engineering studies – aeronautical engineering.pdf
TRANSCRIPT
8/19/2019 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.
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Bryan HoENGINEERING STUDIES – AERONAUTICAL ENGINEERING
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.)
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Bryan HoENGINEERING STUDIES – AERONAUTICAL ENGINEERING
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
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Bryan HoENGINEERING STUDIES – AERONAUTICAL ENGINEERING
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)
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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
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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
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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.
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Bryan HoENGINEERING STUDIES – AERONAUTICAL ENGINEERING
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
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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
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Bryan HoENGINEERING STUDIES – AERONAUTICAL ENGINEERING
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)
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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.
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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
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Bryan HoENGINEERING STUDIES – AERONAUTICAL ENGINEERING
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
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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.
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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)