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With aviation expecting to join the EU
emissions trading scheme in 2010 there
is now an economic driver to reduce
emissions in addition to the social and
technical pressures to reduce its
environmental impact. With aviation
there are two main ways to reduce
emissions – by reducing the overall fuel
burnt and increasing engine efficiency.
To reduce the amount of fuel burnt you
can reduce both aircraft weight and its
parasitic drag (drag due to the non-lift
component i.e. the fuselage).
For a large turbojet aircraft a weight
reduction of 1,000kg cuts fuel use by
about 1.1-1.5 per cent. To improve
engine efficiency, the engine has to run
at a higher turbine inlet temperature,
with a 50°C increase relating to a 1 to
1.33 per cent increase in engine
efficiency, allowing less fuel to be burnt
for the same thrust output. As CO2
emissions are in a 1:1 ratio with fuel
burn, these reductions relate directly to
a decrease in carbon dioxide emissions.
Since fuel costs are the largest
operating expense for airlines,
technologies which reduce fuel use
have a favourable effect on the bottom
line.
Losing weight
During the pioneering period of
aviation (1903-1930) the minimum
weight possible was of utmost impor-
tance due to the poor performance of
propulsion systems (the Wright Flyer
had about 8hp). This led to the use of
wood covered with varnished fabric,
which had limited strength and loading
capacities. Aluminium alloys became
the baseline for aircraft structures after
corrosion issues were overcome in
1927. Initial advancement concentrated
on the refining of aluminium alloys and
the development of new materials,
such as composite systems which
Advanced aerospace
materials: past, present
and future
With the Intergovernmental Panel on Climate Change (IPCC) reporting that up to 15 per cent of total
greenhouse gas emissions could be caused by aviation by 2050, it is important to review how the past,
current and future use of advanced materials and design could help prevent this scenario. Sir David King,
Dr Oliver Inderwildi and Chris Carey, of Oxford University’s Smith School of Enterprise and the Environment,
discuss improvements being made to existing materials, and review the new materials that we could
soon see flying on aircraft.
consists of two or more phases on the
macroscopic scale. The mechanical
performance and properties of the
combined system are superior to those
of the constituent materials. These
materials were first applied on civil
aircraft with the Boeing 707 in 1957,
with approximately 20m2of polymeric
composites in mainly tertiary roles, such
as cabin structures.
Increasing use of composite materials
was limited, with only a three per cent
increase observed from the A300 to
A310. However much larger structural
parts, such as the vertical stabiliser (8.3m
by 7.8 m at the base), were now being
fabricated entirely from carbon com-
posites. This gives a weight saving of
more than 400kg over an aluminium
alloy structure, resulting in approx-
imately 0.5 per cent reduction in fuel
burn per hour. Aluminium/lithium alloys,
first proposed in the 1950s, were also
introduced to reduce the density of
components (one per cent of lithium
reduces the density of aluminium alloys
by three per cent). Production issues
initially restricted their use but they are
now utilised in a variety of structural
applications.
A material advantage?
Since Orville and Wilbur Wright first decided to power their Flyer with a
purpose built, cast aluminium engine to meet the specific requirements for
power to weight ratio, new materials have been necessary to improve and
advance aviation. This improvement in material properties has helped us to
travel quickly and inexpensively around the world, by improving the
performance and operations of modern aircraft.
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The Smith School of Enterprise and the Environment is a unique
interdisciplinary hub where academics from around the world
work with the private sector and government to pioneer solutions
to the major environmental challenges of the 21st century. Its
work on aviation is part of the “Future of Mobility” project being
undertaken by the school to give a comprehensive perspective of
the future of transportation and mobility up to the year 2050
(http://www.future-of-mobility.org).
Sir David King Dr Oliver Inderwildi Chris Carey
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the reduction in fuel consumption per
revenue passenger mile (Figure 2).
Whilst advanced materials are not solely
responsible for this reduction, they have
contributed significantly to this overall
improvement in fuel burn.
Getting hotter
The introduction of turbines required
a development of a new family of
materials to cope with the high
temperatures and stresses present in
the turbine, particularly the so-called
‘hot’ or combustor/turbine stages
where temperatures can reach over
1500°C in modern engines. Initial
engines, such as Sir Frank Whittles W1,
used a variety of stainless steels but
these were soon replaced with the first
super alloy systems, nickel-chromium
alloys such as Nimonic and Inconel. The
development of high strength mat-
erials, resistant to the corrosive en-
vironment in the jet turbine, called for
improvements in production as well as
new materials and alloys.
Development of vacuum induction
melting (VIM) technology allowed a
much greater control over the
composition of superalloys, which
increased the component reliability.
Commercial production of titanium was
also an important development: Not
only did it find many applications in
turbine components, such as the
compressor stage, it also allowed for
the development of ducted bypass fans.
These work by using excess energy
produced during combustion to bypass
an amount of air past the core of the
engine giving an overall increase in
thrust and improvement in specific fuel
cost (SFC) at the cost of top speed and
overall engine weight.
The mid 1950s also saw a radical
change in the technology of turbine
blade production – the use of invest-
ment casting. This process allowed the
casting of fine channels within the blade,
which, with laser drilling, allowed air-
cooling of the turbine blade, increasing
blade-operating temperature.
The casting of the blade led to the
next leap in turbine technology, the
removal of grain boundaries. Standard
cast blades contain a large number of
grain boundaries where a number of
undesirable events occur. The intro-
duction of directionally solidified (DS)
blade (produced by slowly withdrawing
the blade from the furnace in one
direction) gives no grain boundaries
perpendicular to the major stress axis.
This improves reliability and maximum
temperature by up to 25°C and
therefore engine efficiency.
This was further developed to single
crystal (SX) casting (first used in Pratt &
Whitney’s JT9D-7R4 in 1982) where the
use of directional solidification and
crystal removal (via a helix) led to the
production of turbine blades containing
no grain boundaries, again increasing
maximum operating temperature by
25°C. Thermal barrier coatings (TBC)
is another technique used to reduce
the relative temperature of engine parts
by applying ceramic coatings to hot
section parts. The mid 1980s saw the
application of polymeric composite
materials in engines, in many non-core
applications such as fan blades and
casings. These have the benefit of
reducing the overall mass of an engine
and therefore the aircraft, improving
efficiency.
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The latest development in the field of
aerospace materials arises from the use
of application-specific materials. The
A380, which at 61 per cent has the
lowest percentage of aluminium by
weight of all flying Airbus models, has
20 different alloys and tempers
compared to the six utilised on the
A320/330 aircraft. The A380 also saw
the application of a new material,
GLARE, for fuselage skins which shows
improved fatigue and impact properties
at a lower density than incumbent
materials.
The composition of the A380 (Figure
1) illustrates the variation in materials
used in modern airliners, in order to
ensure that the best material is used for
the application, allowing for weight
reduction. Significant increases in the
amount of composite systems have
occurred, with the 787 and proposed
A350 XWB each having a primarily
composite structure (over 50 per cent),
with carbon fibre reinforced polymer
being used. These material develop-
ments have led to the overall reduction
of aircraft weight, which is reflected in
Best match between material and design drivers
Glass Fibre Reinforced Plastic
Quartz Fibre Reinforced Plastic
Carbon Fibre Reinforced Plastic
Metal
Glare
A380 firsts
Result in a 15 tonnes
weight saving
Figure 1. Airbus A380 material composition Source: Airbus
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The majority of these advancements
have led to a vast improvement in
engine efficiency by increasing the
turbine inlet temperature. Figure 3
overleaf shows the turbine inlet
temperature of a selection of Rolls-
Royce turbines and corresponding
material developments, where a
significant proportion of the
temperature increase can be attributed
to advanced materials.
What does the future
hold?
The improvement and development of
materials for aviation applications is
developing on three main fronts: the
development of new materials; the
improvement of current material
properties by refining composition and
novel processing methods for new
applications; and the application of
current materials in new and novel
structures.
New materials
New materials can be defined as
materials which have yet to be applied
in an ‘as-designed’ application in
aviation. Some of these materials,
particularly metal matrix composites
(MMC) and ceramic matrix composites
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(CMC) have seen some in-flight testing
and are approaching military use but
have yet to gain wide ranging accep-
tance by OEMs for various reasons.
The following discussion briefly
introduces a number of materials that
have a potential for applications in next
generation aircraft.
Ceramic matrix composites (CMCs):
While consisting of purely ceramic
constituents, CMCs utilise a ceramic
matrix with reinforcing ceramic fibres
and are accepted as a composite
system. This creates a material with the
excellent thermal properties and with
improved mechanical properties, over-
coming the limitations of monolithic
ceramic (i.e. toughness) and displaying
other benefits. The possible applications
of CMCs in aviation are generally in the
hot section of the aero engines and
include turbine disks, combustor linear,
turbine aerofoils, transition duct
convergent flags and acoustic liners.
The use of CMCs would allow an
increase in turbine inlet temperature
from the current 1200°C to 1500°C,
which would lead to a 6-8 per cent
increase in fuel efficiency.
Metal matrix composites (MMCs):
These consist of an aluminium or
titanium matrix with oxide, nitride or
carbide reinforcement and have many
advantages over monolithic materials.
But they are not as tough, are more
expensive and are difficult to machine.
A major issue for MMCs is their
production and manufacturing cost and
current research is focused in this area.
Possible applications include highly
loaded surfaces such as helicopter rotor
blades, turbine fan blades and floor
supports.
Nanocomposites: As with macro-scale
composites, a number of matrix/-
reinforcement combinations are possible
with CMC, MMC and PMC all under
investigation. Nano-composites utilise
the huge surface area per mass and
high length-to-width ratios of nanoscale
objects to improve material properties.
Current development issues include pro-
ducing the necessary quantity of
nanoparticles at a commercially attract-
ive price and various production issues,
such as filler dispersion.
Shape memory metals (SSM): When
SSMs are heated they revert to a pre-
deformation shape. They usually consist
of copper/nickel based alloys, though
other materials can be used. The
simplicity of SSM actuators is that they
can be used for hybrid applications such
as variable jet intake and morphing
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
1960 1965 1970 1975 1980 1985 1990 1995 2000
Year of Introduction
Short Range Aircraft
Long Range Aircraft
Fuel Consumption (L/Revenue Passenger Miles)
Figure 2. Fuel consumption per revenue passenger mile of short and long range civil aircraft
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mechanical properties so ceramics can
now compete with metals in
applications for which they where
previously unsuitable. Development of
ceramic materials has led to the use of
these highly thermal stable materials in
a variety of applications, such as main
shaft bearings, engine seals and thermal
barrier coating on turbine blades. The
use of ceramics in these applications
allows engines to work at a higher
temperature, increasing their thermo-
dynamic efficiency.
New structures
A number of new structures have been
investigated for a variety of materials
and are at varying stages of develop-
ment. Some, such as fibre metal lamin-
ates, have already been applied to
aviation, whilst others are still at the
laboratory stage.
Lattice: One area of particular interest
is lattice block, which works on either
pyramidal or tetragonal truss arrange-
ments and is produced using investment
casting. These structures weigh approx-
imately 15 per cent of a solid plate of
the same external dimensions, whilst
still exhibiting good strength and
damage architecture.
Foams: Another major development in
the use of aluminium alloys is the
production of foam or cellular systems.
These are produced by a number of
methods such as direct foaming using
gas and investment casting, but all
methods produce a material containing
a number of voids. The size, density and
structure of the void produced depends
on a number of variables, and partic-
ularly the production method. The
Smith School of Enterprise and the
Environment believes that foam
structures will replace honeycomb
structures and could lead to higher
performance at reduced cost. The use
of low density super-alloy foam in noise
abatement applications, replacing
acoustic liners, would allow for an
increase in engine burn efficiency, again
reducing fuel burn and emissions.
Laminate structures: A number of
laminate systems are under investig-
ation with a variety of constituents. The
Super-alloys: Current research in this
area is focused on fourth generation
super-alloys containing ruthenium to
improve microstructural stability and in-
crease high temperature creep strength.
Titanium: The main area of research
with titanium is in improvements to the
production process to lower costs. A
number of development projects are
being carried out with the potential to
reduce the cost of final titanium
products by very significant amounts, in
the region of 30 per cent or more.
Steels: Advances in steel alloys have
concentrated on improvements in ultra-
high strength and toughness. The
AerMet family of alloys are a significant
development in this area, with similar
specific strengths (UTS/density) to
common Ti alloys, but with a vastly
improved ductility and much higher
yield strength. Applications are in safety
critical structures, such as transmission
gears and parts which require the
structural efficiency that steel can offer.
Ceramics: Ceramics exhibit superior
thermal properties and major progress
has been achieved in improving the
26
variable geometry chevrons (Figure 4)
where traditional systems are too large
and complex when compared with the
savings possible.
Material
Improvements
A continuing trend in material
development is the improvement in
processing and production of incum-
bent materials to either improve
physical properties or to allow their
application in new areas and roles.
Aluminium alloys: As the most
common of aviation materials, it is
unsurprising that a large number of
developments are in the pipeline for
aluminium alloys. These include further
refinement of current alloys to improve
specific strength and corrosion resis-
tance, as well as developing alloys for
specific manufacturing processes such
as friction stir welding and laser
welding. These advancements will
continue the trend for much larger
numbers of alloys in aircraft (the A380F
has three planed alloys for wing panels)
leading to lighter structures with
location specific properties.
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0
200
400
600
800
1000
1200
1400
1600
1800
1940 1950 1960 1 970 1980 1990 2000
Year of Introduction
Casting
VIM
TBC
DS
SX
Turbine Inlet Temperature C
0
Figure 3. Turbine inlet temperature for a selection of Rolls-Royce turbines with major material developments indicated.
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laminate structure prevents catas-
trophic failure and exhibits improved
impact characteristics. One such
material is fibre metal laminate, which
consists of layers of composite and
aluminium and provides high impact
strength and directional strength at
a low density. A number of different
composites have been investigated,
such as aramid, glass fibre and carbon
fires with a variety of metal layers such
as aluminium, titanium and steel. New
approaches are investigating asym-
metrical lay-up approaches, such as
CENTRAL, tailoring the panel properties
to the application requirements.
What now?
All these developments have created
one of two things, either a lighter
overall weight for parts of the same
properties, in the case of structural
materials, or a higher thermodynamic
efficiency of the engine with higher
temperatures within the engine. If we
consider the latest aircraft to be
launched, the A380, a single kilogram
of weight saved equates to a 50ml
reduction in fuel burn per hour. This
might not sound much, but assuming
a 75,000 hour life of the aircraft,
it equates to 3,750 litres of fuel. The
hypothetical replacement of steel
within the A380 (approximately
11,500kg) with titanium alloy would
reduce the overall weight by 5,750kg,
saving 288 litres of fuel per hour (22
million litres over the life time) equating
to a two per cent drop in fuel burn and
emissions.
With turbine material improve-
ments, an increase in turbine inlet
temperature from the current 1,200°C
to 1,500°C would lead to a 6-8 per
cent increase in fuel burn efficiency,
equating to a 588 million litre reduction
in fuel use over the life of the aircraft.
This is the equivalent of approximately
300 A380s filled with fuel. And with
Jet-A1 prices exceeding $1.10/L last
year, these developments offer sig-
nificant economic, as well as environ-
mental, benefits in the operation of
airliners – even when the economic and
environmental cost of producing
advanced materials is taken into
account, as discussed in the last issue
of Aviation and the Environment.
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“
”
The use of low density super-alloy
foam in noise abatement
applications, replacing acoustic
liners, would allow for an increase
in engine burn efficiency, again
reducing fuel burn and emissions.
Figure 4. Variable Geometry Chevrons
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