UAVs for ABL research Report by WG 1

Transcription

UAVs for ABL research
Report by WG 1
Burkhard Wrenger
Costas Soutis
Roland von Glasow
Thomas Krüger
Morten Bisgaard
Eduardo Silva
September 9, 2011
Contents
1 Introduction
1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 COST Action ES 0802 . . . . . . . . . . . . . . . . . . . . . . . . .
2 Requirements from the ABL science
2.1 Description of the Small Working Groups within WG 3
2.2 Air Chemistry . . . . . . . . . . . . . . . . . . . . . .
2.3 Stable Boundary Layer . . . . . . . . . . . . . . . . . .
2.4 Convective Boundary Layer . . . . . . . . . . . . . . .
2.5 Spatial Transitions Including Leads and Coastline . . .
2.6 In-Cloud Flights . . . . . . . . . . . . . . . . . . . . . .
2.7 Wind Energy and Wind Profiles . . . . . . . . . . . . .
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3 Modern Composite Material in Manned and Unmanned
Vehicles
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Fibre reinforced Plastics . . . . . . . . . . . . . . . . . . . .
3.3 Design and Analysis . . . . . . . . . . . . . . . . . . . . . .
3.4 Manufacturing Techniques . . . . . . . . . . . . . . . . . . .
3.5 Application in Aircraft Construction . . . . . . . . . . . . .
3.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.7 References . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4 Avionics for Small Unmanned Aircraft Systems
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 General Aspects of Flight Control for Unmanned Systems .
4.3 Basic Flight Control Architecture . . . . . . . . . . . . . .
4.4 Attitude and Heading Refernce System . . . . . . . . . . .
4.5 Ground Station and Telemetry . . . . . . . . . . . . . . . .
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Aerial
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5 Helicopters
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5.1 Using Helicopters in ABL Science . . . . . . . . . . . . . . . . . . . 46
5.2 State Of The Art . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
5.3 Avilable Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
6 Sense and Avoid Techniques
6.1 Towards UAV flight in Civil Airspace
6.2 Sense and Avoid for Small UAVs . .
6.3 Current S&A systems . . . . . . . . .
6.4 Visual Sense . . . . . . . . . . . . . .
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List of Figures
3.1
3.2
3.3
3.4
3.5
Aelius, unmanned aerial vehicle, entirely built out of composite materials (Aeroart/USFD) . . . . . . . . . . . . . . . . . . . . . . . .
V22-Osprey Tilt-rotor plane . . . . . . . . . . . . . . . . . . . . .
An aircraft composite wing rib element produced by RTM . . . .
A380 Glare fuselage crown (Courtesy of Airbus S.A.S.) . . . . . .
Hunter 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1
4.2
4.3
Basic design of a cascaded flight control loop. . . . . . . . . . . . . 40
Basic design of a cascaded flight control loop. . . . . . . . . . . . . 42
Example of a ground station software and its main components. . . 44
5.1
5.2
5.3
5.4
5.5
5.6
The
The
The
The
The
The
6.1
6.2
6.3
Sense & Avoid Background . . . . . . . . . . . . . . . . . . . . . . . 58
Airplane sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
Visual Sense . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
Schieble CamCopter . .
CybAero APID . . . .
Swiss UAV Neo . . . .
Saab Skeldar . . . . . .
Adaptive Fligth Hornet
weControl Bicopt . . .
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List of Tables
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
Requirements for UAS platform and operation for investigations of
passively degssing volcanos . . . . . . . . . . . . . . . . . . . . . . .
Requirements for UAS platform and operation for flights in polar
regions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Requirements for UAS platform and operation for Marine Boundary
Layer investigations. . . . . . . . . . . . . . . . . . . . . . . . . . .
Requirements for UAS platform and operation for urban area investigations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Requirements for UAS platform and operation for variability mapping by long endurance flights. . . . . . . . . . . . . . . . . . . . . .
Requirements for UAS platform and operation for Stable Boundary
Layer investigations. . . . . . . . . . . . . . . . . . . . . . . . . . .
Requirements for UAS platform and operation for Convective Boundary Layer investigations. . . . . . . . . . . . . . . . . . . . . . . . .
Requirements for UAS platform and operation for spatial transition
investigations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Requirements for UAS platform and operation for wind energy and
wind profile investigations. . . . . . . . . . . . . . . . . . . . . . . .
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Chapter 1
Introduction
1.1
Motivation
The monitoring of expected climate change and the provision of reliable climate
scenarios will be one of the vital research challenges during the next decades. In
this context the atmospheric boundary layer (ABL) is of particular interest. On
the one hand it is the part of the atmosphere which is directly related to human
life and activity. On the other hand it is the layer close to the surface of the earth,
where most of the exchange processes of energy and matter in the climate system
take place. The detailed information on the structure of the lower atmosphere,
especially with respect to vertical distribution and horizontal variability of temperature, humidity, wind and trace gases, e.g. CO2 , is the key to the understanding
of exchange processes in the ABL. Up to now there is a lack of cost efficient measurement systems, applicable for ABL phenomena covering relevant processes as
turbulence (typical scale: sub-meter to several hundred meters), waves and sheets
in stable stratification, convection and meso-scale flow processes (up to several
km) that are not resolved appropriate by recent routine observation systems.
Recent investigations indicate that the main portion of uncertainty resulting
from numerical atmospheric modelling has to be attributed to an insufficient representation of such boundary layer processes in the corresponding weather prediction
and climate models. Further improvement of our understanding of these processes
requires innovative measurement strategies to collect atmospheric data with high
spatial and temporal resolution. These data sets will be crucial for the validation
and improvement of boundary layer parameterisation schemes in the numerical
models used both for climate simulation and weather prediction. Furthermore
it is an essential component in dispersion studies which have a broad range of
application from air pollution monitoring and prediction to emergency response
systems.
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It turns out that UAS will be an excellent choice by offering a lot of features that
ideally complement the actually available data from ground observation stations
and ground- or space-based remote-sensing systems, as they provide:
• high-resolution in-situ data that does not rely on similarity or propagation
assumptions
• real area-representative data at user-defined altitude (even at ground level)
in short time
• fast / instantaneous data delivery not disturbed by clouds, at a time defined
by the operating scientists
• high flexibility and mobility.
In addition to these highly relevant features, unmanned systems offer cost-efficient
data acquisition options in regions that are hard to reach or too dangerous for
manned operations (e.g. polar regions, off-shore wind parks, active volcanoes, forest fires, dangerous pollution events or urban areas). The complexity of scientific,
technical and legal aspects, related to the proposed objectives of the Action, requires a multi-disciplinary approach on at least European level. COST Action
ES 0802 influences and initiates a wide spectrum of scientific and commercial applications in meteorology and climatology in the future and therefore has a large
social-economic impact for Europe, particularly in the long term. The participants
of this Action build up a unique team in Europe with large expertise on airborne
systems, atmospheric research and sensor technology creating very fast large values
in the field of science and technology.
The objective of this report is to provide essential informations for the scientific
community and enterprises to start their own UAV projects. It describes the state
of the art technology of UAV airframes and related components. The report is
written by members of Working Group 1 (WG1) of COST Action ES 0802.
1.2
COST Action ES 0802: Unmanned aerial
systems (UAS) in atmospheric research
Unmanned aerial systems (UAS) are of large and increasing importance for environmental monitoring nowadays and in the future, e.g. under the aspects of
climate change and sustainable development. COST Action ES 0802 coordinates
ongoing and conceives future research on the development and application of UAS
as a cost-efficient, trans-boundary method for the monitoring of the atmospheric
boundary layer and the underlying surface of the Earth. These systems help to
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close the recent observational gap between established ground based and satellite based measurements and provide relevant atmospheric data both with high
temporal and spatial resolution and an unique data coverage in space and time.
This distinctly increases the understanding of the atmospheric boundary layer and
related surface- atmosphere exchange processes which is crucial for future improvements in numerical weather prediction and climate simulation. First prototypes of
UAS systems of different size, complexity and equipped with different instrumentation, have successfully proven their functionality. Based on this, COST Action
ES 0802 promotes the conception and further development of prototypes for a fleet
of UAS of different size, instrumentation, and operation range with respect to various specific observational requirements. Finally this interdisciplinary approach is
establishing a forum on the European level for the coordination of the relevant
scientific, technical and legal aspects connected to a safe and permanent operation
of UAS for routine environmental monitoring purposes.
The main intention of the COST Action is to set up a scientific and engineering cluster of competence in order to combine innovative sensor technology and
autonomous UAS and provide a new and cost- efficient measurement concept for
atmospheric boundary layer investigations and environmental monitoring. By that
this cutting edge project is of large importance for a wide spectrum of future applications in meteorology and climatology. From the atmospheric point of view, the
Action improves in the short run considerably the description and understanding
of the atmospheric boundary layer by a largely extended data coverage. This will
later on form the basis for the validation of fine scale numerical simulations and
subsequently lead to improved boundary layer parameterization schemes used in
models for numerical weather forecast and climate simulations.
1.2.1
Scientific focus and Working Groups of COST Action
ES 0802
Due to the interdisciplinary nature of the project two different, but strongly interacting, scientific foci can be defined for the action.
• Atmospheric Research
– Boundary layer turbulence
– Verification of remotely sensed (sodar, large aperture scintillometer)
turbulence and fine structure parameters with in-situ measured data
– The entrainment zone and the capping inversion of the ABL
– The stable boundary layer (SBL) and the nocturnal ABL
– The polar boundary layer
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– Atmospheric pollution issues
• UAV airframes and sensors
These scientific foci are strongly correlated to the operation of UAS and the legals
aspects of UAS operation.
In the follwoing sections, the Working Groups (WG) of COST Action ES 0802
and their relationship will be described in more details. WG 1 deals with Unmanned Aerial Systems, WG 2 with sensors suitable for ABL investigations, WG
3 with high resolution 3D investigations of the ABL and WG 4 with legal issues.
1.2.2
WG 1: UA Systems
A knowledge base with information about UAS is crucial to integrate more groups
and institutions and to avoid redundant research and tests. WG 1 therefore compiles information about UAS, possible and best practise configurations, auto-pilot
systems and flight control techniques and systems. In order to grant access to a
wide public, the database containing these information has a web interface1 and
therefore is acomponent of this COST Actions web site. Furthermore, the WG
discusses the management of data compiled with UAS including structure, deployment, evaluation and processing and proposes a suitable standard. State of the
art ground station software is often targeted to a specific UAS and therefore noninteroperable with other systems. The development of suitable UAS is critical to
successful atmospheric research as proposed in the Action. WG 1 of COST Action
ES 0802 investigates UAS platforms and operation with specific emphasis on atmospheric research capabilities. Current use of airborne systems for atmospheric
research relies on (remotely) piloted aircraft which are often based on model RC
designs. This presents significant restrictions on the available range, endurance
and number of aircraft that can be deployed. By exploiting recent advances in
lightweight composites, aerodynamics, propulsion and automatic control a new
range of high performance UAS specifically suited to atmospheric research can
be developed. This working group brings together such research in a co-ordinated
way to propose the next generation of UAS designs that can operate autonomously
and meet the required performance specifications. Intended deliverables therefore
include
• Novel airframe configurations which utilise latest advances in composite materials and aerodynamics.
• Assessment of clean propulsion technologies for UAS.
1
See www.cost-uas.net or www.cost0802.net
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• Low cost, light weight, autonomous flight control system designs to minimise
human intervention.
• Assessment and recommendations for high speed data links for telemetry,
mission management, and measurement data transfer.
• Ground station designs which utilise latest advances in human factors to
ensure high quality mission management tools.
The objective of this report is to describe these topics as state of the art
technology of UAVs for atmospheric research.
1.2.3
WG 2: UAS Sensors for atmospheric research
Amospheric research identifies and describes three groups of sensors for ABL investigations:
• Sensors for standard measurements (pressure, temperature, humidity, ways
to retrieve wind components),
• Sensors for turbulence measurements (momentum, sensible and latent heat
fluxes for which high resolution 3-D velocity, temperature and humidity measurements are crucial)
• Sensors for non-standard measurements (e.g. microphysical measurements in
clouds, possibly some aerosol measurements and radiometric measurements,
atmospheric trace gases).
One objective of WG 2 is to compile a database of suitable sensors for ABL research. In the following years this survey will be continued and the database
resulting will be actualized. The following activities will focus on recommendations, based on experience gathered by partners of COST actions and available
literature, for use of particular set of sensors and particular type of UAS for various kinds of measurements (this action is be done in coordination with WG3).
WG2 also compares various methods used to retrieve all three components of the
wind vector along the path of UAS in order to describe structure of turbulence.
1.2.4
WG 3: High resolution 3D atmospheric measurements by UAS
Main focus of this WG is the application of UAS in order to produce detailed and
accurate 3D atmospheric data sets related to different specific research requirements. This task includes the exploitation of novel flight strategies together with
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associated data analysis tools in order to maximise the resources available. In
particular the use of co-operative UAS, such as in swarms and flocks, can provide
the best resolution of atmospheric measurements in limited time. Whilst closely
connected to WG1 and WG2 the work of this WG3 focuses on the application of
UAS and the related requirements concerning mission planning and flight strategies to optimise sensing. Part of this work takes place in close cooperation with
the community of numerical atmospheric modelling (fine-scale and large eddy simulations (LES)). On the one hand model results are used for the planning of flight
patterns and flight strategies. On the other hand the measurements are crucial
for model validation purposes and for the planning and setup of future numerical
simulations.
1.2.5
WG 4: UAS Operation
As the legislation regarding UAS has not yet been developed nor standardized
among different countries, an initial task of this work package is to asses the legal
conditions needed for flying UAS in the participant countries. These conditions
should be taken into account when deciding where to perform the flight tests. UAS
are not a mature technology and they are just being introduced in the civil market.
For this, a new legislation is needed as they are not considered more civil aircrafts
than R/C planes. There are initiatives both in USA (FAA, RTCA) and Europe
(EASA, Eurocontrol) to standardize and legislate such systems, a basic step for
their expansion in the civil market. This work which is currently done at European
and at national levels is followed. The legal study also assess the compliance of the
actual UAS solution with current and future legislation. In particular, out-of-sight
operations and autonomous control strategies are to be considered.
Beyond the legal aspects, one sub-task of this WG is the identification of particular requirements for UAS operation under specific dangerous or hazardous
environments (e.g. extreme temperatures in polar regions; high concentration of
smoke, dust and aggressive gases as in the vicinity of forest fires or volcanoes).
This covers technical means as heating or encapsulating electronics as well as lossflight strategies (e.g. the development of specific low cost solutions with online
data-link which are not expected to be recovered).
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Chapter 2
Requirements from the ABL
science
By Burkhard Wrenger, University of Applied Sciences Ostwestfalen-Lippe (DE),
Roland von Glasow, University of East Anglia (UK)
The informations and dates as given in this chapter reflect the requirements and
discussions within COST Action ES 0802 from 2010. They will probably evolve
within the next years.
2.1
Description of the Small Working Groups
within WG 3
Small Working Groups (SWGs) have been introduced in WG 3 as a means to
give scientists with common objectives a common platform for discussions and
possible joint campaigns. For each SWG, the UAS are provider for data in order
to improve the knowledge on meteorological processes and provider of data for
initializing and validating numerical models. Therefore UAS-based measurements
are used to characterise a process and to initialise and validate numerical models as
used, e.g., for high resolution weather predictions, chemical reactions or industrial
plume desitribution. The SWGs within WG 3 are
• Air Chemistry (section 2.2)
• Stable Boundary Layer (Mid-Latitude, Polar)
• Convective Boundary Layer
• Spatial Transitions (Leads, Coastline)
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• In-Cloud Flights
• Wind Energy
The objectives and their requirements related to UAS andoperations of UAS of
these SWGs are described shortly in the following sections. For more details see
website of COST Action ES 08021 .
2.2
Air Chemistry
The use of UAS for atmospheric chemistry is useful, e.g. for regions that are out of
reach and/or too dangerous for manned aircraft like volcano or industrial plumes
or for small scale structures. The characterisation of the air by UAS includes 3D
measurements of concentrations of gases, aerosol particles, trace gases and particle
fluxes. The intended measurements include, e.g.
• Industrial and urban area plumes including biomass burning
• Ship/vessel plumes
• Volcanos: crater interior, rim and vicinity
• Disaster and accidental release of plutants
The advantage of using UAVs for these applications scenarios is to avoid danger to
manned aircraft and their crew, to have longer samping times due to slower true
air speed of UAS if compared to manned aircraft and to be launched from ships.
For some feature of interest in atmospheric chemistry, the scale is very small. This
excludes the efficient sampling with manned aircraft which are usually much faster
and favours slow fixed wing UAS, helicopters, multicopters or zeppelins. This also
holds for stratifications and local turbulence effects, which lead to spatially very
strong gradients. The requirements are mainly determined by the sensors and the
flight strategy. As air chemistry and aerosol experiments based on UAV flights
strongly vary on the scientific objectives, the following subsections describe some
typical scenarios.
2.2.1
Investigation of passively degassing volcanos
The main objective would be the investigation of the plumes by flying several
sections contributing to a 3D mapping of the plumes. Additionally, it is feasible to
analyse the evolution of the plumes as a function of time. The spatial resolution
1
www.cost-uas.net/...
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should be at least 10 m. It has to be taken into account, that the altitute of the
craters of a volcano can be 3000 m or more. Furthermore, temperatures below
10C and high particle loads have to be taken into account, depending on the
activity of the volcano. As some missions require sampling times of 10-30 min.
at the same location in the plume, rotary wing or lighter-than-air platforms with
hovering capability are required.
Table 2.1: Requirements for UAS platform and operation for investigations of
passively degssing volcanos
Topic
Requirement
Endurance
30-90 min.
Range
several 10 km
Speed
10-20 m/s and hovering
Ceiling
≥ 3000 m
Payload
1-5 kg
Power requirements
10 - 50 W
Night time operations sometimes
Altitude and location low, normal GPS sufficient
precision
Requirement to fly in yes
cloud
Requirement to fly in useful
swarms
Data storage
oboard and transfer by telemetry to check if
inside plume
Line of sight operation yes
possible?
Flight patterns
3D survey pattern to probe plume and nonplume background
Other
Ability to fly in high particle and hazardous
components loads; ability to auto-detect the
origin of the plume; limitations for take off
and landing due to typical volcano terrain
2.2.2
Air chemistry in polar regions
UAS might provide unprecedented data over, e.g., snow and sea-ice. As these areas
are difficult to access on the ground and low level flights are dangerous to perform
for manned aircraft, UAS are suitable tools for data acquisition. However, the
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Table 2.2: Requirements
gions.
Topic
Endurance
Range
Speed
Ceiling
Payload
Power requirements
Night time operations
Altitude and location
precision
Requirement to fly in
cloud
swarms
Data storage
Line of sight operation
possible?
Flight patterns
Other
for UAS platform and operation for flights in polar reRequirement
1-3h
30 - 200 km
10-30 m/s
1000 m
1-5 kg
10 - 50 W
sometimes
GPS sufficient (unless analysis of fluxes
yes (technical challenge due to icing)
for some applications
telemetry preferred
no
constant level flights and vertical profiles
very low temperatures (-10 C - 40 C)
flight conditions are usually harsh, preparating and operating UAS is a challenge
for the operators, handling and maintenance have to be very easy.
2.2.3
Marine Boundary Layer
The (tropical) Marine Boundary Layer is essential for the release of chemical active
gases, high water vapor content, high UV radiation and loads of sea salt and is a
key factor for the determination of the oxidation potential of the atmosphere. As
the investigation requires low level flights of several hours, the corrosive potential
of the atmosphere has to be taken into account.
2.2.4
Micro climate and air chemistry in urban areas
Urban areas have a strong but complex influence on the local mirco climate. Data
acquisition in urban areas is usually limited to a few fixed locations. For a better understanding of the mechanisms and the local or regional influences, three
dimensional data acquisition would be feasible. As manned aircraft is not able
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Table 2.3: Requirements for UAS platform and operation for Marine Boundary
Layer investigations.
Topic
Requirement
Endurance
1-3h
Range
100 - 500 km
Speed
10-30 m/s
Ceiling
1500 m
Payload
1-5 kg
Power requirements
10 - 50 W
Altitude and location GPS sufficient (unless analysis of fluxes
precision
Requirement to fly in yes (technical challenge due to icing)
cloud
Requirement to fly in for some applications
swarms
Data storage
telemetry preferred
Line of sight operation no
possible?
Flight patterns
Other
Corrosion due to heavy salt loads in the atmosphere
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Table 2.4: Requirements
gations.
Topic
Endurance
Range
Speed
Ceiling
Payload
Power requirements
Night time operations
Altitude and location
precision
cloud
swarms
Data storage
Line of sight operation
possible?
Flight patterns
Other
for UAS platform and operation for urban area investiRequirement
0.5 - 6 h
50 km
5 15 m/s and hovering capabilities
150 m
1-5 kg
10 - 50 W
yes
altitude and location precision ≤ 1 m even
under multi-path conditions
no
no, but coordinated multi-UAS flights feasible
onboard storage and telemetry required
depends on mission
horizontal and vertial scanning
rotating wing and lighter-than-air-airframes
are suitable
to fly very low above or within urban areas, UAVS might play an important role
for an improved understanding of the inhabited biosphere. Swarming might play
a minor role, but coordinated simultaneous UAV flights inside and outside urban
areas would support simulations.
2.2.5
Long endurance flights for chemical climatology
Long endurance flights are useful means to probe the variabilty of a region. The
endurance has to be several hours or several hundred kilometers distance. The
flights have to be on a constant altitude level and additional vertical profile flights
up to ≈ 2000 m.
2.3
Stable Boundary Layer
Some details from the Stable Boundary Layer (SBL) during the night and the
morning transition from the Stable Boundary Layer to the Convective Boundary
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Table 2.5: Requirements for UAS platform and operation for variability mapping
by long endurance flights.
Topic
Requirement
Endurance
5 - 10 h
Range
100 - 3000 km
Speed
20-40 m/s
Ceiling
1500 m (boundary layer)
Payload
1-5 kg
Power requirements
10 - 50 W
Altitude and location GPS sufficient (unless analysis of fluxes)
precision
cloud
swarms
Data storage
onboard storage and telemetry messages
possible?
Flight patterns
Other
17
Table 2.6: Requirements for UAS platform and operation for Stable Boundary
Topic
Requirement
Endurance
≥1h
Range
60 - 100 km
Speed
20 m/s
Ceiling
300 m
Payload
1-5 kg
Power requirements
10 - 50 W
Night time operations yes
Altitude and location GPS sufficient
precision
Requirement to fly in no
cloud
swarms
Data storage
onboard storage and telemetry messages
possible?
Flight patterns
race-leg patterns on several altitude levels
Other
Layer (see below) are not very well understood. Noctural low-level jets are common in the stably stratified boundary layer due to the baroclinity at low-levels or
due to inertial oscillations of the wind when approaching the geostrophic values.
The mechanical turbulence is weak and in the surface layer decoupled from the
air above. In order to understand this phenomen, turbulence transport has to be
investigated along the vertical axis and within the surface layer. The morning
transition reveals a shallow CBL shortly after sunrise probably due to string entrainment. The linear profile of the turbulent flux of sensible heat is similar to
the daytime CBL, whereas the parameters descibing the flux profile differ from
the daytime CBL. The flight patterns are mainly race-track patterns on several
altitude levels, i.e. two parallel legs separated about 100-200 m.
2.4
Convective Boundary Layer
The main issues related to the Convective Boundary Layer (CBL) are
• Entrainment
18
• (Daytime) Transisitions
• Surface heterogeneity
• Shallow cumulus clouds
Due to the different phenomena to be studied, there are several flight patterns,
which have been proposed for these investigations, namely
• Vertical structure: The flight patterns consist of several stacked legs in a
vertical plane.
• Surface heterogeneity: This flight pattern requires horizontal explorations
flights close (5 - 50 m) to the ground.
• Vertical profiles: Helicoidal ascent or descent (fiexd wing airframes) or vertial
ascent or descent (rotary wing airframes).
• Purpoising legs
• Cloud investigations: Spirals from outside to inside of the cloulds, below and
above the cloud base.
2.5
Spatial Transitions Including Leads and Coastline
The investigations of land/sea transitions try to characterise the vertical profile
of basic meteorological parameters like temperature, wind speed and direction or
humidity at different locations. Time-dependent studies also reveal the evolution
and penetration during the sea-breeze and driven by the mesoscale forcings. Flight
patterns have to be flow as vertical profiles onshore up to 1000m and inland up to
3000 m to take into account, e.g. mountain slopes. For some applications, constant
level flights are feasible. Two UAS flying onshore and inland simultanously are
essential. Very low altitudes have to be flown for sea ice investigations.
2.6
In-Cloud Flights
2.7
Wind Energy and Wind Profiles
With the upcoming new generations of wind turbines there is a need for accurate
information on wind conditions at the relevant altitudes (≈ 200 m). Coastal
19
Table 2.7: Requirements for UAS platform and operation for Convective Boundary
Topic
Requirement
Endurance
1h
Range
30 - 75 km
Speed
10-20 m/s
Ceiling
5 m - 1500 m
Payload
5-10 kg
Power requirements
10 - 50 W
Altitude and location GPS sufficient but very stabile legs essential
precision
cloud
swarms
Data storage
onboard storage and telemetry
possible?
Flight patterns
constant level flights, vertical profiles,
stacked legs
Other
Lagrangian and Eulerian flights required.
20
Table 2.8: Requirements for UAS platform and operation for spatial transition
investigations.
Topic
Requirement
Endurance
1-2h
Range
30 - 150 km
Speed
10-20 m/s
Ceiling
5 - 3000 m
Payload
1 - 5 kg
Power requirements
10 - 50 W
precision
cloud
Requirement to fly in coordinated flights of two UAS required
swarms
Data storage
possible?
Flight patterns
vertical profiles, constant level flights (legs)
Other
N/A
and offshore regions are good locations for wind power plants. Suitable locations
stringly depend on the all-year wind conditions which requires detailed knowledge
on the spatial variability of the wind speed. Measurment towers of suitable size
and wind remote sensing equipment is usually sparely located, flights with manned
aircraft are seldom due to the risk caused by strong turbulence effects in the
downstream area behind the wind turbines.
A new aspect in wind energy research is the need for accurate information
on wind conditions at altitudes over 200 m, where the coming generation of wind
turbines in the 10-12 MW class have their tip heights. Coastal and offshore regions
are good locations for wind power plants. The decision of the exact location of a
wind power plant requires, however, detailed knowledge on the spatial variability
of the wind speed at altitudes of 50-300 m. Since tall measurement towers and
wind remote sensing equipment are sparsely located, this information is mostly
based on numerical model simulations. Tower and remote sensing data are used
for model validation at certain locations. There are, however, no good means
for validation of the small-scale spatial variability present in the mesoscale model
products at altitudes of 50-300 m. The question is particularly challenging over
21
complex terrain, or at large roughness changes (e.g., the land-sea border), or at
flow displacements (e.g. over forests). UAVs could provide valuable data for
model validation, either on experimental basis or even having an automated station
regularly operating UAVs.
Additionally, vertical wind profiles taken by UAs are a suitable tool and replacement for radiosondes. As the UAS usually have sensors for the basic meteorological
data onboard, they can be lighter than the UAS mentioned above.
Table 2.9: Requirements for UAS platform and operation for wind energy and
wind profile investigations.
Topic
Requirement
Endurance
1h
Range
10 - 100 km
Speed
10-20 m/s
Ceiling
3000 m
Payload
1 kg
Power requirements
10 - 50 W
precision
cloud
Requirement to fly in sometimes
swarms
Data storage
possible?
Flight patterns
vertical profiles, constant level flights (legs),
hovering behind wind turbines
Other
N/A
22
Chapter 3
Modern Composite Material in
Manned and Unmanned Aerial
Vehicles
By Costa Soutis, University of Sheffield (UK)
Aerospace Engineering, The University of Sheffield, Mappin Street, Sheffield S1
3JD, UK
E-mail: [email protected]
3.1
Introduction
Fibrous composites combine the properties of two or more materials (constituents).
Any two materials (metals, ceramics, polymers, elastomers, glasses) could be combined to make a composite. They might be mixed in many geometries (particulate,
chopped-fibre, woven, unidirectional fibrous and laminate composites) to create a
system with a property profile not offered by any monolithic material. In mechanical design it is often to improve the stiffness-to-weight ratio or strength-to-weight
ratio or improve toughness, while in thermo-mechanical design, it is to reduce thermal expansion, or to maximise heat transfer, or to minimise thermal distortion.
Composite materials have gained popularity (despite their generally high cost) in
high-performance products that need to be lightweight, yet strong enough to take
high loads such as aerospace structures (tails, wings and fuselages), boat construction, bicycle frames and racing car bodies. Other uses include storage tanks and
fishing rods. Natural composites (wood and fabrics) have found applications in
aircraft from the first flight of the Wright Brothers Flyer 1, in North Carolina on
December 17th 1903, to the plethora of uses now enjoyed by engineered materials
23
(man made composites) on both military and civil aircraft, in addition to more
exotic applications on unmanned aerial vehicles (UAVs), Figure A, space launchers and satellites. Their growing use has arisen from their high specific strength
Figure 3.1: Aelius, unmanned aerial vehicle, entirely built out of composite materials (Aeroart/USFD)
and stiffness, when compared to the more conventional materials, and the ability to shape and tailor their structure to produce more aerodynamically efficient
structural configurations. In this paper, it is argued that fibre reinforced polymers, especially carbon fibre reinforced plastics (CFRP) can and will in the near
future contribute more than 50% of the structural mass of an aircraft. However,
affordability is the key to survival in aerospace manufacturing, whether civil or
military, manned or unmanned and therefore effort should be devoted to analysis
and computational simulation of the manufacturing and assembly process as well
as the simulation of the performance of the structure, since they are intimately
connected.
3.2
Fibre reinforced Plastics
The adoption of composite materials as a major contribution to aircraft structures
followed on from the discovery of carbon fibre at the Royal Aircraft Establishment
at Farnborough, UK, in 1964. However, not until the late 1960s did these new
composites start to be applied, on a demonstration basis, to military aircraft. Examples of such demonstrators were trim tabs, spoilers, rudders and doors. With
increasing application and experience of their use came improved fibres and matrix materials (thermosets and thermoplastics) resulting in CFRP composites with
improved mechanical properties, allowing them to displace the more conventional
materials, aluminium and titanium alloys, for primary structures. In the following
sections the properties and structure of carbon fibres are discussed together with
thermoplastic and thermoset resins and the significance of the interface between
the fibre and the matrix.
24
3.2.1
Modern Carbon Fibres
High strength, high modulus carbon fibres are about 5 to 6 ?m in diameter and
consist of small crystallites of turbostratic graphite, one of the allotropic forms of
carbon. The graphite structure consists of hexagonal layers, in which the bonding
is covalent and strong (?525 kJ/mol) and there are weak van der Waal forces (¡
10 kJ/mol) between the layers [1,2]. This means that the basic crystal units are
highly anisotropic; the in-plane Youngs modulus parallel to the ?-axis is approximately 1000 GPa and the Youngs modulus parallel to the c-axis normal to the
basal planes is only 30 GPa. Alignment of the basal plane parallel to the fibre axis
give stiff fibres, which, because of the relative low density of around 2 Mg/m3, have
extremely high values of specific stiffness (?200 GPa/((Mg/m3)). Imperfections in
alignment, introduced during the manufacturing process result in complex-shaped
voids elongated parallel to the fibre axis. These act as stress raisers and points of
weakness leading to a reduction in strength properties. Other sources of weakness,
which are often associated with the manufacturing method, include surface pits
and macro-crystallites. The arrangement of the layer planes in the cross-section of
the fibre is also important since it affects the transverse and shear properties of the
fibre. Thus, for example, the normal polyacrylonitrile-based (PAN-based) Type
I carbon fibres have a thin skin of circumferential layer planes and a core with
random crystallites. In contrast, some mesophase pith-based fibres exhibit radially oriented layer structures. These different structures result in some significant
differences in the properties of the fibres and of course those of the composites.
Refinements in fibre process technology over the past twenty years have led to
considerable improvements in tensile strength (?4.5 GPa) and in strain to fracture
(more than 2%) for PAN-based fibres. These can now be supplied in three basic
forms, high modulus (HM, ?380 GPa), intermediate modulus (IM, ?290 GPa) and
high strength (HS, with a modulus of around 230 GPa and tensile strength of 4.5
GPa). The more recent developments of the high strength fibres have led to what
are known as high strain fibres, which have strain values of 2% before fracture.
The tensile stress-strain response is elastic up to failure and a large amount of
energy is released when the fibres break in a brittle manner. The selection of
the appropriate fibre depends very much on the application. For military aircraft
both high modulus and high strength are desirable. Satellite applications, in contrast, benefit from use of high fibre modulus improving stability and stiffness for
reflector dishes, antennas and their supporting structures. Rovings are the basic
forms in which fibres are supplied, a roving being a number of strands or bundles
of filaments wound into a package or creel, the length of the roving being up to
several kilometres, depending on the package size. Rovings or tows can be woven
into fabrics, and a range of fabric constructions are available commercially, such
as plain weave, twills and various satin weave styles, woven with a choice of roving
25
or tow size depending on the weight or areal density of fabric required. Fabrics
can be woven with different kinds of fibre, for example, carbon in the weft and
glass in the warp direction, and this increases the range of properties available to
the designer. One advantage of fabrics for reinforcing purposes is their ability to
drape or conform to curved surfaces without wrinkling. It is now possible, with
certain types of knitting machine, to produce fibre performs tailored to the shape
of the eventual component. Generally speaking, however, the more highly convoluted each filament becomes, as at crossover points in woven fabrics, or as loops
in knitted fabrics, the lower its reinforcing ability.
3.2.2
Fibre-Matrix Interface
The fibres are surface treated during manufacture to prepare adhesion with the
polymer matrix, whether thermosetting (epoxy, polyester, phenolic, polyimide
resins) or thermoplastic (polypropylene, Nylon 6.6, PMMA, PEEK). The fibre
surface is roughened by chemical etching and then coated with an appropriate size
to aid bonding to the specified matrix. Whereas composite strength is primarily a
function of fibre properties, the ability of the matrix to both support the fibres and
provide out-of-plane strength is, in many situations, equally important. The aim
of the material supplier is to provide a system with a balanced set of properties.
While improvements in fibre and matrix properties can lead to improved lamina or
laminate properties, the all-important field of fibre-matrix interface must not be
neglected. The load acting on the matrix has to be transferred to the reinforcement
via the interface. Thus fibres must be strongly bonded to the matrix if their high
strength and stiffness are to be imparted to the composite. The fracture behaviour
is also dependent on the strength of the interface. A weak interface results in a low
stiffness and strength but high resistance to fracture, whereas a strong interface
produces high stiffness and strength but often a low resistance to fracture, i.e.,
brittle behaviour. Conflict therefore exists and the designer must select the material most nearly meeting his requirements. Other properties of a composite, such
as resistance to creep, fatigue and environmental degradation, are also affected by
the characteristics of the interface. In these cases the relationship between properties and interface characteristics are generally complex and analytical/numerical
models supported by extensive experimental evidence are required.
3.2.3
Matrix Materials
Thermoplastic materials are becoming more available, however, the more conventional matrix materials currently used are thermosetting epoxies. The matrix
material is the Achilles heel of the composite system and limits the fibre from
exhibiting its full potential in terms of laminate properties. The matrix performs
26
a number of functions amongst which are stabilising the fibre in compression (providing lateral support), translating the fibre properties into the laminate, minimising damage due to impact by exhibiting plastic deformation and providing outof-plane properties to the laminate. Matrix dominated properties (interlaminar
strength, compressive strength) are reduced when the glass transition temperature is exceeded and whereas with a dry laminate this is close to the cure temperature, the inevitable moisture absorption reduces this temperature and hence
limits the application of most high-temperature-cure thermoset epoxy composites
to less than 120◦ C. Conventional epoxy aerospace resins are designed to cure at
120-135◦ C or 180◦ C usually in an autoclave or close cavity tool at pressures up
to 8 bar, occasionally with a post cure at higher temperature. Systems intended
for high temperature applications maybe undergo curing at temperatures up to
350◦ C. The resins must have a room temperature life beyond the time it takes
to lay-up a part and have time/temperature/viscosity suitable for handling. The
resultant resin characteristics are normally a compromise between certain desirable characteristics. For example improved damage tolerance performance usually
causes a reduction in hot-wet compression properties and if this attained by an
increased thermoplastic content then the resin viscosity can increase significantly.
Increased viscosity is especially not desired for a resin transfer moulding (RTM)
resin where a viscosity of 50 cPs or less is often required, but toughness may also
be imparted by the fabric structure such as a stitched non-crimped fabric (NCF).
The first generation of composites introduced to aircraft construction in the 1960s
and 1970s employed brittle epoxy resin systems leading to laminated structures
with a poor tolerance to low-energy impact caused by runway debris thrown up
by aircraft wheels or the impacts occurring during manufacture and subsequent
servicing operation. Although the newer toughened epoxy systems provide improvements in this respect, they are still not as damaged tolerant as thermoplastic
materials. A measure of damage tolerance is the laminate compression after impact (CAI) and the laminate open hole compressive (OHC) strengths. The ideal
solution is to provide a composite exhibiting equal OHC and CAI strengths and
while the thermoplastics are tougher they have not capitalised on this by yielding higher notched compression properties than the thermoset epoxy composites.
Polyetheretherketone (PEEK) is a relatively costly thermoplastic with good mechanical properties. Carbon fibre reinforced PEEK is a competitor with carbon
fibre/epoxies and Al-Cu and Al-Li alloys in the aircraft industry. On impact at
relatively low energies (5-10 J) carbon fibre-PEEK laminates show only an indentation on the impact site while in carbon fibre-epoxy systems ultrasonic C-scans show
that delamination extends a considerable distance affecting more dramatically the
residual strength and stiffness properties of the composite. An other important
advantage of carbon fibre-PEEK composites is that they possess unlimited shelf-
27
life at ambient temperature; the fabricator does not have to be concerned with
proportioning and mixing resins, hardeners and accelerators as with thermosets;
and the reversible thermal behaviour of thermoplastics means that components
can be fabricated more quickly because the lengthy cure schedules for thermosets,
sometimes extending over several hours, are eliminated. It can be seen that in the
effort to improve the through-the-thickness strength properties and impact resistance the composites industry has moved away from brittle resins and progressed
to thermoplastic resins, toughened epoxies, through damage tolerant methodology,
Z-fibre (carbon, steel or titanium pins driven through the z-direction to improve
the through thickness properties), stitched fabrics, stitched performs and the focus is now on affordability. The current phase is being directed towards affordable
processing methods such as non-autoclave processing, non-thermal electron beam
curing by radiation and cost effective fabrication [?]. NASA Langley in the USA
claims a 100% improvement in damage tolerance performance with stitched fabrics
relative to conventional materials (ref. Advanced Composites Technology, ACT,
programme where non-crimped fabric (NCF) laminates are processed by resin film
infusion, RFI). It is essential that if composites were to become affordable they
must change their basic processes to get away from pre-preg material technology,
which currently results in an expensive solution and hence product. However,
autoclaved continuous fibre composites will still dominate for the high levels of
structural efficiency required.
3.3
Design and Analysis
Aircraft design from the 1940s has been based primarily on the use of aluminium
alloys and as such an enormous amount of data and experience exists to facilitate
the design process. With the introduction of laminated composites that exhibit
anisotropic properties the methodology of design had to be reviewed and in many
cases replaced. It is accepted that designs in composites should not merely replace
the metallic alloy but should take advantage of exceptional composite properties
if the most efficient designs are to evolve. Of course the design should account
for through-thickness effects that are not encountered in the analysis of isotropic
materials. For instance, in a laminated structure since the layers (laminae) are
elastically connected through their faces, shear stresses are developed on the faces
of each lamina. The transverse stresses (?z, ?xz, ?yz) thus produced can be quite
large near a free boundary (free edge, cut-out, an open hole) and may influence
the failure of the laminate [?]. The laminate stacking sequence can significantly
influence the magnitude of the interlaminar normal and shear stresses, and thus
the stacking sequence of plies can be important to a designer. It has been reported
that the fatigue strength of a (15/45)s Boron fibre/epoxy laminate is about 175
28
MPa lower than a (45/15)s laminate of the same system. The interlaminar normal stress, ?zz, changes from tension to compression by changing the stacking
sequence and thus accounts for the difference in strengths. In this case progressive
delamination is the failure mode in fatigue. Approximate analytical methods and
numerical approaches such as finite difference and finite element (FE) techniques
[?] can be used to analyse the interlaminar stress distributions near free edges,
open holes, bolted joints (a complex three-dimensional, 3-D, problem) and help
to identify the optimum fibre orientation and laminate stacking sequence for the
given loading and kinematic boundary conditions. The lay-up geometry of a composite strongly affects not only crack initiation but also crack propagation, with
the result that some laminates appear highly notch sensitive whereas others are
totally insensitive to the presence of stress concentrators. The selection of fibres
and resins, the manner in which they are combined in the lay-up, and the quality
of the manufactured composite must all be carefully controlled if optimum toughness is to be achieved. Furthermore, materials requirements for highest tensile and
shear strengths of laminates are often incompatible with requirements for highest
toughness. Compared with fracture in metals, research into the fracture behaviour
of composites is in its infancy. Much of the necessary theoretical framework is not
yet fully developed and there is no simple recipe for predicting the toughness of
all composites. We are not able yet to design with certainty the structure of any
composite so as to produce the optimum combination of strength and toughness.
In metallic and plastic materials, even relatively brittle ones, energy is dissipated
in non-elastic deformation mechanisms in the region of the crack tip. This energy
is lost in moving dislocations in a metal and in viscoelastic flow or craze formation
in a polymer. In composites, the fibres interfere with crack growth, but their effect
depends on how strongly they are bonded to the matrix (resin). For example, if
the fibre/matrix bond is strong, the crack may run through both fibres and matrix
without deviation, in which case the composite toughness would be low and approximately equal to the sum of the separate component toughness. On the other
hand, if the bond is weak the crack path becomes very complex and many separate damage mechanisms may then contribute to the overall fracture work of the
composite. For example, a brittle polymer or epoxy resin with a fracture energy G
0.1KJm−2 and brittle glass fibres with G 0.01KJm−2 can be combined together in
composites some of which have energies of up to 100 kJm−2 . For an explanation
of such a large effect we must look beyond simple addition. Fracture in composite
materials seldom occurs catastrophically without warning, but tends to be progressive, with substantial damage widely dispersed through the material. Tensile
loading can produce matrix cracking, fibre bridging, fibre pull-out, fibre/matrix
debonding and fibre rupture, which provide extra toughness and delay failure. The
fracture behaviour of the composite can be reasonably well explained in terms of
29
some summation of the contributions from these mechanisms but as said earlier it
is not yet possible to design a laminated composite to have a given toughness. Another important modelling issue is the fatigue life of the composite. In contrast to
homogeneous materials, in which fatigue failure generally occurs by the initiation
and propagation of a single crack, the fatigue process in composite materials is very
complex and involves several damage modes, including fibre/matrix de-bonding,
matrix cracking, delamination and fibre fracture (tensile or compressive failure in
the form of fibre microbuckling or kinking). By a combination of these processes,
widespread damage develops throughout the bulk of the composite and leads to
a permanent degradation in mechanical properties, notably laminate stiffness and
residual strength [?]. Although these complexities (free edge effects, impact damage, joints, fatigue life prediction) lengthen the design process, they are more than
compensated for by the mass savings and improvements in aerodynamic efficiency
that result. The finite element analysis is also a crucial component and the biggest
time saving strides have been in the user-friendly developments in creating the data
and interpreting the results using modern sophisticated graphical user interfaces.
The key is using parametric software to generate the geometry and the meshes.
Apparently it used to take Boeing (Phantom Works) in St Louis, USA, more than
six months to perform the initial FE element stiffness and strength analysis for
a complete aircraft and this now takes less than three weeks with a handful of
engineers, so composites can become more attractive. The majority of aircraft
control-lift surfaces produced has a single degree of curvature due to limitation
of metal fabrication techniques. Improvements in aerodynamic efficiency can be
obtained by moving to double curvature allowing, for example, the production of
variable camber, twisted wings. Composites and modern mould tools allow the
shape to be tailored to meet the required performance targets at various points in
the flying envelope. A further benefit is the ability to tailor the aeroelasticity of
the surface to further improve the aerodynamic performance. This tailoring can
involve adopting laminate configurations that allow the cross-coupling of flexure
and torsion such that wing twist can result from bending and vice-versa. Finite
element analysis allows this process of aeroelastic tailoring, along with strength
and dynamic stiffness (flutter) requirements to be performed automatically with
a minimum of post-analysis engineering yielding a minimum mass solution. Early
composite designs were replicas of those, which employed metallic materials, and
as a result the high material cost and man-hour-intensive laminate production
jeopardised their acceptance. This was compounded by the increase in assembly
costs due to initial difficulties of machining and hole production. The cost is directly proportional to the number of parts in the assembly and, as a consequence,
designs and manufacture techniques had to be modified to integrate parts, thereby
reducing the number of associated fasteners. A number of avenues are available for
30
reducing the parts count, amongst which are the use of integrally stiffened structures, co-curing or co-bonding of substructures onto lift surfaces such as wings and
stabilisers and the use of honeycomb sandwich panels. Hand lay-up techniques and
conventional assembly results in manufacturing costs 60% higher than the datum
and only with the progressive introduction of automated lay-up and advanced assembly techniques composites compete with their metallic counterparts. Also the
Figure 3.2: V22-Osprey Tilt-rotor plane
introduction of virtual reality and virtual manufacturing will play an enormous
role in further reducing the overall cost. The use of virtual reality models in engineering prior to manufacture to identify potential problems is relatively new but
has already demonstrated great potential. Bell Textron in the USA made a significant use of IT during the product definition phase (for the V22 Osprey Tilt-rotor,
Fig.1) to ensure right first time approach. Other manufacturing tools that can
reduce production cost and make composites more attractive are Virtual Fabrication (creating parts from raw materials), Virtual Assembly (creation of assembly
from parts), Virtual Factory (evaluation of the shop floor). Virtual manufacturing
validates the product definition and optimises the product cost; it reduces rework
and improves learning.
3.4
Manufacturing Techniques
The largest proportion of carbon fibre composites used on primary class-one structures is fabricated by placing layer upon layer of unidirectional (UD) material to
the designers requirement in terms of ply profile and fibre orientation. On less
critical items, woven fabrics very often replace the prime unidirectional form. A
number of techniques have been developed for the accurate placement of the material, ranging from labour intensive hand lay-up techniques to those requiring
high capital investment in automatic tape layers (ATLs). Tape-laying machines
operating under numerical control are currently limited in production applications
31
to flat lay-up and significant effort is being directed by machine manufacturers
at overcoming these problems associated with laying on contoured surfaces. The
width of UD tape applied varies considerably from about 150 mm down to a single
tow for complex structures. The cost of machinery is high and deposition rates
low. In 1988 the first Cincinnati tape layer was installed in the Phantom Works
and in 1995 a seven axis Ingersol fibre placement machine was installed. This gave
the capability to steer fibres within an envelope of 40 ft x 20 ft with a 32-tow
capability. An overwing panel had been manufactured where it was able to steer
around cut-outs. Collaboration with DASA on global optimisation software was to
be completed at the end of 1998. This software is claimed to have produced a 13%
weight saving. Other applications include an engine cowling door, ducting with
a complex structure, FA18 E/F and T45 horizontal stabiliser skins. Its capacity
was extended to take a 6-inch wide tape and Boeing 777 has been converted from
hand lay-up to fibre placed (back to back then split) spars with a saving $5000 per
set. Bell Textron has a 10-axis Ingersol, contoured automatic tape laying machine
for the B609 skin lay-up, which is placing a 6 in wide T300 tape onto an inner
mould line Invar tool with pre-installed hat stringers. Fibre placement and filament winding technologies are also being used to manufacture components for the
V22 [?]. Once the component is laid-up on the mould is enclosed in a flexible bag
tailored approximately to the desired shape and the assembly is enclosed usually
in an autoclave, a pressure vessel designed to contain a gas at pressures generally
up to 1.5 MPa and fitted with a means of raising the internal temperature to that
required to cure the resin. The flexible bag is first evacuated, thereby removing
trapped air and organic vapours from the composite, after which the chamber is
pressurised to provide additional consolidation during cure. The process produces
structures of low porosity, less than 1% and high mechanical integrity. Large autoclaves have been installed in the aircraft industry capable of housing complete
wing or tail sections. Alternatively, low cost non-autoclave processing methods can
be used like the vacuum moulding (VM), resin transfer moulding (RTM), Figure
2, vacuum assisted RTM (VARTM) and resin film infusion (RFI). The vacuum
Figure 3.3: An aircraft composite wing rib element produced by RTM
moulding process makes use of atmospheric pressure to consolidate the material
while curing, thereby obviating the need for an autoclave or a hydraulic press.
32
The laminate in the form of pre-impregnated fibres or fabric is placed on a single mould surface and is overlaid by a flexible membrane, which is sealed around
the edges of the mould by a suitable clamping arrangement. The space between
the mould and the membrane is then evacuated and the vacuum is maintained
until the resin has cured. Quite large, thin shell mouldings can be made in this
way at low cost. The majority of systems suitable for vacuum only processing are
cured at 60◦ -120◦ C and then postcured typically at 180◦ C to full develop properties. In 1991 the evaluation of this method started at the Phantom Works using
the resin system LTM10 (low temperature moulding) and they created a small
allowables database for their X36 fighter research aircraft study. In 1996 McDonnel Douglas characterised LTM45 EL for the Joint Strike Force (JSF) prototype
and generated design allowable data. In 1998 Boeing also produced LTM45 EL
data. LTM10 applications demonstrated for complex parts with a 140◦ F cure under vacuum include a serpent inlet duct. A box using LTM10 was shown at the
1998 Farnborough Airshow. A research programme at NASA Langley is looking
at the development of 180◦C material properties using low temperature curing
resins. The main advantages of LTM systems are the potential to use autoclave
free cures, the use of cheaper tooling and reduced springback of parts. RTM and
RFI are the predominant curing processes being developed today of which there
are several variations. In traditional prepreg technology the resin has already infiltrated the fibres and processing mainly removes air and volatiles, consolidates
and cure. RTM in its simplest form involves a fabric preform being placed in an
enclosed cavity and resin forced into the mould to fill the gaps under pressure
and cure. The RFI method utilises precast resin tiles with thickness ranging from
0.125 to 0.25 in. This approach reduces the number of consumables used, but is
very process sensitive relying on the resin being of sufficiently low permeability to
fully impregnate the fabric before cure advances too far. The use of an autoclave
or press to apply pressure varies. The RFI process is being applied within the
ACT (Advanced Composites Technology) Programme in conjunction with traditional autoclave processing. Heat is the energy source to activate the resin cure,
but some resin systems can be activated by radiation. Wright Paterson claim that
thermal oven processing could save 90% of autoclave processing time and energy
and hence 50% cost. There is also a radiation curing process developed jointly by
NASA and ACG (Advanced Composites Group) and of innovative electron beam
cured structures being developed by Foster Miller, Lockheed Martin and Oakridge
National Laboratories in the USA [3]. The vacuum assisted RTM is a liquid resin
infusion process and is currently considered by the aircraft industry to be the
favoured low cost manufacturing process for the future. It is an autoclave-free
process that has been identified as reducing the cost of component processing. It
is reported that dimensional tolerance and mass measurements are comparable
33
with stitched RFI autoclave panels. A conventional blade stiffened test panel (3
ft x 2 ft with a 4 in high blades 0.5 in thick) has been manufactured recently at
NASA by using the VARTM method achieving a reasonable quality. Further cost
reduction when manufacturing with composites will be achieved by reducing the
assembly cost, by moving away from fastening (drilling of thousands of holes followed by fastener insertion and sealing) towards bonding and to assembly with less
or no expensive jigging. Bell Textron among others are building and developing a
number of structures (for the V22 and B609) where they are applying state of the
art composites technologyprocesses to achieve a unitised approach to manufacturing and assembly. There are of course significant certification challenges with an
adhesively bonded joint for a primary aircraft structure application that need to
be addressed.
3.5
Application in Aircraft Construction
In the pioneering days of flight aircraft structures were composite being fabricated
largely of wood (natural composite), wire and fabric. Aluminium alloys took over
in the 1930s and have dominated the industry to the recent time. Wooden structures did however persist until world war II and the de Havilland mosquito aircraft
(DH98) constructed of a plywood-balsa-plywood sandwich laminate probably represents the high point of engineering design with wood. The DH91 Albatross
airliner in 1937 was moulded as a ply-balsa-ply sandwich construction and the
Spitfire fuselage in 1940 was designed and built of Gordon Aerolite material that
was a phenolic resin incorporating untwisted flax fibres that could be regarded as
the precursor of modern fibre reinforced plastics. Current civil aircraft applications
have concentrated on replacing the secondary structure with fibrous composites
where the reinforcement media has either been carbon, glass, Kevlar or hybrids
of these. The matrix material, a thermosetting epoxy system is either a 125C
or 180C curing system with the latter becoming dominant because of its greater
tolerance to environmental degradation. Typical examples of the extensive application of composites in this manner are the Boeing 757, 767 and 777 and from
Europe the Airbus A310, A320, A330 and A340 airliners. The A310 carries a vertical stabiliser (8.3 m high by 7.8 m wide at the base) a primary aerodynamic and
structural member fabricated in its entirety from carbon composite (now 10-20/Kg
for large tow HS fibre) with a total weight saving of almost 400 Kg when compared
with the Al alloy unit previously used. In addition the CFRP fin box comprises
only 95 parts excluding fasteners, compared with 2076 parts in the metal unit,
thus making it easier to produce. The A320 has extended the use of composites to
the horizontal stabiliser in addition to the plethora of panels and secondary control surfaces leading to a weight saving of 800 Kg over Al alloy skin construction.
34
As an indication of the benefit of such weight saving it has been estimated that
1 Kg weight reduction saves over 2900 litres of fuel per year. Larger amounts of
FRPs are used in the bigger A330, A340 models and of course in the A380 super
jumbo airliner built by the European Airbus consortium. The wing trailing edge
panels are made of glass and carbon fibre reinforced plastics using a new resin film
infusion method (RFI), in which resin film, interleaved between glass and carbon
fabric layers, when the laminate is laid up, melts when heat is applied. Melted
low-viscosity resin migrates easily through the thickness of the laminate where it
cures to form the final component. A hybrid aluminium/glass reinforced plastic
Figure 3.4: A380 Glare fuselage crown (Courtesy of Airbus S.A.S.)
system (GLARE) is used for the A380 fuselage crown, Fig.3 that results in reduced
weight, increased damage tolerance and improved fatigue life. The new Boeing 787
Dreamliner structure including the fuselage, wings, tail, doors and interior is made
of over 50% composite materials (80% by volume). However, in April 2008, Boeing announced a fourth delay, shifting the maiden flight to the fourth quarter of
2008, and the initial deliveries to the third quarter of 2009 after problems with
external suppliers and late redesign work. Boeing pursued a model that is 80%
outsourcing ending up with a supply chain that is too cumbersome to manage in
addition to the new design and manufacturing challenges faced when build with
composites. There are major assembly issues with the composites fuselage sections
and there are problems with what is coming out of the autoclave. There are also
still many worries on the safety side with electromagnetic hazards like lightening
strikes, since the material does not conduct away electrical energy. Composites
have been used in Bell helicopters (Dallas Fort Worth, USA) since the 1980s following their ACAP (advanced composites airframe programme) when they were
able to achieve a 20% reduction in weight on metallic airframes. All blades on their
newer vehicles (412, 407, 427, 214, 609, OH58D, V22) are all composite. The V22
Osprey tilt-rotor has an all-composite wing, chosen for its stiffness critical design,
which was only possible in composites at low enough weight. Early demonstrators
(from 1960s onwards) did not meet expectations until composites were available.
35
The skins of the V22 wing are I-stiffened with co-bonded spars and bolted on ribs
(the civil 609 version will use bonded ribs). The pylon support spindle is currently
filament wound but it is planned to fibre place this part. Over 60% of the whole
vehicle weight is carbon composite, plus a further 12% in GRP. The V22 uses tape
laying, hand lay-up and filament winding for most of the composite construction
but is moving to fibre placement for the 609 civil version [3]. Mechanical fastening
features heavily in the composite structure, some 3000 each side of the wing, is
introduced by manual drilling with templates, but they are looking towards the
use of automated drilling and probably involving water jet cutting. Other examples where composites will be extensively applied are the future military cargo
Airbus A400M and the tail of the C17 (USA). A 62 ft C-17 tail demonstrator has
been successfully completed yielding 4300 fewer parts (including fasteners), weight
reduced by 20% (260 Kg) and cost by 50% compared with the existing metal tail.
Without exception all agile fighter aircraft currently being designed or built in
the USA and Europe (e.g., JSF, EFA) contain in the region of 40% of composites in the structural mass, covering some 70% of the surface area of the aircraft.
The essential agility of the a/c would be lost if this amount of composite material
was not used because of the consequential mass increase. Many of the materials,
Figure 3.5: Hunter 1.
processes and manufacturing methods discussed earlier in the paper have been
implemented in their construction and in the construction of UAVs such as the
American Desert Hawk, Pegusus X47, X45, Taranis (UK), Hunter 1, 2 (Figure 4)
and Predator to mention just a few. Another interesting relatively new field of
development in the military a/c sphere is that of stealth, a concept that requires
the designer to achieve the smallest possible radar cross section (RCS), to reduce
the chances of early detection by defending radar sets. The essential compound
curvature of the airframe with constant change of radius is much easier to form in
composites than in metal while radar absorbent material (RAM) can be effectively
produced in composites.
36
3.6
Summary
The application of carbon fibre has developed from small-scale technology demonstrators in the 1970s to large structures today. From being a very expensive
exotic material when first developed relatively few years ago, the price of carbon
fibre has dropped to about 10/Kg, which has increased applications such that the
aerospace market accounts for only 20% of all production. The main advantages
provided by CFRP include mass and part reduction, complex shape manufacture,
reduced scrap, improved fatigue life, design optimisation and generally improved
corrosion resistance. The main challenges restricting their use are material and
processing costs, damage tolerance, repair and inspection, dimensional tolerance
and conservatism associated with uncertainties about relatively new and sometimes variable materials. Carbon fibre composites are here to stay in terms of
future aircraft construction (manned or unmanned) since significant weight savings can be achieved. For secondary structures weight savings approaching 40%
are feasible by using composites instead of light metal alloys, while for primary
structures such as wings and fuselages 20% is more realistic [7]. These figures can
always improve but innovation is key to making composites more affordable.
3.7
References
To be integrated into the overall reference file!
[1] Concise encyclopaedia of composite materials. Ed. A. Kelly, Pergamon,
1994. [2] Hull, D. An Introduction to Composite Materials. Cambridge University Press, 1987. [3] Aerospace composite structures in the USA. Report for the
International Technology Service (Overseas Missions Unit) of the DTI, UK, 1999.
[4] Matthews, F.L and Rawlings, R.D. Composite Materials: Engineering and Science. Chapman & Hall, 1994. [5] Matthews, F.L, Davies, G.A.O., Hitchings, D.
and Soutis, C. Finite Element Modelling of Composite Materials and Structures.
Woodhead Publishing Ltd., 2000. [6] Kashtalyan, M. and Soutis, C. Int. Applied
Mechanics, 38(2002), 641-657. [7] Soutis, C. Carbon fibre reinforced plastics in
aircraft structures. Materials Science & Engineering A, 412 (1-2), (2005), 171-176.
37
Chapter 4
Avionics for Small Unmanned
Aircraft Systems
4.1
Introduction
The relevance of unmanned aircraft systems (UAS) as a sensor platform for scientific experiments has significantly increased over the last years. This is especially
the case regarding small, low-cost UAS, as these systems can be easily equipped
and deployed for various applications. The miniaturisation of electronic hardware
and sensors allows not only for research of flight control strategies for small UAS;
the associated increasing degree of automation of these systems offers new experimental setups, not least in the field of atmospheric research. This is the case for
fixed-wing as well as rotary wing aircraft. Although these systems display different flight mechanical characteristics and therefore should be selected according to
the boundary conditions of the mission profile, several aspects of (linear) flight
control are similar or even identical. This comprises for example the cancellation
of atmospheric disturbences, attitude and flight path control as well as the precise
determination of attitude and spatial position of the system.
At present, there is a multitude of different commercial, academic and open
source autopilot systems for small UAS available. These autopilots not only consist of the actual flight control computer, but also of the necessary sensors for
measuring the control variables. While this can be realised with different sensory equipment, in many cases a simplified attitude and heading reference system
(AHRS) is implemented. These systems utilise Kalman-filters to combine the sensor information of an inertial measurment unit (IMU) with the signals of a global
navigation satellite system (GNSS) to augment attitude and position determination. Regarding the overall system, autopilots exhibit strong interaction with the
38
telemetry and ground station subsystems, as these form the interface between
user and aircraft. Therefore the man-machine interface and the communication
link between ground and aircraft have to be considered carefully. The mentioned
subsystems allow for automatic flight of an unmanned aircraft but it does not
reach the level of autonomous flight. Autonomy of an UAS can be realised, if the
system is capable of detecting moving as well as fixed obstacles and changing the
flight plan independently to avoid collision. This field, also called Sense & Avoid
has therefore become more important recently, especially with increasing numbers
of UASs in use. Due to the weight of the necessary additional sensors, functional
Sense & Avoid systems cannot be implemented on small UASs at present.
4.2
General Aspects of Flight Control for Unmanned Systems
The development of autopilot systems for small unmanned aircraft up to the current state of the art mainly happend within the last ten to fifteen years. Although very similar controller architectures as in manned aircraft are commonly
implemented, the development of the low-cost avionic systems is to a great extent seperated from classical aviation. One reason is, that the construction and
many components of such UAS are derived from radio controlled model airplanes.
This has the advantage, that unexpensive commercial parts are available to implement an aircraft comparatively swift, but on the other hand certification and
airworthiness standards, common in aviation, are mostely neglected. This is also
the case for much of the avionic hardware, like onboard computers and sensors.
For many small- and medium-scale UAS applications in atmospheric research and
other scientific domains this difference is outweight by the advantage of airborne
measurements. Due to the high increase of deployment of unmanned aircraft for
various applications, the civil aviation authorities have started to define rules for
certification and operation of UAS (s. WG 4). While these are still being finalised,
it emerges that the general guidelines will become more strict, whereas the focus
of regimentation lies on systems with a take-off weight (TOW) higher than 25 kg
flying beyond visual line of sight. This suggests, that for micro- and mini-UASs
(micro - up to 2 kg TOW, mini - up to 25 kg TOW) the changes regarding the
system’s components and its operation will be manageable. As this report focusses
on small aircraft systems and controller architectures currently in use, the detailed
aspects concerning certification and airworthines standards of avionic systems are
not further discussed here.
Regarding their characteristics of motion, aircraft are nonlinear dynamic sys-
39
tems with six degrees of freedom - three componets each for the vectors of velocities and angular rates. There are different approaches concerning nonlinear
flight control theory, but the mathematical description and the proof of stability
are more complex compared to linear control. As a complete theory on linear control systems, their composition, stability and optimisation exists, most low-cost
autopilots utilise these control strategies. In order to implement such a control
strategy, it is necessary to linearise the system around a given point of interest
or reference point, where the derived control parameters are valid. For a small
UAS such a reference point is for example straight cruise flight at a given altitude.
In comparision to manned aircraft, where the controller design is done for many
reference points within the whole possible flight envelope, miniature autopilots
normally posses only one set of control parameters. This is possible because on
the one hand, the aircraft is flying mostly at constant reference speed within a
limited scope of altitutdes, where the air density is nearly constant; this leads to
boundary conditions approximately around one reference point. One the other
hand the controllers are generally designed for robust charcteristics and are not
highly optimised with regard to control performance at the given reference point.
Another practical reason for this design is, that commercial autopilot systems of
this class shall be useable for arbitrary aircraft configurations, featuring different aerodynamic coefficients. As the aerodynamics have vast impact on the flight
mechanical characteristics and therefore on the design of the control parameters,
the set of parameters has to be determined for every aircraft anew. In general
a complete set of aerodynamic coefficients from wind tunnel tests would be used
for this design. Since it is quite expensive to derive this complete data set, the
parameter design of low-cost autopilots is often done using limited data sets and
approximations. This also includes that the fine tuning of the control parameters
is realised in flight test, in which one control loop after the other is determined.
4.3
Basic Flight Control Architecture
As stated before, an aircraft is a nonlinear dynamic system with six degrees of
freedom. With the assumption of a linearised system it is possible to divide the
aircraft motion into its lateral and longitudinal components and analyse them
separately. This implies that changes of the state of the aircraft over time are
assumed to be minimal from one time step to another and so the coupling effects
between lateral and longitudinal motion components can be neglected. This means
that for both forms of motion one control loop, as depicted in Fig. 4.1, has to be
designed. It can be seen that the controlled system consists of the blocks aircraft,
sensors and actuators, which is disturbed due to influence of wind and gust.
The controllers for lateral and longitudinal motion are cascaded within three
40
Figure 4.1: Basic design of a cascaded flight control loop.
different loops and consist of four control elements each. The principle functionality of the four control elements is identical for both, lateral and longitudinal
control. The outermost element is the navigation block, where the flight plan of
the mission is being processed. The flight plan yields desired values for altitude
as well as position and derived from this information a desired track angle. These
values which are compared with available sensor signals, for example deliverd by an
inertial navigation system (INS) and a GNSS-receiver. These error signals are fed
into the flight path controller, which in the lateral loop controls the track and in
the longitudinal loop the altitude. The flight path controller delivers commanded
signals, necessary to maintain the desired trajectory, for pitch, yaw and bank angle
to the attitude controller in the next inner loop.
The desired attitude angles are compared with the actual ones normally also
delivered by an INS. Using these measured errors, the attitude controller computes control signals for the existing actuators. In case of a fixed-wing aircraft
this would be the elevator for longitudinal motion and rudder as well as aileron for
lateral control. Many micro UASs are missing the rudder, as the airframe is easier
to build and the missing actuator leaves more payload mass. Nevertheless, larger
aircraft above ten kg TOW are in comparision sigificantly more inert, which leaves
the necessity of a rudder to ensures timely adequate response to input changes.
The attitude controller of a rotary wing UAS in a configuration of a quadrocopter
for example uses the motors of the different rotors as actuators. By altering the
number of rotation of the rotors, incremental moments around the three spatial
axis can be realised to change the aircraft’s attitude. Through the attitude controller, the aircraft acquires the demanded attitude angles to follow the flight path
- this control loop is therefore also called stabiliser.
41
The innermost loop is the so called damper, which minimizes the angular rates
around the three spatial axis due to wind disturbances and therefore reduces the
system’s susceptability towards gust. The longitudinal loop comprises the damper
for pitch rate, while the lateral loop consists of the dampers for yaw and roll rate.
This innermost loop does not get a direct commanded angular rate as an input
from an outer loop; it adds an incremental part to the control signal coming from
the attitude controller, which results from cancelling existing disturbances. The
combination of damper and stabiliser is a functionality indispensable for flight
path control and automatic navigation and also denoted as basic controller.
4.4
Attitude and Heading Refernce System
The precise determination of the attitude angles as well as the aircraft’s position
is not only important for automatic flight control, but is also crucial for the quality of the payload data. Because of the necesseity of miniaturisation the INS is
composed of so called MEMS-sensors (Micro-Electro-Mechanical-System). These
sensors are very small and inexpensive but posess the singnifikant disadvantage of
high sensor drift, which would result in high position and attitude errors after very
short time. In order to account for this problem, the highfrequency measurements
of the INS are therefore combined and augmented with long time stable and precise but less frequent GPS measurements. The so called Kalman filter has become
a quasi standard for the data fusion of sensor measurements of inertial and satellite navigation. There are different Kalman filter concepts to realise the fusion of
INS and GNSS measurements, whereas the most common is the so-called loosely
coupled integration. These navigation algorithms use the position and velocity
information calculated by the GNSS-receiver to augment the INS measurements.
This approach implies the problem, that the GPS aiding fails as soon as signals
from less than four satellites are received, as the GNSS-receiver needs this number
of satellites to compute consistent position and velocity information. The loss of
four satellites can happen due to different reasons. On a fixed-wing UAS the receiver is typically mounted on top of the wing; during curve manoeuvres with high
bank angles it is for example possible, that the wing shadows the receiver from
acquiring signals from certain satellites. Rotary wing UASs can be deployed in
urban environments where the connection to satellites might be lost due to buildings and other obstacles.
A possibility to improve the Kalman filter navigation algorithm is to use the
so-called tightly coupled integration of INS and GNSS measurements. The principle design of such a system is depicted in Fig 4.2 in a closed-loop variant. Tightly
coupled means that, in contrast to the loosly coupled system, not calculated po42
sition and velocity information is used to augment the INS, but rather the raw
data coming from the GNSS-receiver. This raw data consists of: pseudo range, the
measured distance beween satellite and receiver, delta range, the relative velocity between satellite and receiver and/or the time differenced carrier phase signal,
as an additional measure for the relative velocities of receiver and satellite. This
means as a result, that the augmentation of the INS is possible even with less than
four satellites in the field of view. Closed-loop on the other side means that the
corrected navigation solution from the INS is fed back to the navigation filter to
correct the measurements from the GNSS-receiver and that data from the navigation filter is used to correct acceleration and gyro sensor errors. In other words
this can be seen as a online sensor calibration.
Figure 4.2: Basic design of a cascaded flight control loop.
In order to estimate the necessary error signals, the Kalman filter utilises an
error state vector with a certain number of entries. A possible combination of
these entries would be for example: the errors in position, velocity, attitude, gyro
bias, acceleration bias, receiver clock error, and receiver clock drift; the use of
additional information, for example coming from a magnetometer is possible. In
General, the Kalman filter works in two phases prediction and update. The prediction is executed at the measurement frequency of the INS to determine the
state vector of the filter and corresponding data for the next time step. Parallel
to the propagation process, the navigation solution, namely the vectors of attitude, position and velocity, is calculated using the measurements of accalerations
and angular rates coming from the INS. These are processed through a so-called
strapdown algorithm, which allows the computation of navigation data from bodyfixed inertial sensors. The update process begins when new measurements from the
43
GNSS- receiver have arrived. During this update the received measurements, in
case of a tightly couple system, for pseudo range, delta range and if used the time
differenced carrier phase are processed. This measurement vector consists of the
differences between predicted and measured values of pseudo ranges, delta ranges
and carrier phase corresponding to the number of received satellites. The improvement of the navigation data is realised using the weighted information about the
difference between the real measurement and the expected measurement vector,
the so-called innovation or measurement residual.
4.5
Ground Station and Telemetry
While the avionic system is responsible for automatic following of a planned mission, the ground station and the corresponding software in most cases cover two
different aspects: the planning of a mission before flight and the monitoring of
a flight through the operator. The communication link between avionic system
and ground station is realised through a telemetry system, which transmits actual state information of the aircraft to the ground and for example changes of
a mission to the autopilot. In case of micro- and mini-UAS the ground station
system is normally realised with a mobile computer combined with an antenna
for telemetry, which due to small size make operation in the field more comfortable. The process of planning a mission is done by the operator beforehand; the
geographic correctness of the map material used for the planning is important, as
unprecise map data would result in a displaced flight plan. For many autopilot
systems and their corresponding ground station software, the mission planning is
done manually. It is also possible to integrate a digital terrain model of the flight
area to (automatically) ensure that the flight plan does not collide with exisiting
elevations. In general it can be stated that the automisation of flight path planning
with respect to desired mission, payload parameters and environmental conditions
is an object of development in the field of low-cost UAS systems.
An example of a ground station control software and the main components
commonly implemented is shown in Fig. 4.3. The main elements for planning and
monitoring a mission are:
• The mission manager (top left), which allows for setting up a new mission
and the organisation of map material and corresponding flight plans,
• the map (middle), where the flight plan of the actual mission is shown, which
can be generated either by hand or automatically,
44
• the mission settings (right), where all waypoints of a mission are depicted
including altitudes and length of the trajectory
• and the primary flight display (bottom left), where the operator can monitor
the actual state of the aircraft.
Figure 4.3: Example of a ground station software and its main components.
The mission and flight path planning has a strong interaction with the navigation block of the flight controller shown in Fig. 4.1. In general a mission of an
UAS is defined by a number of waypoints, which have to be reached in correct
order to fulfill the mission. The standard procedure assumes that a waypoint is
reached, when the aircraft has a certain distance to it and the next waypoint is
acquired. The limitations of this approach become apparent in harsh cross wind
situations. The aircraft is pushed to the side and the flight controller corrects this
by calculating the track angle and consequently heading anew depending on the
measured orientation error. The disadvantage of this procedure is, that on the
one hand there is no direct control of the trajectory between single waypoints;
the aircraft flies a curved trajectory while pointing in the direction of the next
waypoint. On the other hand rather complex flight patterns can only be realised
with a multitude of waypoints.
45
A possible solution for this problem is to determine the flight plan of a mission
using mathematically defined spline curves. This means that the trajectory of a
mission is composed of multiple three-dimensional spline elements interlinked with
each other. Each spline is defined by a certain number of points in geographic coordinates, of which two mark beginning and end of the spline, while the remaining
control points determine the shape of the spline. The desired altitude between
beginning and end of the spline can be defined either in a linear or nonlinear way.
An example of such a flight path composed by cubic splines is given in Fig. 4.3.
The main advantage of a mathematically definded flight path is, that due to the
position determination from the combination of INS and GNSS a lateral flight
path error, also called cross track error can be determined in every control step.
This cross track error is a direct measure for the flight path accuracy and can
therefore be minimized by the outermost loop of the flight controller. This means
that the trajectory between the waypoints defining starting and end of a spline
can be directly controlled, resulting in a higher flight path accuracy in presence
of stronger wind conditions. The example from Fig. 4.3 also shows that rather
complex flight path patterns can be realised with few interlinked spline curves.
Regarding telemetry systems for small UAS there is a multitude of different
solutions possible and in use, each featuring different ranges and other boundary
conditions. Especially for low-cost UAS, used for many scientific applications,
it is difficult to define a homogeneous architecture, since many systems use free
frequencies which internationally are subject to different regulations. Commonly
used frequency bands used are the Industrial, Scientific and Medical (ISM) as well
as Short Range Devices (SRD) band. These bands are free to use without an
additional assingment of a distinct frequncy by national network agencies, but,
depending on the county, where it is used, are limited in their power output resulting in different telemetry ranges. In the ISM band most telemetry modules
use a frequency of 2.4 GHz and in the SRD band frequencies of 868 − 870 MHz.
The advantage of these solutions is the availability of low-cost radiocummunication devices, which are easily implemented and do not exhibit the need of an
additional license. The downside is, beside the range limitation due to power output limitations, that because of the high increase of devices using the ISM and
SRD bands robustness of the telemetry system towards interferences cannot be
completely guaranteed. Other approaches for telemetry systems, which are more
difficult to implement, use for example satellite communication links or mobile
telecommunications technologies like UMTS. Also, it is possible to acquire a distinct frequnecy from a national network agency, which on the other hand requires
a licensed operator and togehter with the license for the device creates additional
costs.
46
Chapter 5
Helicopters
By Morten Bisgaard, Aalborg University (DK)
5.1
Using Helicopters in ABL Science
Helicopters are highly versatile flying machines capable of hover, slow and fast
flight. Furthermore, it offers true 4 degrees of freedom flight as it can fly forward/backwards, up/down, left/right, and turne independently. These flight characteristics makes them ideal suited to a subset of the tasks identified in chapter
??.
Note that in this report, a distinction has been made between Helicopters and
Multicopters. Helicopters are hereafter defined as a having one large main rotor
and one smaller vertical tail rotor which relies on pitching of rotor blades for
control (see figure ??. Multicopters are hereafter defined as having four or more
main rotors which relies on changing individual rotor speed for control – for further
information about multicopters see chapter ??.
In this context only unmanned helicopters with a certain level of autonomy
will be discussed. As a minimum the helicopter must be capable to autonomous
take off and landing as well as waypoint flight. Furthermore, a distinction between
small (¡10 kg), medium (10 - 100 kg) and large (¿100 kg) helicopters will be made
here.
Advantages of helicopter UAVs
Hovering
The capability of hovering in one place make the helicoper idealy suited for certain
measurement tasks.
47
Agility
Helicopters are highly agility aircrafts and can therefore make rapid transisions
between measurement points.
Payload
The design of most helicopters allow them to carry very large loads compared to
their own mass. Futhermore, payload can be carried in wires (slung loads) which
can be used to seperate measurement equipment from the helicopter.
Disadvantages of helicopter UAVs
Complexity
The mechanical construction of a helicopter is complex and therefore requires a
short maintenance cycle.
Slow
Helicopter UAVs normally have a slower cruise speed compared to fixed wing
UAVs.
In the following, the advantages and disadvantages of using helicopter UAV for
different mission will be discussed.
5.1.1
Helicopters for Air Chemistry: Volcanos
The flight envelope of the helicopter is well suited for the purpose of doing air
chemistry measurements (see ??). The requirements for endurance, flight speed
and payload power and mass on this mission are well withing the range for many
helicopter UAVs. However, the requirement of a operational ceiling of more than
3000 m does present a limiting factor as the service ceiling of many small and
medium helicopters are very close to this value. Larger heliocpter UAVs (¿ 100
kg) typically have ceilings that goes significiantly beyound 3000 m.
High particle loads is also a concern that must be considered when choosing
power plant for the helicopter. An electrical power plant is perhaps the best suited
for this purpose, compared to piston or turbu-prop, if care is taken to dust proof it.
Furthermore, an electrical power plant is well suited for high altitude operations,
but will also result in significantly less endurance compare to a piston engine power
helicopter.
48
5.1.2
Helicopters for Air Chemistry: Polar regions
Requirements for this mission is suited for some medium and many large helicopters. The problem for many medium and small helicopters are endurance and
top speed. Operating any UAV in a polar region can be an advantage and this
is also true for helicopter UAVs. Challenges for a helicopter UAV are similar to
those of fixed wing uavs. Icing can be a consern that must be taken seriously both
with fixed wing and with helicopters. Extreem cold can give problems with engine
and electronics.
5.1.3
Helicopters for Air Chemistry: Marine Boundary
Layer
The problem with undertaking this missing with many avilable helicopter UAVs
are the required range of 500 km which is too far for many medium and even large
helicopters.
5.1.4
Helicopters for Air Chemistry: Urban Areas
For operation in urban areas, the use of helicopters are highly recommended due
to their slow flight and hover capability. An urban area is densely occupied with
obstracles and a helicopter is better suitede that a fixed wing to navigate such
areas. However, endureance times up to 6 hours are only achievable for very few
helicopter UAVs.
5.1.5
Helicopters for Air Chemistry: Long Endurance Flight
for Climatology
Helicopters are not well suited for this mission.
5.1.6
Helicopters for Stable Boundary Layer Investigation
Helicopter UAVs can be utilized for this mission but it does not require the special
flight characteristics of a helicopter.
5.1.7
Helicopters for Wind Profiling
Helicopter UAVS are idealy suited for this mission. The capability to hover and do
rapid vertical translations provides an oppertunity for unique measurements. Very
high wind speed can be encountered during this mission which must be considered
49
when choosing UAV, however, most helicopters can handler very high wind speeds
and gusts compared to fixed wing air crafts.
5.2
State Of The Art
The development of helicopter UAVs have undergone a huge advancement in recent
years, but have traditionally been lacking behind the developments of fixed wing
UAVs. This has been especially clear in the military marked where operational
helicopter UAVs have been very rare. Only the Northrop Grumman MQ-8 Fire
Scout and the Boeing A160 Hummingbird have undergone serios flight trials and
only the Fire Scout have an operational history. However, both academic and
commercial systems can now be considered mature with a wide range to products
avilable in the medium and large range. Small helicopter UAVs have received little
attention and the role of these have almost entirely been taken over by multicopter
type aircrafts.
Today, commercial avilable product feature a high level of reliability and operational capabilities, but still only little autonomy. The primary focus for most
platforms remains survailance of different kinds.
All commercial products discussed have are capable to autonomous take-off
and landings and waypoint type trajectory tracking. More advance features like
Sense-and-Avoid are not found on the typical commercial products.
5.3
Avilable Products
When presenting specifications like endurance, payload, altitude ceiling etc. it is
important to remember that not all maximum values can be achieved simultaneously: Ie. to achieve maximum payload capacity, shorter endurence must be
accepted. To achieve maximum endurance payload capacity must be swapped for
fuel. Price is not taken into acount in this discussion.
Most of the larger helicopter UAVs have custom build airframes but for many
of the medium and most of the smaller helicopters, the airframes are commercial
avilable radio controlled (RC) models.
Schiebel CamCopter S-100
The CamCopter (MTOW 200 kg) is one of the oldest and most mature systems
avilable, capable of day and night operations, adverse weather operations, beyond
line-of-sight operation (¡200km). It can carry 34 kg up to 6 hours and has a service
ceiling of 5500 m. A high level of redundance and quality-control ensures a reliable
product. It is currently being operated by at least 3 different armies around the
50
Figure 5.1: The Schieble CamCopter
world - including the German and United Arab Emirates army.
CybAero APID
The APID (MTOW 160 kg) is powered by a piston engine and has been tested
both in artic and desert environments. It can fly for up to 6 hours with a payload
of up to 40 kg and with a speed of 110 km/h (90 km/h cruise speed).
Figure 5.2: The CybAero APID
Swiss UAV Neo S-300
The Neo S-300 (MTOW 100 kg) is a jet powered helicopter capable of carrying
35 kg payload for up to 1.5 hours and at speeds of up to 120 km/h. The UAV
furthermore features a parachute rescue system that, in case of an emergency
failure can bring the UAV relatively saftely to the ground.
51
Figure 5.3: The Swiss UAV Neo
Saab Skeldar
The Saab Skeldar (MTOW 200 kg) has been deriviative from the CybAero APID
and is today a very mature product and Saab offeres a highly developed ground
control system with the helicopter. The Skeldar has a piston engine and can fly for
Figure 5.4: The Saab Skeldar
up to 5 hours with up to 40 kg payload, up to altitudes of 4500 m and at speeds
of up to 130 km/h.
Rotomotion SR series
Rotomotion offeres a range of four helicopter UAVs from the electrical SR20
(MTOW 12 kg) to the piston engine powered SR200 (MTOW 50 kg). All helicopters can be purchased with Ground Control Software and auto take off and
landings. The SR200 can fly for up to 4 hours with 20 kg payload and at speed
of up to 80 km/h. The SR20 can fly for 20 min. with up to 4 kg payload and at
speeds of up to 50 km/h.
52
Adaptive Fligth Hornet Micro
The Adaptive Flight electrical Hornet Micro (MTOW 1.5 kg) is on of the smallest
helicopter UAVs on the marked and is designed specifically for observations missions. However, it should be possible to fit different sensor on the helicopter. The
Figure 5.5: The Adaptive Fligth Hornet
Hornet can fly for about 20 min. with a maximum speed of 55 km/h.
High Eye HE series
The HE series helicopters from High Eye include the HE 80 (MTOW 35 kg), the
HE 60 (MTOW 22 kg) and the HE 30 (MTOW 15 kg). A large focus is put on
imaging application but also other mission can be accomodated. Specific data are:
HE 60 (Endurance 4 hours, cruise speed 80 km/h), HE 30 (Endurance 2 hours,
speed 80 km/h).
weControl Bicopt
The Bicopt (MTOW 30 kg) features two independent piston engines and autorotation for safty landings. It can fly up to 1.5 hour with 10 kg payload and a
speed of up to 36 km/h.
53
Figure 5.6: The weControl Bicopt
54
Chapter 6
Sense and Avoid Techniques
6.1
Towards UAV flight in Civil Airspace
The development of Sense and Avoid (SAA) systems for Unmanned Aerial Vehicles
(U AV ), it’s still a unsolved problem and a strong research vector, mainly due to
two fundamental reasons:
1. The huge dissemination and increase of UAV applications and their ability
to perform operational activities in a cheaper and most reliable manner than
human piloted aircraft.
2. It’s common knowledge in the field as stated by ([7],[3]) that the main reason
for UAV not having access to civil airspace is the see and avoid requirement.
Stated also in FAA order 7610.4 and NATO [8], that UAV capability must
demonstrate a level of performance that meets or exceeds a human pilot
without the use of cooperative communication with other aircraft or prior
knowledge of the flight plans.
In order to perform SAA operations, UAV (and in the future human piloted
aircrats due to crowed airspace) must be equipped with perception systems that
enables them to make decisions based upon their environment. There are two
types of systems: Cooperative Traffic Avoidance systems (TCAS, ADS-B), that
are usually used in human piloted aircrafts and Non cooperative Traffic Avoidance
Systems: active like Radar [11], sonar and lidar systems [1] or passive like electrooptical/IR system that are more suited for small fixed wing UAVs.
It’s well known that vision systems have limitations, such as: low image quality, rapid attitude changes that together with the lack of a priori knowledge of the
environment, makes visual target tracking task very difficult in UAV platforms.
However, some computer visual technology, can be easily migrated for SAA applications.
55
There is some extensive work on optical flow algorithms, for UAV collisions
avoidance scenarios, such as the one developed by Ortiz[10]), that developed a
color optical flow algorithm, or even [4], that developed a combination of optical
flow techniques with stereo vision in order to avoid obstacles.
Utt [12] describe a vision-based sense and avoid system capable of real-time
detection of a small aircraft (a Beechcraft Bonanza) approaching in different configurations, with sufficient time to perform avoidance maneuvers (though they do
not divulge the actual detection range). Using three cameras, they achieve highresolution (about 0.5milli-radians/pixel), while maintaining a large field of regard
(about 90 degrees) on one side of the aircraft.
There are other approaches to SAA problem, [13] uses a human visual detection
and identification model that intents to use a database to identify UAV targets
in visible and IR spectral range. [5] uses a mixture of radar sensing with vision
cameras, [2] developed a vision based tracking method by combining the use of
multiple UAVs.
Finally what concerns visual techniques, we have the image morphology approach to SAA. Started with [6], later on used by [3] this method is based in use
of image morphology filtering combined with dynamic programming in order to
diminish false positives and track small features through out a sequence of images. The use of morphological filtering is popular techniques in computer vision,
however this approach generates a significant number of false positives.
To overcome this [9] uses image morphological techniques combined with a
Hidden Markov Model in GPU parallel implementation. The obtained results are
very promising an clearly show that hardware parallel implementations are a must
have when dealing with the development of Track Before Detect problems.
6.2
6.2.1
Sense and Avoid for Small UAVs
Regulations
• UAS pilot in charge required
• UAS should behave as all the other flying systems
• ICAO Circular 328:
– Authorizes flight in LOS
– Identifies UAS applications
– Chief in command required
– Visual Flight Rules
56
6.2.2
UAV operation in non-segregated airspace
6.2.3
UAV flight with Visual Flight Rules
Supersede human pilot performance (detection, range)
In-flight obstacle detection and avoidance without the use of cooperative communication or prior knowledge of flight plans.
6.3
6.3.1
Current S&A systems
Cooperative Traffic Avoidance systems
TCAS, ADS-B
6.3.2
Non-cooperative traffic avoidance systems
• Radar
• Lidar
• Sonar
• Electro-optical/Infrared
6.3.3
Small UAV systems
Limited payload and energy Hard to be detected by even small human piloted
aircrafts
6.3.4
For operation in non-segregated airspace
• Must avoid flying obstacles
• Land topology and infrastructures
• Demonstrate high level of SAA reliability for human acceptance
57
6.3.5
S&A requirements
• On board processing and decision
• High performance (range, detection, avoidance maneuvers)
• Low power and weight
• Non- cooperative
• Small size
6.3.6
6.4
Sensors
Visual Sense
• Subpixel image analisys
• Egomotion integration
• False positive filtering
• On-board processing
• Multiple camera support
• Real-time
• Dynamic range image sensors
• Hardware parallelizable (GPU, FPGA)
58
Figure 6.1: Sense & Avoid Background
Figure 6.2: Airplane sensors
59
Figure 6.3: Visual Sense
60
Bibliography
[1]
[2] Andreas Bethke. Persistent Vision Based Search and Tracking using Multiple
UAVs. PhD thesis, 2007.
[3] Ryan Carnie, Rodney Walker, and Peter I. Corke. Image processing algorithms for uav ”sense and avoid”. In ICRA, pages 2848–2853. IEEE, 2006.
[4] Stefan Ellis De Nagy Koves Hrabar. Vision-based three-dimensional navigation for an autonomous helicopter. PhD thesis, Los Angeles, CA, USA, 2006.
AAI3237117.
[5] G. Fasano. Multisensor based Fully Autonomous Non-Cooperative Collision
Avoidance System for UAVs. PhD thesis, 2005.
[6] T. Gandhi, Mau-Tsuen Yang, R. Kasturi, O. Camps, L. Coraor, and J. Mccandless. Detection of obstacles in the flight path of an aircraft. Aerospace
and Electronic Systems, IEEE Transactions on, 39(1):176–191, 2003.
[7] ChristopherM Geyer, Debadeepta Dey, and Sanjiv Singh. Prototype senseandavoid system for uavs. Technical Report CMU-RI-TR-09-09, Robotics
Institute, Pittsburgh, PA, May 2009.
[8] Nato Naval Armament Group. Sense and avoid requirements for unmmaned
aerial vehicle systems operating in non−segregated airspace. Technical report.
[9] John S. Lai, Luis Mejias, and Jason J. Ford. Airborne vision-based collisiondetection system. Journal of Field Robotics, 28(2):137–157, June 2010.
[10] A.E. Ortiz and N. Neogi. Color optic flow: A computer vision approach for
object detection on uavs. In 25th Digital Avionics Systems Conference, 2006
IEEE/AIAA, pages 1 –12, oct. 2006.
[11] M. Skolnik. Opportunities in radar - 2002. Electronics and Communications
Engineering Journal, 14(6):263–272, 2002.
61
[12] J. Deschenes M. Utt, J. McCalmont. Development of a sense and avoid system.
AIAA Infotech at Aerospace, 2005.
[13] Andrew B. Watson. Uav see and avoid systems: Modeling human visual
detection and identification. Technical report.
62

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