1. i
SCHOOL OF ENGINEERING & BUILT ENVIRONMENT
BACHELOR OF ENGINEERING
SESSION: 2015 – 2016
Project Title Analysis of light Aircraft landing gear and a
Redesign
Author Ronan Nicol
Programme Computer Aided Mechanical Engineering
Supervisor(s) Dr Peter Wallace
Non Disclosure Agreement ( N.D.A) : Yes , No Put a cross in the
box.
If Yes, please bind a copy of the N.D.A immediately behind the front cover.
X
3. iii
Abstract
The purpose of this report is to investigate light aircraft which have experienced landing gear
failure and complete analyses and a redesign phase. Using data obtained from the Civil
Aviation Authority, two aircraft, the Piper PA28 and the Grob G115 were chosen to be
investigated further. Once several loading forces had been calculated for both the nose and
main gear of each aircraft, the landing gear underwent numerous unsuccessful attempts at
completing a Fluid Structure Interaction analysis. This forced the analysis to be undertaken
in a static structural manner within ANSYS workbench. The results of this showed that three
of the four gears failed to meet the minimum factor of safety of 2.5 under all loading criteria
with only the G115 nose gear not required to undergo a redesign. By changing materials and
the design of the three landing gears, the results provided showed that only the Piper PA28
nose gear failed to meet the minimum requirements in both the initial design and redesign,
with both main gear successfully being redesigned.
4. iv
Acknowledgements
This honours year project has been undertaken with the help and support of my project
supervisor, Dr Peter Wallace. Along with his help, I have received the help of Andrew Cowell
and also all other lecturers and tutors during my time at Glasgow Caledonian University. I
would also like to extend my thanks to Piper Aircraft who shared some of their information
regarding the Piper Pa28 landing gear and also the Civil Aviation Authority who provided
details on light aircraft incidents.
Finally, I would like acknowledge the support and care which my family have provided me
during my time at university, especially during times of stress.
5. v
Contents:
1. List of Figures & Tables
2. Nomenclature
3. Introduction
4. Literature Review
4.1 Aircraft Safety
4.2 What is a landing gear?
4.2.1 Tailwheel/Taildragger
4.2.2 Tricycle/Nose wheel
4.3 Shock Absorber
4.3.1 Oleo Pneumatic Shock Absorber
4.3.2 Leaf Spring
4.3.3 Landing Gear Materials
4.4 Aircraft to undergo analysis
4.4.1 Piper PA28
4.4.2 Grob G115
4.5 Finite Element Analysis
4.6 Projects with similar aims
5. Fluid Structure Interaction Attempts
5.1 What is Fluid Structure Interaction?
5.2 Attempt 1 – Fluent Static Structural
5.3 Attempt 2 – Fluent Transient Structural
6. Method
6.1 Model Generation
6.2 Loading Calculations
6.3 Model Setup
7. Initial Design Results
8. Initial Design Results Discussion
9. Redesign
9.1 Design Method
9.2 Redesign Details
6. vi
10. Redesign Results
11. Redesign Results Discussion
12. Conclusion
13. Recommendations
i) References
ii) Appendices
A) PA28 Landing Gear Information
B) Landing Gear Loadings
7. 1
1. List of Figures & Tables
Figure 1: Tailwheel Configuration
Figure 2: Tricycle Landing Gear
Figure 3: Oleo Pneumatic Shock Strut
Figure4: Leaf Spring Landing Gear
Figure 5: Alloy steel table
Figure 6: Aluminium Alloy Table
Figure 7: Titanium Alloy Table
Figure 8: Table of data obtained from CAA
Figure 9: Piper PA28 Front View
Figure 10: Piper PA28 Nose Gear
Figure 11: Piper PA28 Main Gear
Figure 12: Table of Piper PA28 Data
Figure 13: Grob G115
Figure 14: Table of Grob G115 Data
Figure 15: FEA Matrix
Figure 16: ANSYS Workbench Deflection
Figure 17: Fluid Flow CFD
Figure 18: Static Structural
Figure 19: System Coupling
Figure 20: Fluid modelled within gear
Figure 21: Fluent & Transient Structural Coupling
Figure 22: Fluent & Transient structural mesh
Figure 23: Model Tree
Figure 24: Boundary Conditions
Figure 25: PA28 Nose gear fork
Figure 26: PA28 Nose Gear Upper cylinder sketch
Figure 27: PA28 Nose Gear Assembly
Figure 28: PA28 Main gear creation
8. 2
Figure 29: PA28 Main gear assembly
Figure 30: Grob G115 Nose gear hidden line
Figure 31: Grob g115 Main gear sketch
Figure 32: Grob G115 Main gear
Figure 33: Engineering Data - Aluminium 7049
Figure 34: Engineering Data - Titanium 6-4
Figure 35: PA28 Nose gear mesh
Figure 36: PA28 Nose Gear Analysis settings
Figure 37: Table of Loadings PA28 Nose gear
Figure 38: PA28 Nose gear model tree
Figure 39: PA28 Main Gear mesh
Figure 40: PA28 Main gear loadings
Figure 41: Grob G115 Nose gear mesh
Figure 42: Grob G115 Nose gear loadings
Figure 43: Grob G115 main gear mesh
Figure 44: Grob G115 Main gear loadings
Figure 45: Table of PA28 Nose gear results
Figure 46: PA28 Nose gear graphic results – loading 1
Figure 47: PA28 Nose gear graphic results – loading 2
Figure 48: PA28 Nose gear graphic results – loading 3
Figure 49: PA28 Nose gear graphic results – loading 4
Figure 50: Table of PA28 Main gear results
Figure 51: PA28 Main gear graphical results – loading 1
Figure 52: PA28 Main gear graphical results – loading 2
Figure 53: PA28 Main gear graphical results – loading 3
Figure 54: PA28 Main gear graphical results – loading 4
Figure 55: Table of G115 Nose gear results
Figure 56: G115 Nose gear graphical results – loading 1
Figure 57: G115 Nose gear graphical results – loading 2
Figure 58: G115 Nose gear graphical results – loading 3
Figure 59: G115 Nose gear graphical results – loading 4
9. 3
Figure 60: Table of G115 Main gear results
Figure 61: G115 Main gear graphical results – loading 1
Figure 62: G115 Main gear graphical results – loading 2
Figure 63: G115 Main gear graphical results – loading 3
Figure 64: G115 Main gear graphical results – loading 4
Figure 65: PA28 Main gear redesign
Figure 66: G115 Main gear redesign
Figure 67: PA28 Nose gear redesign results - Titanium 6-4
Figure 68: PA28 Nose gear redesign result - Titanium 4Al - 4Mo - 2Sn
Figure 69: Pa28 Nose gear redesign graphical results - Titanium 4Ai -4Mo -2Sn
Figure 70: PA28 Main gear redesign results - Titanium 6-4
Figure 71: PA28 Main gear redesign graphical results - Titanium 6-4
Figure 72: PA28 Main gear redesign results - Design change
Figure 73: PA28 Main gear redesign graphical results - Design change
Figure 74: G115 Main gear redesign results -Titanium 6-4
Figure 75: G115 Main gear redesign graphical results - Titanium 6-4
Figure 76: G115 Main gear redesign results - Design change
Figure 77: G115 Main gear redesign graphical results - Design change
10. 4
2. Nomenclature
CAA – Civil Aviation Authority
CoG – Centre of Gravity
FEA – Finite Element Analysis
FoS – Factor of Safety
FSI – Fluid Structure Analysis
a = Acceleration/Deceleration
B = Wheelbase
Bn = Nose wheel to CoG
Bm = Main wheel to CoG
Fn = Force on nose gear
Fm = Force on main gear
g = Gravitational acceleration
H = Height
m = Mass
s = Landing distance
u = initial velocity
v = final velocity
11. 5
3. Introduction
In a safety conscious world where technology is being used to increase the survivability of
passengers who are involved in motor vehicle incidents, it would be naïve to overlook the
field of aviation. Safety in aviation has always been taken extremely seriously, however
accidents still occur and the field continues to learn from the mistakes which have been
made. Numerous different types of accidents can occur with aircraft; loss of power, collision
with terrain or another aircraft, but these could be attributed to pilot error or areas in which
the design of the aircraft is not flawed. As everyone is well aware, what goes up must come
down, and that is of particular importance throughout aviation as every take-off must result
in a safe and successful landing. For a landing to be a success, a suitable and effective
landing gear has to be incorporated into the design of any aircraft. An unsafe or defective
landing gear may not survive normal or abnormal (overweight, bad weight distribution)
loading conditions and therefore may fail, leading to the possibility of fatalities. A failure
which occurs in a landing gear is something which continues to happen frequently and will
continue to occur well into the future.
This report intends to investigate landing gear failures in light aircraft which will lead to a
redesign of areas in which there is failure or where the gear has not performed satisfactorily.
The investigation will begin with information being sourced regarding light aircraft landing
gear incidents. This will be analysed and categorised to provide data into the make and
model of aircraft which are most commonly involved in these events. From there,
background information will have been gathered regarding the loading and design of the
aircraft and its landing gear. Simplified Computer Aided Designs of numerous landing gear
designs will be completed using the information obtained and these will be uploaded into
Finite Element Analysis software, ANSYS. The models will undergo a simulation of the
realistic normal and abnormal loading conditions experienced by the different aircraft, with
reference made to conditions set out in the pilots operating manual for each aircraft. Once
the analysis is complete, a redesign will occur in areas at high risk of failure or which have
failed. The change in design will look into various aspects, ranging from a change in
materials used in the manufacture of the gear through to the inclusion of greater damping
and additional supports. Once this has been achieved, the investigation can be deemed a
success and an appropriate discussion and review of the results can be undertaken.
12. 6
4. Literature Review
Within this section of the report, a review of relevant literature regarding the subject has
been undertaken. This will provide both background knowledge and also information which
is of value to the project.
Ever since the first flight of what we call a conventional aircraft by the Wright brothers in
1903, landing gear have prominently featured in the design of aircraft. In essence the
purpose of a landing gear is to allow an aircraft to return to the ground safely and without
causing any damage to the structure of the aircraft (Gudmundsson, 2014).
4.1 Aircraft Safety
Light aircraft used for general aviation and training purposes can be said to be less safe than
aircraft in other fields, such as military and commercial. This can be seen in the United
States during 2013 where there were a total of 236 aviation related fatalities, of which 222
occurred in the field of general aviation (National Transportation Safety Board, 2016). While
some of these incidents will have been caused by pilot error and others by mechanical
issues, it can be said that making sure that all aircraft are as safe as possible is a priority. An
integral part of a safe flight involving any aircraft, big and small, is the landing. Any landing
you can walk away from is a successful landing, and that could come down to the ability of
the landing gear of an aircraft to absorb a heavy and hard landing. When these landings
occur, a larger than ideal force is applied through the landing gear. If the force is too large,
either due to an overweight aircraft, a high rate of descent or another problem, then the
gear could fail leading to injury or at worst loss of life. Due to this, the landing gear is a vital
aspect of the aircraft and one which, under certain circumstances, could perform better.
This project will therefore involve an Analysis of landing gear of light aircraft which have had
the highest number of reportable incidents in the UK since the start of 2013, according to
data obtained from the Civil Aviation Authority (available upon request).
13. 7
4.2 What is a Landing Gear?
As mentioned before, the landing gear of an aircraft has been common place since the birth
of flight as we know it in 1903. Since not every plane in the sky is of the same shape, size
and weight, it is obvious that there is not one landing gear design which works for them all.
During the First World War, many aircraft took to the skies with rudimentary landing gear,
which were basically struts fitted to the bottom of the aircraft and they offered little in the
way of shock absorption (Currey, 1988). Since modern day air travel revolves around
comfort and practicality, these basic landing gear designs would not be adequate. Shock
absorbers were introduced into landing gear design as a way of damping the effects of
landing the aircraft, with this practice now common place across aviation. While the
introduction of damping to landing gears is common, the orientation of the landing gear can
vary depending upon the aircraft. There are two main ways of incorporating the landing
gear into an aircraft; the tailwheel/taildragger configuration, or the tricycle/nose wheel
layout.
4.2.1 Tailwheel/Taildragger
The use of a tailwheel landing gear, where two main wheels are positioned forward of the
centre of gravity and a smaller wheel positioned at the tail, was common place during the
early years of aviation. This layout provides a larger amount of clearance between the
propeller and the ground which makes the type of aircraft which use this configuration more
suitable to the use of rough and rudimentary airfields (Flight Learnings, 2010). Along with
this, another advantage of having a tailwheel is that having a small wheel at the back of the
aircraft, where there is less weight
and less force experienced during
landing, allows for a lighter and
less drag inducing design
compared with a heavy nose
wheel. Tailwheels do however
have some setbacks due to the
nose high nature of the aircraft not
providing adequate visibility when
Figure 1: Tailwheel configuration (Aerospaceweb.org, 2016)
14. 8
moving on the ground, meaning that to fly an aircraft with this gear configuration will
require additional training (Flight Learnings, 2010).
Tail dragger aircraft not only include the famous DC3 – also known as the Dakota when used
by the Royal Air Force – but also includes civilian gliders and military helicopters such as the
Apache.
4.2.2 Tricycle/Nose wheel
While appearing very similar in design to the tailwheel arrangement, the tricycle landing
gear features two main wheels to the rear of the centre of gravity along with a sturdy nose
wheel. Now featuring heavily in light and commercial aircraft, this layout of landing gear
provides a much higher degree of visibility during ground operations when compared with
tailwheel aeroplanes. The addition of the wheel at the front also allows the pilot to use the
wheel brakes more forcefully during higher speed landings without the fear that the plane
will tip onto its nose – a common problem in tailwheel aircraft (Flight Learnings, 2010). The
nose wheel in this configuration also provides the pilot with steering capabilities through the
use of the rudder pedals as the wheel is free to rotate. Since the nose wheel is required to
take a much larger share of the loading under landing conditions when compared to a
tailwheel. The additional weight and drag of the nose wheel is the main disadvantage of this
arrangement, meaning that it is ideal for a nose wheel layout to have retractable gear
system. This arrangement allows trainee pilots to experience a stable aircraft which is easier
to fly than a taildragger; however, it could be argued that learning to fly a more difficult
aircraft produces a better
equipped pilot. As well as featuring
in light aircraft, such as the PA28
and the Grob G115 which are the
two aircraft undergoing analysis in
this project, the tricycle landing
gear is also common on large
commercial and military aircraft, as
well as having the added benefit of
being able to adopt floats to
perform a sea plane role.Figure 2: Tricycle Landing Gear (Aerospaceweb.org, 2016)
15. 9
4.3 Shock Absorber
As mentioned earlier, not every aircraft in the sky is the same and because of this there is
more than one way of absorbing the force of the landing. When an aircraft is undertaking a
landing the weight, the rate of decent and deceleration have to be considered as all these
forces have a bearing upon the landing gear once the tyres hit the ground. Although the
tyres themselves will absorb some of the impact of the landing, further damping is necessary
to prevent components failing. There are a few different types of shock absorbers which are
implemented into aircraft design however; only two will be discussed here.
4.3.1 Oleo Pneumatic Shock Absorber
With a first patented design back in 1915, shock strut shock absorbers have been present in
aviation ever since (Oleo.co.uk, 2016). These absorbers use gas and oil, which during landing
are compressed. When a landing is taking place, a piston forces the oil/hydraulic fluid
through a small hole (orifice) which creates heat (Boldmethod.com, 2016). This heat then
dissipates through the walls of the strut as the piston proceeds through a downward cycle
allowing the airframe to return to its resting position (FAA, 2016).
A lot of the work being undertaken in this project will revolve around shock struts and how
they behave in the aircraft I have chosen to study. Components which are involved in the
steering of the aircraft will be neglected as they are not involved in the distribution of the
loading during landing.
These shock struts as mentioned before are filled
with gas, typically nitrogen, and also with oil, which
is typically of Mil-h-5606 type, however in this
analysis; hydraulic fluid may be used (Karam and
Mare, 2009). While this is the typical configuration,
it can be suggested that using a different
combination of oil and gas, with differing densities,
may prove to be more efficient. The rate at which
the oil flows through the orifice is dependent upon
the metering pin which is tapered and also a relief
valve which allows gas to escape if necessary. ThisFigure 3: Oleo Pneumatic Shock Strut
(Boldmethod.com, 2016)
16. 10
slows the rate at which the compression of the strut occurs to prevent any damage to the
gear or airframe (Goodrich Corporation, 2015). It must be considered that slowing the rate
of compression too much could lead to damage of the airframe, which is something which
the metering pin is there to prevent. The design of the metering pin could perhaps be
optimised to obtain a flow which under as many circumstances as possible will prevent
damage but will continue to dampen. Aside from the metering pin, the orifice itself has a
part to play in the damping effect of the landing gear. The relationship between the small
diameter of the orifice and the diameter of the strut closely relate to the Reynolds number.
The high Reynolds number obtained means that turbulent flow is present and the damping
force increases at the square of the rate the piston rises (Milwitzky and Cook, 1953).
Assuming that little attempt has been made to obtain a very high Reynolds number for the
shock struts in the aircraft which will be analysed, it could be useful to proceed along this
avenue, however the gain of this may be negligible and other avenues will be explored in
this report.
4.3.2 Leaf Spring
Featured in some light aircraft is the leaf spring shock absorber. The idea behind this is that
the struts will flex, allowing for the impact of the landing to dissipate at a rate which will not
affect the airframe. Landing gear of this particular design is generally non-retractable and is
constructed mainly from light weight composite
materials with great flexibility (FAA, 2016). An
advantage of this design can be said to be the
simplicity, as the leaf spring requires less
maintenance than the oleo pneumatic, while it
also provides a very robust gear for training
aircraft (Boldmethod.com, 2016).Figure 4: Leaf Spring Landing Gear (Zenithair.com,
2016)
17. 11
4.3.3 Landing Gear Materials
An important aspect of the design of anything is the material it is constructed from. In the
case of landing gear, it is important to consider that a great deal of loading is placed upon
the material, while also considering that the material has to be light enough to be practically
incorporated into the design. While steel alloys are common in the majority of landing gear
currently, there is a move towards other alloys, mainly Titanium and Aluminium.
Steel Alloys:
There are numerous stainless steel alloys which meets British standard for landing gear
design (Aircraftmaterials.com, 2016). These steels will all need to possess an ultra-high
strength to make up for the potential weight (Best, K.F. & Sc,1986).
Alloy Density
(kg/m3)
Young’s
Modulus
(GPa)
Yield
Strength
(MPa)
Tensile
Strength
(MPa)
Low Alloy
Steel 300M
7.79e3 200 1.59e3 1.93e3
Low Alloy
Steel AISI
4340
7.8E3 205 420 670
Intermediate
Alloy
7.74e3 207 1.38e3 1.66e3
Low Alloy
Steel AISI
4130
7.79E3 200 483 621
Figure 5: Alloy steel table (Obtained from CES EduPack 2015)
18. 12
Aluminium Alloys:
Aluminium Alloy landing gear have a lower specific strength than the equivalent steel and
titanium designs however they still feature prominently in landing gear design (Best, K.F. &
Sc,1986).
Alloy Density
(kg/m3)
Young’s
Modulus
(GPa)
Yield
Strength
(MPa)
Tensile
Strength
(MPa)
Aluminium
7049
2.84e3 70 379 455
Aluminium
A201.0
2.78e3 71 331 386
Aluminium
2014
2.78e3 72 359 386
Aluminium
5083
2.64e3 70 131 221
Figure 6: Aluminium Alloy Table (Obtained from CES EduPack 2015)
Titanium Alloys:
Titanium Alloys have numerous uses in the aerospace industry. From compressor blades in
jet engines, hydraulic systems and also landing gear (Continental Steel & Tube Company,
2016).
Alloy Density
(kg/m3)
Young’s
Modulus
(GPa)
Yield
Strength
(MPa)
Tensile
Strength
(MPa)
Titanium Ti-
6AI-4V
4.43E3 113 786 869
Titanium TI-
4AI-4Mo-2Sn
4.59e3 110 970 1.06e3
Titanium
Ti15V-3cr-
3Sn-3Al
4.75e3 108 749 770
Figure 7:Titanium Alloy Table (Obtained from CES EduPack 2015)
19. 13
As can be seen from Figures 5, 6 and 7, there is a variety of alloys which are suitable for the
landing gear of aircraft. When comparing the three materials it is clear to see that
aluminium alloys possess the lowest yield strength, however they also have the lowest
density of the three, which does have its advantages when trying to keep the aircraft weight
to a minimum. Steel alloys are the densest of the three, but do possess relatively high yield
strength which is why it is still common on large commercial aircraft. Finally, the titanium
alloy proved to have the superior yield strength of all materials which reaches over 700 MPa,
whilst also maintain a low density which is one of the reasons why titanium alloys are
growing in the aerospace industry. For this project, alloys from all three categories will be
involved when it comes to the redesign aspect.
4.4 Aircraft to undergo analysis
Since this project is orientated around light aircraft landing gear failures, certain aircraft had
to be chosen to be analysed. Instead of simply choosing aircraft at random, data was
obtained from the Civil Aviation Authority (CAA) through a Freedom of Information Act
request. The CAA provided an Excel document, containing information from the beginning
of 2013, which included the
date of an incident, the make,
model and registration of the
aircraft involved and also a
brief report into the findings.
This document was analysed
to determine which aircraft
had featured most frequently
in the incidents:
Make Model Number of
Incidents
Piper PA28 38
Piper PA31 25
Beech 200 17
Diamond DA42 10
Aerospatiale SA365 8
Britten Norman BN2 7
Cessna 402 7
Grob G115 7
Piper PA34 7
Cessna 182 6
Piper PA23 6
Cessna 210 5
Cessna 172 5
Cessna 152 5
Piper PA38 5
Pitts S1 5
Figure 8: Table of data obtained from CAA
20. 14
From Figure 8 above, it is clear that a large number of aircraft manufactured by Piper were
involved in landing gear related incidents. It was determined that only analysing the PA28
aircraft, which was involved in the most incidents, from the manufacturer Piper would
provide the most variation amongst other manufactures. The Grob G115 was also selected
due to it single engine design and prominent use as a training aircraft, meaning the aircraft
will experience a few hard landings. Both aircraft selected have differing landing gear
designs, with the Piper PA28 featuring an Oleo Pneumatic Design, while the Grob G115 uses
a leaf spring main gear.
4.4.1 Piper PA28
The PA28, also known as the Cherokee or now the warrior, is a light aircraft introduced back
in 1961 by Piper Aircraft Inc. Since its inception, this aircraft has been produced in more
than 40 different variants with changes to the engine size and wing area (Archer III, 2012).
The aircraft features a low wing design and metal construction and was initially available
with 150 or 160 horsepower engines (Airliners.net, 2016). From its birth, the aircraft was
designed to be an inexpensive plane that offered safe flight characteristics which made it a
hit with flying schools worldwide (Web.archive.org, 2016). The aircraft is also infamous for
the death of a member of the royal family, when in 1972 the then Prince William was killed
in a crash (News.bbc.co.uk, 2016).
As previously mentioned the Piper PA28 uses the oleo pneumatic shock absorber landing
gear in the tricycle configuration. All three parts of the gear – Nose and main – are of this
shock absorber design and Piper Aircraft has provided several pages of information which
will be of use in this project.
Figure 9: Piper PA28 Front View (Piper.com, 2016)
21. 15
Figure 11 and Figure 12 above are assembly information regarding the main and nose gear
of the aircraft respectively. These assembly drawings will be used during the creation of 3D
models to maintain a level of accuracy. The figures show the different components which
make up the main gear strut and the nose gear, however many of the parts shown however
will not be analysed due to them not undertaking any of the loading or damping. Further
diagrams and information regarding the landing gear of the PA28 can be found in Appendix
A.
Every flight of the Piper PA28 has a checklist which contains a set of aircraft limitations,
specifications and also emergency drills. These vary from fuel capacity to take off speed and
also to variables which can have an effect on the landing of the aircraft. Below is a table
containing parameters for this aircraft.
Maximum Take-off weight 1156.6 Kg
Final Approach Speed 32.41 m/s
Landing Roll 243 m
Figure 12: Table of Piper PA28 Data
These parameters will be incorporated into calculations further on in this report, involving
the landing forces and dynamic forces.
Figure 11: Piper PA28 Main Gear Figure 10: Piper PA28 Nose Gear
22. 16
4.4.2 Grob G115
Built by the Grob Company of Germany, the G115 is a two seater training aircraft which is
used by the Royal Air Force as a basic trainer and is also used to teach aerobatics. With
around 200 aircraft of this type produced to date, there are far fewer of these aircraft in the
skies of the UK compared with the PA28 (Airliners.net, 2016). As well as featuring for the
Royal Air Force, this aircraft is also used by Tayside Aviation flying school where the plane is
used to teach both civil pilots and cadets of the Air Training Corps.
In recent years the aircraft has been involved in numerous incidents which have been
reported on the news, including an instance of a mid-air collision between two Grob G115’s.
This aircraft features a tricycle landing gear layout and from inspection of numerous
available images, it can be assumed that the nose gear of the aircraft features a shock strut
while the main gear features a spring leaf system. Information requested about the landing
gear of this aircraft has not been obtained, however numerous sources including the
manufacturer Grob, Tayside Aviation and also West of Scotland University Air Squadron have
been contacted.
The G115 aircraft, also known as the Tutor, features a similar style checklist to that of the
Piper, with information extracted shown below in Figure 14
Figure 13: Grob G115 (Skybrary.aero, 2016)
23. 17
Maximum Take-off Weight 990 Kg
Final Approach Speed Max 35 m/s
Landing Roll 457 m
Figure 14: Table of Grob G115 Data
4.5Finite Element Analysis
Finite Element Analysis (FEA) stems from a numerical way of approximately calculating
solutions to problems which have numerous and complex loadings. Equations can be
determined for structures to be analysed, for example an analysis of a simple beam may
lead to 5 equations being produced which will then generate a 5x5 matrix. These simple
matrices are able to be solved using software such as Microsoft Excel, however once larger
and more complex matrices – up to 1 million x 1 million- are produced, dedicated FEA
software is required (ANSYS).
[
𝑘11 ⋯ 𝑘1𝑛
⋮ ⋱ ⋮
𝑘 𝑛1 ⋯ 𝑘 𝑛𝑛
] [
𝑢1
⋮
𝑢 𝑛
] = [
𝑓1
⋮
𝑓𝑛
]
Figure 15: FEA Matrix (MIT, 2001)
Using dedicated software may well be beneficial for solving stresses with regards to the
complexity of the structure and also the speed at which FEA software can be used; however,
the results given are not necessarily the most accurate. To verify the accuracy of the model,
a convergence study will need to be undertaken. This involves calculating the stress at a
point by hand calculations and comparing the result to the output from the model. Once
done, the model inputs can be adjusted to match the nodal figure with that of the
calculation, achieving an accurate result for the rest of the model.
The use of FEA software in
this report will revolve
around the use of ANSYS
16.1 workbench. This
software allows for 3
dimensional parts to either
be generated in software or
uploaded from a CADFigure 16: ANSYS Workbench Deflection
24. 18
programme. ANSYS workbench can perform numerous different analyses ranging from static
structural analysis, to thermal analysis to computational fluid dynamics. In this project the
software is intended to be used to perform a Fluid Structure interaction between the gear
structure and also the liquid and gas present in the oleo pneumatic shock absorbers. If this
is unable to work as planned, a static structural analysis will be performed.
4.5 Projects with similar aims
As part of the literature review, research was undertaken into previous projects which have
similar aims or the same undertaking of the analysis. One report which was of particular
significance was titled; Redesign and Stress Analysis of Simplified Landing Gear. This report
approached the problem in a single large commercial aircraft which as mentioned previously
is involved in far fewer serious incidents than light aircraft. Another area of difference
between the undertaking of that report and this is that no fluid structure interaction was
involved during analysis; instead the author chose to model a spring damping system which
will neglect the forces that the fluids will exert upon the landing gear. Although both
projects aim to solve a similar problem, this shows that there is more than one way of
approaching the problem, and that thinking is something to keep in mind (Redesign and
stress analysis of simplified landing gear, 2014).
Another report which was reviewed involved a 2-way FSI of a shock absorber part. This
Report provides a detailed amount of background information regarding Fluid Structure
Interaction which could prove useful in the undertaking of this project. It can be said that
the vast majority of the information provided in the reviewed project is far beyond the level
of training received and the difficulty of attempting a Fluid Structure Interaction is apparent
(2-way FSI simulations on a shock absorber check valve, 2015).
25. 19
5. Fluid Structure Interaction Attempts
This section of the report will go into detail regarding the attempts to use Fluid Structure
interaction to successfully analyse the stress in the landing gear which stems from the
pressure coming from the fluid and the force acting upon the solid structure. Unfortunately,
all attempts at analysing using this method were unsuccessful and all three main attempts
will be discussed.
5.1 What is Fluid Structure Interaction?
Put simply, Fluid Structure Interaction takes place when a fluid creates a change in a
structure which in turn changes the boundary conditions of the fluid (Adina.com, 2016). If a
deformation in a structure is small, then the fluid can effectively be ignored, however if the
deformations are large then pressure waves can present themselves within the fluid
(Comsol.com, 2016).
Using software to undertake an analysis of this type will involve system coupling of a fluid
dynamic portion of software (Fluent) and a mechanical portion (Static Structural). There
however is not only one way of doing a FSI analysis, but three.
Rigid Body Fluid Structure Interaction
Rigid body is the simplest form of FSI, where the assumption is made that no
deformation occurs in the structure. This analysis is undertaken in Computational fluid
dynamics alone and only the motion of the structure in the fluid are considered.
One –Way FSI
This type of analysis can be undertaken where very small deformations occur in the solid
structure due to the fluid. Results obtained from one –way FSI can be passed straight
into the FEA simulation.
Two – Way FSI
Two – Way Fluid Structure Interaction incorporates large deformations within the
structure and is iterated between CFD and FEA software.
26. 20
5.2 Attempt 1 – Fluent & Static Structural
The first attempt to model a Fluid Structure Interaction involved coupling both the
computational fluid dynamic software – Fluent – with the mechanical aspect – Static
Structural. To begin with, both aspects of the software had to be brought into the project
schematic and then linked. This allows for the geometry which is generated in Fluent to be
shared with Static Structural. As well as sharing the geometry, the solution from the CFD can
be used as input parameter in the setup of the mechanical portion. This coupling is shown
below in Figure 19, with the lines representing the coupling.
With the system coupled, the geometry is brought into ANSYS. This is done by saving the
created 3D model as an IGES file then uploading the geometry. Once this is done, the fluid
has to be modelled in the geometry window. The fluid will occupy any space within the
landing gear however for simplicity of the analysis a symmetry plane will be used. This is
done by simply slicing the parts along a plane, and suppressing half of the remaining parts to
Figure 17: Fluid Flow CFD Figure 18: Static Structural
Figure 19: System Coupling
27. 21
reveal the hollow upper and lower tubes of the nose
gear. Once done, the modelling of the fluid can take
place through the use of extrusion using surface faces
which are situated at various points inside the gear.
With the fluid successfully created within the landing
gear, the next step is to create a mesh of both the
structure of the gear itself and also the fluid. Due to
Fluid Structure Interactions being computationally
intensive, it was decided that the mesh be left
unrefined and also on the coarse sizing setting. As well as
creating the mesh, it is essential to create named selections. A named selection is a face of
either a fluid or solid which is of importance to the analysis undertaken. In this instance, the
top of the fluid is used as a named selection as this is where the pressure from the force of
the aircraft landing is going to be transferred into the liquid. As well as stating the top of the
fluid, the area of interaction between the fluid and the structure was also set as a named
selection along with the symmetry plane.
The next stage of the analysis involves setting up the analysis. There are several areas which
need to be completed before a solution can be run. Firstly, the materials used in the solution
need to be defined, with key characteristic being prompted for both fluid and solid
materials. These materials can then be attached to certain cell zone conditions within the
software, in this case, the bottom cylinder being setup as an aluminium alloy. With regards
to boundary conditions, the top of the fluid, which was generated as a named selection
earlier, is setup as a pressure inlet and the appropriate pressure applied. The area of
interaction named selection is also setup as a wall boundary condition due to the fluid being
bound by the gear structure. To continue the setup, it is required to set reference values
which include where the analysis is to compute from and also an area of reference.
The next step is to run the solution once the solution has been initialised. The solution is run
for a number of iterations to allow the solution to achieve convergence. If the solution does
not achieve convergence, the number of iterations must be increased, however doing so
increase the time taken to achieve a result.
The solution in this case does not run properly and no results obtained. An error message
states that no application is active. Since no solution is obtained using Fluent, the data
Figure 20: Fluid Modelled within gear
28. 22
cannot be shared with Static Structural and no Fluid Structure Analysis was successfully
completed.
5.3 Attempt 2 – Fluent & Transient Structural
Since the above attempt was unsuccessful, it is necessary to attempt a different approach to
the problem. This attempt at completing a Fluid Structure Interaction involves coupling
Transient Structural analysis with Fluent with the help of an online tutorial on simulating
blood flow in an artery (Two way FSI using ANSYS Fluent Part-1, 2015).
Similar in approach to the first FSI attempt, the systems to be used have to be coupled
together, however in this case, a third system is introduced to couple both the setup for
both Fluent and Transient structural. This allows for a joint solution to be run incorporating
both the fluid pressure and the force acting upon the gear.
With the system successfully coupled, the geometry is created in the same manner as the
first attempt. The geometry is imported and then the fluid created through the use of
extrusion. The next step, since transient is being dealt with first in this instance, is to
generate the mesh that is to be used for analysing only the structure. The fluid is supressed
in the model tree and using a coarse sizing, the mesh is generated. From Figure 22 it can be
seen that around the orifice of the landing gear – the area with the small hole – there is a
greater concentration of nodes allowing for greater accuracy in the solution here.
Figure 21: Fluent & Transient Structural Coupling
29. 23
With the mesh generation successful, the next stage
is to setup the analysis of the structure. In the
transient branch of the model tree, certain aspects
can be added to the environment. The first aspect
added to the environment a load in the way of a
force which is acting straight down the landing gear.
Next the gear is fixed as to simulate the wheel of the
aircraft stopping further movement downward by
the gear, and finally a Fluid Solid Interface is
incorporated where there are surfaces which are in
contact with both solid and fluid.
With the setup for the Transient structural complete, the fluent aspect of the coupling has to
be setup. Since the geometry is already updated as it is the same as the transient geometry,
all that needs to be done is to supress the structure and to leave only the fluid present.
Similar to with attempt 1 a mesh is created, only for the fluid, and then named selections are
created. These are Pressure Inlet at the top of the fluid, fixed end at the bottom and the
gear wall which is where the fluid interacts with the structure.
Figure 232: Fluent & Transient structural mesh
Figure 223: Model Tree
30. 24
Setting up the analysis in this coupling system is
very similar to that of attempt 1, however the
solver time is immediately changed from steady to
transient. With this done, the materials are defined
and then assigned. Once again the boundary
conditions are set with the pressure inlet being set
as a pressure inlet and the pressure defined.
Once the boundary conditions are set and the reference values stated, the calculation
cannot be run. This is down to the solution being generated in the system coupling system,
although time steps and the number of these steps have to be input.
Within the system coupling system, all the setup input should be available to use. To run the
solution, a data transfer has to be completed which sources an input, in this attempt it is the
fluid force, and applies it to a target, structure. The solution should then be free to run to
obtain results. This was not the case and an error warning appeared to state that the
solution update failed and that the data threw an exception. This was unexpected and was
essentially the last avenue to explore regarding Fluid Structure interaction without further
training being undertaken.
Figure 24: Boundary Conditions
31. 25
6. Method
6.1 Model Generation:
The generation of all models used in the analysis is undertaken using the Creo Parametric
modelling software.
PA28 Nose Gear
For the creation of the Nose gear of the Piper PA28, use is made of the data provided by the
Piper aircraft company. The assembly drawing, although not featuring any actual
dimensions, show the shape and positioning of the key components within the gear.
For the nose gear of this aircraft, 7 main parts were modelled in Creo;
The fork is situated as such to allow a connection between the main shock strut and the
wheel and axle. The fork is of a similar style to that of a bicycle however will be undergoing
much higher stresses and deflections. This part is generated by sketching on a plane and
extruding. An identical part of the fork was
created on a plane created a distance from
the initial extrusion. Both parts are then
joined by a third extrusion which creates the
top curved section of the fork. To allow for an
axle to be inserted, a cylindrical extrusion was
subtracted from the part.
With the fork created, a connecting part has to be generated to allow for the fork, lower
cylinder and the bottom link arm to be joined together. This involved simple geometry to
begin with, extruding a rectangle which had been sketch on the x-y axis. A further sketch
was included to generate the part where a pin would connect with the link arm. Similar to
the fork, a subtraction of material took place to create the area for the lower cylinder to sit
and also further subtractions where included to pin the cylinder in position.
Figure 25: PA28 Nose gear fork
32. 26
The generation of the lower cylinder was simple as it only involved
extruding a circle in the z axis and then a subtraction for the area where
the fluid would be situated within the strut. The top cylinder was more
challenging as the orifice had to be modelled within the tube. This was
solved by sketching on the z plane and then revolving through 360
degrees to obtain the part. The upper cylinder had an internal diameter
which was slightly larger than the outer diameter of the lower tube. This
allows for the bottom cylinder to sit within the top.
The generation of the link arms involved calculating the angle at which
both would need to be sketched as to allow for the assembly of the parts
to be successful. The parts were modelled using simple extrusion and
subtraction commands, allowing for both parts to assemble with one
another.
With these main parts created, the next step was to
assemble the finished model. This involved constraining all
parts within the software which allows for them all to be
fixed in position in relation to the planes. Initially the fork
was assembled, followed by the connection part, the lower
then upper cylinders then finally the link arms. Simple pins
were created and assembled to act as supports.
Figure 26: PA28
Nose Gear Upper
cylinder sketch
Figure 27: PA28 Nose Gear Assembly
33. 27
PA28 Main Gear
The creation of the main gear for the PA28 aircraft follows a very similar approach as to that
of the nose gear however this gear only consists of two main parts.
The bottom cylinder is created in the same manner as with the nose gear, extruding a circle
through the height of the tube, and then
subtracting material from the inner part. From
there, additional modification takes place with
the inclusion of a broad lower section which is
used to connect the cylinder to the axle and also
a support for a linkage. The linkage support was
generated using the extrusion and as it can be
seen form figure 28, subtraction of material also
took place.
Again with the generation of the upper cylinder for the PA28 main gear, the revolution
modelling command was used, this time including a taper. This taper creates a larger area at
the top of the cylinder compared with the bottom, meaning that there is also an increase in
the thickness of the material. As well as the taper, there is the inclusion of two supports
which are used to attach the landing gear to the aircraft. These were again created using
extrusions, with new planes being created for the sketches. Figure 29 features the
assembled gear and it is clear to see both the supports and also the axle.
Figure 28: PA28 Main gear creation
Figure 29: PA28 Main gear assembly
34. 28
Grob G115 Nose Gear
Since no information was obtained regarding the Grob G115 landing gear, it had to be
assumed that the nose landing gear is of a similar design of that of the PA28. Taking that into
account, as well as the fact that the G115 is a much smaller aircraft, it can be assumed that
the nose gear is smaller too. The creation of the nose
gear follows the same idea of the Piper, with the
generation of the fork being completed through the
use of extrusions. Once this is done, the lower cylinder
is again created using extrusion and removing material.
The upper cylinder of the nose gear is once again
created using a revolution with a hole being inserted to
model the orifice. Similar to the design of the PA28
gear, the nose gear of the Grob will feature a
connection part to hold the lower cylinder into
position in the final assembly.
Grob G115 Main Gear
The main gear of the aircraft has to be modelled in a different way to the other landing gear.
Since the gear is of the leaf spring type and not the oleo pneumatic shock absorber, this
landing gear will be modelled as a whole part which will span across the aircraft. The
generation involved using yet another extrusion, however in this case, only one attempt is
required. This is due to the initial sketch including all parts of the gear as one and not
individual entities.
Figure 30: Grob G115 Nose gear hidden line
Figure 31: Grob g115 Main gear sketch
35. 29
Once the sketch had been created and extruded, the straight edges which were featured
had to be smoothed as the actual landing gear of the aircraft has a smooth leading and
trailing edge.
6.2 Loading Calculations
Within this section calculations relating to the loading which the landing gear of both aircraft
are undergoing to be shown. The calculations range from determining the centre of gravity
which has use when it comes to what percentage of the loading is on the nose gear or the
main gear. Also calculated in the deceleration of the aircraft at both normal and fast landing
speeds and then finally static and dynamic forces are combined to determine the overall
force exerted upon each of the landing gear. It could be said that a worst case scenario will
result in all the force being exerted through one gear, which would simulate an aircraft
landing on only one wheel.
6.2.1 Piper PA28
Centre of Gravity:
𝐶𝑜𝐺 = (
𝐵
2
+ 0.5) − 𝐶𝑜𝐺 𝐷 + 𝐶𝑜𝐺 𝑀𝑎𝑥𝐴
𝐶𝑜𝐺 = (
2.3
2
+ 0.5) − 1.99 + 2.36
𝐶𝑜𝐺 = 2.02𝑚
Figure 32: Grob G115 Main gear
36. 30
Deceleration:
@ 32.41 m/s approach speed
𝑎 =
𝑣2
− 𝑢2
2𝑠
𝑎 =
02
− 32.412
2 ∗ 243
𝑎 = 2.162
𝑚
𝑠2
𝐷𝑒𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛
@ 40 m/s approach speed (too fast)
𝑎 =
𝑣2
− 𝑢2
2𝑠
𝑎 =
02
− 402
2 ∗ 243
= 3.292
𝑚
𝑠2
𝐷𝑒𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛
Force on Nose Gear:
Four separate forces have been calculated for the nose, however only the first case will be
shown here, with the rest of the results available in appendix B.
𝐹 𝑁 = ((
𝑚∗𝑔∗𝐵 𝑚
𝐵
) + (
((𝑚∗𝑔)∗𝑎∗𝐻)
𝑔∗𝐵
)
𝐹 𝑁 = ((
1156.6∗9.81∗0.28
2.3
) + (
((1156.6∗9.81)∗2.161∗1)
9.81∗2.3
)
𝐹 𝑁 = 2468.451𝑁
Force on Main Gear:
Similar to above, only one of the four cases will be calculated with the rest of the results
available in appendix B.
38. 32
𝑎 =
02
− 482
2 ∗ 457
𝑎 = 2.520 𝑚/𝑠2
𝐷𝑒𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛
Force on Nose Gear:
Four separate forces have been calculated for the nose, however only the first case will be
shown here, with the rest of the results available in APPENDICE
𝐹 𝑁 = ((
𝑚∗𝑔∗𝐵 𝑚
𝐵
) + (
((𝑚∗𝑔)∗𝑎∗𝐻)
𝑔∗𝐵
)
𝐹 𝑁 = ((
990∗9.81∗0.15
2.9
) + (
((990∗9.81)∗1.340∗1)
9.81∗2.9
)
𝐹 𝑁 = 959.87 𝑁
Force on Main Gear
Four separate forces have been calculated for the nose, however only the first case will be
shown here, with the rest of the results available in APPENDICE
𝐹 𝑀 = (((𝑚 ∗ 𝑔) − 𝐹 𝑁) + (
((𝑚 ∗ 𝑔) ∗ 𝑎 ∗ 𝐻)
𝑔 ∗ 𝐵
)
𝐹 𝑀 = (((990 ∗ 9.81) − 959.87) + (
((990 ∗ 9.81) ∗ 1.340 ∗ 1)
9.81 ∗ 2.9
)
𝐹 𝑀 = 9209.56 𝑁
39. 33
6.3 Model Setup
For successful analysis to take place, each of the landing gear have to be setup properly. The
first step of this for each gear is to generate a mesh, followed by the application of the
loading and supports, and also a setup of the materials used during analysis. For the landing
gear to meet suitable requirements for this project, a minimum factor of safety of 2.5 must
be achieved for each loading condition.
Before analysis can be started on the model, material data must be setup in the project
schematic. This allows for the proper properties of each material to be defined. Since the
materials used in the static analysis will be common for all four landing gear, the engineering
data can be coupled across all four analyses. The materials which will be defined in the
engineering data segment include Aluminium 7047 and Titanium 6-4, with the aluminium
alloy being used in initial analysis. The data which is required to be implemented includes
the density for each material and also Young’s Modulus, Poisson’s ratio and also the material
strength.
Figure 33: Engineering Data - Aluminium 7049
Figure 34: Engineering Data - Titanium 6-4
40. 34
Figure 33 and 34 show how this materials data is organised in ANSYS with the Young’s
Modulus clearly shown. Once the data has been updated, analysis of each individual gear
can be undertaken.
6.3.1 Piper PA28 Nose Gear
To start the analysis on the PA28 nose gear, the geometry file firstly needs to be uploaded
into ANSYS workbench. Once the model has been generated and the analysis saved, the
mesh can be generated for the part. In this case since the model cause the calculation to be
computationally intensive; it is better to reach a compromise with regards to the mesh. The
mesh was generated by simply changing the relevance centre under the sizing aspect of the
mesh details, from coarse to fine.
Figure 35: PA28 Nose gear mesh
41. 35
With the mesh setup, the next step is to update the
analysis settings. Under analysis, all loading and
supports can be introduced to the model. In this case
there will be a force present on the top surface of the
gear, with the landing gear being supported at the axle
(where the tyre would be present).
Figure 36 shows a graphical representation of the
loadings and support which the nose gear is undergoing.
All the forces that will be used in the analysis of the gear
have been defined by the calculations in section 6.1 of
this report. Figure 37 below contains all the loading
scenarios undertaken in this analysis.
F (N)
max aft Cog, 32.41m/s, 243m, 1156.6 kg 2468.151
max aft Cog, 40m/s, 200m, 1156.6 kg 3036.82
max aft Cog, 32.41m/s, 243m, 1300kg 2774.162
max aft Cog, 40m/s, 243m, 1300kg 3413.337
Figure 37: Table of Loadings PA28 Nose gear
With these loadings being used in the
analysis, the next step is to set up which
results are viewable when the solution has
been run. In this case, and for all the landing
gear, the results which are required include
the Total Deformation, The Equivalent
Stress (Von Mises) and the factor of safety.
Figure 36: PA28 Nose Gear Analysis
settings
Figure 38: PA28 Nose gear model tree
42. 36
6.3.2 Piper PA28 Main Gear
Like with the nose gear of the PA28, the first step involved in the analysis setup requires the
geometry IGES file to be uploaded. Again with this done then the next stage is to create a
suitable mesh for the gear. Similar to the nose gear, due to the complexity of the part, a
simple mesh will be used to keep the computational complexity to a minimum. Figure 38
below shows the mesh generated and how the sizing of the mesh decreases in areas where
this is either a joint or a hole.
To undergo the structural analysis, the landing gear must
be setup for the analysis. The force once again is
positioned on the top surface of the gear, with a fixed
support being positioned upon the axle which is out of
the side of the bottom cylinder.
Since there are two main gears on this type of aircraft,
the total force which is experienced by the main landing
gear must be halved as shown in figure 40.
Once the loading has been setup and a fixed support defined, the solution information must
once again be updated to allow for the Deformation, Equivalent stress and the Factor of
safety.
F (N)
max aft Cog, 32.41m/s, 243m, 1156.6 kg 4982.482
max aft Cog, 40m/s, 200m, 1156.6 kg 5266.816
max aft Cog, 32.41m/s, 243m, 1300kg 5753.236
max aft Cog, 40m/s, 243m, 1300kg 6072.824
Figure 40: PA28 Main gear loadings
Figure 39: PA28 Main Gear mesh
43. 37
6.3.3 Grob G115 Nose Gear
To analyse the nose gear of the Grob G115, the geometry once
again is uploaded and generated in ANSYS. Once the model is
successfully uploaded and saved, the mesh generation is
undertaken. This follows the same procedure as the previous two
landing gear, with the mesh sizing being set to fine to allow the
computer to undertake the solution without an intensive mesh.
Figure 41 shows the final mesh used on this landing gear before the
loadings and supports have been added.
As with the Piper landing gear the force being exerted upon the
gear is placed upon the top surface, with the force acting vertically
along the Z-Axis.
F (N)
max aft GoG, 35m/s, 457m, 990 kg 959.8776
max aft Cog, 48m/s, 457m, 990 kg 1362.884
max aft Cog, 35m/s, 457m, 1100kg 1066.531
max aft Cog, 48m/s, 457m,1100kg 1514.316
Figure 42: Grob G115 Nose gear loadings
Once the loadings have been input and the fixed support defined in the software as the axle,
the solution information is setup.
Figure 41: Grob G115 Nose
gear mesh
44. 38
6.3.4 Grob G115 Main Gear
Analysis of the main gear of the Grob G115 aircraft firstly involves uploading the sole part
which makes up the landing gear. Once generated this piece is to undergo mesh generation
to allow for the analysis to be undertaken. Figure 43 below shows the mesh generation for
the main gear.
The mesh once again has a fine relevance centre and the next stage of undertaking the
analysis is the setup of the loads and supports. In this instance, since the whole of the main
gear has been modelled, compared with only one of the two gears in the PA28, the total
force which this gear will experience does not need to be halved. As well as this, the gear
will require two fixed supports, positioned where the main wheels would be.
F (N)
max aft GoG, 35m/s, 457m, 990 kg 9209.56
max aft Cog, 48m/s, 457m, 990 kg 9612.567
max aft Cog, 35m/s, 457m, 1100kg 10339.5
max aft Cog, 48m/s, 457m,1100kg 10787.28
Figure 44: Grob G115 Main gear loadings
Figure 43: Grob G115 main gear mesh
45. 39
Figure 46: PA28 Nose gear graphic results – loading 1
Figure 47: PA28 Nose gear graphical results - loading 2
7. Initial Design Results
PA28 Nose Gear:
Deflection
(mm)
Max
Stress
(MPa)
Factor of
Safety
Loading 1: 2468.151 N 8.1681 355.42 1.0663
Loading 2: 3036.82 N 10.05 437.31 0.86665
Loading 3: 2774.162 N 9.1808 399.49 0.9487
Loading 4: 3413.337 N 11.296 491.53 0.77105
Figure 45: Table of PA28 Nose gear results
Loading 1 Graphical Results:
Loading 2 Graphical Results:
46. 40
Figure 48: PA28 Nose gear graphical results - loading 3
Figure 49: PA28 Nose gear graphical results - loading 4
Loading 3 Graphical Results:
Loading 4 Graphical Results:
PA28 Main Gear:
Deflection
(mm)
Max
Stress
(MPa)
Factor of
Safety
Loading 1: 4982.482 N 1.751 145.26 2.609
Loading 2: 5266.816 N 1.8509 153.55 2.4682
Loading 3: 5753.236 N 2.0219 167.74 2.2595
Loading 4: 6072.824 N 2.1342 177.05 2.1406
Figure 50: Table of PA28 Main gear results
52. 46
8. Initial Design Results Discussion
As can be seen from the results above, three of the four landing gear which underwent
analysis failed to maintain a minimum factor of safety of 2.5 for all the loading conditions.
For the nose gear of the Piper PA28, the maximum factor of safety achieved under the
analysis was 1.0663, achieved with a loading of 2468.15N. This is significantly below the
minimum FoS of 2.5 which had been set as the lower limit. It can also be said that while this
factor of safety means that the component did not fail, for the other three loading
conditions, a failure did occur. With a loading of 3413.337, the Maximum Equivalent (Von
Mises) Stress was recorded at 491.53 MPa, which exceeds the maximum yield strength of
the material. Taking this into account, when undertaking a redesign of the Nose gear of this
aircraft, it would be beneficial to incorporate a stronger material and also to look at
changing the design of the critical component.
With the main gear of the Piper PA28, the main gear managed to achieve a passable factor
of safety of 2.609 for the lowest loading of 4982.482 N. This means that under that loading
condition, the gear would not require a redesign, however for the three remaining loadings,
the factor of safety did not meet the requirements. With a loading of 6072.824 N, a safety
factor of only 2.1406 was achieved. The gear however did not fail, but owing to the
possibilities of any uncertainty in the quality of the material used and any other external
factors, maintain a 2.5 factor of safety is a must.
Moving on to the Grob G115, the nose gear of this aircraft performed extremely well under
all loading conditions. Achieving a minimum factor of safety of 6.0603 with the maximum
loading on the gear being 1514.316 N, this gear will not be required to undergo any
redesign. It can be added that due to the lack of design information regarding the Grob G115
landing gear, the design used may vary from the one that is used on the aircraft, and that
the actual landing gear design of the plane will obtain different results to this, including the
possibility of an unsatisfactory factor of safety.
With the nose gear of the G115 obtaining high factors of safety for the loading conditions,
the main gear did not fare as well. Achieving a maximum factor of safety of 2.1871 for a
53. 47
loading of 9209.56 N, this result is to low and therefore this aspect of the gear will need a
redesign. Under a larger loading the factor of safety drops further to 1.8672.
In summary for these results, three of the landing gear will require a redesign process with
only the nose gear of the Grob G115 achieving the minimum factor of safety of 2.5. Although
only one gear actually failed under the loading conditions, the minimum factor of safety is in
place to maintain a safe level in the event of unforeseen circumstances. The redesign will
mainly look at the components which did not meet requirements and also the use of
materials for the gears.
54. 48
9. Redesign
Since there are areas of improvement which can be seen from the results obtained for all
landing gear, a redesign process must be undertaken. This will involve identifying the issue,
changing the requirements, selecting a solution to implement then obtain the relevant
results.
9.1 Design Method
At a basic level, design processes in engineering generally follow the same five stages to
achieve a suitable design.
Firstly, the problem needs to be defined. For all cases with the landing gear, the factor of
safety is too low, with a factor of safety of less than one meaning that the stress has
exceeded the yield strength of the material. Of course it is better to have a high factor of
safety, with a factor of safety of over 2.5 generally being acceptable.
With the problem defined, it is essential to gather information which is appropriate to the
problem encountered. Obtaining information at an early stage of the design process allows
for a quicker and smoother transition into a redesign. In this case, relevant information
regarding materials commonly used in landing gears has already been gathered at an earlier
stage of this project.
The next stage of the design process involved in redesigning the landing gear revolves
around generating numerous solutions to the problem. In this case, one main way of solving
the problem of the landing gear failures is to use a higher strength material. This along with
improving the design of specific components can be deemed two ways in which the redesign
can take place.
Step four of the redesign process involves selecting a solution to implement. This will be
undertaken once both of the design solutions have been analysed and then compared to
determine which would be more beneficial to include. For instance, if y changing the design
of components the factor of safety reaches the minimum of 2.5, however changing the
material vastly improves the results, a compromise may be made as it may prove to be
financially beneficial to choose the design change over the material.
55. 49
Figure 65: PA28 Main gear redesign
Once a suitable design has been chosen as the solution to the problem, then this has to be
fully implemented and also fully tested so that a comparison can be made between the
initial failed design and the solution design.
9.2 Redesign details
At this stage, all three landing gear which either failed or did not meet the minimum factor
of safety requirements will undergo two different design changes. All three will have their
material changed from Aluminium 7049 to Titanium 6-4. This change will be the first to be
analysed, then for each of the three gears the component dimensions will be changed, or
additional components added.
PA28 Nose Gear Component Change
An attempt at the redesign for the PA28 nose gear involved initially looking at the graphical
results for the gear. This showed the area of highest stress was situated around both the
linkages arms of the gear. An attempt was made at redesigning the link arm however this led
to a vastly changed gear and did not provide the required decrease in stress. The design was
then changed by smoothing the link arms while also incorporating the Titanium 6-4 material.
This bizarrely decreased the factor of safety of the gear, however another titanium alloy
with higher strength could replace the Titanium 6-4.
PA28 Main Gear Component Change
For the PA28 main gear, after reviewing the
graphical results for the deflection, stress and the
factor of safety, the main area where a focus on
changing the design was noted. These areas
where mainly situated where there were sharp
corners in the lower cylinder and this was
rectified by introducing a fillet radius between
the two parts.
56. 50
Figure 66: G115 Main gear redesign
Grob G115 Main Gear Component Change
For the redesign of the main gear of the Grob G115, the graphical results for the initial
analysis were reviewed. The graphics show that the highest levels of stress and deflection
occur in the middle of the gear where there is no support.
It was decided to create two support struts which should alleviate some of the stress from
the gear, while also increasing the thickness of the material at the midpoint of the gear and
also increasing the length of the gear to have a greater are to spread the force.
57. 51
Figure 69: Pa28 Nose gear redesign graphical results - Titanium 4Ai -4Mo -2Sn
10. Redesign Results
PA28 Nose Gear:
Material changed to Titanium 6-4
Deflection
(mm)
Max
Stress
(MPa)
Factor of
Safety
Loading 1: 2468.151 N 5.0152 354.32 2.4836
Loading 2: 3036.82 N 6.1707 435.96 2.0185
Loading 3: 2774.162 N 5.637 398.25 2.2097
Loading 4: 3413.337 N 6.9558 490.01 1.7959
Figure 67: PA28 Nose gear redesign results - Titanium 6-4
Material changed to titanium 4AI-4Mo-2Sn
Deflection
(mm)
Max
Stress
(MPa)
Factor of
Safety
Loading 1: 2468.151 N 5.6814 343.91 2.8205
Loading 2: 3036.82 N 6.9904 423.15 2.2923
Loading 3: 2774.162 N 6.3858 386.55 2.5094
Loading 4: 3413.337 N 7.851 475.61 2.0395
Figure 68:PA28 Nose gear redesign result - Titanium 4Al - 4Mo - 2Sn
58. 52
Figure 71: PA28 Main gear redesign graphical results - Titanium 6-4
PA28 Main Gear
Material changed to Titanium 6-4
Deflection
(mm)
Max
Stress
(MPa)
Factor of
Safety
Loading 1: 4982.482 N 1.0756 143.53 6.131
Loading 2: 5266.816 N 1.1369 151.72 5.8
Loading 3: 5753.236 N 1.2419 165.74 5.3097
Loading 4: 6072.824 N 1.3109 174.94 4
Figure 70: PA28 Main gear redesign results - Titanium 6-4
59. 53
Figure 73: PA28 Main gear redesign graphical results - Design change
Design Change
Deflection
(mm)
Max
Stress
(MPa)
Factor of
Safety
Loading 1: 4982.482 N 1.294 95.469 3.9699
Loading 2: 5266.816 N 1.3678 100.92 3.7556
Loading 3: 5753.236 N 1.4942 110.24 3.438
Loading 4: 6072.824 N 1.5772 116.36 3.2571
Figure 72: PA28 Main gear redesign results - Design change
60. 54
Figure 75: G115 Main gear redesign graphical results - Titanium 6-4
Grob G115 Main Gear
Material changed to Titanium 6-4
Deflection
(mm)
Max
Stress
(MPa)
Factor of
Safety
Loading 1: 9209.56 N 13.74 173.23 5.08
Loading 2: 9612.567 N 14.342 180.81 4.867
Loading 3 10339.5 N 15.426 194.48 4.5249
Loading 4: 10787.280 N 16.094 202.9 4.337
Figure 74: G115 Main gear redesign results -Titanium 6-4
61. 55
Figure 77: G115 Main gear redesign graphical results - Design change
Design Change
Deflection
(mm)
Max
Stress
(MPa)
Factor of
Safety
Loading 1: 9209.56 N 1.5844 43.719 8.669
Loading 2: 9612.567 N 1.6537 46.632 8.3056
Loading 3 10339.5 N 1.7788 49.083 7.7216
Loading 4: 10787.280 N 1.8558 51.208 7.4011
Figure 76: G115 Main gear redesign results - Design change
62. 56
11. Redesign Discussion
With the redesign completed, from the results detailed above it is clear that not all of the
landing gear were redesigned to meet the minimum factor of safety of 2.5.
The PA28 Nose gear was the first of the three landing gear to undergo a redesign. From the
initial results obtained, it was clear from the graphics that the link arms where were the
areas of highest stress were, leading to a small factor of safety of only 0.7705 for the highest
loading. To attempt to achieve a higher Fos, the link arms were redesigned, however this
proved to be unsuccessful and the Factor of safety actually dropped. Numerous attempts at
redesign this aspect of the landing gear did not prove to be fruitful, so the next stage was to
change the material. Changing the material from the Aluminium alloy to Titanium 6-4
improved the safety factor to 1.7959 from the previously stated 0.0705. This means that the
part would not fail however the requirements are still not met. A final material was used in
hope to increase the Factor of safety above the required level, with another Titanium alloy
use, Titanium 4AI-4Mo-2Sn. This once again increased the factor of safety for the maximum
loading, but unfortunately the minimum was not met as only 2.0395 was achieved. It can
however be said that this is for an unlikely loading criteria, and that the two lowest loading
used, although still high compared to normal operation, met the minimum FoS
requirements.
Regarding the main gain of the Piper PA28 aircraft, the initial results stated that a minimum
safety factor of 2.1406 was achieved, with areas of highest stress occurring where the axle
came into contact with the lower end of the cylinder. Taking that into account, the area
around this point of contact was initially redesigned with a fillet radius. This had a
detrimental effect upon the factor of safety, lowering it even further. It was decided to
create a fillet between the thick and narrow sections of the cylinder, and then increase the
radius of the axle. This approach was successful and obtained a minimum safety factor of
63. 57
3.2571 with a loading of 6072.824 N. This can be compared with the results obtained by
changing the material to the Titanium 6-4. The minimum safety factor that the solution
provided was 4, which was higher than the Factor of safety obtained with the lowest loading
under the design change.
Finally, the main landing gear for the Grob G115 was redesigned. It was noted from the
results of the initial design undergoing loadings, that large deflections and stresses were
occurring at the centre point of the gear. This is where the focus of the redesign was aimed,
with the inclusion of support struts being used to reduce the stress upon this area. As well as
these struts, the initial design was also thickened along the body with a priority given to the
centre of the geometry. From this change, the minimum safety factor obtained, for the
highest loading of 10787.280 N was 7.4011. This is a very high factor of safety and Is
extremely suitable. As well as this redesign the material was once again changed to the
Titanium alloy to provide comparison. In this case the titanium under the same loading of
10787.280N achieved a FoS of 4.337.
It can be said that for the redesign that changing the material generally offered the greatest
improvement in increasing the factor of safety, except in the case of the Grob G115 main
gear. Unfortunately for the PA28 nose gear the minimum factor of safety was not obtained
for all the loadings undertaken, however the loadings are abnormal.
64. 58
12. Conclusion
To conclude this project and report, it would have been ideal to suggest that everything
undertaken and attempted worked as planned. That however is not the case. The numerous
attempts at undertaking the fluid structure interaction as described earlier in this report
were done to try and achieve as accurate a result possible, to relate the findings to real
world situations and scenarios. Completing a Fluid Structure Interaction analysis would have
been ideal, however using the static structural analysis the project through up some
findings. Firstly, before an analysis can even be undertaken, it is necessary to have access to
relevant material regarding the design and layout of landing gear being modelled. This led to
guesswork however results were obtained regarding each the landing gear for the Piper
PA28 and the Grob G115. With regard to the initial designs of the landing gear, when
undergoing abnormal loading situations, three out of the four did not meet the minimum
Factor of safety requirement of 2.5 The nose gear of the Grob achieved this and was
therefore not subject to any redesign. When it came to the redesign, this ran fairly smoothly,
with both the G115 main gear and the PA28 main gear achieving the minimum 2.5 for both
the material change and the design change. The optimum design for the main gear of the
G115 would utilise the change of design, incorporating the struts and the increased
thickness of the part, while for the PA28 main gear, improving the material to the Titanium
6-4 alloy provides the highest factor of safety. The redesign was not all smooth as the Piper
nose gear, even though two different materials were analysed and design change
attempted, did not achieve the minimum factor of safety requirements.
The aim of this project from the outset was to analyse incidents in light aircraft involving
their landing gear. Since the landing gear has to be designed safely, any mean s of increasing
65. 59
the factor of safety in the gear is worthwhile, and from the analysis undertaken, three of the
four landing gear that were modelled are now safer than they were at the outset.
13. Recommendations
If this project or a project with similar aims was to be undertaken in the future, it would be
absolutely necessary that relevant design information had been collected. This would
remove a lot of the guess work involved in the project, especially relating to the dimensions
of parts within each of the landing gear. It can however be said that companies within the
industry should offer more support when students are undertaking projects such as this.
Another recommendation which can be given is that prior knowledge of undertaking Fluid
Structure Interaction analysis would prove to be beneficial. Knowing how to successful
complete a FSI analysis would provide a higher degree of accuracy to the answer which
would allow the project undertaken to be more relevant to a real life scenario.
66. 60
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