Final Year Project Report

University of Hertfordshire
School of Engineering and Technology
BACHELOR OF SCIENCE
DEGREE WITH HONOURS
IN BSc AEROSPACE TECHNOLOGY WITH PILOT STUDIES
BSc Final Year Project Report
THE DESIGN OF A POWER UNIT FOR A BASIC ORNITHOPTER
Report by
Josh Hammond
Supervisor
Rachael Cunliffe
Date
17/04/2013

i
DECLARATION STATEMENT
I certify that the work submitted is my own and that any material derived or
quoted from the published or unpublished work of other persons has been duly
acknowledged (ref. UPR AS/C/6.1, Appendix I, Section 2 – Section on cheating
and plagiarism)
Student Full Name: Josh Hammond
Student Registration Number: 10232042
Signed: …………………………………………………
Date: 17/04/2013

School of Engineering and Technology BSc Final Year Project Report
THE DESIGN OF A POWER UNIT FOR A BASIC ORNITHOPTER ii
ABSTRACT
This report focuses on the project objective to design a power unit for a basic
ornithopter. The primary consideration during the design phase of the
ornithopter was to minimise weight to enable sustained flight. The power unit
consists of small electrical and mechanical components that all contribute to the
flapping motion of the ornithopter.

THE DESIGN OF A POWER UNIT FOR A BASIC ORNITHOPTER iii
ACKNOWLEDGEMENTS
In order to achieve the completion of this project I have received guidance and
advice from many of my peers, in particular I would like to thank the following
people:
− Rachael Cunliffe for liaising with me to offer her expertise in the area of
ornithopters and small scale electronics
− The staff at Addlestone Model Centre for aiding me in tracking down the
appropriate components required for the project

THE DESIGN OF A POWER UNIT FOR A BASIC ORNITHOPTER iv
TABLE OF CONTENTS
DECLERATION STATEMENT………………………………………………. i
ABSTARCT……………………………………………………………………. ii
ACKNOWLEDGEMENTS……………………………………………………. iii
LIST OF FIGURES……………………………………………………………. v
GLOSSARY……………………………………………………………………. vii
1. Introduction…………………………………………………………………..1
1.2. Project Aims………………………………………………………………. 1
1.3. Project Objectives………………………………………………………... 2
2. Subject Review……………………………………………………………... 2
2.1. Introduction……………………………………………………………….. 2
2.2. Literature Review………………………………………………………… 3
2.2.1. Origin of the Ornithopter………………………………………………. 3
2.2.2. Flapping Wing Flight……………………………………………………3
2.2.3. Flapping Wing Science………………………………………………... 4
2.2.4. Wing Shape and Size…………………………………………………. 6
2.2.5. Practical Applications………………………………………………….. 7
2.2.6. Ornithopter Case Studies……………………………………………... 8
2.2.7. Materials………………………………………………………………… 9
2.3. Component Investigation………………………………………………... 10
2.3.1. Brushed Motors………………………………………………………… 10
2.3.2. Brushless Motors………………………………………………………. 11
2.3.3. ESC……………………………………………………………………… 13
2.3.4. Battery……………………………………………………………………13
2.3.5. Servo……………………………………………………………………..13
2.3.6. Gearing…………………………………………………………………..15
2.3.7. Transmitters and Receivers…………………………………………... 17
2.4. Literature Review Conclusion……………………………………………17
3. Conceptual Sketches………………………………………………………. 18
4. Prototypes……………………………………………………………………19
5. Final Components………………………………………………………….. 20

THE DESIGN OF A POWER UNIT FOR A BASIC ORNITHOPTER v
5.1. Motor………………………………………………………………………. 21
5.2. ESC and Receiver……………………………………………………….. 22
5.3. Transmitter and Receiver Crystal………………………………………. 23
5.4. Battery…………………………………………………………………….. 24
5.5. Servo………………………………………………………………………. 25
5.6. Crankshaft and Gearing…………………………………………………. 26
6. Theoretical Calculations…………………………………………………… 27
6.1. Motor RPM………………………………………………………………... 27
6.2. Output RPM………………………………………………………………. 28
7. CATIA Design………………………………………………………………. 29
7.1. Component Visualisation………………………………………………... 30
7.2. Component Assembly…………………………………………………… 33
8. Fabrication – Key Processes……………………………………………… 35
9. Discussion……………………………………………………………………38
10. Conclusion………………………………………………………………….39
11. Further Developments……………………………………………………. 39
11.1. Weight Reduction………………………………………………………..39
11.2. Flapping Symmetry……………………………………………………...39
11.3. Autonomous Flight……………………………………………………… 40
REFERENCES………………………………………………………………… 41
BIBLIOGRAPHY………………………………………………………………. 42
APENDICIES…………………………………………………………………...43
Battery Instructions……………………………………………………………. 43
Ornithopter Drawing……………………………………………………………44

THE DESIGN OF A POWER UNIT FOR A BASIC ORNITHOPTER vi
LIST OF FIGURES
Figure 1 – Bird Wing Aerofoil
Figure 2 – Upstroke with negative lift
Figure 3 – Downstroke
Figure 4 – Force and speed for the downstroke
Figure 5 – Dr Von Holst’s Swan plan
Figure 6 – Balsa and Spruce
Figure 7 – Aluminium and Steel
Figure 8 – Brushed Motor Schematic
Figure 9 – Brushless Motor Schematic
Figure 10 – Basic Servo System
Figure 11 - Typical control configuration of a servo-system
Figure 12 – In-line Gearing
Figure 13 – Transverse Gearing
Figure 14 – Conceptual Sketches
Figure 15 – Prototype fabricated from household materials
Figure 16 – Prototype bearings mounted to the main body
Figure 17 – Brushed Motor
Figure 18 – ESC and Receiver
Figure 19 – Transmitter
Figure 20 – Receiver Crystal
Figure 21 – Transmitter Crystal
Figure 22 – Initial Battery Intended For Use
Figure 23 – Final Battery
Figure 24 – Initial Battery Weight
Figure 25 – Final Battery Weight
Figure 26 – Servo
Figure 27 – Main Gear Mounted On Crank
Figure 29 – Pinion Gear
Figure 30 – Gear Parameters
Figure 31 – CATIA Design of the Ornithopter
Figure 32 – Battery and Battery Holder
Figure 33 – ESC and Transmitter Unit with Mounting Unit

THE DESIGN OF A POWER UNIT FOR A BASIC ORNITHOPTER vii
Figure 34 – Motor and Pinion Gear
Figure 35 – Servo Components
Figure 36 – Crank Shaft and Main Gear
Figure 37 – Connecting Rod and Fasteners
Figure 38 – Crystal
Figure 39 – Wing Spar, Connector, Hinge, Skin and Bearing
Figure 40 – ESC Mounting Plate
Figure 41 – Motor Mounting Spar, Bearing, Lower Motor Support and Main
Body Spar
Figure 42 – Tail and Rotating Tail Mounting Plate
Figure 43 – Battery Arrangement
Figure 44 – Motor, Crank and Bearing Arrangement
Figure 45 – Receiver and ESC combination
Figure 46 – ESC Arrangement
Figure 47 – Gearing Arrangement (Front View)
Figure 48 – Gearing Arrangement (Side View)
Figure 49 – Wing Spar and Hinge Arrangement
Figure 50 – Servo and Rotating Plate Arrangement
Figure 51 – Angle between main body spar and motor mounting spar
Figure 52 – Securing the bearings
Figure 53 – Hand held drill
Figure 54 – Essential tools
Figure 55 – Final Model Ornithopter

THE DESIGN OF A POWER UNIT FOR A BASIC ORNITHOPTER viii
GLOSSARY
Ornithopter Flapping wing aircraft
DC Direct Current
ESC Electronic Speed Control
UAV Unmanned Air Vehicle
RPM Revolutions per Minute
V Voltage
mm Millimetre
CAD Computer Aided Design

THE DESIGN OF A POWER UNIT FOR A BASIC ORNITHOPTER 1
1. Introduction
The focus of this report is to detail how the design process was undertaken to
reach the completion of making a physical model ornithopter. Despite the lack
of development allocated to ornithopters compared to their fixed and rotary
wing counterparts there is still a demand for flapping wing vehicles. Modern
technologies and materials are gradually filtering their way down to the UAV
branch of engineering thus re-invigorating experimentation with ornithopters.
The predominant challenge in designing mechanical flapping wing air vehicles
is emulating the natural muscular movement of birds and insects, designs
must also consider the sensitivity of weight implications on such vehicles, as
their stability is greatly decreased when subjected to uneven distributions of
weight and forces. A primary asset of the ornithopter is the ability to sustain
hovering flight like helicopters and hummingbirds whilst also being capable of
gliding like aeroplanes and albatrosses. Perhaps the most exciting and rapidly
growing aspect of ornithopters is their use in military and environmental
applications where innovative uses include recognisant missions and bird
population control respectively.
1.2. Project Aims
! To design an electrically powered ornithopter
! Capable of self-sustained flight
! Develop concept sketches into a physical model through a design
process
! Identify appropriate components

1.3. Project Objectives
! Investigate all forms of the components possibly required
! Investigate the characteristics of ornithopter UAVs
2. Subject Review
2.1. Introduction
The following literature reviews outline the various sources of information
gathered during the research phase at the beginning of the project. The
research focused on, but was not limited to, the fundamental aspects of
ornithopter mechanics and the basic principles of electric motors, servos,
speed controllers, batteries and radio transmitters/receivers. The relatively
niche market for ornithopters consequently means that information and
documentation is confined to the internet as opposed to being published in
books or journals, having said that, it was possible to combine information
from a variety of wider sources, not obviously associated with ornithopters, to
gain a substantial understanding of the flapping flight discipline. Initial
research swiftly ruled out two possible options for sustaining power to an
ornithopter during flight, the first being an elastic band and the second being a
combustion engine. The elastic band method relies on the potential energy
stored in the winding of the band; once the whole of this energy has been
exerted the ornithopter can no longer sustain flight. Alternatively the
combustion engine would be capable of sustaining flight for a substantial
period of time, however the ancillary parts required for the engine to function,
such as a fuel tank, would violate the weight restriction imposed by wing size
and flapping frequency. Therefore the electrical DC motor, drawing power
from a battery, is the most suitable application for sustaining an ornithopter in
flight due to its lightweight characteristics and reliability.

2.2. Literature Review
2.2.1.Origin of the Ornithopter
Leonardo da Vinci and Otto Lilienthal are two of the most notable names
associated with the primitive stages of human developed and human powered
flapping wing flight. Ornithopters can be traced back even further than da
Vinci and Lilienthal to Greek mythology where Homer’s Daedalus constructed
bird like wings from goose feathers and beeswax to escape imprisonment
enforced by King Minos, the word ‘ornithopter’ actually derives from Greek
where ‘ornithos’ translates to ‘bird’ and ‘pteron’ to ‘wing’ [1]
, however da Vinci
and Lilienthal were pioneers of progressing ornithopters into useful vehicles
by studying the natural flight of birds and wing shapes. Consequently for
ornithopters the development of aeroplanes and helicopters gradually
diminished the interest, and indeed requirement, for flapping wing flight. Yet,
as the development of aircraft in use today begins to peak and environmental
awareness grows ever more poignant interest in flapping wing flight has been
re-kindled.
2.2.2.Flapping Wing Flight
Similarly to avian vertebrates, ornithopters can only produce the thrust
required to fly through the flapping of wings, a unique motion that does
without the requirement of a propeller or rotating wing installed on more
traditional aircraft. The complexity of bird wings have occupied scientists and
engineers for decades; the shape, size, angle of attack, flap frequency,
layering of feathers and muscular construction of a bird’s wing have been the
subject of numerous studies.

2.2.3.Flapping Wing Science
Much like an aeroplanes aerofoil a bird’s wing creates high-pressure regions
around the lower surface and low-pressure regions along the upper surface
thus allowing the bird to glide [2]
. In order to gain height the bird increases the
flapping frequency of its wings, which forces larger deflections of air
downwards resulting in the reactive force of lift.
The complex nature of a bird’s wing occurs during the transition from a down-
stroke, to produce lift, to an up-stroke, where the cycle restarts. A common
misconception is that the up-stroke should produce a force similar to that of
the down-stroke, thus equalling the forces. YouTube video maker
SmarterEveryDay [3]
uses slow motion film to illustrate how bird wings function
and do not conform to the misconceptions. He explains that during the up-
stroke feathers separate to allow air to flow through the wing, additionally the
bird is capable of altering the angle of their wing to form the most streamline
and aerodynamically efficient shape, both of these actions therefore enable
the bird to ‘provide downward thrust on a backward stroke [3]
. Current
technology does not have the capability to replicate birds’ wings in such
delicate detail; instead ornithopters are resigned to mirroring bird wing
shapes, sizes and flapping frequencies. The forces acting on a flapping wing
can be quantified and calculated when using a simpler aerofoil as opposed to
attempting to replicate the complicated natural mechanics of a real avian
wing, for example, the extensive research that has already been conducted
on aeroplane aerofoils can be implemented for the theoretical reckoning of
flapping wing dynamics.
Figure 1 – Bird Wing Aerofoil [2]

Figures 2 and 3 illustrate how the thrust of a flapping wing can be related to
vectors of resultant forces where U∞ is the airflow due to forward motion, Uz is
the airflow due to the flapping motion and Ur is the relative airflow.
Figure 4 demonstrates in further detail how the downstoke of a birds’ wing
produces lift and thrust in the same movement.
Figure 2 – Upstroke with negative lift [4]
Figure 3 – Downstroke [4]
Figure 4 – Force and speed for the downstroke [4]

In Figure 4 it becomes apparent that the aerodynamic force (K) is almost
perpendicular to the track of the birds’ wing, furthermore the downstroke
generates lift (L) and thrust (T), this concept has also been explored by Henk
Tennekes who concluded that ‘the thrust to lift ratio is equal to the ratio of the
downward speed of the wing and the forward speed of the bird (w/V)’ [4]
. As
well as identifying speed and force vectors for a flapping wing it is also
possible to distinguish physical deformations that are imposed on the wing
during flight. In particular, bending and twisting forces can be detrimental to
any wing, especially man-made ones that are constructed from material that
eventually fatigues and fails. The flapping motion make the wing susceptible
to bending forces at the root where the power of the motor is being transferred
and converted to thrust. In addition to the fore mentioned forces, vibration can
also cause distress to an ornithopter in the shape of a force that will gradually
lessen the rigidity of the structure. Vibration forces can form from asymmetric
wing flapping, where the wings are not flapping in sync (symmetrically), but
the most common source of vibration is from the motor and gearing system
that will be rotating at high speeds. Such vibration can be controlled by
balancing the rotation, problems occur when an unbalanced object is rapidly
rotated and eventually ‘shakes’ itself apart.
2.2.4.Wing Shape and Size
As mentioned in section 2.2.3 scientific research has diligently explored the
reasons behind the differing shapes and sizes of avian vertebrates wings. The
behaviour and lifestyle of birds dictates the type of wing they have evolved to
bear. Research has revealed that predatory birds, like Hawks and Eagles,
have developed wings capable of enabling them to fly quickly and change
direction rapidly, whereas birds that traverse large distances over landmass
and oceans typically have wings suited to gliding, resulting in energy efficient
flight. Ornithopters tend to adopt wing shapes that provide the most stable
platform for stable flight and as previously mentioned the intricate detail of a
bird’s wing is not directly mirrored but instead the outline shape and span is

more closely observed. As well as endeavouring to provide stability for the
ornithopter the wing design also considers the required lift generation to
sustain it in flight. Heavier ornithopters require larger wings or more frequent
wing beats, but because the stresses of rapid wing flapping can be
problematic a larger wing is usually preferred. Consequently larger wings
require more powerful motors to provide enough leverage for the downward
and upward strokes. Martin Simons discusses model aerodynamics in his
book Model Aircraft Aerodynamics and explains that a semi-elliptical wing has
been mathematically proven to be the most stable because at all speeds it
provides constant downwash, equal load distribution along the wing and stalls
simultaneously at each point [5]
. Stability is an imperative attribute to make the
ornithopters handling characteristics more forgiving for the pilot.
2.2.5.Practical Applications
This section will diverge slightly from the scientific analysis of ornithopters and
focus on the reasons for their use and the environment in which they are
deployed. Military interest in ornithopters has re-focused the interest on
flapping wing UAVs, particularly where there is an opportunity for companies
to gain capital from developing and delivering ornithopters for practical uses.
The United States Darpa department (the Pentagon’s advanced research
unit) is an exceptional example. Darpa were commissioned to manufacture a
micro-ornithopter, capable of hovering and carrying its own power source,
with the intention of using it for covert indoor missions such as bugging a
room with hidden listening devices or transmitting audio and video to locations
up to one kilometre away. Darpa claim that the micro-ornithopter is more
aerodynamically efficient than remote control helicopters/aeroplanes, a claim
measured by the low Reynolds Number (airborne efficiency) [6]
. Moreover,
military interest extends beyond small-scale ornithopters, they also intend to
capitalise on the ornithopters resemblance to birds and insects for
inconspicuous recognisance missions. Natural environmentalists are also
beginning to understand the advantages of ornithopters, for example, the

Colorado Division of Wildlife has used ornithopters to aid the survival of an
endangered bird species, the Gunnison Sage Grouse. Further commercial
uses extend to the protection of commercial aircraft from bird strikes at take-
off and landing, Schiphol Airport in Holland has seen success with this
method.
2.2.6.Ornithopter Case Studies
To gain a further understanding of how ornithopters work it was important to
research existing models to identify what made them successful and attempt
to identify the key components required for a successful design. Dr Erich Von
Holst created some of the early successful ornithopters in the 1940s, his
Swan, Brimstone Butterfly and Thrust
Wing models incorporated wings
derived from his research on biological
and bird flight. However his models
were powered by elastic bands, as the
micro-electronic components available
nowadays were not present in the
1940s. Jean-Louis Solignac’s design
(2000) draws attention because he
moved away from the tradition of
incorporating a tail unit. Battens in the
aerofoil create a downward camber
allowing the model to fly in a stable
attitude [7]
. More advanced
ornithopters have come in the form of Sean Kinkade’s Park Hawk and Slow
Hawk. Both models have been designed with mass production in mind,
therefore they are relatively simple to construct and are made up of
replaceable parts interchangeable with new parts in case of breakages. By
using modern components Sean Kinkade has created an efficient design
capable of being used by ornithopter hobby enthusiast. The models are
powered by a ‘high KV brushless outrunner motor and brushless ESC and is
Figure 5 – Dr Von Holst’s Swan plan

designed to fly on 2 cell lipo packs’ [8]
. Consequently the use of cutting edge
electrical components, materials and fabrication techniques (CNC) comes at a
cost to the consumer with the Park Hawk retailing at £188.
2.2.7.Materials
Traditional model building materials have always catered for designs of a
lightweight nature whilst providing acceptable levels of strength. UAVs will
always be susceptible to damage from crashes and manoeuvring stresses
because they can operate at considerable speeds at altitude. Materials such
as woods, plastics and metals can be purchased from hobby shops and DIY
stores, making them the material of choice for most aero modellers. Balsa is
typically the most popular wood for its
diversity for model building; cuts come in a
variety of sizes and grain directions. Balsa
strength is acknowledge to be better than
steel when comparing the tensile strength to
density ratio, however, like all fibrous
materials the best loading strength is
obtained when the load is with the grain as
opposed to across it. Spruce wood is a
denser material than Balsa and is therefore
heavier but has the advantage of being
stronger. Spruce wood is suitable for
applications where stresses may overcome
Balsa and eventually damage it. The plastics
available to model builders are usually
moulded into pre-determined shapes as
most builders do not have the facilities such
as injection moulders and vacuum formers
to fabricate their own designs. Aluminium
and steel are favoured for their attributes of
strength and workability where they can be
Figure 6 – Balsa and Spruce
Figure 7 – Aluminium and Steel

reworked into a desired shape. Research has shown that metal is typically
used on large-scale ornithopters that are capable of producing enough thrust
to lift more weight, whereas small-scale ornithopters utilise the lightweight
characteristics of woods such as Balsa and Spruce. When designing an
ornithopter materials must be selected that are suitable for different aspects of
the design, careful consideration must be made during the trade-off between
weight and strength.
2.3. Component Investigation
2.3.1.Brushed Motors
Brushed DC electric motors are the most commonly used variant of the DC
motor family. They have been in use for over a century and have proved to
offer many favourable characteristics. The relatively simple workings of this
motor allow the user to be confident about its predictable performance.
Brushed motors in their simplistic form have four main components: a stator, a
rotor, brushes and a commutator.
The polarity of the magnets forms the core principles of how brushed motors
work. By polarising the windings coiled around the armature a north and south
pole is created thus creating an attracting force between opposite poles. As
the armature reaches a set point on the rotation the brushes are triggered to
alter their position on the commutator, as a result the polarity is reversed,
rotating the armature in the opposite direction in an attempt to reassert itself
with the opposite pole. This cycle is rapidly repeated to create the direct
Figure 8 – Brushed Motor Schematic

current that forms the foundation of the motors power. The most appropriate
uses for brushed motors can be a topic of debate; the advantages and
disadvantages of such motors are as follows:
Brushed Motor
Advantages Disadvantages
! Replaceable brushes for an
extended life
! Low cost
! Simple construction
! Easily controlled
! No controller required for fixed
speeds
! Operates in extreme
environments
! Periodic maintenance required
! Speed/torque is moderately flat
! High speeds increase brush
friction resulting in less useful
torque
! Poor heat dissipation
The advantages of the brushed motor favour the design concepts of an
ornithopter, in particular the simple construction (weight reduction) and
simplicity (reduced number of components required). On the contrary it cannot
be ignored that over a period of time brushed motors require brush changes
and periodical resurfacing of the commutator to maintain the motor’s
efficiency.
2.3.2.Brushless Motors
Brushless motors are inherently more complex than brushed motors because
they rely on electronic commutator sensors to sense the position of the rotor.
Brushless motor coils are activated in sequence by an electronic speed
controller on cue from the signals being transmitted from the rotor position
sensors. Similarly to brushed motors, brushless motors still utilise the reliable
mechanics of repelling magnetic poles but houses the magnets at the edge of
the stationary casing of the motor.

Brushless motors are measured by Kilo Volt (Kv) ratings, which indicates the
motors maximum rotational speed for a given input voltage. The motors shaft
rotation generates back EMF, the term used to describe the build up of
voltage on the windings. Back EMF gradually reduces the effect the voltage
has on the windings, resulting in the EMF eventually matching the voltage
being applied and the motor reaches its maximum rotational speed. Brushless
motors have some favourable characteristics over brushed motors such as
the reduced winding friction within the motor because of the removal of
physically interacting brushes and commutators. Having said that the
brushless motor does have pros and cons that need to be identified as
problematic or acceptable for the chosen application:
Brushless Motors
Advantages Disadvantages
! Less maintenance required
! Efficient
! High power to weight ratio
! Good heat dissipation
! Higher speed range
! Low electrical noise
! Costly construction
! Control is complex and
expensive
! Open casing allows friction with
air
! Requires electronic controller
Figure 9 – Brushless Motor Schematic

2.3.3.ESC
Electronic speed controls are a vital safeguard against damaging electrical
motors by regulating motor rotation speeds. ESCs are often defined as stand-
alone units or are married to the remote control receiver. Brushless motors
use the back EMF to sense rotary position whereas brushed motors tend to
rely on Hall Effect or optical sensors. ESCs are programmed by the user to
define required voltage cut-off values, acceleration rates and rotational
direction. In order to provide voltage to an ESC a battery is required, this
supply of voltage is regulated by the ESC and passes through to the motor.
2.3.4.Batteries
Lightweight batteries suitable for ornithopters are identified as the lithium
polymer (lipo) variety. Lipo batteries are the most recent addition to the
battery family and benefit from a steady discharge of voltage and a higher
average voltage when compared to older nickel-metal hydride and nickel-
cadmium batteries. Batteries are required to be sized to the voltage
consumption of motors and their charge capacity can be measured in milliamp
hours (mAh). A particular safety concern regarding a battery mounted to an
ornithopter is the possibility for damage to occur from a crash landing, battery
instruction manuals clearly stress that puncturing battery cells will release
toxic chemicals and create a viable fire hazard. Manuals also state that the
voltages applied to the battery during re-charging cycles should be monitored
and voltage overcharging or discharging will impair the lifespan of the battery.
2.3.5.Servos
A servo is basically a small electric motor that responds to control inputs by
applying torque to a shaft connected to an output arm in order to rotate it in a
very calculated manner. Servos are constructed from four main components:

gears, a motor, a feedback potentiometer and an amplifier [9]
. Firstly, gears
are fabricated form plastic or metal depending on their intended use. Plastic
gears are suitable for smaller models requiring less powerful servos to
manipulate control surfaces under less stress whereas metal gears are suited
to servos mounted on models weighing over 2.5kg. Secondly, brushed motors
make a re-appearance for use in servos as they are robust and can be
obtained at low costs. The iron armature at the core of the motor can be
problematic when a servo with extremely fast responses to control inputs are
required because of the inertial/momentum forces of a heavy material. The
feedback potentiometer is an ingenious component mounted to a small circuit
board in the servo casing that can determine the position of the output arm by
using a variable resistor to create a voltage determining its precise location.
This is an important act that has to be carried out so that the user can
manipulate the output arm in the desired manner during all phases of the
operation. Finally, amplifiers are digital microcontrollers that control the
torque, speed and accuracy of the servo. A. W. Richmond characterises servo
schematics (Figure 10 & 11) in his book Servos and Steppers that illustrate
how servo motors are connected to their loads, among other components.
Figure 10 – Basic Servo System [10]

2.3.6.Gearing
Gears on ornithopters work the same way as those found in automobiles and
aircraft turboprop engines. They are required to reduce the shaft power of the
motor and convert it into controllable RPMs required to drive the axle of a car,
rotate the propeller of an aircraft or flap the wings of an ornithopter. The
number and size of gears is referred to as a gearing ration and determines the
final power output being delivered by the motor after the power has been
transferred through the gears. Torque is also an important factor when
considering gearing ratios as larger gears provide more torque to overcome
inertial forces. Metallic gears are less favoured over plastic gears for
ornithopters as they are too heavy; instead plastic gears are utilised for their
weight, wear resistance and ability to absorb backlash shocks. Ornithopter
weight restrictions dictate that gears should be kept to a minimum and remain
as simple as possible, they should also be installed as close to the centre of
lift as possible in an attempt to balance the overall weight of the ornithopter.
Research has yielded that in-line and transverse configurations are the most
suitable arrangement for minimising the space required and the number of
gears necessary.
Figure 11 – Typical control configuration of a servo-system [10]

Connecting
rod
Figure 13 – Transverse gearing
In-line gearing illustrated in Figure 12 is the more simplistic of the two. The
weight of the gearing is concentrated under the wing spars and allows other
components to be mounted further down the main body spar.
Figure 13 depicts transverse gearing which has the benefit of being mounted
parallel to the main body spar allowing for opportunity to select a variety of
gearing ratios.
Figure 12 – In-line gearing
Gear connected
to motor
Main gear

2.3.7.Transmitters and Receivers
Crucially, UAVs must be capable of being controlled when in flight; otherwise
their possible applications discussed in section 2.2.5 become null and void.
The pilot remotely controls modern UAVs such as scale aeroplanes,
helicopters and ornithopters using a hand held transmitter that communicates
with a receiver mounted onboard the vehicle. To ensure signal reliability,
range, power and stability the 2.4 GHz technology is preferred. 2.4 GHz
systems rely on piezoelectric materials, often referred to as crystals, which
create an electrical signal at a precise frequency, via the vibration of the
crystal, to transmit frequencies on an MHz band. Each remote control system
requires the same frequency rated crystal to be installed on the transmitter
and the receiver so that they can communicate with one another; this also
restricts interference from other remote control systems that may be in the
same vicinity, for example, two 35.100 MHz crystals, one in the receiver and
one in the transmitter, will not be interfered with by a different set of crystals
operating on a frequency of 35.150 MHz. Crystals, typically fabricated from
quartz, are available in a wide range of frequencies resulting in a low
probability of interference where large concentrations of remote control
operations occur.
2.4. Literature Review Conclusion
The research phase formed a vital role that was pivotal in progressing the
project into the design phase. Understanding the fundamentals of ornithopters
as well as the key aspects of electrical components that were specified as
being relevant meant that conceptual designs could be sketched and the
components of interest could be sourced. After the information from the
research phase had been considered it was decided that the simplest
components and design aspects would be incorporated into the ornithopter to
conform to the project title where the emphasis was on a ‘Basic Ornithopter’.
The following sections will further justify the final design and component
choices.

3. Conceptual Sketches
The Project Logbook often became a sketchbook of ideas that related to the
research carried out. Referring to information logged in the book it was
possible to incorporate ideas into a design with the intention of developing
prototypes that could be tested to justify the design choices. The main
purpose of the conceptual sketches was to gauge the most suitable layout for
the ornithopter; from the layout it could be determined what the maximum size
of the components could be.
Figure 14 – Conceptual Sketches

4. Prototypes
Building prototypes is a recognised phase of the design process in order to
familiarise with the characteristics and build difficulties of ornithopters. At this
point it was important to experiment with the significance of weight and the
geometric constraints of component size.
Using mere household items such as bamboo skewers, paper clips and
supermarket carrier bags a simple elastic band powered ornithopter could be
made. This prototype offered a chance to explore the implications of wing
size, crank geometry and component interaction. The conclusions drawn from
the first prototype, Figure 15, provided a foundation from which to improve
future designs. The most problematic aspects were recorded; firstly, the
interaction between the wing spar hinges and the bearings, Figure 16,
encountered to much interference during the flapping motion because the
hinges were not securely held in place by the oversized bearings, thus
allowing the hinges to penetrate the gaps between the windings, inhibiting the
rotation.
Figure 15 – Prototype fabricated from household materials

Secondly, the weight of the individual parts were minimal, the bamboo
skewers weighing just 3 grams, however the sum of all the parts became
considerably problematic when the ornithopter had been assembled. Previous
research suggested two solutions for such a problem, the first being
increasing the wing size and flapping frequency, however this solution could
quickly be disregarded, as the elastic band power unit cannot provide enough
energy to agitate larger or faster wing movements. The second solution,
weight reduction, became an apparent option to carry forward for prospective
designs. Aside from the problems encountered the first prototype also yielded
lesson that would prove useful in a later stage of the design process. Lessons
such as the relation between the crank geometry and the maximum angle
created by the vertical travel of the wings and the forces imposed on the wing
hinges during flight.
5. Final Components
Following on from the research conducted in section 2.3, Component
Investigation, it was possible to derive a list of suitable components that would
be incorporated into the ornithopter design. Sourcing the components was
achieved by visiting various model shops and exploring the realm of online
hobby stores, both of which stock a plethora of items.
Figure 16 – Prototype bearings mounted to the main body
Bearings
Wing hinge
diameter

5.1. Motor
The decision to use a brushed DC motor was backed up by the supporting
evidence that they are simple and the less costly than their brushless
counterparts. Also, the robust design of the chosen 180 SE brushed motor
with its internal workings protected by a solid outer shell would be advantages
in the event of a crash landing. Mounting options are also more extensive
than that of the brushless motors researched due to the motor casing having
large, relatively flat surfaces that allow the motor to be mounted in close
proximity to other flat surfaces.
The additional advantages of the brushed motor in Figure 17 include: a 10mm
motor shaft, allowing for more mounting position flexibility, a high quality
soldering to establish a firm electrical interaction between the wire for the ESC
connection and the connection ports on the motor, and finally a 90mm
connection wire allowing the motor to be mounted further from the ESC if
required. The brushed motor in question also has a power rating of 2500 Kv
and weighs 25 grams, making it one of the heavier components on the
ornithopter.
Motor shaft
and pinion
gear
Motor casing
Connection to
ESC
Female
connector
Figure 17 – Brushed Motor

5.2. ESC and Receiver
During the sourcing of the components it was discovered that an ESC could
be combined with a receiver to make one unit, something that was not
brought up during the research phase. The combination of these two
components simplifies the implication of their mounting on the ornithopter
because the units can be concentrated in one position. The two circuits are
simply connected via the connecting pins on the receiver circuit and a female
receiver on the back of the ESC circuit. The ESC forms the centre of the
power unit; the energy form the battery is drawn via the battery connector that
relies on a male and female plug to transfer the power. The motor and servo
are then connected to the ESC via the relevant pins. Incredibly for such a
sophisticated unit the ESC and receiver, when combined, only weigh a total of
10 grams.
ESC Circuit
(Front)
ESC Circuit
(Back)
Receiver
Circuit (Front)
Receiver
Circuit (Back)
Circuit connector pins
Battery
connector
Pin receiver
Figure 18 – ESC and Receiver

Figure 20 –
Receiver
Crystal
Figure 21 –
Transmitter
Crystal
5.3. Transmitter and Receiver Crystal
Observing Figure 19 it can be seen that the hand held control transmission
unit consists of joysticks to dictate the movement of the ornithopter, trim tabs
to govern the continued position or power settings of certain components and
a crystal on the same frequency as the crystal found in the receiver unit.
The receiver crystal (Figure 20) corresponds to the 35.010 MHz frequencies
on which the transmitter crystal (Figure 21) works, the importance of which
was discussed in section 2.3.7.
Figure 19 - Transmitter
Transmitter
crystal
Throttle
Longitudinal
control
Throttle trim
Longitudinal
trim

5.4. Battery
Research revealed that the crucial characteristic of the battery should be that
it is capable of providing enough voltage to power the components to which it
is connected. For the motor to perform at its maximum rating of 2500 Kv a
battery that could provide 7.4V would be necessary. Initially the battery shown
in Figure 22 was chosen as the ornithopters power supply, however after
installing it on a prototype it was found to be too heavy. A smaller battery
(Figure 23) with the same voltage was sourced to overcome the weight issue.
The disadvantage of this was that the smaller battery would reduce the flight
time of the ornithopter, as it could not store as much energy.
The flight time can be theoretically calculated from the mAh rating of the
battery and the loads imposed on it. By almost halving the mAh rating from
800 to 450 the flight time was almost halved from roughly 8 minutes to 4
minutes. However this was a necessary step to take in order to save weight.
Figure 22 – Initial Battery
Intended For Use
Figure 23 – Final Battery

By reducing the weight of the battery and sacrificing the flight time capability a
total weight saving of 9 grams has been achieved. This may seem an
insignificant figure, however any weight that can be shed should when
working with small scale UAVs. The final battery chosen for installation can be
recharged when all of its energy has been exerted, making redundant the
need to purchase a new power source at the end of each batteries discharge
cycle.
5.5. Servo
The intended use for the servo was to control the longitudinal direction of the
ornithopter via the tail unit.
Figure 24 – Initial Battery Weight Figure 25 – Final Battery Weight
Figure 26 – Direction Control

Figure 28 – Main
Gear Mounted On
Crank
The servo chosen to control the tail is simple yet effective. Depicted in Figure
27, the servo has a power rating of 5.3V, a reaction time to control inputs of
0.12 seconds, a torque rating of 1.17 kg per cm and weighs only 6 grams. The
fast reaction time to control inputs is particularly important for complete and
instantaneous control of the ornithopters direction. The torque that the servo
is capable of generating is more than that required to move the light tail
structure form left to right, however the extra torque has no consequence on
the performance or the weight of the unit. The servo’s arm extender is a
crucial component for connecting external parts to the rotating motion
provided by the servo.
5.6. Crank Shaft and Gearing
During the prototype builds the size of the crank was
experimented with to determine the best size of each
segment with regards to the dihedral of the wing flap.
The length of each section the crank impacted the
vertical distance travelled by the wings. If the crank
sections became too long the wings would over rotate
and become stuck at the top of their rotation. Too
short and the wings would not travel far enough to
produce a significant flap to sustain flight.
Figure 27 - Servo
Arm extender

An optimal size was determined for the crank size, more of which will be
discussed in section 5.7.
The two gears chosen for the ornithopter can be seen in Figures 28 and 29.
The main has a diameter of 35 mm and bears 72 teeth; the pinion gear has a
diameter of 5 mm and bears 8 teeth. The total RPM of the motor is reduced
by the gearing system; the theoretical values of this reduction will be explored
in the next section.
6. Theoretical Calculations
6.1. Motor RPM
The RPM of the motor can be calculated using the following equation:
Equation 1
Using the known values from the components acquired for installation
Equation 1 can be calculated as:
Equation 2
Figure 29 – Pinion Gear
€
MotorRPM = MotorKv × BatteryVoltage
€
MotorRPM = 2500Kv × 7.4V =18500RPM

6.2. Output RPM
The output RPM is a measure of the RPM left after the motor shaft rotation
has been transferred through the gear mechanism.
Figure 30 – Gear Parameters
Equation 3
The tooth ratio of both gears, 2/18, can be inserted into Equation 3 to make:
Equation 4
Equation 4 indicates that the total RPM after taking the gears into account
equates to 2055 RPM. It is important to note that although the RPM is high for
a small-scale ornithopter the motor power can be reduced via the hand held
transmitter (section 5.3).
Pinion
Main
Gear
5mm 35mm
72 Teeth
8 Teeth
€
PinionTeeth
MainGearTeeth
€
18500Kv ×
2
18
= 2055RPM

7. CATIA Design
CATIA is an extremely valuable CAD tool for designers that allow them to
model scaled objects using computer software. For this project the final
design of the ornithopter was constructed entirely using this CAD package.
The software is particularly useful for tweaking designs without having to
manually re-draw them on paper, saving vast amounts of time. The software
also allows for 3D manipulation to ensure components will not interfere with
one another during or after fabrication.
Figure 31 – CATIA Design of the Ornithopter

7.1. Component Visualisation
In order to arrive at the stage of a fully assembled model (Figure 31) each
individual component must be modelled.
Figure 32 – Battery and Battery Holder
Figure 33 – ESC and Transmitter Unit with Mounting Unit
Figure 34 – Motor and
Pinion Gear

Figure 35 – Servo Components
Figure 36 – Crank Shaft and Main Gear
Figure 37 – Connecting Rod and Fasteners
Figure 38 - Crystal

Figure 39 – Wing Spar, Connector, Hinge, Skin and Bearing
Figure 40 – ESC Mounting Plate
Figure 41 – Motor Mounting Spar, Bearing, Lower Motor Support
and Main Body Spar
Figure 42 – Tail and Rotating
Tail Mounting Plate

7.2. Component Assembly
Figure 43 – Battery
Arrangement
Figure 44 – Motor, Crank and
Bearing Arrangement
Figure 45 – Receiver and ESC
Combination
Figure 46 – ESC
Arrangement
Figure 47 – Gearing Arrangement (Front View)

Figure 48 – Gearing Arrangement
(Side View)
Figure 49 – Wing Spar and Hinge
Arrangement
Figure 50 – Servo and Rotating
Plate Arrangement

8. Fabrication – Key Processes
After extensive alterations to the CATIA design the final CAD model was
assembled, marking the beginning of the fabrication phase where a design is
transformed from a theoretical drawing to a physical object. At this stage all
the components and materials required had been gathered and a clear design
had been set out. Unfortunately, due to a lack of foresight for the compilation
of this report, only a few photos were captured during the fabrication phase,
therefore this section will be mostly descriptive rather than illustrative. Only
the key fabrication processes will be discussed as many components are
simply attached using adhesive substances.
! After gathering the Balsa and Spruce wood required for the fabrication
of the ornithopter they required cutting down to their desired length.
Fortunately a range of tools were available during the fabrication stage
making the processes easier to conduct and resulting in a more
clinically executed parts conforming to the CATIA design specifications.
The main body spar (Figure 41) was securely fastened in a vice and
cut to size with a handsaw. The same process was carried out for the
motor mounting spar (Figure 41), two 5mm holes were then drilled to
contain the bearings for the crankshaft and motor shaft as seen in
Figure 44. A single finger joint was then created to secure the main
body spar to the motor mounting spar. An additional part, not included
in the CATIA design, was to strengthen the motor by adding a screw to
securely fix the motor to its mounting spar.
Figure 51 - 90° angle between the main body spar and the
motor mounting spar

! The cuff around the shaft on the motor is the same diameter as the
bearing; the combined length of the motor cuff and the bearing equals
the thickness of the motor mounting spar, therefore the cuff can be
counter-sunk into the hole along with the baring to create a flush
surface and improve the rigidity of the motor.
Figure 52 – Securing the bearings
Figure 53 – Hand held drill Figure 54 – Essential Tools
C-Clamp

! Securing the ESC and receiver was problematic due to the protruding
items on the circuit boards. To overcome this problem a balsa wood
mounting plate (Figure 40) was extended from the main body allowing
the ESC and receiver mounting unit (Figure 33) to be suspended as
close the centre of gravity as possible.
! To attach the servo unit to the main spar it was decided that double
sided tape should be used to avoid damaging the plastic casing of the
servo.
! Like Solignac’s ornithopter discussed in section 2.2.6, battens were
added to the skin of the wing and tail unit to provide more lift and
rigidity. The skin was cut slightly larger than necessary and the excess
was wrapped over the batten and glued into place.
! It was decided that the connecting rods should be threaded the entire
length of the shaft allowing for more options in the total length of the
rod when the fasteners were screwed on.
! In order to achieve the 90° turns in the crankshaft it was heated slightly
to make it more pliable and then swiftly bent around the 90° corner of a
metal block.

9. Discussion
Upon completion and after flying the final model it can be that the power unit
designed has achieved the overall object of designing a power unit for a basic
ornithopter. Arguably the most challenging part of any process where a design
is developed into a working model is the testing and subsequent tweaking of
the design that follows, that was no exception in the case of this project. It is
often acknowledged that testing is a small part of the overall design phase but
consumes the most amount of time. The theoretical calculations and their
implementation into the CATIA model enabled the final design to work at its
optimum capability. The performance of the ornithopter conformed to the
preconceptions of how it would perform with regards to the previous
prototypes, CATIA modelling and theoretical calculations. During the design
phase the friction from the bearings was not taken into account, however the
friction that is encountered does not noticeably affect the performance of the
ornithopter.

10. Conclusion
The power unit designed has conformed to the projects aims and objectives.
A power unit has been successfully designed and assembled to sustain an
ornithopter in flight. The appropriate components for the power unit and the
entire ornithopters were researched and subsequently sourced from model
suppliers. In addition to component research a broader knowledge was
gained about ornithopters, this information proved essential for gaining a
better understanding of flapping wing UAVs. Finally, a design process was
implemented in a chronological order starting with conceptual designs,
prototypes, CAD modelling, testing and fabrication.
11. Further Developments
11.1. Weight Reduction
The sum of all the parts equates to 114 grams. A reduction in weight could be
achieved by reducing the size of the more complex components such as the
motor, ESC and servo. However the smaller the components the more costly
they are to purchase due to the cost of more modern manufacturing
techniques being required to make them being passed on to the consumer.
Additionally microelectronics are harder to source as they are typically
associated with more specialist and exotic models, ornithopters fall into that
category.
11.2. Flapping Symmetry
The current design has an asymmetrical wing flapping motion inherent from
using the crank design. Although not problematic and perfectly acceptable for
this application, a symmetrical motion would provide more stability during
flight. In order to achieve symmetry the gear and connecting rod design would

have to be revisited with the possibility of adding more gears or changing the
gear arrangement to a traverse configuration.
11.3. Autonomous Flight
Devices are available that would make a hand held transmitter redundant.
Through the use of computer software a user would be able to plot the flight
path of the ornithopter on a map, the unit onboard the ornithopter would then
use a global positioning technique to follow the plotted path. This type of
control would be useful for situations where the ornithopter may be out of
sight and a hand held transmitter becomes unviable because the pilot is no
longer in visual sight of the ornithopter. This method however requires an
additional component to be mounted to the ornithopter thus adding weight.

REFERENCES
[1] Wikipedia, Ornithopter, [Online] Available at:
http://en.wikipedia.org/wiki/Ornithopter [Accessed 7th October 2012]
[2] Avian Biology, Ornithology – Lecture Notes 2 – Bird Flight 1, [Online] Available at:
http://people.eku.edu/ritchisong/554notes2.html [Accessed 12th October 2012]
[3] SmarterEveryDay, How Bird Wings Work, [Online] Available at:
http://www.youtube.com/watch?v=4jKokxPRtck [Accessed 20th
October 2012]
[4] Tennekes, H. (1997) The Simple Science of Flight: From Insects to Jumbo Jets,
Massachusetts: The MIT Press, Page 68
[5] Simons, M. (1987) Model Aircraft Aerodynamics, Second Edition, London: Argus
Books Ltd, Page 63-66
[6] Popular Science (2009) Darpa’s First Robotic Ornithopter, Flies Like a
Hummingbird, [Online] Available at: http://www.popsci.com/military-aviation-amp-
space/article/2009-07/darpa-tests-first-robotic-ornithopters [Accessed 2nd December
2012]
[7] Jean-Louis Solignac (2000) Aux Ornithoptères à Élastiques (The Elastic
Ornithopter), [Online] Available at: http://ovirc1.free.fr/solignac-ornitho.htm [Accesses
12th December 2012]
[8] Hobby Technik, Park Hawk 4, [Online] Available at:
http://flappingflight.com/inc/sdetail/777 [Accessed 11th December]
[9] RC Model Reviews, How Do RC Servos Work?, [Online] Available at:
http://www.rcmodelreviews.com/howservoswork.shtml [Accessed 18th December
2012]
[10] Richmond, A. W. (1999) A Practical Engineers Handbook: Servos and Steppers,
Croydon: Kamtech Publishing Ltd, Pages iv & 12

BIBLIOGRAPHY
Tennekes, H. (1997) The Simple Science of Flight: From Insects to Jumbo Jets,
Massachusetts: The MIT Press
Simons, M. (1987) Model Aircraft Aerodynamics, Second Edition, London: Argus
Books Ltd
Richmond, A. W. (1999) A Practical Engineers Handbook: Servos and Steppers,
Croydon: Kamtech Publishing Ltd
Gottlieb, I. (1997) Practical Electric Motor Handbook, Oxford: Reed Elsevier Plc
Lozano, R. (2010) Unmanned Aerial Vehicles: Embedded Control, London: ISTE Ltd
Moczala, H., Draeger, J., Kraus, H., Schock, H., Tillner, S. (1998) Small Electric
Motors, London: IEE
Moulton, R. G. (1974) Modern Aeromodelling, Second Revised Edition, Kent:
Whitstable Litho Ltd

APPENDICIES
Battery Instructions

Ornithopter Drawing

Final Year Project Report

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