24

An air-fuel mix enters when the piston is at the top of the cyl, and is forced down into the crackcase as the piston decents. When the piston gets to BDC, the air fuel mix goes through the transfer port and enters the top. It is then compressed and at TDC, then ignited. This reaction forced the piston down, exposing the exhaust port and the combustion reactants are blown out. This cycle continues every stroke, meaning the suck-squeeze-bang-blow occurs once every rotation of the crackshaft

Question 25

EQ: Describe the operation of the 4 stroke engine

Answer 25

Suck-squeeze-bang-blow
An air fuel mix enters through the open inlet port as the piston travels down. Once piston is near BDC, the valve closes. The air is then compressed into combustion space when the piston travels up. At TDC, ignition occurs and forces the piston down on the power stroke. The exhaust valve opens as the piston moves back up, and the combustion reactants are forced out.

Question 26

What is propulsion efficiency and describe the graph of propulsive efficiency against Ve/V0

Answer 26

A measure of how effectively the engine power is used to power the aircraft.
Is equal to the ratio of aircraft power (T * V0 ) to the power out, Wdot, out
Graph is exponentially inversely proportionate

Question 27

EQ: Describe how flow in a CD nozzle changes as back pressure decreases from stagnation P0

Answer 27

A. When Pb = P0, no pressure or velocity changes occur

B. When Pb>Pc, flow is subsonic. v increases in converging then decreases in diverging, pressure is opposite, minimum at throat.

C. When Pb=Pc, throat pressure is critical and flow is sonic at throat. Velocity still decreases in diverging

D. When Pe<Pb<Pc, fluid is sonic at throat and keeps accelerating in diverging nozzle as pressure decreases. Acceleration stops suddenly between throat and exit, normal shock occurs, velocity drops and pressure instantly increases. deceleration continues until exit. Normal shock gets closer to exit as Pb approaches Pe

E. When Pb=Pe, normal shock forms at exit plane of nozzle. Flow is supersonic through entire diverging section, can be approximated as isentropic. Velocity drops to subsonic as it crosses shock

F. When Pb < Pe, flow in diverging nozzle is supersonic, fluid expands to pf at exit with no shock but oblique shock and expansion waves form outside shock. Still isentropic.

G. When Pb=pf, no shocks occur, best performance. Pf is optimal pb.

H. When pb<pf, pressure increases from pf to pb, oblique shock and expansion waves occur in wake

Question 28

EQ: Explain the difference between Froude Efficiency and actual propulsive efficiency

Answer 28

The propulsive efficiency of a propeller is the ratio of the useful thrust power to the power expended on the air
The Froude efficiency is ideal of these two powers, ignoring losses of the ideal disk model

Question 29

What do the symbols N, n, K and k represent?

Answer 29

N is number of pistons
n is the revolutions per second
K represents the number of working strokes per engine revolution. K = N for two stroke, K = N/2 for four stroke
k number of useful cycles per piston per unit time. k=n for two stroke, k=n/2 for four stroke

Question 30

Describe the 3 times of solid propellent geometries

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

EQ: Explain the principle of a converging-diverging nozzle and discuss why rockets have such large area expansion ratios

Answer 31

A CD nozzle are sequential nozzles that allow for supersonic flow after the throat due to rapid fluid density decrease
In outer space rockets produce maximum thrust because the ambient pressure is zero. To operate at optimum thrust, the exhaust pressure must also be 0. This requires an infinitely large nozzle area expansion.

Question 32

List losses ignored by the Froude Efficiency

Answer 32

Froude Efficiency ignoes losses in the ideal actuator disk model
Vortex Shedding
Compressibility effects
Rotational Flow
Interference effects
Non-uniform Radial Loading
Profile Drag

Question 33

EQ: What is the difference between static and stagnation properties of a fluid?

Answer 33

Static properties are characteristics of the fluid measured at a point where the fluid is not moving relative to the observer
Stagnation properties are characteristics of the fluid when it is brought to rest isentropically. Accounts for fluid KE

Question 34

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

Answer 34

Using forced induction such as turbo and superchargers, increases mechanical loading on engine
Evaportative cooling - raises density by injecting cooled fluids
Increasing Engine speeds - power output proportional to n, increased engine wear
Increase compression ratio - bmep increases but so does mech loading
Improving volumetric efficiency - optimisation of intake/exhaust manifolds, more valves. Engine ‘breathes’ easier
Increasing cylinder number - cyl volume normally 1-3L, more cyl = more power, but increased weight, complexity and

Question 35

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

Answer 35

The bypass ratio is the mass flow rate of air through the fans over the mass flow rate through the gas generator. A high bypass ratio indicates a larger mass of air has been accelerated to a lower velocity for a higher propulsive efficiency