The document analyzes materials for the landing gear strut of a Cessna TTX light aircraft. It translates design requirements into constraints and objectives, screens materials based on compressive strength and corrosion resistance, ranks materials based on indices that minimize weight and maximize buckling resistance and fatigue endurance. Magnesium alloy is selected as it is lightweight, inexpensive, and easier to manufacture than aluminum alloys, though composites are increasingly common in aircraft.

1. Materials 2: Properties of Materials
Materials Selection Group Assignment
Light Aircraft Landing Gear Strut
Group 6
19 December 2014
1 Introduction
Aircraft landing gears can be split into 2 parts: the solid cylindrical strut to support the
plane on the ground and withstand the forces during landing, and the damping element
to reduce those forces. This analysis seeks to determine the most suitable material the
landing gear strut of a light aircraft. A light aircraft is classified as an aircraft that has a
maximum gross takeoff weight of 5670 Kg or less. Consequently, the aircraft selected was
the Cessna TTX, a 4 passenger high performance aircraft with a maximum gross takeoff
weight of 1633 Kg.
Figure 1: Cessna TTX
Figure 1 shows the Cessna TTX with its landing gear highlighted in red. It uses a
tricycle type configuration with one gear at the front of the plane and 2 gears further back
near the wings of the plane. The landing gear of the Cessna TTX is non-retractable, which
means that it will be exposed to the elements during operation. However, the following
analysis will only consider compressive axial loading of the strut and environmental factors
such as corrosion, as other forces resulting from particulate impacts and air resistance
will be assumed to be negligible. The requirements will first be translated into a table
of constraints and objectives and then material indices will be derived for screening and
ranking. As there are multiple objectives the final ranking will be done with a penalty
function.
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2. Materials selection group 6
2 Translation
Function Support the weight of a Cessna TTX aircraft during landing and
while the plane is on the ground
Constraints Specified length, L
Minimum compressive strength
Resist corrosion in water
Resist buckling during landing
Minimum fatigue endurance strength
Moderately easy to manufacture
Objectives Minimise weight
Minimise material cost
Free Variable Choice of material
Cross-sectional area, A
Table 1: Translation
Many constraints and objectives were considered for our translation table. For ex-
ample, the minimum service temperature of the material was considered but was not
implemented because it proved unnecessary seeing as the constraint did not remove any
materials. Hardness was also considered but was also rejected because any collision with
airborne particles will be unlikely to cause localised damage that will harm the integrity
of the structure.
The primary objective of this design is to minimise weight as it affects the fuel con-
sumption of the aircraft which leads to higher long-term running costs. The balance is
that the savings due to a cheaper material might outweight the future fuel savings.
3 Screening
3.1 Compressive Strength & Corrosion
After some research it was found that alloy steels and aluminium are the most common
materials for this type of strut. Therefore, in order to immediately screen out materials
that would be unsuitable for use on landing gear, a minimum compressive strength of
180MPa was applied, the lower limit stainless steels strength. This removed the weaker
materials in the Edupack library such as bamboo and removed all foams and elastomers.
Materials that were rated Limited Use or Unacceptable in fresh water we also screened
out, since the Cessna TTX would most likely experience a damp environment during its
life use, however, this only removed a very small number of materials.
3.2 Machinability
In stage 4 of the selection process another screening was applied to remove anything
with a rating for Formability, the ability of a material to be shaped or experience plastic
deformation without being damaged, lower than 3 in order to cut down the remaining
materials and so that the chosen material would be moderately easy to shape into the
required form. Again, this only removed a small number of materials. The result of
screening is shown below in Figure 2.
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3. Materials selection group 6
Figure 2: Screening based on minimum compressive strength
4 Ranking
4.1 Buckling
By using the equations for the mass of a column, the minimum buckling load of a column
and the fatigue equation, two material indices can be calculated by eliminating the free
variable of the cross-sectional area. To find an index that will minimise the mass of the
strut while maximising its minimum bucking load, it is necessary to find an expression
for the cross-sectional area as a function of the mass, and rearrange the column buckling
equation to find another expression for the area.
The mass of a column is defined as:
m = ρAL, (1)
and therefore:
A =
m
ρL
(2)
Meanwhile, the expression for the minimum buckling load of a column, according to
the Euler Column Buckling formula, is
F =
π2
EI
L2
, (3)
where I is the area moment of inertia which, for a solid cylinder is defined as:
I =
πr4
4
=
A2
4π
(4)
Therefore,
F =
πEA2
4L2
(5)
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4. Materials selection group 6
A =
4FL2
πE
(6)
m =
ρ
√
E
2L
F
π
(7)
Therefore, the required material index is:
M1 =
ρ
√
E
, (8)
4.2 Fatigue Strength
In order to calculate the material index to minimise the mass of the strut while maximising
the endurance toughness, it was necessary to first rearrange the fatigue equation to find
an expression for the cross-sectional area as a function of the fatigue endurance, and then
equate it with the expression for area as a function of mass.
σe =
F
A
(9)
A =
A
σe
(10)
m =
ρ
σe
F
L
(11)
This produces the material index:
M2 =
ρ
σe
, (12)
Then, the two indices are plotted together on the bubble plot in Figure 3, with M2
on the X-axis and M1 on the Y-axis. In order to minimise the mass while maximising
the buckling resistance and fatigue endurance, materials in the lower left of the chart
must be selected, as lower values of M1 and M2 indicate a high endurance and buckling
resistance relative to the density of the material. The selection curve was moved into
a position where stainless steel is on the outer edge, meaning that only other equal or
superior materials are left for selection.
4.3 Cost
More materials were removed by forming a second bubble plot that shows the material
cost per kilogram in relation to buckling strength and fatigue endurance. The material
indices for cost were the same as for mass but with ρ replaced by ρCM . The lower left
corner of the bubble plot is shown in Figure 4. The pink and yellow bubbles represent
several varieties of glass and ceramics respectively, both of which were screened out in
stage 4 by removing materials with poor manufacturability. A selection line was drawn
and moved to screen out several less desirable materials: such as Zirconia, which requires
a long and costly machining process to achieve the desired shape; Titanium alloys, which
are too brittle and expensive; and CFRP, which has low performance in moist and high
fatigue environments.
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5. Materials selection group 6
Figure 3: Ranking based on specific fatgiue endurance and specific buckling strength
Figure 4: Ranking based on cost, fatigue endurance and buckling strength
The final step was to minimise the cost and weight of the material. Figure 5 shows
dotted lines to indicate potential penalty function lines where α = 300. The clear winning
material was cast magnesium alloy, since it is one of the lightest and cheapest materials,
and is easier to manufacture compared to wrought magnesium alloy.
5 Discussion
Magnesium alloys are already used in the aerospace industry, but in limited amounts.
They account for 1.0-1.5% of the average aircraft’s weight, whereas the most common
materials used for landing gear are strong and ductile specialist steels [1] and aluminium
alloys [2]. In fact, materials for aerospace are usually selected for having a high strength-
to-weight ratio and a high specific stiffness [3]. These are properties which were ranked
for in this project, meaning that the method by which the magnesium alloy was selected is
similar to that by which steels were chosen in industry. It is possible that errors were made
in this selection process. For example, it is important to note that the sources used in this
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6. Materials selection group 6
Figure 5: Ranking based on cost and density
analysis of the elected material predate the development of many composite materials, and
there is a strong trend in the aerospace industry towards the use of composite materials.
It is also significant that Edupack measures the fatigue endurance as the fatigue stress
for a fatigue life of 107
cycles, whereas many forged aluminium landing gears demonstrate
lifespans of less than 6000 cycles, although these tests were carried out on much larger
commercial aircraft [2]. However, magnesium alloys are becoming increasingly prevalent
in aerospace applications and their limited use is more likely a result of implementational
inertia. Their many advantages include that they are cheaper than steel and lighter than
both steel and aluminium [4]. It is also easier to produce than aluminium alloys; which
must be heat treated, quenched and then aged by heat. The process by which aluminium
alloys are produced often distorts the metal, requiring correction by rolling [4].
6 Conclusion
In conclusion, the material best suited for use in a light aircraft landing strut is a cast
magnesium alloy, due to its superior mechanical properties, low weight and cost. Although
it is a newer material that is relatively untested for such an application, the data recording
its advantages is clear and it will likely form a much larger percentage of the weight of
aircraft in the future.
References
[1] Alexandru Nica Mechanics of Aerospace Materials. Elsevier Scientific Publishing Com-
pany, 1st edition, 1981.
[2] Alfred M Freudenthal Fatigue in Aircraft Structures. Academic Press Inc, 1st edition,
1956.
[3] Darrol Stinton The Anatomy of the Aeroplane. Blackwell Science Ltd, 2nd edition,
1998.
[4] CES Edupack datahseets. CES, 2014.
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