ENGINEERING STUDIES – AERONAUTICAL ENGINEERING.pdf

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Bryan HoENGINEERING STUDIES – AERONAUTICAL ENGINEERING

AERONAUTICAL ENGINEERING

SCOPE OF THE PROFESSION

Responsible for design and development of new aircraft and modify existing ideas

Use latest technology to fulfil design specifications

Designing and maintaining systems for tracking and controlling the movement of aircraft, passengers

and cargo in airspace and on the ground

Visually inspect aircraft in service and develop airport operational systems

Office OHS – Office Designers

RSI – Repetitive Stress Injury

Lighting

Ergonomics

Field OHS – Field Managers

Manufacturing

Testing

4 key material properties in Aero:

Strength to Weight ratio (S:W)

Formability

Durability/Fatigue

Corrosion resistance

Most engine designs require stability at high temp. eg/ Ti alloys, Nimonic (Ni based superalloys)

Use composites (good specific str.) and adhesive tech. (avoid bolts, rivets. No weak pts)

Polymer adhesives are used instead of rivets as they provide a smooth surface, but fail catastrophically

Effect on Society:

Greater accessibility to further locations, allowing time shortages

More rapid overseas commerce, postal and freight

Can be used save lives, in a military sense (reduce casualties) ambulances, fire-fighters

Boosting tourism

Residential areas under flight paths and near airports are subject to air and noise pollution as

airplanes pass

Opening new flight paths or new airports are subject to much criticism due to environmentalism,

actual necessity, NIMBYism, etc (see Sydney’s second airport, Badgerys Creek)

Unique Technologies of Aeronautics

Advanced composite materials, computerised design, calculation and drawing systems, wind tunnel

testing of airframes

Note that these technologies are not exactly exclusive to aeronautics, they are also used in other

fields of engineering, such as naval design

As aeronautical engineers, they are expected to consider and calculate complex moments and forces

on a 3D airframe, in flight or not. Whilst programs can aid this process, engineers must consider as

many points of failure as possible, to root out these points of danger whilst still in design.

Environmental impacts of aviation

Large amounts of noise and air pollution. Despite new and more efficient engines (such as turbofan

and turboprop engines) being utilised, the rapid growth of air travel has increased its impact

Biofuels, and other alternative fuels are being researched and developed to reduce the environmental

impact of flight. Commercial flight tests have been undertaken successfully (In December 2008, an

Air New Zealand jet completed the world's first commercial aviation test flight partially using

jatropha-based fuel.) but biofuels aren’t yet sustainable economically to be used worldwide.




Brief History of Aeronautics

1783 Montgolfier Brothers construct the first lighter-than-air vehicle (a balloon). First tethered balloon

flight with humans on board

1797 André-Jacques Garnerin carried out the first jump with a silk parachute

1877 Enrico Forlanini developed an unmanned helicopter powered by a steam engine. It rose to a heightof 13 meters, where it remained for some 20 seconds,

1785 First flight over the English Channel, traveling from Dover to France in a balloon

1903 Orville and Wilbur Wright fly first successful self-propelled airplane

1911 The Italian-Turkish war (September 1911 – October 1912), in Libya was the first military use of an

aircraft, for both reconnaissance and bombing runs.

1918 United States Post Office establishes airmail service

1924 First flight around the world

1926 Air Commerce Act marks first federal attempt to set safety regulations for civil aeronautics and

requires the registration and licensing of pilots and planes

1930S Development of the jet engine began in Germany and in England

1950S Technologies such as long-range missiles, computer systems, electronic controls, combustion

chemistry, and new composite structures made possible by the aerospace industry

1969 Neil Armstrong and Buzz Aldrin become the first persons to walk on the moon

1976 Concorde flies

The last quarter of the 20th century saw a slowing of the pace of advancement. No longer was

revolutionary progress made in flight speeds, distances and technology. This part of the century saw

the steady improvement of flight avionics, and a few minor milestones in flight progress. In general,

aviation has progressed through failed experiments since the 18th century.

Notable People in Aviation

Sir George Cayley (1773 – 1857) First successful gliders

Understood importance of separating lift and propulsion

Developed whirling-arm apparatus to measure forces on aerofoils and wings

Understood importance of camber

Otto Lilienthal (1848 – 1896)

“Father" of hang-gliding

Understood importance of control

Developed extensive tables of lift and drag forces based on (flawed) whirling-arm experiments

Died as a result of injuries sustained in a glider crash Wilbur (1867 – 1912) & Orville (1871 – 1948) Wright

Understood importance of 3-axis control (but not stability) – learned to control flight in extensive

glider experiments

Discovered errors in Lilienthal’s whirling-arm data

Built wind-tunnel for aerodynamic testing

Developed first theory for propellers (and built one that had better than 80% efficiency)

(PLEASE NOTE THAT THIS IS NOT THE MOST COMPLETE TIMELINE REGARDING THE HISTORY OF AVIATION, IT IS

MERELY A SUMMARY OF WHAT ARE RELATIVELY MAJOR MILESTONES. IT IS ALSO NOT ENTIRELY EXPECTED FOR

STUDENTS TO MEMORISE THE DATES, BUT MORE SO TO UNDERSTAND THE PROGRESSION AND DEVELOPMENT OF

AVIATION. FURTHER RESEARCH TO UNDERSTAND THE PROCESS AND APPLICATION OF AFOREMENTIONED AND

OTHER INNOVATIONS IS RECOMMENDED.)




ENGINEERING MECHANICS AND HYDRAULICS

Flight Mechanics

1.

Level Flighta. Weight – Gravitational pull

b.

Lift – Net force generated by the airflow over wings and tailplane

c.

Thrust – Forward force generated by the engines

d.

Drag – Air resistance. There are two components:

i.

Induced Drag – as a result of lift

ii.

Parasitic Drag – moving aircraft through air (ie friction)

2. Level Flight (complex)

a.

In normal flight, the actual forces acting do not act through the

centre of gravity. The following factors influence the point of application of various forces

i.

Weight force acts through CoG (centre of gravity)

ii. Line of thrust force is inclined to the direction of flight – AoA (angle of attack)

iii. Forces of lift are generated at the aerodynamic centre of wing and tailplane

iv. CoG of aircraft moves in flight due to changes in

cargo, fuel usage

Basic Aerodynamics

The design of aero foils and their passage through air governs the basic principles of flight. The aerofoil

refers to the cross sectional shape of a plane’s wings, or anything that creates lift. The asymmetry of the

aerofoil is called camber.

A lift-to-drag ratio (L/D ratio) is simply the amount of lift generated divided by the drag it creates. A high

L/D ratio a major goal in aircraft design since an aircraft’s required lift is set by its weight, delivering that

lift with lower drag leads directly to better fuel economy, climb performance, and glide ratio.

Bernoulli’s Principle

Air travels faster across top surface and slower across lower surface

Creates low pressure on the top surface and therefore high pressure at the bottom

Pressure differential results in an upward lifting force to act on the wing

Planes travel on the runway at high speed to produce adequate lifting force to overcome gravity and drag

Stalling refers to the situation when the wing no longer produces lift

a) Lower airspeed does not produce adequate pressure difference

between the upper and lower wing surface, therefore, not

producing the necessary lifting forces

b) High AoA will cause air turbulence on the top surface resulting

in increased pressure which in turn lifts downwards to oppose lift

Stalls may occur during tight turns, steep climbs or landings

ie// Airflow over the top of the aerofoils is broken




Bending Stresses (Airframe)

The airframe will have to withstand the compressive loading due to drag and acceleration, withstand

moment forces at the connection between wing and body due to lift as well as cyclic loadings on all

components due to pressure differentials and varying forces during no flight (on ground) and steady flight.

At the aircraft wings during flight, a UDL is applied along the length of the beam, similar to simply

supported beams (cantilevered beams).

Fluid Mechanics

Pascal’s Principle

“Pressure applied to an enclosed liquid is transmitted undiminished to every point in the fluid and to thewalls of the container.”

Hydrostatic pressure is applied to cylinder with a moving piston. Pressure acts at right angles to every

surface w/in the cylinder, including the piston. Therefore, force is created to move the piston. Also, some

hydraulic rams are two-way, like in diagram. It is able to provide movement and force in 2 directions.

This is particularly useful in aeronautics as using mechanical linkages and levers to move control surfaces

on an aircraft, such as the flaps and rudder, from the cockpit would be quite difficult and nigh impossible.

With hydraulics, the force from a lever in the cockpit can be efficiently transferred through pipelines to

where it is needed. Furthermore, given Pascal’s principle that pressure is constant throughout the pipelines,input forces can be magnified into a far larger force (eg/ the pilot pushes a lever in the cockpit to move large

flaps on the wing)

Mathematically: As P (pressure) is constant, ()

(), if the output piston is 5 times the

area of the input piston, the output force has to be correspondingly 5 times larger

Hydrostatic and Dynamic Pressure

Hydrostatic Pressure (P S ) – pressure resulting from a static fluid, such as air pressure in an air cabin

Dynamic Pressure (P D ) – pressure resulting from moving fluids, such as airflow over an aerofoil.Pressure is created by moving fluids, because of the velocity involved.

√ () (pressure changes with speed)

D (drag)




Ventur i Ef fect is the reduction in fluid pressure that results when a fluid flows through a constricted section

of pipe. Velocity of the fluid increases as the cross sectional area decreases, with the static pressure

correspondingly decreasing.

Effects on Aircraft Structures

Both PS and PD have a large effect on the plane’s structure

a) Planes flying at high elevations are pressurised, ie, the air pressure

inside is far greater than the air pressure outside. This results in

forces pushing outwards on the plane’s superstructure as well as

windows, doors and seals. Metal fatigue is of concern due to the

cyclic nature of this pressure

eg/ planes have “life span” / safe operation time

b)

Pressure is also exerted on the outer plane surface, and therefore the

airframe, by the fast moving air, ie, PD. The jet engines are also

exposed to large amounts of PD because of the intake of air and the

thrust produced. The pressure from the thrust is two-directional as

jet aircraft use reverse thrust vanes during the braking procedures.

Applications to Aircraft Instruments

Vital flight info is obtained from gauging the velocity and air pressure surrounding a plane using

instruments such as altimeter and speed indicators.

Pitot Tube

Placed under the wing or in the nose. Gauges diff between PS and PD.

Air entering tube has velocity, therefore PD. Other openings connected

to the inside (not pressure) of the plane allow PS to surround the tube.

By measuring the PS and the total pressure, the plane’s airspeed can be

found.

Airspeed Indicator

Total pressure entering pitot tube acts on inside of diaphragm

Outside of diaphragm is surrounded by PS

Diaphragm connected to linkage that controls airspeed indicator and

positions itself according to the difference between PT and PS

Altimeter

Uses a small expandable vessel or air, called an aneroid, surrounded by static AP.

As aircraft ascents, the static AP falls, allowing the aneroid to expand

This acts on a linkage system, controlling the needles on the altimeter




Propulsion Systems

Piston Engines

Generally used for smaller aircraft and resemble simple car type internal combustion engines

These engines can be turbo or supercharged to improve performance. Both force air into the engine

under greater pressure, resulting in a boost of power

Super – driven by a belt working off the crankshaft of the actual engine (better at higher speeds)

Turbo – works off exhaust gases of the engine (better fuel consumption)

Dual ignition systems are used in these engines which provide safety and better efficiency

Jet Engines – must have high fatigue, abrasion, oxidation and corrosion resistance

Last and faster aircraft use Jet Engines. There are 4 basic types:

Turbojets (TJ) – original type. Very loud. Inlet – Air is compressed slightly

Compressor – Air is heated and compressed by turning blades

Combustor – A mixture of compressed air and injected fuel is

burnt in the combustion chamber

Turbine – Small amount of the energy from the burning gases is used to drive the turbine out the

back of the engine, which provides energy to drive the compressor

Nozzle – Very hot outlet and high velocity gases, expanding

on combustion, leaving through the nozzle to provide thrust

Turboprop (TP) – better at slower speeds and slower altitudes Similar to TJ except turbine is used to drive propeller

Most of energy produced is used the turbine, and therefore the

propeller, leaving a small volume of exhaust to provide thrust

The propeller provides most of the thrust

Turbofan (TF) – successfully developed in response to reduce

noise from TJ.

Known as the bypass engine because most of the air entering

the engine ‘nacelle’ passes around the main engine/combustion

chamber The fan produces most of the thrust from the air bypassing the

engine, whilst the engine still produces some of the thrust

TF jets are more efficient that TJs and the bypass air reduces

noise significantly by shielding the engine core gases

Having less moving parts than TP means more power from

afterburners

Ramjet (RJ)

Simple design that isn’t good at low speed

Air is compressed and therefore heated by the shape of engine interior before it is mixed withfuel and ignited. Again, the expanding gases from burning fuel provide the thrust

Scramjet is a variant of ramjet in which combustion takes place in supersonic airflow.

Rockets – a large thrust is produced from burning fuel dedicated to escaping earth’s gravity




ENGINEERING MATERIALS

Specialised testing of aircraft materials

Destructive testing can only be tested on specimen material, not actual component.

Fatigue Testing (Fatigue is a major structural consideration in aircraft, as weakening structural components in

aircraft are generally not desired, especially mid flight)

1.

Initiation – many microscopic crack forms due to slip along shear planes. Impossible to detect.

2.

Stable Growth – Visible cracks develop perp. to the local tensile stresses. Detected through non-

destructive testing.

3.

Unstable Growth – As crack grows, the structure remaining to carry load decreases. At critical

length, it becomes unstable and grows at near the speed of sound, leading to sudden failure.

4 conditions necessary for fatigue crack development and growth:

1.

Material is prone to stress cracking

2.

Tensile stress must be present

3.

Stress, at least at the crack tip, must be in plastic range of material

4.

Stress with cyclically varying intensity (the basis of fatigue)

Diff manu processes can directly influence component fatigue life. Even machining/grinding marks/burrs

can concentrate stress for fatigue cracking.

Processes increasing fatigue life Processes reducing fatigue lifeCase hardening (induction heating), nitriding Cladding of aluminium (diff materials, diff

expansion rates)

Cold rolling, cold working Cadmium plating

Shot peening and grit blasting (compresses surface

layer)

Decarburising of steel (using oxygen to reduce

carbon in steel)

Good quality machining (sharp precision tools) Chrome plating (more brittle)

Galvanising (hot working

Final design requires a safety factor of 4 times so requires accelerated testing equivalent to at least 16

lifetimes, even under the worst environmental conditions.

Modern aircraft design allows for serious fatigue cracking, corrosion or accidental damage, and still be able

to carry reasonable loads. This affects the design of critical airframe components and determines the critical

fatigue crack allowed in each. For aircraft to remain airworthy, aircraft structural integrity must be

maintained, achieved through full-scale fatigue testing under controlled, simulated operating conditions and

coupled with actual flight data, predictions on component life expectancy can be made, as well as a

development of inspection schedules and component replacements. Thus, techniques can be implemented to

extend component life, such as extra reinforcement, component replacement, specialised repairs (composite

repair kits can be used on primary structural members, even metal ones).

Non Destructive Testing

Design phase – wind tunnels used along with models of new designs = predict in-flight performance

X-ray, Dye Penetrant, Ultrasonic have been previously discussed. Gamma ray works similarly to x-ray.




Visual

Inspection

Magnifying glass to identify external flaws. Fill

tubular structures and under pressure, hot oil will

seep through cracks

To check rewelded and repaired

structures

Magnetic

Particle

Inspection

Useful on only ferrous materials (irons, steels). Item

is magnetised, flaws/cracks are seen to accumulate

magnetic particles when applied, either dry orsuspended in oil. Sides of crack become magnetic.

Dry: Find subsurface defects in

heavy welds, forgings and castings

Wet: Use on more complex shapesto allow better particle distribution

Aluminium and its alloys

Pure Al is has high corrosion resistance, but it is unsuitable as it is too soft and lacks strength. Thus Al

alloys such as duralumin were developed to increase strength and hardness, but these lacked corrosion

resistance and so a layer of pure Al was pressure welded onto both sides, resulting in Alclad, commonly

used on airframe skins.

Copper (2xxx) Enhances ductility and malleability. Prevents stress crack formation. Makes some alloys more shock

resistant. Strength and hardness increases with age.

Duralumin. 2017. High tensile strength, S:W. Strengthen by precipitation hardening.

Manganese (3xxx)

Provides wear resistance, corrosion resistance, increases strength

Silicon (4xxx)

Non-metal. Harder alloy, but not brittle. Reduces melting point, so easier to cast

Magnesium (5xxx)

2/3rds weight of Al. Can be used structurally when alloyed with Al, Zn, Mn. Tensile strength isincrease, as is corrosion resistance, hardness and weldability. Often in sheet form, but Al-Mg 5056

rivets are commonly used to hold skins to Mg surfaces.

Zinc

Creates stiffer and more brittle alloy than pure Al and with a bit of Mg, gives a higher strength. Often

used for skin applications, but doesn’t have as much corrosion resistance as pure Al

Other metals to alloy with

Titanium. Higher melting point than steel, therefore good for high speed aircraft especially at hot

sections (nose). Possesses high strength, thus used in load bearing applications (landing gear). High

expense means that it is only used for critical load bearing application that require superior strength Steel. Heavy and brittle at low temperatures at high altitudes, so used sparingly in restricted areas

where strength is needed (carriageways)

Stainless Steel (Fe/C/Cr/Ni). High corrosion resistant due to chromium oxide. Commonly cold

rolled to increase strength. (firewalls, skins, structural parts, fasteners.)

CroMo. High shock and corrosion resistant. (engine mounts and shock struts)

Mg alloys. Castings – landing wheels. Sheet alloy/forgings/pressings – tanks, wings

Metal manufacturers are forced to develop new alloys that improve on the properties of composites

in order to make all designs lighter and stronger

eg/ Ti alloys, new Al/composite structures, superalloys




Heat Treatment of Applicable Alloys

Heat treatable

These alloys commonly harden by precipitation hardening

Heat, then soak material, then quench to hold it in soften state = solid solution hardening (very important!)

Then age it; that is to let it rest in room or elevated temperature for some period of time, to precipitate

submicroscopic particles out the alloying element which inhibit dislocation movements and causing

internal stress. This will increase its hardness and (yield) strength.

Solution treated parts may be refrigerated to prevent age hardening

Duralumin is a common alloy to be heat treated, so copper is predominant alloying element

Natural aging

Room temp for 5-7 days. Submicroscopic particles

precipitate out of the structure, inhibiting dislocation

movements and causing internal stress, therefore

increase hardness and strength

Artificial aging/Precipitation hardening

Soak in oven between 100-200°C for 4-24hrs.

Increase strength, stability, corrosion resistance,

hardness. Reduce malleability and ductility.

This is often chosen over natural aging as it is

markedly faster but produces very similar results

Non heat treatable

May be hardened by alloying or cold working (anything elongating the grains; rolling)

Al alloy 1100. Small diameter low pressure tubing, rivets

5052. Low pressure tubing, storage tanks for hydraulic fluids, fuel, oil.

5056. Rivet stock for Mg control surface skins

Other heat treatment processes

Stabilising (>UCT)

Tempering for aluminium. Relieves residual stresses when soaked at 250° (for some Al alloys) under5hrs. Retain majority of strength and hardness.

Annealing (>UCT)

Soak at 360° for 1 hour, cool in air. Cool slower to further soften alloy. Too rapid cooling may

produce conditions that will lead to age hardening (quench like)

When annealing Al clad materials, soaking for too long will allow some of alloying elements to

diffuse into pure Al and consequently reduce corrosion resistance.

Localised annealing can be used in work hardened materials, simply with gas torch. As Al doesn’t

change appearance when heated, use crayon that melts at certain temp.

Reheat treatment

If the alloy was solution treated at too low a temperature, precipitation occurred at too high a

temperature, or aging for too long, can be solution treated again to get full desired properties. Do

NOT re-heat treat clad materials




Polymers

Properties of Polymers

Low specific gravities ie lightweight

Good thermal and electrical resistance

Good surface finish from forming dies

Generally easy to form

Flexibility allows for versatility in applications

Low strength compared to metals

Unsuitable for service where temp exceed

several hundred degrees

Poor - fair dimensional stability, especially in

moist conditions

Thermosetting Polymers

Once formed this type of polymers cannot be reheated or softened. The chains have covalent bonds along

and across the molecules. Heating will char and burn them.

Manufacturing processes

Compression moulding has been mentioned in P&PT.

Hand Lay-up

1. Release Agent: A wax/non-binding polymer is first coated

onto the mould. This allows the finished cured part to be

easily removed

2. Laminate: A resin (typically a 2-part polyester, vinyl or

epoxy) is mixed with its hardener and applied to the surface.3. Reinforcement: Sheets of fibreglass matting are laid into the mould, then more resin mixture is added

using a brush or roller. This is all done by hand.

a. Additional resin is applied and possibly additional sheets of fiberglass.

4. Removing gas bubbles: The material must conform to the mould and air must not be trapped between

the fiberglass and the mould and so hand pressure, vacuum (ie vacuum lay-up) or rollers are used to

make sure the resin saturates and fully wets all layers, and any air pockets are removed

a. In some cases, the work is covered with plastic sheets and vacuumed to remove air bubbles

and press the fiberglass to the shape of the mould.

Thermoplastic Polymers (whilst thermoplastic polymers are not included in this section of the syllabus, it would be useful toknow some types and their applications in aeronautics)

Polyethylene Excellent electrical insulator. Easily

formed by extrusion or injection moulding

Coating on electrical wiring. Ventilation

fans

Acrylic/Perspex Can be transparent Windows

Nylon Good strength, heat and wear resistant,

low coefficient of friction

Gears and brushes in instruments

Teflon Very low coefficient of friction.

Chemically inert.

Wing ball bearings. Inner hose on

hydraulic lines

Polyurethane Foamed polymer that can be either flexible

or rigid

Insulation, filler in sandwich construction

(see below)




Most common use for polymers is to provide the matrix in composite materials. The polymer binds the

reinforcing fibres together and transfers the load to and between the fibres. This polymer matrix also keeps

reinforcement fibres in correct orientation, distributes load evenly, provides crack resistance and inter-

laminar shear strength. Also determines overall shape, service temp limitations, and may control corrosion.

Composites

Airframe Composites (eg matrix material + reinforcing materials)

Advantages

High S:W and stiffness

Tailored directional properties

Non-corroding in salt environment

Excellent fatigue resistance

Dimensional stability

Less parts required

Disadvantages

Hard to inspect for flaws

Sudden and catastrophic failure

Labor intensive and expensive to fabricate

Moisture pickup

Susceptible to lightning strikes

Susceptible to extreme temperatures

Early composite – Plywood – propellers, airframes.

Boeing. Integrated fibreglass in 1958, when fibreglass skins were used to cover Al honeycomb cores on

a few secondary control surfaces.

Early 1960s. Filament fibres (boron, carbon) mixed in an epoxy resin matrix. High strength and stiffness.

New materials (Kevlar), matrix materials (thermoplastics), metals (Al, Ti, Ni)

Composite materials allow for up to a 30% reduction in mass whilst having the same strength

Performance of composite depends on:

Composition, direction, length and shape of fibres Properties of matrix material

Bond between fibres and matrix

Fibres

To carry load in the composite

Provide tensile strength, flexural strength and stiffness

Determine electrical and thermal properties

Mostly circular cross section. Hollow fibres increase compressive strength.

Glass Relatively low cost, light weight, high

strength, non-metallic characteristics

Used for aircraft parts that don’t carry heavy

loads. Eg/ fuselage interior, trailing edge panels on

larger craft. Used extensively in primary structures

of small aircraft, helicopter rotor blades.

Kevlar High toughness, tensile strength,

stiffness with low density. Low

compressive strength, but overcome

with Kevlar/Carbon hybrid. Good

fatigue properties, chemical resistance,

high temp strength

Poor compressive strength has prevented its use in

primary aircraft structures.

Kevlar/Phenolic skins – lower surfaces of some

military craft – damage resistance

Polyethylene Better impact resistance thanglass/carbon fibres, stronger than

Kevlar. Melts at lower temp, absorbs

little moisture

Difficult to combine with matrix. Still indevelopmental stage.




Carbon or

Graphite

Careful placement can produce

composites stronger and stiffer than

steel at have the weight. Have fatigue

limits > Al./Steel with low thermal

expansion. Best balance of properties

and cost.

Most widely used.

Eg/ stabilisers, rudders (most control surfaces),

sections of fuselages – ribs, struts, skins.

Quartz Can be used up to 1040°, >500° greater than glass fibres. Strongest of the high temperature

fibres, also has good S:W, good radar transparency (like glass)

Fabrics can be woven from a mixture of fibres to provide a blending of properties

Two directional woven fabrics are stronger, tougher and less likely to delaminate and are thinner

Matrices

Binds the fibres together

Transfers load between the fibres and keeps them in correct orientation

Protects fibres from abrasion and oxidation/corrosion

Provides overall dimensions of the component

Determines the service temp and compressive strength

Thermoset Matrix Thermoplastic Matrix

Epoxy Polyethylene

Polyester Polystyrene

Phenolics Polyurethane

Thermosets

Can be used to form complex shapes, easily bond to different fibres. Provide a high strength and stiff

structure when cured.

Polyester

Secondary structures, cabin interiors with glass fibres. Low cost, processes easily, but not very

tough or strong

Epoxy

Most widely used. Principal resin used in carbon fibre structures. Excellent mechanical

properties, good toughness, fairly low cost

Phenolics

Also used in secondary structures often with glass fibres. Good for cabin interiors as it has low

smoke generation in case of a fire. Poor toughness, fair mechanical properties but low cost. Used

for dimensional stability at high temp and pressures

Thermoplastics

Used more extensively recently. Excellent strain capabilities, high moisture resistance. Major

advantages over thermosets are the shorter fabrication cycle, ability to weld and ease in

machining/drilling

Military aircraft are one of the major catalysts in their development, requiring 3 things:

High temp capabilities under severe hot/wet conditions

Better damage control in structural members

Easy mass production to reduce costs




Metal Matrices – strength offsets the extra weight. Greater strength and stiffness than polymers, superior

fracture toughness, greater S:W

Aluminium

Principal metal matrix. Improved properties when reinforced. Light and easily processed

Eg/ reo with carbon. Use for structures of missile, helicopters. Boron fibres are used in compressor

blades and structural supports

Titanium

Light and good resistance to high temp. Difficult to reinforce, quite expensive

Eg/ Reo with boron fibres. Use in jet engine fan blades.

Magnesium

Bonds well with the reinforcing. Light but poor corrosion resistance.

Eg/ Boron fibres are used in antenna structures. Alumina fibres are used for helicopter transmission

structures.

Copper

Improved shear strength over aluminium at elevated temperatures but denser.

Carbon Matrices

Excellent S:W and high stiffness but also possesses high temperature capabilities.

Carbon matrices with carbon fibre reinforcing (carbon/carbon composites) are sometimes used for nose

zones, jet engine turbine wheels

Also aircraft brakes. Outwear steel up to twice as long, high heat absorption rate (heat sink) and maintain

consistent performance with no reduction in stopping ability

Ceramic Matrices

Already used in braking systems. But impossible to machine or join with conventional fasteners so

components must be made in one piece.

Designing to retain high temp properties whilst improving toughness and impact strength

Carbon Fibre

Composite of carbon fibres embedded in an epoxy resin matrix.

Overall very lightweight, high S:W, high modulus of elasticity Resistance to cyclic stress = good for aircraft. Eg/ control surfaces, wingtips

Sudden/catastrophic failure. Damage detection and repair is more complex




Sandwich Core Materials (whilst not

specifically part of the syllabus, this is a very common

manufacturing technique and it would not

unreasonable for the examiners to blindside students

with such a question)

Suitable in aircraft as the thin surface skins

separated by the core combines light-weight with

strength. Used since 1940s. Cells now often made from composites (Kevlar, fibreglass, CF). Outer skins are

composite or metals. These materials are rigid and show low deflection. Eg/ nose cones, wing leading and

trailing edge panels, fuselage floor panels.

Syntactic cores – combine microspheres with resin matrix as filler. Will fit

to contoured shapes. Denser, so used in thinner panels. Provides greater

strength, continuous support of face material, little moisture seeping into

core.

Corrosion

Frames and skins are already stressed, therefore weakening via corrosion is

a concern. Although composites resist electrochemical corrosion, UV and weather may degrade. But as

many aircraft use metal airframes and skins, problems arise when carbon composites are coupled with

metals as part of an aircraft structure eg/ metal rivet used to hold composite skins to airframe.

This form of metal to composite corrosion can be reduced by:

Excluding moisture from the structure

Using a layer of inert cloth (Kevlar, fibreglass) as an insulator between the materials

Anodising Al parts

Finish external surfaces of both Al and composite with paint (epoxy)

Corrosion must be identified early before costly replacements or repairs are needed

Al alloys. White powdery deposits with a surface dulling of unpainted parts.

Alloy and plain steels. Red dust deposits on surface and some pitting of affected area

Stainless steels. Black pits or a uniform reddish-brown surface

Forms of Corrosion

Pitting. Occurs to unprotected metals when acids/alkalis/saline solutions chemically react with the metal

= Small holes/pits form = Losses in ductility and strength. Keep clean and keep surface coating in good

condition

Uniform etch. Frosty appearance resulting from general corrosion over the entire surface

Fretting corrosion. Rapid form attacking ferrous metals. Occurs at the junction between two highly

loaded components subject to vibration. Use lubrication.

Intergranular corrosion. Greater concentration of impurities at grain boundaries, resulting in a

potential difference with the centre of grain = Loss of strength and ductility. Use plating of cladding of

metal eg/ Alclad. Coating is anodic relative to core = electrolytic protection as well as physical

protection.




Conditions Causing Corrosion

Dissimilar metals Likely to cause EC reaction, even with similar metals with diff. heat treatmentconditions.

Eg/ steel bolts through Al alloy structural members

Eg/ Copper and steel hydraulic lines attached to Al alloy members

Incorrect heat treatment may lower a material’s corrosion resistance.

Heat treatment High strength alumium alloy is quenched too slowly = more susceptible to

intergranular corrosion

Welding Heated strip around the join is anodic and will corrode in preference tosurrounding metal. Can be reduced if part is annealed after welding.

Fretting Heating caused by localised friction promotes oxidation of steel and greatly

reduces the fatigue strength of the metal.

Overcome by plating structural assembly bolts with non-ferrous metal (cadmium).

As tight as possible. Use lubricating grease.

Stress

Corrode more readily than unstressed metals. Can also crack protective coatings.High temperatures Oxidise more quickly than unheated parts. Minimise using alloys containing

nickel or chromium

Electrical

Equipment Electrical insulation should be kept in good condition as leakage of current may

lead to the corrosion of both the electrical equipment and surrounding metal parts

Damaged

Protective Coatings

and Surfaces

Scratching/abrasion here may become starting pts for corrosion

Any foreign particles embedded into the surface may initiate corrosion, AWAscratches.

Crevice corrosion Concentration cell that occurs due to diff oxygen levels at top/bottom of crevice.

Low Oxygen = Anodic. Often occurs at fine gaps that should be riveted.

All enclosed areas in aircraft should be vented to prevent oxygen deprivation anddrained to remove the electrolyte (water) necessary for corrosion to proceed

Prevention and Control of Corrosion

Keep all surfaces clean (dirt, mud, acids)

Minimise moisture accumulation (drain out, condensation, rain)

ENGINEERING STUDIES – AERONAUTICAL ENGINEERING.pdf

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