Design, simulation and analysis of a semi-dirigible Micro

Design, simulation and analysis of a
semi-dirigible Micro Aerial Vehicle
T.M.W. van Tilburg (520538)
DCT 2009.083
Traineeship report
Supervisor:
J. Page (University of New South Wales)
prof. dr. H. Nijmeijer (TU/e)
Technische Universiteit Eindhoven
Faculty of Mechanical Engineering
Dynamics and Control
Eindhoven, August, 2009
Acknowledgements
I would like to thank Mr John Page for giving me the opportunity to work on this unconventional type of aerospace design as well as for all the insight and discussions about it.
I want to acknowledge Elisse Zarimis and Colin Schwecke from the University of New
South Wales for their work and help with the CATIA design and movies and Stephan Keller
from the Darmstadt University of Technology for his help with X-Plane.
Also I would like to thank the support I received from my parents and friends while I was
studying on the other side of the world.
1
Contents
1 Introduction
4
2 Literature Study
2.1 Blimps . . . . . . . . . .
2.2 Heavier-than-air aircraft
2.3 Hovering MAV . . . . .
2.4 Concluding remarks . .
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3 Preliminary model
3.1 General . . . . . . . . . . . . . . . . . . . . . . . .
3.2 CATIA . . . . . . . . . . . . . . . . . . . . . . . . .
3.3 The preliminary model . . . . . . . . . . . . . . . .
3.4 Preliminary mass estimation . . . . . . . . . . . . .
3.5 Balloon . . . . . . . . . . . . . . . . . . . . . . . .
3.6 Fans . . . . . . . . . . . . . . . . . . . . . . . . . .
3.7 Battery . . . . . . . . . . . . . . . . . . . . . . . .
3.8 Payload . . . . . . . . . . . . . . . . . . . . . . . .
3.9 Frame . . . . . . . . . . . . . . . . . . . . . . . . .
3.10 Sensors . . . . . . . . . . . . . . . . . . . . . . . .
3.10.1 Sensors for measuring position, velocity and
3.10.2 Sensor for visual feedback . . . . . . . . . .
3.11 Summary . . . . . . . . . . . . . . . . . . . . . . .
4 Simulations
4.1 X-Plane . .
4.2 Experiments
4.3 Decisions .
4.4 Yaw . . . .
4.5 Roll . . . .
4.6 Pitch . . . .
4.7 Summary .
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5 Post Simulation Model
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attitude
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6 Conclusions and Recommendations
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6.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
6.2 Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
2
Bibliography
32
A Buoyancy of the balloon
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B GWS Electric Ducted Fan
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C Thunder Power LiPo Battery Pack
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D Post Simulation Catia Model Draftings
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3
Chapter 1
Introduction
Micro Aerial Vehicles or MAVs are small unmanned aerial vehicles, which can be used for
different purposes, like reconnaissance or mapping applications. These aircraft can be remotely
controlled or fly autonomously, depending on the purpose of the vehicle [1].
All aircraft can be subdivided into two main groups, namely heavier-than-air and lighterthan-air aircraft. Lighter-than-air aircraft, also known as dirigibles, use buoyancy to lift
the aircraft. Heavier-than-air aircraft use aerodynamics to produce lift by moving wings or
airfoils through air [2]. A semi-dirigible MAV is an aircraft which uses both aerodynamics and
buoyancy. An advantage dirigibles have over aerodynamic aircraft, is the fact that they are
more stable and require none or little speed to stay airborne. A disadvantage is that dirigibles
are less maneuverable. A semi-dirigible MAV is a small unmanned aerial vehicle which uses
both advantages of stability and maneuverability.
A feasibility study on a semi-dirigible MAV, done by an undergraduate design group at the
University of New South Wales, concluded that it is possible to create such an aircraft [3]. The
literature study also concluded that lighter-than-air aircraft can be used for multiple purposes.
The purpose of this semi-dirigible MAV is to send it in areas where humans would be at risk
or for reconnaissance of unknown territory. Collapsed buildings and toxic gassed areas are
examples of these kind of situations. The feasibility study concluded that a balloon/airship
type of aircraft is a suitable starting design. This vehicle will consist of a balloon filled with a
lighter-than-air gas to provide buoyancy. A payload with instruments and controllers is placed
beneath the balloon. The buoyancy of the balloon and the payload beneath the balloon are
actually creating a pendulum effect. This effect creates a stabilizing mechanism. At least two
fans are required to fly and steer the vehicle. Although the feasibility study concluded that this
type of vehicle is suitable, the vehicle dimensions chosen in this study are not representative
and the choice of several components are not realistic or optimal. This report will discuss an
advanced study based on the feasibility study.
The goal for this project is to redesign, simulate and analyze a semi-dirigible micro aerial
vehicle. The specifications of this MAV are
• The vehicle uses buoyancy and aerodynamic forces to create lift.
• The maximum dimensions of the vehicle should not exceed 350x350x350mm.
• The vehicle design is based on a balloon and carries a payload, which could consist of
batteries, a camera, speed controllers and other.
4
• The vehicle is able to fly forward at a velocity of at least 1 m/s (hovering not required).
• The vehicle is able to turn, ascend and descend.
• The vehicle should be mechanically stabilized as much as possible.
• The vehicle is able to fly inside "unsuitable" buildings/areas.
• The model is tested in flight simulations.
This report is organised as follows. The second chapter contains a literature study which
describes several different approaches of MAV applications and setups achieved by others.
Then a starting model is described in the third chapter, where estimations are made and
the required parts are explained. Using this model, experiments in a simulation program
are executed in order to optimize the mechanical stability. The experimental setup and the
stability and damping of the three rotations of the vehicle are explained in the fourth chapter.
From the experiments, the model is updated which is described in chapter five. Conclusions
and recommendations are described in the last chapter.
5
Chapter 2
Literature Study
This literature study will discuss different types of Unmanned Aerial Vehicles, with each its
own advantages and disadvantages. Both lighter-than-air and heavier-than-air aircraft are
discussed, with different types of propulsion and control strategy.
2.1
Blimps
The Atalanta Wingman Project aims for the development of small and lightweight Unmanned
Aerial Vehicles (UAVs). J. v.d. Loo demonstrates a platform for formation flight with two or
more autonomous mechatronic UAVs for the Atalanta Project [6]. His master thesis demonstrates that blimps are preferred, because they have major advantages over other types of
flying vehicles, such as robustness, safety, stability and energy usage. Because blimps do
not require any power to stay airborne, since they are buoyant, the dynamical model can be
simplified. Since the center of gravity is below the center of buoyancy in the same plane of
symmetry, roll and pitch are mechanically stabilized and thus need no active stabilization.
Therefore the dynamical model is reduced to four degrees of freedom, and can be fully actuated with four propellers. The thesis then proposes a controller based on computed torque
method to deal with nonlinearities with its feedback linearization control strategy. A tracking
controller can now be realized to follow a desired trajectory.
Figure 2.1: The blimp used by J. v.d. Loo, [6]
6
2.2
Heavier-than-air aircraft
An entirely different platform is used by S. Winkler et al. [8]. Winkler et al. presents a twin
fixed wing MAV which is stabilized by aerodynamics and has no buoyancy. The purpose of this
project is the development of a MAV for observing airports and other critical facilities. The
MAV should be very agile and fully autonomous. An optimal configuration, with minimized
weight, is realized and it is capable of take-off and landing at low velocities and flight with
low drag. The prototype has a wing span of 38cm, a weight of about 400 grams and operates
at a velocity of 65km/h for more than 30 mins. Navigation is achieved on-board by a GPS
and INS (inertial navigation).
Figure 2.2: Twin fixed wing MAV by Winkler et al. [8]
Another heavier-than-air aircraft model is presented by L. Marconi et al. [5]. This paper
presents preliminary results of a small size reduced-complexity ducted MAV. The main feature
is to combine the characteristics of a VTOL aircraft (vertical take-off and landing) and fixed
wing air vehicle. The paper describes a possible architecture and the associated model with
its zero dynamics with respect to the natural outputs of the system. The goal for the future
is to design a nonlinear controller for such a system. The architecture is divided into two
subsystems, the fixed pitch rotor engined system and the four active flaps or vanes below the
ducted fan to compensate for engine torque and to generate the forces and torques necessary
to control the system. After presenting preliminary results, conclusions show that a simple
linear quadratic frequency-shaped regulator is sufficient to control the MAV.
2.3
Hovering MAV
J.M. Pflimlin et al. propose a hovering flight vehicle suitable for surveillance applications
despite of wind perturbations [7]. The MAV is also based on a VTOL with ducted fan.
Automatic control design enables easy operation by an inexperienced user. The vehicle is
uses adaptive control to stabilize the MAV, although robustness is still marginal with respect
to fluctuations in the aerodynamic efforts and sensor noise. The MAV uses two counter
rotating propellers in-line in single duct. The yaw rate is decoupled from the rest of the
system dynamics to overcome the problem of the gyroscopic effect. The controller is based
7
Figure 2.3: Ducted MAV by L. Marconi et al. [5]
on backstepping techniques allowing stabilization of the vehicle’s position. Due to the low
Reynolds number and atypical form of the body, aerodynamic effects are strongly nonlinear.
Figure 2.4: Ducted MAV by Pflimlin et al. [7]
B. Motazed et al. present a concept for a 15-cm hovering MAV [4]. The concept is based
on a two fixed pitch counter-rotating rotors, driven by a single electric motor. Net torque en
gyroscopic effects on the MAV body are minimized by the counter-rotation. Stabilization of
the MAV in roll, pitch and site to side translation are achieved by via control vanes attached to
the body. The vanes are actuated with micro brushless motors. Attitude damping is achieved
by a separate set of fixe vanes. A pendulous based stabilization mechanism was investigated
for minimizing power consumptions and weight. Concluded was that the concept would not
work due to the fact that the pendulum acts on the local acceleration vector with bandwidth
of the natural frequency of the pendulum. These conclusions were verified by experiments.
8
Active control of the vanes is the alternate method.
2.4
Concluding remarks
The literature study shows that different types of Micro Aerial Vehicles are possible for indoor
and outdoor flight. Lightweight sensors, structures and fans make a flight time up to 30
minutes possible. Although no semi-dirigibles are discussed, dirigibles prove it is possible to
fly an lighter-than-air aircraft which is mechanically stabilized in pitch and roll. This project
aims to use properties of both dirigibles and heavier-than-air aircraft, namely maneuverability
and stability, which should be balanced in a semi-dirigible. A semi-dirigible could prove to fill
the gap between the lighter- and heavier-than-air aircraft.
9
Chapter 3
Preliminary model
3.1
General
A semi-dirigible type aircraft is used as starting model for this project. This means that the
aircraft uses both aerodynamics and buoyancy to lift the aircraft. The feasibility study showed
that merely using buoyancy is not sufficient to lift the MAV, because the balloon would be
become too large. This is the reason why this MAV will use both buoyancy and aerodynamics.
Another reason for using this type of aircraft is mainly for stability reasons. An aircraft which
only uses aerodynamics to lift is highly unstable, therefor a combination of buoyancy and
aerodynamics is preferred. When the semi-dirigible is presented in a static case, the aircraft
will act like a pendulum, with the payload as its majority of mass and the balloon filled with
lighter-than-air gas will have a lift component. Assuming in this scenario that the balloon
will stay at the same height, the payload will swing around a certain point, which is depicted
in figure 3.1. Because the gravity acting on the payload will always create a vertical force
towards the center of the earth, a restoring moment is created. However, a simple pendulum
does not have any damping.
Figure 3.1: Pendulum effect with a dirigible
With
B :
w :
Buoyancy [g]
Weight of MAV [g]
10
Because this is a new and relatively unconventional type of aircraft, simulation will play an
important role in this project. Because of the complexity of the dynamics and ever changing
circumstances, it is preferred that the MAV is optimized for mechanically stability. This
involves optimizing the layout and the design of the MAV. For conventional type of aircrafts,
much knowledge and data is available to design a reasonably stabilized vehicle. For this new
semi-dirigible aircraft, a lot of variables and parameters are uncertain and therefore simulation
can assist in improving the stability of the MAV. It is unwise to specify and research all parts
before the simulation experiments are executed; for now only the required parts (like fans and
batteries) are researched in order to get a concept model which can be analyzed in simulations.
Essential parts for this vehicle are:
• The balloon
• Two ducted fans including motors to create the necessary lift
• One fan which provides the thrust
• One battery pack for supplying power
• A payload which could consist of several instruments, like a camera and speed controllers
• A frame to mount the fans, payload, battery pack and others
3.2
CATIA
All the design work on the model has been done in CATIA. This software bundle of CAD/CAM/CAE
is very extensive and used by aerospace industry [10]. Before the design process, an introduction course was executed in order to get familiar with the program. When sufficient experience
was gained in the program, research on the preliminary model was started and a design was
constructed in CATIA.
3.3
The preliminary model
A model in CATIA was created, as depicted in figure 3.2. For now, not much thought to
manufacturing has been included in this model. Also the forward thrust fan is not included,
because the location has yet to be located. The amount of drag is very small when the MAV
is flying forward, which means that even a small propeller would be capable of thrusting this
aircraft. As can be seen in the figure, the design consists of different parts. All parts in the
current design are separately explained in the next sections. As can be seen in the figure, the
MAV consists mainly of a balloon with ducted fans, a payload which could include a camera,
batteries, speed controllers, etc. The coupling between the payload (which will act as the
mass in the pendulum) is achieved with a frame. The frame also serves a second purpose; to
increase stability by adding some damping against rotations of the aircraft.
3.4
Preliminary mass estimation
Before it is possible to make any choices about fans, batteries and a frame, a preliminary
estimation of the total mass of the vehicle was made. The feasibility study and some early
11
Figure 3.2: Pre-simulation model
research indicate that it should be possible to design a vehicle with a total mass of approximately 350 gram. This estimation is visible in table 3.1. Trying to achieve a lower total
mass will eventually increase the percentage of buoyancy, which results in less powerful fans
requirements. Although the mass might be reduced in improved designs, starting with an
estimated mass of 350g seems a defendable amount for the fans to lift the MAV.
Mass estimation
2 Lift fans 150g
Thrust fan
15g
Battery
80g
Payload
50g
Frame
50g
Balloon
5g
Total
350g
Table 3.1: Mass estimation for pre-simulation model
3.5
Balloon
One of the most important parts of the vehicle, is the balloon. One reason why the balloon is
an important part, is that the balloon can add buoyancy to the aircraft. Buoyancy increases
stability. It is a force which always points upwards, which also creates a restoring moment
against excitations, see also figure 3.1. The lift fans do not add any restoring moment, due
to the fact that they always create lift in the vertical direction of the vehicle. This concludes
12
that adding buoyancy increases stability of the MAV. Designing this aircraft as a semi-dirigible
with an as high as possible amount of buoyancy could provide reasonable results.
The second reason is that an inflated balloon is reasonably resistant against collisions. In
comparison to for example a helicopter, the balloon would simply bounce against obstacles,
where the helicopter could destroy its propeller or would at least crash. This advantage makes
the balloon type ideally for indoor flight, where it can be flown without risking potentially
crashing.
The balloon is donut shaped as shown in 3.3, with an outer diameter of 300mm and height
of 120mm. The reason for a donut shaped balloon is the fact this aircraft requires fans in
order to lift it. It is, after all, a heavier-than-air aircraft. Placing the fans in the center of the
balloon makes it a symmetrical design, which greatly improves the simplicity. A zeppelin type
of aircraft, with a sigar-like shape, is not chosen due to the fact it uses rigid rings inside the
balloon and therefor has a skeleton. This property increases the overall weight of the aircraft
which is preferred as low as possible. The balloon is filled with helium gas, which is lighter
than air. The buoyancy of the balloon is approximately 7.1g as calculated in appendix A.
The only material which provides more buoyancy than helium is hydrogen. Since hydrogen
is highly flammable, the inert gas helium is preferable. Also hydrogen merely produces 7.9%
more buoyancy than helium as shown in appendix A, which is small in comparison to the total
weight of the vehicle.
Figure 3.3: Balloon
The balloon itself can be made of thin sheets of biaxially orientated PET or polyamide,
also known as BoPET or BoPA or trade names such as Mylar [11]. The sheets need to be
metallized in order to achieve the low permeability to helium, against leaking. Because the
thickness of these sheets is smaller than 1mm, the balloon adds almost negligible weight (5
grams according to CATIA) to the vehicle.
3.6
Fans
Since the buoyancy of the helium in the balloon is only a small percentage of the total lift, one
or more fans are required to lift the aircraft. Using only one fan to lift the MAV, the aircraft
will start rotating around its z-axis, because the fan produces a torque. Also simulations with
a single-fanned MAV concluded that a gyroscopic effect occurs [3]. This effect is countered by
13
using two fans. Placing the fans vertically in line, pointing in opposite directions, like shown
in the section view in figure 3.4. Because the thrust of the fans both require to point upwards,
the impeller of the bottom fan is mirrored upside down. With this two-fan setup it is possible
to yaw (rotate around z-axis) the vehicle simply by changing the speed of one of the fans. The
second fan requires to increase or decrease its speed to be able to adjust the total amount of
thrust required to stay at the same height.
Figure 3.4: Top and section view of two lift fans mounted in the balloon
An unknown variable is the vertical distance between the ducted fans. This distance could
influence the amount of lift they produce. Also the efficiency drops because the fans are in
line, also here, experiments could conclude how much the efficiency is influenced.
The use of ducted fans seems a logical choice, since they will be mounted inside the balloon.
Exposing the balloon to the tip of a rotating fan could rupture the balloon. The duct provides
protection of the balloon.
Increasing the diameter of the fan would increase the amount of lift, but would also decrease
the volume of the balloon. A smaller diameter fan would require more speed and thus more
power to get the same amount of lift than a large diameter fan. More power requires more
energy from batteries, which increases total weight. This creates a dilemma in which direction
to find an suitable solution. This is the reason why the efficiency of the fans (and battery) is
important. For this aircraft at least 350g of lift is required. Having the diameter of the fans
to be as small as possible with the efficiency as high as possible is preferable.
Efficiency of a fan is given by the amount of thrust a fan produces divided by the amount
of power it requires, as shown in equation 3.1. Because the ducted fans are used to lift the
14
vehicle, the thrust parameter can be seen as lift.
ηf an =
With:
ηf an
T
P
:
:
:
T
P
(3.1)
Fan efficiency [g/W]
Thrust [g]
Power [W]
Research showed that a 55 mm fan of GWS [12] with a reasonably efficiency of ηf an =
2.5 − 4 g/W is able to lift half of the 350g MAV. This would conclude that around 115 − 140
W at 11.1 V would be required to generate enough lift for the aircraft to stay airborne. The
ducted fan is designed in CATIA, visible in figure 3.5 and its specifications are shown in
appendix B. For further calculations, this specific ducted fan is chosen for the rest of the
calculations. The GWS EDF-55 fan consists of three parts; a motor, impeller and a duct
where all parts are assembled in.
Figure 3.5: GWS EDF-55 Electric Ducted Fan
3.7
Battery
A battery pack is necessary to power the fans and some electrical instruments. The type
Lithium-ion Polymer (short LiPo) of battery pack is chosen for several reasons. This type
is relatively new and is being used in remote controlled aircraft, such as model planes and
helicopters. The battery is capable of delivering high currents which are required for the
15
motors of the fans. Also the efficiency of these battery packs has increased in the last few
years, which makes this an excellent choice for the lightweight aircraft. Also in comparison to
other types of battery packs, the energy/weight efficiency is a lot better [3].
The energy/weight efficiency of a battery pack is given by formula 3.2.
ηbatt =
With
ηbatt
E
wbatt
:
:
:
E
wbatt
(3.2)
Efficiency of the battery [Wh/g]
Energy [Wh]
Weight of the battery [g]
The efficiency of the LiPo battery packs vary between 0.15 and 0.20 Wh/g, depending on
size and capacity. Clearly the efficiency influences the amount of flight time of the vehicle. The
flight time can be estimated after choosing a battery pack with a specific capacity. Obviously,
a larger/heavier battery pack will supply more energy. Increasing the amount of weight,
increases the amount of power required by the lift fans, which will drain the battery pack
faster. This causes a trade-off between the endurance (flight time) and total weight of the
MAV. For now, a minimum flight time of 5 minutes is sufficient for this design. An optimization
after realization of the design will be examined.
Also the voltage of the battery should match the operation voltage of the fans. With these
parameters a battery pack is chosen. The Thunder Power RC TP-1320-3SPL provides 180g of
thrust at 11.1V, and provides a flight time of approximately 7 minutes. Further specifications
and calculations are given in appendix C.
The battery pack will be placed in the payload, so that as much weight as possible is as
low as possible, to increase the pendulum effect.
3.8
Payload
The payload will consists of several parts. For the lift fans speed controllers are necessary
for changing the thrust of the lift fans. Also a camera is required, to make snapshots of the
environment, in order to look for objects or humans. A small comment about this camera is,
is that this camera is not used for the navigation of the vehicle. Using still shots, the camera
does not require a continuous light source. Using flashes of light instead of a continuous
light source will decrease power consumption. Using this method, endurance is increased by
reducing power. Also an acoustic height sensor is required and positioned in the payload.
The most important fact is that the payload will act as the mass in the pendulum. Lowering
the payload in the design, increases the amount of restoring moment and is therefore placed
as low as possible in the design. This way, if for unknown reasons more parts are required
than originally thought of, placing these in the payload will increase stability, although more
lift is required of the fans.
For now, the payload is represented in the model as a box below the balloon.
3.9
Frame
After the choice of battery and fans is made, a simple frame can be designed to assemble all
parts together. For this pre-simulation model, manufacturability is not taken into account.
16
This frame consisted mainly of two vertical fins and could be glued against the lower ducted
fan. These fins add drag when flying forward and add damping against pitching, rolling and
yawing. At the lower end of the plates, rods could be used to attach the payload. With these
rods it should be possible to position the payload in x-direction, so that the center of gravity
is below the lift fans. Placing the center of gravity below the lift fans ensures that the lift fans
do not create any pitching or rolling moment. The same is true for forward thrust fan, which
will be mounted at the rear of the frame. When the thrust fan is exactly behind the center of
drag, the fan will not cause any pitching moment when thrust is increased or decreased. The
location of the center of drag when flying forward is determined by the frontal surface areas
and the shape of the surfaces. Because the balloon has a large frontal surface area, the center
of drag is relatively high (inside the balloon). In order to reduce this height, some surface area
should be placed beneath the balloon (near the payload). Increasing the height and surface
area of the fins will decrease the height of the center of drag. Now, the thrust fan can be
positioned behind the center of drag below the balloon. Experiments will have to show where
this center exactly is, for that reason the thrust fan’s location is not yet fixed. The thrust
fan will be mounted inside the block at the rear of the frame and is currently not designed in
CATIA, because many variables have to be decided from simulations.
Figure 3.6: Frame
3.10
Sensors
Sensors can serve multiple functions on an aircraft such as this MAV. First of all, sensors are
required for determining accurate position, velocity and attitude information of the aircraft.
Not only is this data essential for evaluating real life flight experiments, it is also necessary
feedback for the control system. The last argument poses that a certain amount of sensors with
high sensitivity for all three rotations and translations is required. Second, visual feedback
for the remote operator is a requirement, since it makes it possible for the operator to search
17
for people in rescue operations and also has the ability to scout hazardous regions without
actually being there himself.
3.10.1
Sensors for measuring position, velocity and attitude
A lot of different sensor systems are widespread available, such as static pressure sensors for
measuring altitude, dynamic pressure sensors for measuring airspeed, micro-electromechanical
system based accelerometers for measuring angular and linear accelerations. Also global positioning system based sensors, ultrasonic and acoustic sensors can be used for positioning.
Since this project is still in its early stages, an in-depth review of all sensor systems is
not performed. Though there are some remarks which can be made at this stage of the
project. Since the MAV will be controlled autonomously, accurate knowledge about its position, velocity and attitude need to be available and updated frequently. This means that high
bandwidth sensors are required. Since the MAV will be carrying all the sensors, they should
be lightweight. As shown before, reducing the weight of the payload is essential. The sensors
should be small, which also means the amount of power needed is small.
For now, there is little to say how much bandwidth is required. To get some further insight
in a sensor system, a setup from another MAV project is discussed.
S. Winkler et al. use a small Inertial Measurement Unit they developed which is based
on micro-electromechanical system. This IMU uses three angular rate sensors with a range of
±300 deg/s and two accelerometers with both two axes. This setup is chosen because of the
high dynamic behaviour of small aircraft, where high-frequency attitude information is most
important. Also a static and dynamic pressure sensor is used for respectively altitude and air
speed measurements. For all sensors a nominal rate of 100 Hz is obtained. The total weight
of the IMU has a weight less than 15 grams, making it very suitable for MAVs [8].
3.10.2
Sensor for visual feedback
Visual feedback can also be used to control the MAV, but for now we assume it is only used as
feedback for the remote operator. A small CCD or CMOS based camera with remote frequency
transmitter can be used to send captured frames to a RF receiver outside the hazardous zone.
For now, an CMOS camera with low power consumption, like the CSS-59C [17], seems a
suitable solution. This camera has a power consumption of 65mA at 12V, 330 lines resolution
and 300 meters range. It can easily be mounted in the payload zone and connected to the
batterypack which is also in the payload zone.
3.11
Summary
This chapter describes all the required parts for the semi-dirigible MAV based on a mass
estimation. Lift and thrust fans, battery, payload, frame and balloon are now chosen and
a preliminary model is made. The weight of the aircraft is an essential part of the design,
since this parameter will greatly influence the flight time and amount of lift required. Also
the balloon, filled with helium, adds buoyancy, but this provides only a percentage of the lift
required. In the center of balloon, two counter rotating ducted fans provide the rest of the
lifting forces. Below the balloon, a frame is mounted, which will also carry the payload, with
the battery, visual feedback and positioning sensors.
18
Figure 3.7: RF-Links CSS-59C CMOS camera with RF transmitter
19
Chapter 4
Simulations
After the preliminary model has been created in CATIA, the design is implemented into a
simulation program. For this project, the program X-Plane is used for simulations. The
purpose of the simulations is to analyze the design and improve where necessary. Then the
modifications can be implemented in CATIA. The last part is the manufacturing of the MAV,
which involves the design in CATIA.
The flight experiments conducted mainly consists of the MAV ascending, descending, flying
forward and making turns. Different speeds are used to test the behavior of the aircraft. In
the beginning, qualitative data and results were more important to stabilize flight behavior.
4.1
X-Plane
X-plane is a commercial available flight simulator. This simulator uses the so called ’blade
element theory’, which means that the aircraft is broken down to small elements on which
the forces are being determined. These forces are rewritten to accelerations, which can be
integrated into velocities and position. Using this method, even the most sophisticated aircraft
can be simulated [13]. X-plane provides extensive data during the simulation, which can be
saved in logfiles and/or shown on display realtime. Figure 4.1(a) and 4.1(b) shows a flight
simulation of a Boeing 747-400 in X-Plane. As visible in the top left corner, several instrument
and experimental data can be analyzed during the flight.
(a) Boeing 747-400 in X-Plane, cockpit view
(b) Boeing 747-400 in X-Plane, external view
Figure 4.1: X-Plane
20
Plane-Maker is the program which X-plane uses to build the aircraft. After creating the
aircraft in Plane-Maker, it can be loaded in X-plane to start the simulation. A disadvantage
is that models created in CAD programs, such as CATIA, can not be loaded into X-Plane
or imported in Plane-Maker. It is necessary to rebuild the aircraft in Plane-Maker. Because
X-plane is not meant for CAD design, Plane-Maker uses an entirely different methodology for
creating aircrafts. This is the main reason two separate design programs are used, CATIA for
the CAD modeling and Plane-Maker for the simulations. A screenshot of a Boeing 747-400 in
Plane-Maker is visible in figure 4.2.
Figure 4.2: Boeing 747-400 in Plane-Maker
Because there was no introductory course for X-plane or Plane-Maker, the way of learning
to work with both programs is by using existing airplanes and changing parameters in both
X-plane and Plane-Maker. When sufficient insight is gained in both programs, the MAV is
created in Plane-Maker and flight simulations are executed in X-plane.
Plane-Maker seems a bit complicated in the beginning because of the abundance of options,
buttons and knobs available. It took quite some time before all necessary options and methods
were explored. For this particular model, the balloon shape has to be modeled. Fortunately
Plane-Maker has several methods for designing freeform objects and one of these methods is
used to create the shape of the balloon. Adding fans, buoyancy and several other adjustments
can easily be achieved with the program.
4.2
Experiments
The model building concluded that the dimensions of the MAV are pushing the lower boundaries of X-plane. In order to maintain the proportions of each part of the MAV, the design
had to be scaled up 10 times. The main purpose of X-Plane was for testing large airplanes and
vehicles. Scaling up the design of the MAV for testing purposes means that the reality of the
simulation changes, but still observations and decisions can be made on a qualitative basis.
To test a 1:1 scale model, real life wind tunnel experiments can be done when a prototype has
been made.
The first simulations with the scaled model showed that the MAV is very unstable. It is
hard to conclude which of the motions (pitch, roll or yaw) is unstable, because the instability
21
of one rotation could cause instability in another rotation.
Continuing experiments showed that merely using the pendulum effect together with the
buoyancy of the helium is not sufficient for the MAV to stabilize. In order to stabilize this
model, more damping is required. The frame constructed provided little, so experimenting
with more conventional methods could provide more damping to stabilize the MAV.
Figure 4.3: The MAV in X-Plane
Changes included using a horizontal stabilizer (wing) and a vertical stabilizer. Since conventional aircraft use both, more knowledge can be gained by experimenting with both stabilizers. With these wings a nicely stable design is constructed as shown in figure 4.3. The
vertical and horizontal stabilizer proved very useful for the model in the simulations and for
the creating of the necessary damping in all three rotations. The analysis and tuning of the
design made in X-plane and Plane-Maker will be elaborated in the next sections.
4.3
Decisions
Before starting the analysis of the simulations, some decisions concerning annotations are
made. One of the important properties of the the aircraft is the center of gravity. The center
of gravity is the point where the gravity only causes a force on the body and no moments.
Likewise, the center of drag or center of thrust is a fictional point on the body where if a
thrusting force is applied, no moments occur. In the following explanations, neither of these
simplifications are used to explain the figures. A simple center of rotation is merely used to
show a rotation of the body. These rotations resemble unwanted instabilities of the MAV
which could occur during flight or hovering. Further more, this fictional center does not
correspond to the real center of rotation, simply because of the fact that such a fixed point
on the flying MAV does not exist. It is depending on the horizontal and vertical speed, angles
and dynamics of the aircraft, and will vary during flight.
For every rotation, two different properties should be distinguished: the effect of damping
and forces which create stability. Both can be present when the MAV is flying or when it is
22
hovering (or simply swinging around the center of rotation). Airfoils and wings can create
drag which will increase damping of the MAV. The stability and damping of all three rotations
will be explained in both (forward) flight and the hovering case.
4.4
Yaw
Yaw, the rotation around the vertical axis, is the rotation used to turn the MAV. For stability
and damping of yaw, the vertical stabilizer proved to be very useful. Yaw stability is achieved
with the stabilizer due the lift it creates when the airflow is under a small angle. A bottom
view sketch of the MAV with a vertical stabilizer is shown in 4.4(a). The lift (L) creates a
counteracting moment around the center of rotation which yaws the aircraft into the direction
of the flow. When the forward thruster is placed in line with the vertical stabilizer, no lift is
created by to forward thrust.
(a) Lift due to vertical stabilizer
(b) Damping of yaw
Figure 4.4: Bottom view of the MAV
With
T :
F :
L :
Torque
Force (drag)
Lift
For the MAV, the vertical stabilizer also causes air resistance (drag) when rotation around
the z-axis is enforced by a moment. This is a damping effect against the rotation. This is
shown in figure 4.4(b). Clearly, the drag (F) counters when a torque (T) is applied.
Increasing the distance of the vertical stabilizer from the center of rotation increases both
stabilizing lift and damping, due to a larger moment-arm. Also increasing the chord length
of the wing increases damping, because the drag force is linear with the wing surface area.
Experiments with these parameters confirmed both properties. With these properties known,
separate fine tuning of yaw stability and damping is possible.
In the case of this MAV, the lift fans are used to create a torque around the vertical
axis. This requires the lift fans to rotate at different speeds which creates a torque difference.
Increasing the torque difference and decreasing the amount of damping will improve the ma23
neuverability in yaw, but will decrease the stability, and vice versa. This is a trade-off between
stability and maneuverability which often exists in aerospace design.
4.5
Roll
Roll is the rotation around the x-axis. In this case, the x-axis defines the forward direction,
which makes roll swinging sideways. Roll appeared a more sophisticated instability to control.
Figure 4.5 shows a rear view of the MAV. Note that the horizontal stabilizer in this figure is
depicted with no angle of attack, in order to simplify the figure. Both stabilizers only add
damping caused by drag forces. The figure shows that if a torque (T) is applied, the horizontal
stabilizer produces forces (H1 and H2) with create a moment counteracting the effect of the
torque. Also the vertical stabilizer counteracts the torque with the force (V) it produces.
Damping can be increased by changing several parameters:
• Increase the surface area of one or both stabilizers.
• Increase the moment-arm of a stabilizer by increasing its length.
• Decrease the distance of the horizontal stabilizer with respect to the center of rotation.
• Decrease the angle of attack of the horizontal stabilizer. This results in more surface
area and thus more drag.
The last change suggests that using no angle of attack would be optimal for roll, however
for damping the pitching motion, this is not desired. This will be further investigated in the
next section. Note that no aerodynamic forces (like lift) are created when rolling, in contrast
to conventional aircraft.
The actual force which creates stability in roll is purely caused by the pendulum effect,
which is described in section 3.1.
4.6
Pitch
Pitch, the rotation around the y-axis is the most critical stability issue. The thrust fan is
constantly applying (varying) force on the MAV, which could cause moments if it is not applied
to the center of drag. Because the height of the center of drag varies while flying and/or while
swinging, the thrust fan will automatically cause instability. Also when no forward thrust is
applied, constant varying circumstances will create instability. These variables make pitching
stability a complex rotation to stabilize.
The most important part which could influence stability is the horizontal stabilizer. The
horizontal stabilizer should always be placed behind the location(s) around which the MAV
could start rotating (which from now on is only represented by a center of rotation). The
horizontal stabilizer is a wing which adds lift in flight. Like with the vertical stabilizer in yaw,
when the stabilizer receives flow of air under an angle, it will create lift which will induce
a restoring moment. Hence, this lift will always try to get the stabilizer in line with the
airflow.This is possible for angles up to approximately 15 degrees, after which the lift will
drop significantly (stall angle). This is schematically depicted in figure 4.6(a). As shown, the
lift (H) produced by the horizontal stabilizer will create a restoring moment with respect to the
24
Figure 4.5: Damping of roll
With
T
H1
H2
V
:
:
:
:
Torque
Force (horizontal stabilizer)(drag)
Force (horizontal stabilizer)(drag)
Force (vertical stabilizer) (drag)
center of rotation. As depicted in grey, the vertical placement of the stabilizer does not make
a difference for the amount of lift created in forward flight (without an angle of attack). Also
the pendulum effect creates a moment because the weight (w) has a perpendicular component
with respect to center of rotation.
The damping of the horizontal stabilizer is depicted in figure 4.6(b). Again, the drag (D)
causes the damping when a torque (T) is applied. As shown in grey, if the stabilizer is beneath
the balloon it adds the same amount of damping.
Placing the horizontal stabilizer behind the balloon is not optimal solution, because a compact structure is preferable. Also more drag in forward flight is required, since the balloon adds
a lot of drag above the possible center(s) of rotation. Placing the horizontal stabilizer straight
under the balloon as depicted grey in figure 4.6(b), creates the same amount of damping. This
reduces the height of the center of drag, which is now (depending on the size) located beneath
the balloon. Having this lower center of drag, the thrust fan can now be positioned somewhere
beneath the balloon as well, which is an advantage for structural design. A disadvantage of
placing the stabilizer here, is that it does not act as a horizontal stabilizer any more (no lift is
created when flying), it merely acts as a damping plate. A compromise is made between both
situations, by placing the stabilizer just beneath the balloon, see figure 4.7(a). Now reducing
the angle of attack of the wing with respect to the airflow, it will add not only damping, but
also lift, which increases stability when flying, visible in figure 4.7(b). Note that if the angle
of attack is increased instead of decreased, the same amount of damping will occur, but the
amount of lift when flying forward will decrease. A trade-off exists between damping and lift,
which is all depending on the forward velocity. If more damping is required, which is usually
required at low speeds, the angle of attack can be increased. If more stabilizing lift is required
25
(a) Lift produced by horizontal stabilizer
(b) Damping produced by horizontal stabilizer
Figure 4.6: Effects of the horizontal stabilizer
With
L :
T :
H :
D :
w :
Thrust (fans)
Torque
Lift (horizontal stabilizer)
Drag (damping)
Weight of MAV
and damping should be reduced (at high speeds), the angle of attack should be decreased.
Simulations can be used to find a suitable compromise between damping and aerodynamic lift
by varying the angle of attack.
4.7
Summary
The experiments which are explained in this chapter are executed on 10 times scaled up model,
due to the fact that the program used, X-Plane, is unable to work with the small dimensions
of the model. The analysis of the simulations shows that the framework achieves inadequate
damping and stability in pitch, roll and yaw movements. The simulations conclude that a
horizontal and vertical stabilizer are required. These two parts are now the most important
parts of the frame beneath the balloon.
26
(a) Damping produced by horizontal stabi- (b) Lift and damping produced by horizontal
lizer
stabilizer
Figure 4.7: Effects of the horizontal stabilizer
27
Chapter 5
Post Simulation Model
Using the data gathered from the simulations in X-plane, the drastically changed model is
designed in CATIA. The new design is visible figure 5.1.
Figure 5.1: Post simulation CATIA model
28
The new frame consists of the horizontal and vertical stabilizer. Again, this frame serves
two purposes. First, the frame is the stabilizing factor for the MAV. Second, the frame has
to connect the balloon and its fans to the payload. The distance between the payload and
the balloon influences the amount of pendulum effect. Also for the height of the frame, which
influences the stability and damping, the placement of the payload is influencing the stability
of the MAV.
To construct this MAV the fans have to be mounted to the balloon. By gluing both sheets
of the balloon on a flanged tube, shown in figure 5.2(a), the balloon can be sealed airtight.
This tube is used to mount the two ducted lift fans in.
(a) Flanged tube
(b) Horizontal and vertical Stabilizer
Figure 5.2: CATIA parts
The horizontal and vertical stabilizer can be made out of lightweight plastic or carbon
fibre and should be fixed together as shown in figure 5.2(b). The vertical stabilizer is under
an angle of 18◦ with the vertical axis and the horizontal stabilizer has an angle of attack with
respect to the x-direction of 40◦ . The lengths, angles and chord lengths are based on the
results from the simulations in X-plane and are shown in figure D.2 in appendix D.
There are several methods of attaching the stabilizers to the flanged tube. These methods
should be further investigated. For this model, a large ring with two spurs is glued against the
lower flange from the tube. The stabilizers can be clamped in or glued between the two spurs.
The payload (which is represented by the box) should now be positioned. Two horizontal
position of the payload is fixed by the two rods visualized in figure 5.1. One vertical rod
from the ring under the balloon positions the height. Using these three rods, it is possible to
position the payload. This setup is routing the vertical force through the vertical rod. Also
the positioning of the payload can be fine tuned in x, y and z-axis with these three rods. This
is important, because the real prototype should have the center of gravity beneath the lift
fans.
For the thrust, simulation showed that small amounts of drag are produced by the MAV.
29
Therefore, a very light fan can be used, preferably with a large propeller. For now, the thrust
fan chosen is the GWS electric direct drive propeller and is mounted at the rear of the spurs
[16]. The GWS EDP-05 is capable of delivering 15g of thrust, which is able to overcome the
amount of drag at a forward velocity of 1 m/s. A representation of the forward thrust fan is
shown in 5.1 as well.
For this design, the important parts are the balloon, the stabilizers and the represented
payload. The lift fans, forward fans and other structures can be chosen arbitrarily, as long as
the specifications allow it. CATIA estimated the total weight for this model approximately
350 grams.
30
Chapter 6
Conclusions and Recommendations
6.1
Conclusions
After creating a model of a semi-dirigible MAV in CATIA, flight experiments in X-Plane were
executed to test the model. Simulations showed that the model which was expected to fly
reasonably stable was actually very unstable and required more buoyancy and damping to
stabilize it. In Plane-Maker an entire new frame consisting of stabilizers was designed based
on flight simulations. The new proposed design is still based on a pendulum. The weight and
buoyancy of the MAV create the stabilizing forces. A vertical stabilizer is used to increase
stability of yaw and adds damping to yaw and roll. A horizontal stabilizer is used to increase
stability of pitch and adds damping to roll and pitch. By placing the horizontal stabilizer
under an angle of attack also some aerodynamic lift is created which increases stability during
flight. The result from the simulations are a stable flying and controllable semi-dirigible MAV.
After simulations, a new design is created in CATIA, with specifications resulting from the
flight simulations.
For this unconventional type of aircraft, this project showed that using the conventional
method of CAD modeling then simulating was not very useful. Since the aerodynamic and
flight stability mathematics can become very complex, this design is optimized for stability
without the usage of controllers. Simulations proved to be a valuable tool for designing an
unconventional type of aircraft such as this MAV. Within a short amount of time, changes in
the design can be simulated and analyzed, which can result in major improvements.
It became clear that the simulation program used, X-Plane, is being pushed to its boundaries due to the fact that the MAV is so small. These lower bounds resulted in creating a
scaled model of the semi-dirigible in Plane-Maker. This could cause inaccurate results, but
still qualitative changes in the model can be analyzed.
6.2
Recommendations
It is recommended that a prototype should be built and analyzed. Wind tunnel experiments
can provide more realistic and detailed results and can also verify if X-Plane is a realistic
simulation program for this type of aircraft. Also a control strategy and controllers for the
fans should be made. This report is aimed at achieving an optimal design for static stability,
but dynamic and flight stability should be achieved by using and tuning the fan controllers.
31
Bibliography
[1] Wikipedia,
Micro air vehicle,
http://en.wikipedia.org/wiki/Micro_air_vehicle
[2] Wikipedia,
Airship,
http://en.wikipedia.org/wiki/Dirigible
[3] Survivor seeker 07,
AERO 4102, Aero Vehicle Design, 2007
School of Mechanical and Manufacturing Engineering,
University of New South Wales, Sydney, Australia
[4] Aerodynamics and Flight Control Design for Hovering Micro Air Vehicles,
B. Motazed, Ph.D, D. Vos, Ph.D, Aurora Flight Sciences, M. Drehla, Ph.D, MIT,
Proceedings of the American Control Conference, Philidelphia, Pennsylvania, june 1998
[5] Modeling and analysis of a reduced-complexity ducted MAV,
L.Marconi, R. Naldi, A. Sala,
Center for research of complex Automated Systems (CASY), DEIS-University of Bologna,
Italy
[6] Formation Flight of two Autonomous blimps,
J. v.d. Loo,
TU Eindhoven, Department of Mechanical Engineering, Dynamics and Control Group,
October 2007
[7] Hovering flight stabilization in wind gusts for ducted fan UAV,
J.M. Pflimlin, P. Soueres, T. Hamel,
43rd IEEE Conference on Decision and control, December 2004
[8] AutoMAV - Micro Aerial Vehicles for Airport Surveillance,
S. Winkler, M. Buschmann, L. Kruger, H.-W. Schulz, P. Vorsmann,
Technical University of Braunschweig, Germany
[9] Micro Air Vehcile: Configuration, Analysis, Fabrication, and Test,
H. Wu, D. Sun, Z. Zhou,
IEEE/ASME transactions on mechatronics, vol.9 no 1, march 2004
[10] CATIA,
http://www.3ds.com/products/catia/
32
[11] Mylar, BoPET,
http://www.mylar.com/,
http://en.wikipedia.org/wiki/Mylar
[12] Grand wing servo tech co.,
GW/EDF-55 Electric ducted fan power system,
http://www.gwsus.com/english/product/powersystem/edf55.htm
[13] X-Plane,
http://www.x-plane.com
[14] Wikipedia,
Buoyant mass
http://en.wikipedia.org/wiki/Buoyant_mass
[15] Thunder Power RC Pro Lite
Thunder Power RC Pro Lite 1320 mAh 11.1 V
http://www.rctoys.com/rc-toys-and-parts/TP-1320-3SPL/
RC-PARTS-THUNDER-POWER-PRO-LITE-LI-POLY-BATTERY.html
[16] Grand wing servo tech co.,
GW/EDP-05 Electric direct-drive power system,
http://www.gws.com.tw/english/product/powersystem/edp5.htm
[17] RF-Links CSS-59C,
http://rf-links.com/newsite/pdf/css-59c.pdf
33
Appendix A
Buoyancy of the balloon
This buoyant mass is the effective mass of the object with respect to the gravity and is given
in kilograms [14].
mb = mo · (1 −
ρf
)
ρo
(A.1)
The buoyant mass of the balloon can be calculated by equation A.1.
(A.2)
mb = Vo · (ρo − ρf )
where
mb
mo
ρf
ρo
Vo
is
is
is
is
is
the
the
the
the
the
buoyant mass [kg]
true mass of the object [kg]
density of the object [kg/m3 ]
density of the surrounding fluid [kg/m3 ]
volume of the object [m3 ]
Equation A.1 can be simplified by substituting the mass of the object by the density times
its volume. Equation A.2 shows the result of this simplification.
Helium has a density of ρo = 0.1663 kg/m3 and air a density of ρf = 1.204 kg/m3 at
20◦ C. The volume of the balloon is V = 6.861 · 10−3 m3 which is calculated by CATIA after
designing the balloon. The buoyant mass becomes mb = −7.12 g, the negative sign indicating
that it is opposite to the gravity acceleration.
If hydrogen is used instead of helium, the difference in buoyancy can be calculated. The
density of hydrogen is ρo = 0.0838 kg/m3 , which will make the buoyant mass mb = −7.69 g.
This means an increase of 7.9% with respect to helium.
34
Appendix B
GWS Electric Ducted Fan
There are many manufacturers of ducted fans [3]. It is also possible to get a duct, motor and
impeller from separate manufacturers, but for this project a total package has been chosen.
The reason is that the manufacturer can provide the specifications and data of the combination
of duct, motor and impeller. The data which includes the amount of thrust provided when
an amount of power is supplied, can be used for the efficiency calculations and the choice of
capacity of the battery pack. Table B.1 shows the data provided by GWS for the EDF-55
used with the GWBLM005B motor [12].
Volts [V]
7.4
11.1
14.8
EDF-55, GWBLM005B motor, 57 grams weight
Amps [A] Thrust [g] Power [W] Efficiency [g/W]
3
97.00
22.20
4.37
5.7
180.00
63.27
2.84
9.3
326.00
137.64
2.37
Table B.1: Specifications of the GWS EDF-55
35
RPM [-]
15500
22900
30250
Appendix C
Thunder Power LiPo Battery Pack
Specifications for Thunder Power RC TP-1320-3SPL LiPo [15] are shown in table C.1.
Thunder Power RC TP-1320-3SPL
Voltage
11.1 V
Cells
3
Capacity
1320 mAh
Max continuous current
17 A
Max burst current
27 A
Weight
84 g
Dimensions
19x34x65 mm
Table C.1: Specifications of the LiPo battery pack
Using this battery pack and the GWS EDF-55 ducted fan with specifications shown in
appendix B the flight time can be calculated. Equation C.1 shows that the amount of flight
time can be calculated by dividing the capacity of the battery pack by the total amount of
current required for lifting 350 g. When the amount of capacity of the battery pack is increased, the flight time will increase, however the increase of capacity will increase the weight
of the battery. Also if the efficiency of the fan is increased (eg. less current required for the
same lift) the flight time will increase as well.
t =
t =
With
t
:
C :
At :
C
At
1.32
= 0.116
2 ∗ 5.7
(C.1)
Flight time [h]
Capacity of the battery pack [Ah]
Total current required by the lift fans [A]
36
Appendix D
Post Simulation Catia Model
Draftings
Figure D.1: Horizontal and vertical stabilizer
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Figure D.2: Assembly of the post simulation CATIA model
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