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DESIGN AND ANALYSIS OF COMPOSITE
LANDING STRUT FOR A VERTICAL
TAKEOFF AND LANDING UNMANNED
AERIAL VEHICLE 1M Upesh Kumar Rao, 2Dr. R.J.V Anil Kumar,
1PG Student, 2Assistant Professor, 1Department of Mechanical Engineering, Anantapuramu, India, 2Department of Mechanical Engineering, Anantapuramu, India.
Abstract: Landing strut is the critical component of Unmanned Aerial Vehicle (UAV), which plays a crucial role in landing the
UAV safely and gives support to the UAV structure in the rest position. In this paper, landing strut designed according to
requirements of the UAV. The landing strut can be sepatared into two parts one is bridge cradle, and the other is a skid. The skid
can be designed with two different cross-sections, one is rectangular skid, and the other is a circular skid. The landing strut model
is designed in CATIA V5. The modelling and analysis for both the model carried out by Hyperworks software and compared with
the theoretical values. The objective of this study was to compare the results of CFRP and GFRP materials by static, dynamic and
buckling analysis using the computational tool.
Keywords: Landing strut, skid, dynamic and buckling analysis, Catia V5, Hyperworks.
I. INTRODUCTION:
UAV has a unique structural design of a landing strut and supports to land on the ground surface safely while taxiing,
takeoff, landing, and protect the UAV from colliding to ground. The landing struts can be designed with wheels or skids; the wheels
have more weight compared to skids, so most of the helicopters designed with skids. Skid landing strut is fundamental and lighter
weight, so it is the best choice for little UAVs as weight is reliably an idea. Moreover, skid landing strut needs practically no help,
yet the drawback is that ground dealing with is dynamically problematic. In small UAVs ground dealing with wheels can be annexed
to the slides and the helicopter moved around by one person. The structural design depends on the required characteristics of a
UAV. The landing struts are less expensive and can design with innovative ideas. The landing strut consists of two different parts,
one is bridge cradle connected to the UAV’s fuselage and another, is skid, attached to cradle end, and fixed. In this paper, the
structure of the landing strut designed with two different skids, applying composite material properties for analysis to determine the
structural characteristics of landing strut.
II. LITERATURE SURVEY:
The project presents a static, dynamic and buckling analysis of impact loading landing strut in Hypermesh software. The
improvement of the landing strut requires several references and is developed from the Hypermesh Software.
Rasees Fifa Swati1, Dr. Abid Ali Khan2 et al [1] have investigated on optimizing procedure of designed landing strut model and
carried out the static analysis with impact analysis. Finally, they concluded that the model weight was optimized from the original
model weight.
R. Arravind1, M. Saravanan2, R. Mohamed rijuvan3 et al. [ 2 ] performed on the analysis of landing strut cast-off composite
material to optimize the weight design by taking the iterations as thickness to minimize the weight of the landing strut. They
concluded that the optimized model is safe for fabrication.
Xinyu Zhu1, Junwen Lu2 et al. [3] had created a FEM model by assigning composite materials on the landing gear for estimating
the static analysis and buckling analysis. They showed the damaging locations in the landing gear model.
S. Naresh1, J. Abdul Shukur2, K. Sriker3, A. Lavanya4 et al. [4] studied on landing gear skid and analysed by assigning different
composite alloy properties to evaluate the deformations in landing gear skud. Lastly concluded that material which can to resist
maximum stress, it has high strength.
Mandeep Chetry1, Han Dong2 et al. [5] conducted the static analysis on helicopter skid landing gear on both the composite
materials and aluminium alloy. They compare the results of materials from the stress and displacements to reduce the deformation
in the skid landing gear.
S. A. Mikhailov1, L. V. Korotkov2, S. A. Alimov3 et al. [6] have modelled on the landing of a helicopter with skid under-carriage
in regards for the second landing impact. They presented a comparison between the analysis results and the experimental data.
Benazir Zia1, Hafiz Sana Ullah Butt2 et al. [7] was analysed the dynamic response of a composite strut of landing gear by using
ANSYS LS-DYNA software. They concluded on the effect of impact velocity on the landing gear and finalized the results having
higher the impact velocity and impact reactions on the strut.
G Krishnaveni1, E.S Elumalai2, S.Mayakkannan3 et al. [8] performed on buckling analysis of landing gear under static condition.
They calculated the model and verified using the software. The landing gear will not buckle for any load below limit load under
static static conditions.
Jerzy Józwik1, Jaroslaw Pytka2, Arkadiusz Tofil3 et al. [9] analysed on dynamic analysis of aircraft landing gear wheel. They
performed the test with the use of one aircraft on two different surfaces. The results considered the time course of displacement,
velocity and acceleration of one selected point on the sidewall of the tire during a drop from a kilometre ramp.
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III. MODELING:
The modelling of the landing strut is designed according to the design requirements of the UAV. The landing struts consist
of a horizontal cradle connected to the fuselage bottom of the UAV to sustain the impact load while taxiing, takeoff, and landing.
The skid connected to another end of the bridge cradle that gives safe landing from the damage of the UAV body, and has less stress
distribution from skid to strut vice versa. Here landing strut designed with two different skid models one is rectangular skid, and
the other is circular skid landing strut.
Fig 1: Rectangular Skid Landing Gear Fig 2: Circular Skid Landing Gear
IV. MATERIAL PROPERTIES:
Most of the UAVs and Light Helicopter (LH), where the aerial body and landing strut are fabricated with the composite
material, which has advantage strength to weight ratio and strength to volume ratio. Fibre-reinforced polymer (FRP) mostly used
for fabrication because of high rigidity, corrosion resistance, electric conductivity, fatigue resistance, and excellent tensile strength
but brittle. CFRP and GFRP materials are using in this project for the analysis of the landing strut. CFRP materials and GFRP
materials commonly used in military, automotive, submarines, ships, etc. Materials properties of CFRP and GFRP are taken the
NAIR AGATE website https://www.niar.wichita.edu/agate/. The values are considered in report on Carbon Plain Weave Fabric
3K70P/NB321 and E-Glass Fabric 7781/NB321.
Table 1: Mechanical Properties of CFRP
Property Tensile
Strength
Mpa
Young’s
Modulus
Mpa
Compressive
Strength
Mpa
Density
Kg/m3
Poisson
Ratio
Tensile 620.183 66327.45 - 1500 0.058
Compression - 4247.47 114.97 1490 0.058
Table 2: Mechanical Properties of GFRP
Property Tensile
Strength
Mpa
Young’s
Modulus
Mpa
Compressive
Strength
Mpa
Density
Kg/m3
Poisson
Ratio
Tensile 438.27 28957.93 - 1740 0.138
Compression - 4198.9 131.73 1860 0.138
V. PROCEDURE:
1. Firstly, define the geometry and dimensions of the landing strut; design the 3D model with the acquired geometry by using
CATIA software. After that model should export into IGES or STP file for further steps.
2. Import the model in the Hypermesh workbench by setting the profile as bulk data in Radioss solver, meshing to be done
to model by giving the element size and mesh type.
3. Next, the preprocessing setup carried out by creating materials, properties, loads, and boundary conditions.
4. Solve the analysis in Radioss solver, view the results, and contour in Hyperview.
VI. Model Meshing:
Meshing plays a crucial role in finite element analysis that gives a direct output of the process. Meshing was created by
setting up the mesh type to quad that creates the QUAD4 elements through the nodes and to restrict the size of the element; the
element size value is given here as 5. With the help of element size, nodes, and elements are created according to the given element
size value. After creating the mesh, the preprocessing arrangement to be assigned to the meshed component.
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Fig 3: Mesh Model of Rectangular Skid Landing Gear Fig 4: Mesh Model of Circular Skid Landing Gear
VII. LANDING LOADS:
The landing loads that include gravity and body weight. These loads were considered for landing strut to examine structural
strength. After the meshing of components, the next step is assigning boundary conditions and applying forces to the components.
Table 1 shows the landing loads applied to land strut. The landing load is applied on the landing strut bridge cradle (the connection
between fuselage and strut), which was applied in the downward z-direction, and skids are constrained in all directions. By applying
these loads, the strut deforms in a negative z-direction.
Table3: Landing Loads
Load Type Value
Landing load 130 N
Fig 5: Landing Loads of Fig 6: Landing Loads of
Rectangular Skid Landing Circular Skid Landing Gear
VIII. STRUCTURAL STATIC ANALYSIS:
The landing strut model was imported from CATIA, which consists of fuselage, strut, and skid. This imported model
meshed and applied a landing load of UAV weight as 130 N to analyse the structural deformation. Before analysis, the structure
assigned to boundary conditions in all directions. In this study, landing loads have applied on the bridge cradle which can be seen
in fig (5, 6), i.e. connected to the bottom of the fuselage and the skids were constrained in all degrees of freedom. The static analysis
shows the results of vonmises stresses and displacements under applied static loads. The deformation contour of the structure
visually seen in hyperview software. The results of the stress and displacement shown in table 4 and 5.
8.1 Displacement:
a. CFRP:
Fig 7.a: Displacement Contour of Tension Properties for Rectangular Skid Landing Strut
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Fig 7.b: Displacement Contour of Tension Properties for Circular Skid Landing Strut
Fig 8.a: Displacement Contour of Compression Properties for Rectangular Skid Landing Strut
Fig 8.b: Displacement Contour of Compression Properties for Circular Skid Landing Strut
b. GFRP:
Fig 9.a: Displacement Contour of Tension Properties for Rectangular Skid Landing Strut
Fig 9.b: Displacement Contour of Tension Properties for Circular Skid Landing Strut
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Fig 10.a: Displacement Contour of Compression Properties for Rectangular Skid Landing Strut
Fig 10.b: Displacement Contour of Compression Properties for Circular Skid Landing Strut
8.2 Stress:
a. CFRP Material:
Fig 11.a: Vonmises Stress Contour of Tension Properties for Rectangular Skid Landing Strut
Fig 11.b: Vonmises Stress Contour of Tension Properties for Circular Skid Landing Strut
Fig 12.a: Vonmises Stress Contour of Compression Properties for Rectangular Skid Landing Strut
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Fig 12.b: Vonmises Stress Contour of Compression Properties for Circular Skid Landing Strut
b. GFRP Material:
Fig 13.a: Vonmises Stress Contour of Tension Properties for Rectangular Skid Landing Strut
Fig 13.b: Vonmises Stress Contour of Tension Properties for Circular Skid Landing Strut
Fig 14.a: Vonmises Stress Contour of Compression Properties for Rectangular Skid Landing Strut
Fig 14.b: Vonmises Stress Contour of Compression Properties for Circular Skid Landing Strut
Table 4: Results of Tensile Properties in Static Analysis
Materials
Rectangular Skid Circular Skid
Displacement
Mm
Stress
Mpa
Displacement
Mm
Stress
Mpa
CFRP 8.52 98.16 8.42 101.1
GFRP 19.46 95.43 19.31 98.02
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Table 5: Results of Compressive Properties in Static Analysis
Materials
Rectangular Skid Circular Skid
Displacement
Mm
Stress
Mpa
Displacement
Mm
Stress
Mpa
CFRP 133.0 98.16 131.6 101.1
GFRP 134.2 95.43 133.1 98.02
IX. DYANAMIC ANALYSIS:
FEM model used in Hypermesh consist of, 2D (Quad-shell and Tria shell elements) elements, considering the boundary
conditions at one end of a skid as fixed and free at the other end of landing strut and fuselage. Normal mode analysis is carried out
for both landing strut models using Radioss as the solver, where the mode shapes and frequency are obtained.
9.1 Mode Shape 1:
a. CFRP Material:
Fig 15.a: Fuselage Nose Pitching Mode for Rectangular Skid Landing Strut
Fig 15.b: Fuselage Nose Pitching Mode for Circular Skid Landing Strut
b. GFRP Material:
Fig 16.a: Fuselage Nose Pitching Mode for Rectangular Skid Landing Strut
Fig 16.b: Fuselage Nose Pitching Mode for Circular Skid Landing Strut
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9.2 Mode Shape 2:
a. CFRP Material:
Fig 17.a: Bending of Landing Strut for Rectangular Skid Landing Strut
Fig 17.b: Bending of Landing Strut for Circular Skid Landing Strut
b. GFRP Material:
Fig 18.a: Bending of Landing Strut for Rectangular Skid Landing Strut
Fig 18.b: Bending of Landing Strut for Circular Skid Landing Strut
9.3 Mode Shape 3:
a. CFRP Material:
Fig 19.a: Rear Fuselage Pitching Mode for Rectangular Skid Landing Strut
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Fig 19.b: Rear Fuselage Pitching Mode for Circular Skid Landing Strut
b. GFRP Material:
Fig 20.a: Rear Fuselage Pitching Mode for Rectangular Skid Landing Strut
Fig 20.b: Rear Fuselage Pitching Mode for Circular Skid Landing Strut
Table 6: Dynamic Analysis using Tensile properties
Materials
Frequency (Hz)
Rectangular Skid Circular Skid
1 2 3 1 2 3
CFRP 6.397 11.943 12.620 6.441 12.539 12.782
GFRP 3.903 7.694 7.845 3.775 7.442 7.588
Table 7: Dynamic Analysis using Compressive properties
Materials
Frequency (Hz)
Rectangular Skid Circular Skid
1 2 3 1 2 3
CFRP 1.624 3.032 3.204 1.635 3.184 3.204
GFRP 1.427 2.699 2.853 1.438 2.834 2.853
X. BUCKLING ANALYSIS:
Buckling was characterised by failure of the structure due to compression loading, where the load is taken as 130 N. The
load was applied at the connection between the bridge cradle and base of the fuselage. At the bottom, the landing strut was
constrained in all degree of freedom. Finally, Critical loads were determined to the skid landing gear models by conducting iterations
on loads until we get mode frequency 1.
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a. CFRP Material:
Fig 21.a: Buckling Contour of Tension Properties for Rectangular Skid Landing Strut
Fig 21.b: Buckling Contour of Tension Properties for Circular Skid Landing Strut
Fig 22.a: Buckling Contour of Compression Properties for Rectangular Skid Landing Strut
Fig 22.b: Buckling Contour of Compression Properties for Circular Skid Landing Strut
b. GFRP Material:
Fig 23.a: Buckling Contour of Tension Properties for Rectangular Skid Landing Strut
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Fig 23.b: Buckling Contour of Tension Properties for Circular Skid Landing Strut
Fig 24.a: Buckling Contour of Compression Properties for Rectangular Skid Landing Strut
Fig 24.b: Buckling Contour of Compression Properties for Circular Skid Landing Strut
Table 8: Buckling Analysis using Tensile properties
Materials Mode Frequency Critical Load (N)
Rectangular skid Circular Skid Rectangular skid Circular Skid
CFRP 9.114 9.15 1286.72 19888.84
GFRP 3.981 3.998 645.15 19256.68
Table 9: Buckling Analysis using Compressive properties
Materials Mode Frequency Critical Load (N)
Rectangular skid Circular Skid Rectangular skid Circular Skid
CFRP 0.587 0.585 204.74 207.32
GFRP 0.579 0.575 202.48 200.12
XI. Factor of safety: The factor of safety is a significant concern in structure and landing strut. It is determined after a point by point assessment
of stresses along with the structure for loading conditions. Here the factor of safety is calculated by dividing with the tensile and
compression strength of CFRP and GFRP material with the Vonmises stresses generated in rectangular and circular skid landing
strut.
The factor of safety for UAV’s landing struts is considered as 1.2.
Table 10: Results of Factor of safety Material
Rectangular Skid Circular Skid
Tensile Compression Tensile Compression
CFRP 6.32 1.17 6.13 1.13
GFRP 4.59 1.38 4.47 1.34
XII. CONCLUSION: This paper concludes the landing strut of CAD model designed in CATIA software, discretized in finite element analysis
by Hypermesh software and landing loads were applied in Radioss solver to estimate the structural deformations by performing the
static, dynamic and buckling analysis.
1. Comparing the results of stresses and displacements, the landing strut of CFRP material is better than GFRP material.
2. In every analysis, there is a slight deviation in results on both rectangular skid landing strut and circular skid landing strut.
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3. The tensile properties and compressive properties are applied to observe the behaviour of landing strut while loading within the
constrained conditions.
4. The circular skid has less stress distribution with less deformations in the landing strut in both the tensile properties and
compression properties.
5. From the results of dynamic analysis, the frequency of the landing strut is 6.441 Hz in tensile properties and 1.635 Hz in
compression properties.
6. In buckling analysis, buckling critical loads are estimated for the models of CFRP and GFRP materials. It can be concluded that
for tensile properties and 1.03 times safer than GFRP material in a circular skid while considering the compression properties,
CFRP material is 1.01 times safer than GFRP material in rectangular skid landing strut and 1.04 times safer than GFRP material in
circular skid landing strut.
7. A load of 130 N on the landing strut has a lesser chance of failure in tensile properties, whereas landing strut is weak in
compressive properties due to factor of safety is not close to 1.2.
It can be concluded that the design methodology for landing strut through FEM considers that the compressive properties for
the design are crucial to finalise the design of the landing strut due to the compressive behaviour. The buckling studies show that
CFRP based circular skid-landing strut can be fabricated.
XIII. ACKNOWLEDGMENT:
I am thankful to my guide Mr. D. Dwarakanathan, Principal Scientist, STTD, and my mentor Dr. S. Raja, Head, STTD,
for guiding me in my project work. I also would like to thank Director of NAL for providing the opportunity to do my dissertation
work at CSIR-NAL, Bengaluru.
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