Modelling and Autonomous Flight Simulation of a Small Unmanned ...

AbstractThis project describes the use of FlightGear, an open-source flight simulator, andJSBSim, an open-source flight dynamics model, to model and simulate a smallautonomous Unmanned Aerial Vehicle (UAV).A small commercial electric engine Cessna-182 radio controlled (RC) aircraft waschosen to represent the UAV. The first step was to create the required JSBSim aircraftconfiguration files by using the Aeromatic v0.8, a free web application to createaircraft configuration files for use with the JSBSim. The next step was to makeeducated guesses to refine important sections in the created configuration files with theassistance of available data of similar UAV.In order to perform a visual simulation, a 3D model for the Cessna-182 (RC) wascreated using AC3D, a commercial 3D modelling software tool.To fly the modelled UAV autonomously a tuning process was made for the built-ingeneric PID (proportional, integral, and derivative) autopilot of FlightGear, which hasthe ability to hold aircraft velocity, vertical aircraft speed, altitude, pitch angle, angleof attack, bank angle, and true heading.Finally, a flight path, which contains a number of waypoints chosen over a selectedarea using Google Earth map, was constructed. In order to use the chosen waypointswith FlightGear navigation system, a unique ID was assigned to each waypoint, andthe FlightGear database was altered to include the new waypoints with their IDs.The outcome of the project was a complete JSBSim flight dynamic model for theCessna-182 (RC), with 3D model for visual simulation and an effective autopilot. Agood autonomous flight simulation was performed.This project concluded that modelling and simulating a UAV accurately is not an easytask, due to the need to calculate many parameters either by physical measurements,experiments, or estimation from available data of similar UAV, or by software tools.iii

AcknowledgmentsThis project is dedicated to my family for their unconditional support and sacrifice,which has enabled me to reach the position where I am. Special thanks to mysupervisor Dr Gorden Manson, who helped me in every possible way. Lastly, I wouldlike to thank my friends and well-wishers, who encouraged me and supported me inany way during this development time.iv

ContentsMODELLING AND AUTONOMOUS FLIGHT SIMULATION OF A SMALL UNMANNEDAERIAL VEHICLE..........................................................................................................................1SIGNED DECLARATION...............................................................................................................IIABSTRACT.....................................................................................................................................IIIACKNOWLEDGMENTS................................................................................................................IVCONTENTS......................................................................................................................................VLIST OF FIGURES........................................................................................................................VIICHAPTER 1 : INTRODUCTION.....................................................................................................11.1 AIMS OF THE PROJECT.................................................................................................................11.2 PROJECT DELIVERABLES.............................................................................................................11.3 STRUCTURE OF THE PROJECT ......................................................................................................2CHAPTER 2 : UAV SYSTEM TECHNOLOGY..............................................................................32.1 PAYLOADS.................................................................................................................................32.1.1 Sensors..............................................................................................................................32.1.2 Non Sensor Payloads.........................................................................................................32.1.3 Payload Stabilization.........................................................................................................32.2 DATA LINK ................................................................................................................................32.2.1 Functions...........................................................................................................................42.2.2 Data Link Implementation..................................................................................................42.2.3 Data Compression..............................................................................................................42.3 FLIGHT PATH MANAGEMENT ......................................................................................................42.3.1 Navigation.........................................................................................................................42.3.2 UAV Control System..........................................................................................................52.3.3 Guidance...........................................................................................................................52.4 POWER PLANT............................................................................................................................62.4.1 Propeller Efficiency...........................................................................................................62.4.2 Internal Combustion Engines.............................................................................................62.4.3 Electric Propulsion............................................................................................................62.5 CONFIGURATION ........................................................................................................................62.5.1 Fixed Wings.......................................................................................................................62.5.2 Rotary Wings.....................................................................................................................72.5.3 Lighter than Air.................................................................................................................72.6 STRUCTURE................................................................................................................................72.6.1 Fixed Wings.......................................................................................................................72.6.2 Rotary Wings.....................................................................................................................72.6.3 Materials...........................................................................................................................72.7 LAUNCH AND RECOVERY............................................................................................................72.7.1 Launch...............................................................................................................................72.7.2 Recovery............................................................................................................................7CHAPTER 3 : BASIC AERONAUTICAL CONCEPTS AND DEFINITIONS...............................93.1 WING GEOMETRY .......................................................................................................................93.1.1 Wing Span..........................................................................................................................93.1.2 Chords...............................................................................................................................93.1.3 Wing Area..........................................................................................................................93.1.4 Mean Chords...................................................................................................................103.1.5 Aspect Ratio.....................................................................................................................103.1.6 Sweep Back......................................................................................................................103.1.7 Dihedral Angle.................................................................................................................103.1.8 Airfoil..............................................................................................................................113.1.9 Angle of Attack.................................................................................................................123.2 THE AIRCRAFT AERODYNAMICS ...............................................................................................123.2.1 Lift...................................................................................................................................12v

3.2.2 Drag................................................................................................................................133.2.2 Side Force........................................................................................................................143.2.3 Pitch Moment...................................................................................................................153.2.4 Yaw Moment....................................................................................................................153.2.5 Roll Moment....................................................................................................................163.3 THE AIRCRAFT MAIN COMPONENTS..........................................................................................16CHAPTER 4 : A REVIEW OF FLIGHT SIMULATORS.............................................................184.1 AEROSIM BLOCKSET................................................................................................................184.2 JSBSIM FLIGHT DYNAMIC MODEL VERSION 2.0........................................................................184.3 FLIGHTGEAR FLIGHT SIMULATOR VERSION 0.9.10.....................................................................194.4 MICROSOFT FLIGHT SIMULATOR 2004......................................................................................194.5 THE CHOSEN FLIGHT SIMULATORS ...........................................................................................204.5.1 Flight Dynamic Model......................................................................................................204.5.2 Flight Simulator...............................................................................................................20CHAPTER 5 : DESCRIPTION AND CONSTRUCTION OF THE REQUIRED FILES TOMODEL THE UAV AND PERFORM THE SIMULATION.........................................................215.1 FLIGHTGEAR FLIGHT SIMULATOR REQUIRED FILES ...................................................................215.1.1 UAV Main Folder............................................................................................................215.1.2 Engines Folder.................................................................................................................225.1.3 Systems Folder.................................................................................................................235.1.4 Models Folder..................................................................................................................235.2 JSBSIM FLIGHT DYNAMIC MODEL REQUIRED FILES..................................................................245.2.1 MAIN AIRCRAFT CONFIGURATION FILE ...................................................................................245.1.2 ENGINE CONFIGURATION FILE ...............................................................................................335.1.3 Propeller Configuration File............................................................................................345.1.4 Electrical system Configuration File.................................................................................355.1.5 3D Graphical model Configuration File...........................................................................365.1.6 Top level Configuration File.............................................................................................38CHAPTER 6 : FLIGHTGEAR AUTOPILOT AND FLIGHT PATH...........................................416.1 PID CONTROLLER.....................................................................................................................416.1.1 PROPORTIONAL .....................................................................................................................416.1.2 INTEGRAL .............................................................................................................................416.1.3 DERIVATIVE..........................................................................................................................426.1.4 COMBINING P + I + D.............................................................................................................426.2 FLIGHTGEAR AUTOPILOT..........................................................................................................426.3 CONSTRUCTING THE UAV FLIGHT PATH (ROUTE).....................................................................506.7.1 Calculating the Cessna 182 RC route...............................................................................506.7.2 Deploying the Calculated Waypoints with FlightGear Database.......................................51CHAPTER 7 : RUNNING AND TESTING THE SIMULATION.................................................527.1 STARTING FLIGHTGEAR FLIGHT SIMULATOR.............................................................................527.2 SELECTING THE CESSNA 182 RC...............................................................................................527.3 SELECTING THE TAKE OFF LOCATION........................................................................................537.4 SETTING THE SIMULATION PARAMETERS...................................................................................547.4.1 Setting the Flight Dynamic Model....................................................................................547.4.2 Setting Latitude and Longitude Coordinates of the Take off Point.....................................557.5 RUNNING THE SIMULATION ......................................................................................................567.5.1 Settings for the Autopilot..................................................................................................567.5.2 Specifying the Waypoints..................................................................................................577.6 TESTING THE SIMULATION........................................................................................................57CHAPTER 8 : CONCLUSION AND FUTURE WORK................................................................59REFERENCES................................................................................................................................61vi

List of FiguresFigure 1.1: Cessna 182 RC..........................................................................................1Figure 3.1:Wing geometry...........................................................................................9Figure 3.2: Dihedral angle.........................................................................................11Figure 3.3: Airfoil and Angle of attack......................................................................11Figure 3.4: The forces and moments acting on an aircraft..........................................12Figure 3.5 Traditional Aircraft Components..............................................................17Figure 5.1: Main UAV folder contents.......................................................................22Figure 5.2: Engines folder contents............................................................................22Figure 5.3: Systems folder contents...........................................................................23Figure 5.4: Models folder contents............................................................................23Figure 5.5: Sections of the main aircraft’s configuration file......................................24Figure 5.6: Metrics section of the main aircraft’s configuration file...........................25Figure 5.7: Mass balance section of the main aircraft’s configuration file..................25Figure 5.8: Ground reactions section of the main aircraft’s configuration file............26Figure 5.9: Propulsion section of the main aircraft’s configuration file......................27Figure 5.10: Generating the JSBSim main configuration file using Aeromatic...........28Figure 5.11: JSBSim version 1.65 configuration file format.......................................29Figure 5.12: Flight control section of the main aircraft’s configuration file................30Figure 5.13: Aerodynamics section of the main aircraft’s configuration file (a).........31Figure 5.14: Aerodynamics section of the main aircraft’s configuration file (b).........32Figure 5.15: Aerodynamics section of the main aircraft’s configuration file (c).........33Figure 5.16: Electric engine specification file............................................................33Figure 5.17: Propeller configuration file....................................................................34Figure 5.18: Generating propeller configuration file using Aeromatic v 8.0...............35Figure 5.19: Electrical system configuration file........................................................36Figure 5.20: 3D graphical model configuration file....................................................37Figure 5.21: 3D graphical model of Cessna 182 RC...................................................38Figure 5.22: Set configuration file.............................................................................39Figure 6.1: Autopilot configuration file (a)................................................................43Figure 6.2: Autopilot configuration file (b)................................................................44Figure 6.3: Autopilot configuration file (c)................................................................45Figure 6.4: Autopilot configuration file (d)................................................................46Figure 6.5: Autopilot configuration file (e)................................................................47Figure 6.6: Autopilot configuration file (f).................................................................48Figure 6.7: Autopilot configuration file (g)................................................................49Figure 6.8: Cessna 182 RC flight path.......................................................................51Figure 6.9: Modifying the FlightGear fix database.....................................................51Figure 7.1: FlightGear desktop short cut....................................................................52Figure 7.2: Selecting Cessna 182 RC in FlightGear wizard........................................53Figure 7.3: Selecting a take off location in FlightGear wizard....................................53Figure 7.4: Main settings window in FlightGear wizard.............................................54Figure 7.5: Setting Flight Dynamic Model in FlightGear wizard................................55Figure 7.6: Settings of the Cessna 182 RC initial position in FlightGear wizard.........55Figure 7.7: Cessna 182 RC before take off.................................................................56Figure 7.8: Autopilot settings window.......................................................................56Figure 7.9: Add waypoints window..........................................................................57Figure 7.10: Cessna 182 RC in hold altitude flight....................................................57Figure 7.11: Side view for Cessna 182 RC altitude hold flight...................................58Figure 7.12: Cessna 182 RC in True heading flight....................................................58vii

Chapter 1: IntroductionChapter 1 : IntroductionSmall Unmanned Aerial Vehicles (UAVs) are increasingly used by researchers,hobbyists, civilian organizations, and the military for different purposes, due to thelack of risk to a pilot and their low cost. In some cases, these UAVs are flownautonomously by on-board autopilots, with highly integrated systems and complexcontrol laws. The use of simulation environments to test the UAVs design and controlsystems is an important phase in their development cycle, since it reduces the time andthe risks associated with the real flight (Sorton and Hammaker , 2005).The new iUAV (intelligent Unmanned Aerial Vehicle) research group in the faculty ofEngineering in Sheffield University has started to investigate how to simulate swarmsof UAVs, and it is also interested in high-level UAV intelligence such as monitoring aground target or recognizing certain targets. Therefore, a sample UAV model andsimulation is required. This project is also useful for the students of Aerospace andControl engineering degrees, who use simulators as a design tool.1.1 Aims of the projectThe aims of this project are to investigate how to create a Flight Dynamic Model fora given small UAV, and how to visualise that model in a Flight Simulator with theability to perform autonomous flight simulation. The approach followed in this projectshould be clear and relatively easy in order to allow the students and researchers tofollow it in their research.1.2 Project DeliverablesA commercial small Cessna 182 radio controlled aircraft shown in Figure 1.1 wasconsidered to be the starting point of the project.Figure 1.1: Cessna 182 RCTo achieve the aims of the project the following related items to Cessna 182 RC mustbe delivered:• A complete Flight Dynamic Model• A 3D graphical model• An effective autopilot• A way to identify flight route• Autonomous flight simulation1

Chapter 1: Introduction1.3 Structure of the ProjectThis project outlines the approaches taken to model and simulate a small UAV. Theproject starts with a summary of the available UAV system technology in Chapter 2 .Then it introduces the basic aeronautical concepts and definitions required to beunderstood in order to deal with the flight dynamic model in Chapter 3. Chapter 4reviews the possible flight dynamic models and flight simulators related to the project,and justifies the chosen simulators. Chapter 5 describes the files required for thechosen simulators to model and simulate the Cessna 182 RC and explains the tools andtechniques used to construct those files. Chapter 6 gives an overview of the PIDcontroller used in the FlightGear Simulator and describes the autopilot configurationfile, it also explains how the autopilot file for Cessna 182 RC was constructed, andfinally, it explains how the flight route can be calculated. Running and testing of themodelled Cessna 182 RC are described in Chapter 7. Finally, Chapter 8 concludes theproject by evaluating the work carried out.2

Chapter 2: UAV System TechnologyChapter 2 : UAV System TechnologyThe UAV system depends on its mission and range, however, most UAV systemsinclude: airframe and propulsion systems for the UAV, control systems and sensors tofly the UAV, sensors to collect information, launch and recovery systems, data linksto get collected information from the UAV and send commands to it, and a groundcontrol station.The purpose of this chapter is to give the reader (who has no knowledge in this field)enough information about the UAV system technology. The information in this chapterwas taken from (Cameron, 1995).2.1 PayloadsThe payload represents the most vital part of the UAV system. The objective of therest of the system is to deliver the payload to its targeted position and collect,manipulate, and spread the gathered information by the payload. The payloadmass/size, along with mission requirements, operator requirements, and operationalrequirements are used to determine the UAV specification. Generally, the total mass ofthe UAV is around four times the total mass of the payload according to anexamination of the manufacture’s data on UAVs. Large UAVs need large systems,which increases the cost and complexity. Consequently, a balance between therequirements of the payload performance and its mass must be considered.2.1.1 SensorsCommonly electro-optical sensors, which include daylight video and/or forwardlooking infrared, are used with the UAV to return a real video image to the groundcontrol station. Other sensors may be used to collect different types of data, forinstance, radar, nuclear, biological and chemical. The chosen sensors depend on themission requirements, the UAV carrying capacity, and costs.2.1.2 Non Sensor PayloadsUAV can transport equipment such as radio relays, which are used to broadencommunication ranges between the UAVs and the ground control station, andjammers, which are used to interfere with the enemy’s communication and othersystems.2.1.3 Payload StabilizationThe UAV motion and vibrations have negative effects on the outcome of the Electroopticalsensors; therefore, stabilisation is required to reduce these effects, and providesthe ability to steer the sensor around two axes, which means a complex mechanicalstabilisation system. However, this system increases the overall UAV weight.Alternatively, image processing and optical stabilisation may be used to reduce themass by comparison with the mechanical stabilisation system.2.2 Data LinkThe communications between the UAV and the ground control station are provided bythe data link. In most cases the data link represents the foundation of the UAV’snavigation system.3

Chapter 2: UAV System Technology2.2.1 FunctionsThe main functions of the data link are: transmitting the control signals to control theUAV and its payload from the ground station, sending the data gathered by thepayload and the UAV condition parameters back to the ground control station. In thecase of the autonomous UAV the data link may be shut down temporarily.2.2.2 Data Link ImplementationThere are many implementation methods for the data link methods such as highfrequency (HF) radio, very high frequency (VHF) band, ultra high frequency (UHF)band, optical fibre, and satellite.2.2.3 Data CompressionThe use of data compression reduces the bandwidth used for transmission, andincreases the stored data in a given storage capacity. The data compression is speciallyused with electro-optical sensors, which require high bandwidth. However, in somecases such as target detection mission, the quality of images after decompression is animportant factor, thus, special compression techniques are needed.2.3 Flight Path ManagementThe UAV’s position and the sensor pointing angles must be known by the UAVground operator in order to direct the UAV and the sensors in the required direction. Insome sophisticated systems, it may be done automatically.2.3.1 NavigationThe long range UAV requires a navigation system to determine its position. Therequired accuracy is mission dependent, which plays a major factor in deciding whichsystem to implement. The following systems are used:• Sensor Information: The operator can use the video image of a well known areareturned by the electro-optical sensor to determine the UAV position. However,this may not be practical in most cases due to the partial field view andabnormal viewing angles. Radar payload may be used in determining theUAV’s position; however, it can not be used as a stand-alone navigationsystem.• Dead Reckoning: The UAV’s position is determined by using its air speed anddirection in combination with the information of the electro-optical sensor,which is reliable over a limited area. However, it can not be used as a generalarea navigation system.• Data Link: The UAV’s range and bearing from the ground station can beobtained using the data link. This navigation system is commonly used in smallUAVs. However, loss of data link denies the UAV the navigation information,and therefore, a fall back system is required.• Satellite Navigation System: The Global Positioning System (GPS) based ona group of American satellites can be used to determine position and timeinformation. Simple light GPS sensors can be used in small UAVs, allowingthe operator to track the UAV in the ground control station.• Inertial: Inertial navigation systems do not need to send out or receive signals.The main disadvantages of these systems are: they are expensive, consumelarge power, heavy, and the position errors increase with time.4

Chapter 2: UAV System Technology• Terrain Matching: Terrain matching uses a radio altimeter together with abarometric altimeter to compare the ground shape being crossed with a shapeobtained from a digital map of the area. The combination of the providedinformation with a primary navigation system produces an accurate low driftnavigation system. However, this system is expensive and heavy for a smallUAV.• Scene Matching: The accuracy of a primary navigation system can beimproved by the matching of a picture obtained by the primary sensor with aknown view. However, this is practical only for limited views, and requiresformer knowledge of scenes.• Radio Location: The UAV’s position can be determined from observation of anumber of known radio transmitters, for instance commercial radio stations.However, this requires the stations to be working during the UAV’s flight.2.3.2 UAV Control SystemThe UAV control system stabilises the UAV and controls its speed, altitude, anddirection. In most UAVs this can be done automatically (using Autopilot). However, inshort range UAVs it can be done manually using radio control. The autopilot requiresinformation on the current UAV speed, attitude, height, and flight plan. Theseparameters can be measured using sensors or onboard systems. The autopilot forcesthe measured parameters to follow the demanded values.2.3.3 GuidanceGuidance is the element of the system that guarantees that the UAV follows therequired flight path. Therefore, information about the required flight path and the UAVposition must be available. The alterations required in the UAV’s parameters to keepthe UAV on course may then be worked out and adjustments passed to the controlsystem.The guidance of the UAV has three major phases:• Launch phase: the type and period of the launch phase decide whether thecontrol is required during the launch phase or not. Accurate guidance over asignificant distance and time is required when taking off from a runway. Themain problem is to get a suitably accurate distance reference for the controlsystem. However, physical restrictions in the form of guide rails during thelaunch phase are provided in the case of taking off from launchers. There arethree ways to control the UAV in the launch phase, direct operator, groundstation, and fully automatic controls. In direct operator control, the operatorwatches the UAV visually and sends commands directly to its control surfacesto change the flight path. In the case of launchers, the ground control stationcan control the UAV take off quite easily, because the flight speed is achievedclose to the end of the launcher. However, it is more difficult to control the takeoff from a runway due to the partial information offered to the GCS operatorsand close view of the ground from the UAV sensors. Fully automatic take offcontrol is possible in the case of taking off from launchers, however it is quiteimpractical in the case of the runway without support, due to the shortage ofinformation such as spatial references appropriate for accurate guidance of theUAV on/or close to the runway.• Mission phase: The mission phase starts once the UAV has been launched andends before the recovery phase. It may include flying outside the data link5

Chapter 2: UAV System Technologyrange under automatic control, which executes pre-planned procedures torecover from losing the data link. The direct operator control is limited byvisual range. The process requires two operators at least, one controls the UAVusing a directional and an altitude control. The second operator directs the datalink antenna at the UAV. Most ground control stations support many UAVfunctions such as mission planning, data analysis, and UAV control. The directcontrol may include entering speed, direction, altitude, and waypointscommands; it may also include steering the sensors to gather information overthe targeted area. If the mission requires the UAV to operate over a large areathat can not be covered by one GCS, auxiliary station(s) may be used to coverthe area. In fully automatic control, the UAV is flown according to the preplannedflight path installed in the onboard computer.• Recovery Phase: UAV recovery without any damage to its body or equipmenthas been a major problem, because it requires accurate guidance and control,which is very complicated in the presence of wind. In the direct operatorcontrol, the operator controls the UAV landing in the same way as the take off.The ground control station uses the UAV sensors and data link to control thelanding. However, it is very difficult to perform the landing due to the lack ofinformation. A fully automatic landing is still impractical, instead, a netrecovery is used, in which the UAV is flown into the net.2.4 Power PlantPower plant is one of the important aspects in the UAV development, since it affectsother aspects such as UAV range.2.4.1 Propeller EfficiencyThe propellers used in the UAV operate at low Reynolds numbers. They are small incomparison with manned aircraft. Therefore, their efficiencies are low, which leads togreater fuel consumption and installed power.2.4.2 Internal Combustion EnginesInternal combustion engines are used in most UAVs. They vary from two stroke, fourstroke petrol engines to gas turbine engines.2.4.3 Electric PropulsionElectric propulsion engines are used in small UAVs, as they are cheap and easy toinstall. However, the lack of appropriate power source represents a major problem.2.5 ConfigurationMost UAVs have the same configuration as manned aircraft with either fixed wingwith tail or rotary wing. However, some UAVs are just flying wings, and some arelighter than air.2.5.1 Fixed WingsMost currently working UAVs are in this group. The requirement of forward view,imposed by the primary sensor, leads to the use of pusher propellers at the back of theUAV.6

Chapter 2: UAV System Technology2.5.2 Rotary WingsThe rotary wing configuration solves some of the fixed wings UAVs’ problems suchas launch and recovery problems. Moreover, it gives the ability to hover over the targetand point the sensor in any direction. However, it has many problems such as flightperformance, the need for greater power, and the effect of altitude conditions in theperformance.2.5.3 Lighter than AirThe lighter than air UAVs, for instance balloons, have the ability to remain in the airwithout consuming much power. However, they have many problems such asdifficulty controlling the altitude due to the temperature effect, are huge and heavy todeploy, and have a low speed.2.6 StructureThe UAV’s sub systems are protected by the structure of its body. The UAV structuremay have many restrictions, such as minimising the radar reflections.2.6.1 Fixed WingsThe structure of small UAVs required to resist the aerodynamic forces is very light.However, it should be strong enough to cope with the launch and recovery phases.2.6.2 Rotary WingsIn the rotary wing UAVs, the lift and control forces are generated by the rotor(s). Therotor(s) and their connections are critical parts of the UAV. Material fatigue is a majorproblem for these parts due to the cyclic behaviour of the forces related to the rotor.The capability of rotary wings UAVs to manoeuvre is limited, which reduces theaerodynamics forces applied to the structure. The forces generated during the take offand landing are small.2.6.3 MaterialsThe materials in UAVs include aluminium or composites, for instance glass fibre.Because the composites are light, strong and damage resistant, they are used inconstructing most UAVs.2.7 Launch and Recovery2.7.1 LaunchLaunching the UAV is one of the important issues. In most cases, the operationrequires the UAV to be launched from limited areas, which makes the launch difficult.A variety of methods have been used such as hand launch, mechanical launch, rocketassisted take off, runway, vertical take off, and air launch.2.7.2 RecoveryThe recovery of the UAV is the most difficult and dangerous part of its operations.Many methods have been used such as crash landing, parachute, net, arrester, runway,and vertical landing.7

Chapter 2: UAV System TechnologyThis chapter indicates that there is a wide range of different technologies involved inthe UAV system, which must all be considered. The system of autonomous UAV ismore complex than the systems of some basic manned aircraft. Small aircraft havebeen used as the starting point for the development of UAVs. In order to develop aUAV, subsystems must be developed from scratch, or estimated from manned aircraft.8

Chapter 3: Basic Aeronautical Concepts and DefinitionsChapter 3 : Basic Aeronautical Concepts and DefinitionsIn order to calculate the forces and moments present on the aircraft, which are due tothe aircraft’s surfaces, engine, and wind, flight dynamics simulators consist of acomplicated flight model.This chapter gives information about the aircraft wing geometry, aircraft components,and the aircraft aerodynamic coefficients used by most dynamic flight simulators.These coefficients, which represent the classical linear aerodynamic forces andmoments of aircraft, are used with the dynamic pressure, aircraft geometry and mass,to find the forces and moments acting on the aircraft (Zyskowski, 2003).3.1 Wing geometryThis section illustrates the various parameters of the wing geometry shown in Figure3.1.Figure 3.1:Wing geometry3.1.1 Wing SpanThe wing span (b) is the distance between the wing tips (b). The wing semi-span (s) isthe distance from each tip to the centre line (Houghton and Carpenter, 2003).3.1.2 ChordsThe distance between the intersections of the leading and trailing edges with thefuselage centre line is the root chord co. The length CT is the tip chord. The taper ratioλ is the ratio CT/ co. (Houghton and Carpenter, 2003).3.1.3 Wing AreaThe gross wing area SGis the plan area of the wing including the continuation withinthe fuselage. The net wing area SNis the plan area of the wing without thecontinuation within the fuselage (Houghton and Carpenter, 2003).9

Chapter 3: Basic Aeronautical Concepts and Definitions3.1.4 Mean ChordsThe standard mean chord c (often abbreviated to SMC) is defined by c = S G/b orIt is also called geometric mean chord. It also can be redefined as+ s∫−s=+ s∫−scdyc 3.1dyWhere y is the distance measured from the centre line towards the right tip.The aerodynamic mean chord cA(AMC) is defined bySN/b.cA+ s∫c−s=+ s∫−s2dycdy3.2(Houghton and Carpenter, 2003)3.1.5 Aspect RatioThe aspect ratio is a measure of the narrowness of the wing plan form. It is referred toby (A) or (AR), and is presented asspan bA = =3.3SMC c(Houghton and Carpenter, 2003)3.1.6 Sweep BackThe sweep-back angle of a wing is the angle between a line perpendicular to the centreline, and a line drawn along the span at a constant part of the chord from the leadingedge or trailing edge. It is referred to by either ΛLEorφLE, if it is measured from theleading edge, and by either ΛTEor φTE, if it is measured from the trailing edge(Houghton and Carpenter, 2003).3.1.7 Dihedral AngleThe dihedral angle is the angle between the lines drawn on the wings along the locusintersections between the chord lines and the nose section as shown in Figure 3.2. It isdenoted by Γ . It is said they are dihedral if the wings are inclined upwards, andanhedral if they inclined downwards (Houghton and Carpenter, 2003).10

Chapter 3: Basic Aeronautical Concepts and DefinitionsFigure 3.2: Dihedral angle3.1.8 AirfoilAn airfoil is a shape designed to create lift. It is a cross section of the wing as shown inFigure 3.3.The chord is an imaginary direct line connecting the leading edge with the trailingedge. The mean camber line is an imaginary line that forms an equal distance from theupper and lower surfaces of the wing. The camber is the curvature of the mean camberline (Anderson and Eberhardt, 2001).Figure 3.3: Airfoil and Angle of attack11

Chapter 3: Basic Aeronautical Concepts and Definitions3.1.9 Angle of AttackThe angle of attack is “the angle between the mean chord of the airfoil and thedirection of the relative wind ”as shown in Figure 3.3. It is denoted by α (Andersonand Eberhardt, 2001:4).3.2 The Aircraft AerodynamicsUnderstanding the forces and moments acting on the aircraft and their act directions isimportant to be able to model an aircraft in a flight simulator. The term 6 DOF meanssix degrees of freedom. The aircraft can move in three dimensions in space and canrotate about three axes. Figure 3.4 shows the three axes and the forces and momentsacting on an aircraft (Anderson and Eberhardt, 2001).Figure 3.4: The forces and moments acting on an aircraftThe forces of lift, weight, drag, thrust, and side force act along the axes, forcing theaircraft to move in the axes direction. On the other hand, the three moments, yaw, roll,and pitch force the aircraft to turn around the axes.3.2.1 LiftThe lift is the component of force perpendicular to the flight direction, and actingupwards. It is denoted by L (Houghton and Carpenter, 2003).The lift produced by the wing is dependent on the speed of flight, the air density, wingarea, and the lift coefficient. The lift coefficient depends on the aerofoil shape, angle pfattack, and aspect ratio. The lift can be calculated from the equation:12

Chapter 3: Basic Aeronautical Concepts and Definitionswhere:2VL = CL × ρ × × S3.42C L is the lift coefficient,ρ is the air density ,V is the free stream velocity,S is the wing surface area,L is the lift force produced.Since the dynamic pressure2ρVq = , the equation 3.4 can be rewritten as2L = CL× q × S3.5Despite the wing, the lift force is contributed from other surfaces such as elevator andhorizontal tail. The total lift coefficient is calculated from the following equationCL= C + C α + Cδδ + C i + ∆C+ ∆C3.6LOLaL eeLih hLflapLspoilerwhereC LO is the lift coefficient at zero angle of attackC La is the lift coefficient due to angle of attackα is the angle of attackC Lδe is the lift coefficient due to elevator deflectionδ e is the elevator deflectionC Lih is the lift coefficient due to horizontal tail incidencei h is the horizontal tail incidence angle.C Lflap is the lift coefficient due to flapsC Lspoiler is the lift coefficient due to spoiler(Zyskowski, 2003)3.2.2 DragThe drag is the component of force acting in the opposite direction to the line of flight.In other words, it is the force that resists the motion of the aircraft (Houghton andCarpenter, 2003)Drag can be calculated from the equationwhere:D = CD × q × S3.7CDis the coefficient of drag,q is the dynamic pressure,S is the wing surface area,D is the drag force produced.13

Chapter 3: Basic Aeronautical Concepts and DefinitionsThe total drag coefficient is calculated from the equationCD= C + C α + Cδδ + C i + ∆C+ ∆C3.8DODaD eeDih hDflapDspoilerWhere:C DO is the drag coefficient at zero angle of attackC Da is the drag coefficient due to angle of attackα is the angle of attackC Dδe is the drag coefficient due to elevator deflectionδ e is the elevator deflectionC Dih is the drag coefficient due to horizontal tail incidencei h is the horizontal tail incidence angle.C Dflap is the drag coefficient due to flapsC Dspoiler is the drag coefficient due to spoiler(Zyskowski, 2003)3.2.2 Side ForceThe side force (or cross-wind force) is the component of force acting on the aircraftperpendicular to the lift and drag (Houghton and Carpenter, 2003).It can be calculated from the equation:Y = CY × q×S3.9where:C Y is the coefficient of side force,q is the dynamic pressure,S is the wing surface area,Y is the side force produced.The total side force coefficient is calculated from the equationCY= C + CC δ3.10YOYβ β + CYδaδa+YδrrWhere:C YO is the side force coefficient at zero angle of attack,C Yβ is the side force coefficient due to sideslip angle,β is the angle of sideslip,C Yδα is the side force coefficient due to aileron deflection,δ α is the aileron deflection,C Yδr is the lift coefficient due to rudder deflection,δ r is the rudder deflection.(Zyskowski, 2003)14

Chapter 3: Basic Aeronautical Concepts and Definitions3.2.3 Pitch MomentPitch moment is the moment which tends to rotate aircraft around the pitch axis (lateralaxis) as shown in Figure 3.4. In this rotation, the aircraft nose and tail pitch up ordown. The pilot pitches up to climb, and pitches down to dive (Houghton andCarpenter, 2003).The pitch moment can be calculated from the equation:where:M = CM × q × S × c3.11C M is the coefficient of pitch,q is the dynamic pressure,S is the wing surface area,c is the chord length,M is the pitching moment produced.The total pitch moment coefficient can be calculated from the equation:CM= C + C δMOMα α + CMδee+ CMihih+ ∆CMflap+ ∆CMspoiler3.12Where:C MO is the pitch moment coefficient at zero angle of attackC Ma is the pitch moment coefficient due to angle of attackα is the angle of attackC Mδe is the pitch moment coefficient due to elevator deflectionδ e is the elevator deflectionC Mih is the pitch moment coefficient due to horizontal tail incidencei h is the horizontal tail incidence angle.C Mflap is the pitch moment coefficient due to flapsC Mspoiler is the pitch moment coefficient due to spoiler(Zyskowski, 2003)3.2.4 Yaw MomentThe yaw moment is the moment which tends to rotate the aircraft about the yaw axis(vertical axis) (Houghton and Carpenter, 2003).The yaw moment can be calculated from the equation:Where:N = CN× q × S × b3.13C N is the coefficient of yaw moment,q is the dynamic pressure,S is the wing surface area,b is the wing span,N is the yaw moment produced.15

Chapter 3: Basic Aeronautical Concepts and DefinitionsFigure 3.5 Traditional Aircraft ComponentsThe elevator is used to control the pitch of the aircraft. The rudder is used to controlthe yaw of the aircraft. The ailerons, which are movable surfaces on the outer trailingedge of the wings, are used to control the roll of the aircraft. The flaps, which arehinged parts on the inner trailing edge of the wings, are used to produce higher lift atlow speed, and to increase drag on landing to get the required landing speed andapproach angle. The landing gears configurations are either tricycle landing gear,which has the main landing gears just behind the aircraft centre of gravity and asteerable nose gear, or tail dragger, which has the main landing gear forward of theaircraft centre of gravity and a small steerable gear at the tail (Anderson andEberhardt, 2001).In order to model an aircraft in any Flight Dynamic Model, the aircraft geometry, thelift, drag, and side forces, and the roll, pitch, and yaw moments acting on the aircraftmust be understood. It is also important to know the aircraft control surfaces and theireffects on the aircraft motion.17

Chapter 4: A Review of Flight SimulatorsChapter 4 : A Review of Flight SimulatorsA flight simulator is a system that attempts to imitate the experience of flying anaircraft as practically as possible. The types of flight simulators vary from personalcomputer video games to full size cockpit replicas controlled by sophisticatedcomputer systems. Flight simulators are widely used by academic researchers, theaviation industry, and air forces for pilot training and aircraft development (WWW1).This chapter reviews some of the most relevant flight simulators available for use inthis project, explaining the advantages and disadvantages of using each, and justifiesthe chosen flight simulators. Two flight dynamic model (FDM) simulators, AeroSimblockSet, and JSBSim, (FDM is “the physics/math model that defines the movement ofan aircraft under the forces and moments applied to it using the various controlmechanisms and from the forces of nature”)(Berndt, 2000), and two visualisation flightsimulators, FlightGear and Microsoft Flight Simulator, have been reviewed.4.1 AeroSim BlockSetThe AeroSim blockSet is a Matlab/Simulink block library designed by flight controlengineers, and provides elements for fast development of nonlinear 6-degree-offreedom(6-DOF) aircraft dynamic models. Academic and non-commercial users candownload the AeroSim blockSet freely for use.Aerosim blockSet Features (WWW2):• Non-linear 6-DOF aircraft dynamics are implemented in various referenceframes.• Linear aerodynamics, piston-engine propulsion, and time-varying inertiamodels are provided.• The model can read the aerodynamic, propulsion, and inertia data from userdefinedsources.• Standard atmosphere model, wind shear, background wing, and turbulencemodels are available.• Detailed earth models are provided.• Transformations to and from multiple reference frames are provided.• Conversions between engineering units are provided.• XML aircraft configuration files of JSBSim flight dynamic model can be used.• Pilot interface blocks for joystick input and for visual output in FlightGearFlight Simulator or Microsoft Flight Simulator are provided.• Pre-built aircraft models, which can be reused by changing the parameters file,are provided.• By using the Real-Time Workshop, which is one of the Matlab tools, Clanguage code can be generated automatically from the model.The main disadvantage of AeroSim blockSet is that it does not model aircraft with anelectric propulsion system.4.2 JSBSim Flight Dynamic Model version 2.0JSBSim is an open source flight dynamic model (FDM) written in the C++programming language. It is the default flight dynamic model for the FlightGear flightsimulator.JSBSim Features:18

Chapter 4: A Review of Flight Simulators:• Compiles and runs under many operating systems.• Can be run as a stand-alone programme, or it can be run as an integrated partof the flight simulator, which provides visual output.• The code is generic, and particular aircraft flight control systems, propulsion,aerodynamics, landing gears, and autopilot are defined in XML formatconfiguration files.• Can be used to model many propulsion systems including an electricpropulsion system.• Models the rotational earth effects on the equation of motion.• Many data output formats such as to screen, socket, file.One of the strongest advantages of the JSBSim is that it is an open source project,which gives a very flexible environment for academic researchers and volunteersaround the world to use it as a tool in their research or to develop it itself (Berndt,2000)4.3 FlightGear Flight Simulator version 0.9.10FlightGear is an open-source flight simulator written in the C++ programminglanguage, available on the World Web Wide. It is used “to create a sophisticated flightsimulator framework for use in research or academic environments, for thedevelopment and pursuit of other interesting flight simulation ideas, and as an end-userapplication”(WWW3).FlightGear features:• High Degree of Freedom: FlightGear is open-source, its source code can bedownloaded and altered to meet a specific research purpose.• Flight Dynamics Models: FlightGear has three primary flight dynamicsmodels; JSBSim, YASim and UIUC- the researchers can use any one accordingto their requirements. In addition, it is possible to add a new dynamic model orinterface with an external model.• Extensive and accurate world scenery database.• Accurate and Detailed Sky Model: FlightGear provides accurate sun, moonstars and planets, positions for a certain time and date.• Flexible and Open Aircraft Modelling System: FlightGear can model a widevariety of aircrafts. It has very good instrument animation and infrastructure tobuild 3d cockpits. Moreover, it models real world instrument behaviour, andsystem failures.• Moderate Hardware Requirements: FlightGear can be run on a personalcomputer.• Internal Properties Exposed: FlightGear allows the user to access the internalproperties too and monitor any of its internal state variables. By editingconfiguration files it is possible to create sound effects, model animations,instrument animations and network protocols for approximately any situation.• Networking Options: By setting its networking options, FlightGear cancommunicate with external flight dynamics models, GPS receivers, otherinstances of FlightGear, and other software. (WWW4)4.4 Microsoft Flight Simulator 2004Microsoft flight simulator (MSFS) 2004 is a popular commercial programme; it isavailable in most computer games stores.19

Chapter 4: A Review of Flight SimulatorsMSFS 2004 Features:• Contains many aircraft models.• Provides a tool to generate scenery automatically.• Provides detailed visual effects.• Provides detailed flight information using flight analysis maps and graphs.• Provides 3D interactive cockpits.• Provides moving maps and GPS positioning system.• Provides multiplayer capability.The main disadvantage of Microsoft Flight Simulator is that it is only for the Windowsoperating system. (WWW5)4.5 The Chosen Flight Simulators4.5.1 Flight Dynamic ModelAeroSim blockSet was chosen at the beginning of the project, and a lot of time wasspent in learning how to use it. However, later on, JSBSim was chosen to be the flightdynamic model due to the following reasons:• The UAV to be modelled has an electric propulsion system, which theAeroSim blockSet does not model.• The JSBSim is the default flight dynamic model for the FlightGear flightsimulator, which was chosen to visualise the output, which is an importantfactor in terms of computer performance.• Using AeroSim blockSet requires good experience of using Matlab andSimulink, which is not the case for the author. On the other hand, usingJSBSim is easier.4.5.2 Flight SimulatorFlightGear was chosen to be the flight simulator instead of Microsoft flight simulatordue to the following reasons:• FlightGear is an open source, can be downloaded from the internet and usedfreely.• The FlightGear source code can be altered according to the requirements of theproject.• FlightGear uses the JSBSim flight dynamic model as a default model. JSBSimis installed automatically with FlightGear.JSBSim and FlightGear are described in more details in the next chapter.20

Chapter 5: Description and Construction of the Required Files to Model the UAV andPerform the SimulationChapter 5 : Description and Construction of the Required Filesto Model the UAV and Perform the SimulationIn the previous chapter the relevant flight dynamic models and flight simulators thatcan be used in the project to model the small commercial Cessna 182 Radio Controlledaircraft, which was chosen to represent the UAV, were reviewed . The decision wastaken to use the JSBSim as a Flight dynamic model to model the Cessna-182 (RC), andthe FlightGear flight simulator as flight simulator to visualise the modelled UAV.Many issues were involved in modelling the UAV. One of the big challenges wascalculating its aerodynamics coefficients. Usually these coefficients are calculated byexperiments, by using computer software, or from available data of similar aircraft.Many approaches were investigated such as using DATCOM+, which is a computerprogramme to estimate the design aerodynamics coefficients of an aircraft; however,this approach was ignored due to the complexity of the programme and the need formany refinements to the programme output.The next approach was to use Aeromatic version 0.8, a web application to createaircraft configuration files for use with the JSBSim Flight Dynamics Model.According to the Aeromatic web site (WWW6), the output aircraft configuration filesformat is JSBSim version 2.0. However, this is not the case at the time of constructingthe Cessna-182 (RC) configuration files. The output format was JSBSim version 1.65,consequently, the UAV configuration files were rewritten in version 2.0 format.Although, the Aeromatic gives a fast and effective way to construct the UAVconfiguration files, some important refinements are required. Due to that, anotherassistance approach had to be found.The use of available data of similar UAV was the missing part that was required to geta good JSBSim model. Unfortunately, there are not many UAV JSBSim modelsavailable, however, the Rascal 110 R/C UAV, which is quite similar to the Cessna-182(RC) in shape, and bigger in size, was very helpful in refining the Cessna-182 (RC)configuration files.This chapter describes the architecture of FlightGear and JSBSim and explains the filesstructure required to model Cessna 182 (RC) and perform a visual simulation. It alsogives explanations about how these files were constructed using the Aeromatic andRascal 110 RC Model, and how the 3D graphical model was created by using thecommercial 3D modelling software AC3D. The architecture of FlightGear autopilot,Autopilot tuning process, and flight path construction are discussed in chapter 6.5.1 FlightGear Flight Simulator Required FilesThe FlightGear flight simulator needs essential files arranged in folders in order toperform successful simulation. This section represents the top level of all requiredfiles.5.1.1 UAV Main FolderThe name of this folder is the name of the modelled UAV (RC-Cessna-182). Itcontains the UAV main (c182.xml) and Set (c182-set.xml) JSBSim configuration files,Engines folder, Models folder, and systems folder as shown in Figure 5.1 . The mainUAV folder should be the directory: FG_ROOT\FlightGear\data\Aircraft.21

Chapter 5: Description and Construction of the Required Files to Model the UAV andPerform the SimulationFigure 5.1: Main UAV folder contents5.1.2 Engines FolderThe Engines folder contains the engine (Cessna_RC_Engine.xml), and the thruster(Cessna_RC_Propeller.xml) JSBSim configuration files as shown in Figure 5.2.Figure 5.2: Engines folder contents22

Chapter 5: Description and Construction of the Required Files to Model the UAV andPerform the Simulation5.1.3 Systems FolderThe systems folder contains the autopilot (Cessna182-auropilot.xml) and the electricsystem (electrical.xml) JSBSim files as shown in Figure 5.3.Figure 5.3: Systems folder contents5.1.4 Models FolderThe Models folder contains the JSBSim 3D graphical model configuration file (c182-dpm.xml), the 3D graphical model (c182.ac), and textures (c182-01.rgb and c182-02.rgb) used by the 3D model as shown in Figure 5.4.FlightGear supports many 3D file formats such as AC3D, DXF, VRML1, and MDL.Megginson (2002)Figure 5.4: Models folder contents23

Chapter 5: Description and Construction of the Required Files to Model the UAV andPerform the SimulationThe required files and their arrangement have been defined in this section. Thefollowing section goes through each file and describe its structure and explains how itwas constructed.5.2 JSBSim Flight Dynamic Model Required FilesJSBSim is an open source project written in the C++ programming language. The useof an object-orientated approach makes JSBSim a generic simulator, and allows themodelling of any aircraft, missile, or rotorcraft without the need to change theprogramme code. The modelling process can be done by the use of specification fileswritten in eXtensible Markup Language (XML) format. The JSBSim version used inthis project is version 2.0, which is the current available version.Several files are involved in modelling the aircraft. The aircraft’s metrics andaerodynamics characteristics are specified in the main aircraft configuration file. Theaircraft’s propulsion system is specified in two files, one for the engine, and the otherfor the thruster (a thruster is the device that turns the power of the engine into a thrustforce such as a propeller or a nozzle). Other files are required for use with FlightGearflight simulator, which include the autopilot file, electric system file, and 3D graphicalmodel specification file. The final file required is a file to tie the previous files togetherin order to perform complete simulation using FlightGear flight simulator. Due to thelack of clear documentation of FlightGear and JSBSim, and to make these files clear toread, comments were added to the XML files to explain every parameter.5.2.1 Main aircraft configuration fileThe main aircraft configuration file specifies the aircraft’s airframe geometry, massand inertia properties, landing gear positions and their ground reactions, propulsionsystem, flight control system, and aerodynamic characteristics. Figure 5.5 showsdifferent sections of the main aircraft configuration file. These sections will bedescribed one by one.Figure 5.5: Sections of the main aircraft’s configuration file24

Chapter 5: Description and Construction of the Required Files to Model the UAV andPerform the SimulationMetrics SectionThe metrics section specifies the aircraft’s wing and tail characteristics, and severallocations of important points as shown in Figure 5.6. The imperial unit system is usedin all measurements. The original point for all measurements in the files is at the noseof the aircraft, X-axis is along the aircraft body (positive towards the tail), Y-axis isalong the wings (positive towards the right wing tip, and Z-axis is in the vertical axis(positive downward).Figure 5.6: Metrics section of the main aircraft’s configuration fileIn this section some values were measured physically from the Cessna 182 (RC) suchas wing span, wing area, wing chord, horizontal tail area, horizontal tail arm, andvertical tail arm. Other values were estimated from the available data of Rascal 110RC.Mass Balance SectionThe mass balance section specifies the aircraft’s moment of inertia and the location ofthe centre of gravity as shown in Figure 5.7.Figure 5.7: Mass balance section of the main aircraft’s configuration fileThe values in this section were difficult to calculate, due to the need for experimentalresults. Therefore, with the help of Rascal 110 RC data, moments of inertia and centre25

Chapter 5: Description and Construction of the Required Files to Model the UAV andPerform the Simulationof gravity were estimated and altered many times by changing the value and runningthe simulation until a good aircraft balance was achieved.Ground Reactions SectionGround reactions section specifies the wheels location, and the coefficients associatedwith each wheel, as shown in Figure 5.8.Figure 5.8: Ground reactions section of the main aircraft’s configuration fileSome values in the section were measured physically from the Cessna 182 RC such asthe location of gears. Other values were obvious, such as the type of brake, andwhether or not the gear was retractable. Static friction, dynamic friction, rollingfriction, spring, and damping coefficients values need to be calculated by experiment,which was not available, therefore, they were estimated from the Rascal 110 RC.Propulsion SectionJSBSim can model different types of engines, for instance electric, piston, rocket andturbine engines. Engine and thruster characteristics are specified in separate files.These files are referred to in the propulsion section in the main aircraft specificationfile, which allows the researcher to assign different kinds of engines and thrusters tothe aircraft. The propulsion section specifies information about engine, thruster, andfuel tank, as shown in Figure 5.9.26

Chapter 5: Description and Construction of the Required Files to Model the UAV andPerform the SimulationThe locations of engine and propeller were measured physically. JSBSim ignores thetank section in Cessna 182 RC because the engine used is an electric engine.Figure 5.9: Propulsion section of the main aircraft’s configuration fileFlight Control SectionJSBSim provides components which can be connected together to model a flightcontrol system for an aircraft. The flight control surfaces are elevator, right and leftailerons, and rudder.This section was generated by Aeromatic v 8.0. As shown in Figure 5.10, theAeromatic v 8.0 takes inputs from the user such as system of measurements used, theaircraft name, type of aircraft, maximum take off weight, wing span, length, wing area,landing gear layout, number of engines, engine type, and engine layout. By clicking onthe Generate button, the main JSBSim configuration file will be generated in anotherwindow. The next step is to save the generated file by using save as, and giving the filea name (for instance, c182.xml). The saved xml file is in JSBSim version 1.65 format,as shown in Figure 5.11. Therefore, it should be rewritten in version 2.0. The controlsection is shown in Figure 5.12.27

Chapter 5: Description and Construction of the Required Files to Model the UAV andPerform the SimulationFigure 5.10: Generating the JSBSim main configuration file using Aeromatic28

Chapter 5: Description and Construction of the Required Files to Model the UAV andPerform the SimulationFigure 5.11: JSBSim version 1.65 configuration file format29

Chapter 5: Description and Construction of the Required Files to Model the UAV andPerform the SimulationFigure 5.12: Flight control section of the main aircraft’s configuration file30

Chapter 5: Description and Construction of the Required Files to Model the UAV andPerform the SimulationAerodynamics SectionThe aerodynamic section specifies the aerodynamic characteristics of the aircraft. Theclassic build up method is used to model the aerodynamic forces and moments (drag,side force, and lift forces, and roll, pitch, and yaw moments) acting on the aircraft.Many factors affect each of the forces and moments. The total force or moment is thesum of the individual effect (Berndt, 2005). This section was generated automaticallyby Aeromatic v8.0.The complete aerodynamic section is shown in Figure 5.13, Figure 5.14, and Figure5.15 respectively.Figure 5.13: Aerodynamics section of the main aircraft’s configuration file (a)31

Chapter 5: Description and Construction of the Required Files to Model the UAV andPerform the SimulationFigure 5.14: Aerodynamics section of the main aircraft’s configuration file (b)32

Chapter 5: Description and Construction of the Required Files to Model the UAV andPerform the SimulationFigure 5.15: Aerodynamics section of the main aircraft’s configuration file (c)5.1.2 Engine Configuration FileThe engine file specifies the specific engine characteristics used. JSBSim can modelmany kinds of engines such as piston engine, rocket engine, and electric engine. Sincethe Rascal 110 RC has an electric propulsion system, it was easy to know how tomodel the electric engine, which includes just the engine power as shown in Figure5.16 . The engine power for Cessna 182 RC was estimated from the Rascal 110 RC.Figure 5.16: Electric engine specification file33

Chapter 5: Description and Construction of the Required Files to Model the UAV andPerform the Simulation5.1.3 Propeller Configuration FileThe propeller file specifies the propeller measurements and characteristics as shown inFigure 5.17Figure 5.17: Propeller configuration fileThis file was generated automatically by Aeromatic v8.0. As shown in Figure 5.18,Aeromatic takes the engine power, maximum engine RPM, pitch condition, andpropeller diameter from the user.34

Chapter 5: Description and Construction of the Required Files to Model the UAV andPerform the SimulationFigure 5.18: Generating propeller configuration file using Aeromatic v 8.05.1.4 Electrical system Configuration FileThe electrical system file specifies the battery characteristics, and the lights and otherparameters as shown in Figure 5.19.The electrical system configuration file is required to perform a simulation inFlightGear flight simulator. Aeromatic does not generate this file. Therefore, a similarconfiguration to Rascal 110 RC electrical system file was used.35

Chapter 5: Description and Construction of the Required Files to Model the UAV andPerform the SimulationFigure 5.19: Electrical system configuration file5.1.5 3D Graphical model Configuration FileIn order to perform a visual simulation, a 3D graphical model should be specified. Theanimated control surfaces and their kind of animation are specified in this file asshown in Figure 5.20.36

Chapter 5: Description and Construction of the Required Files to Model the UAV andPerform the SimulationFigure 5.20: 3D graphical model configuration fileFlightGear supports many 3D formats. The common extension used in FlightGear is acextension. The AC3D is commercial 3D modelling software and can be used for manypurposes such as game model creation and virtual reality simulators (WWW7).The modelled Cessna 182 RC is a scale model of the real Cessna 182, which meansthat the body geometry is exactly the same. Therefore, the real Cessna 182 model,which is modelled by Stuart Buchanan and available to download from the FlightGearaircrafts download web page, was the starting point to construct the 3D model of theCessna 182 RC UAV. The constructing process included resizing the real model toadapt to the UAV dimensions. It included adapting the position of the new modelinside the FlightGear during the simulation. The 3D graphical model of Cessna 182RC UAV is shown in Figure 5.21.37

Chapter 5: Description and Construction of the Required Files to Model the UAV andPerform the SimulationFigure 5.21: 3D graphical model of Cessna 182 RC5.1.6 Top level Configuration FileThe set file ties all the previous files together by specifying their names and paths. Theset file is the first file processed in the simulation. Figure 5.22 shows an example of theset file.38

Chapter 5: Description and Construction of the Required Files to Model the UAV andPerform the SimulationFigure 5.22: Set configuration file39

Chapter 5: Description and Construction of the Required Files to Model the UAV andPerform the SimulationIt was clear that the modelling of an aircraft in general and the Cessna 182 RC UAVby using JSBSim is not an easy task. It involved many calculations and required muchexponential data that in most cases was not available. However, the Cessna 182 RCUAV was modelled accurately to some extent with the help of the available data of thesimilar aircraft, the Rascal 110 RC.40

Chapter 6: FlightGear Autopilot and Flight PathChapter 6 : FlightGear Autopilot and Flight PathIn the previous chapter the architecture of the required files to model and simulateCessna 182 RC without autonomous flight and the ways used to construct them weredescribed. However, to perform autonomous flight simulation autopilot configurationfile must be added and a waypoint must be calculated to construct the UAV flight path.This chapter starts by giving a brief overview of PID (Proportional, Integral, andDerivative) controller used in the FlightGear autopilot, then it describes the Autopilotconfiguration file and the Autopilot tuning process, and finally, it explains how theflight path was constructed and deployed within the FlightGear navigation system.6.1 PID controllerTypical autopilots are built using a PID algorithm. Typically a PID controllermanipulates one control output to force a current value (or process value) towards atarget value (or reference point). The information in this section was taken fromOlson, 2004.6.1.1 ProportionalAll three components of the PID algorithm are driven by the difference between thecurrent value and the target value. This difference (or error) is called for one particulartime step en. For that same time step, the current value is ynand the target point r n.en= r − y6.1nnThe output value is called un.The proportional component simply calculates unbased on the size of the error enbymultiplying it by a constant, K .nPnPu = K . e6.26.1.2 IntegralIntegral refers to the area under a curve. The integral of a function produces a secondfunction, which gives the area under curve of the first function.By multiplying the en, which is the difference between the process value and thereference point at each step time, by dt(the time step) the area, which approximates theerror under the curve for this step time, can be calculated. . An approximation of thearea under the curve can be calculated by adding those areas up over time.41

Chapter 6: FlightGear Autopilot and Flight Path6.1.3 DerivativeThe derivative of a function is the rate of change of its output. If the function is known,the derivative of that function can be taken to produce a second function. For any pointin time, the derivative function will give the rate of change (or slope) of the firstfunction.In the context of controller, the rate of change from each time step to the next is veryimportant information, which can be helpful in building a more stable system that canachieve the target value quickly.6.1.4 Combining P + I + DThe proportional component is very stable. The Integral and Derivative componentsare very unstable.If the proportional is the only controller used, it will be very stable but will stabilize tothe wrong value.If the integral is the only controller used, it will quickly achieve the target value, butwill overshoot, then overcompensate, and will oscillate wildly around the target value.It is very unstable.The trick then is to combine these components together by summing them. The actualoutput is equal to the P component output plus the I component output plus the Doutput. A weighting value should be assigned to each component to increase ordecrease its relative power to influence the final output value.u = K . uP + K . uI + K . uD6.3nPnInDnThe PID controller is quite simple to implement. The real trick in creating a wellbehaved PID controller and a well behaved autopilot is tuning the relative weights ofeach of the P, I, and D components.6.2 FlightGear autopilotFlightGear uses a PID algorithm designed by Roy Ovesen. FlightGear implements thealgorithm in a flexible way, which makes it reusable with similar aircrafts. Anynumber of PID controllers can be defined in the autopilot configuration file. Eachcontroller can be assigned a process value, reference point, any number of outputvalues, and other tuning constants. Moreover, cascading controllers can beimplemented by specifying multiple PID controllers, in which the output of the currentstage is used as the input to the next stage (Olson, 2004).The complete generic autopilot configuration file is shown in Figure 6.1, Figure 6.2,Figure 6.3, Figure 6.4, Figure 6.5, Figure 6.6, and Figure 6.7 respectively.42

Chapter 6: FlightGear Autopilot and Flight PathFigure 6.1: Autopilot configuration file (a)43

Chapter 6: FlightGear Autopilot and Flight PathFigure 6.2: Autopilot configuration file (b)44

Chapter 6: FlightGear Autopilot and Flight PathFigure 6.3: Autopilot configuration file (c)45

Chapter 6: FlightGear Autopilot and Flight PathFigure 6.4: Autopilot configuration file (d)46

Chapter 6: FlightGear Autopilot and Flight PathFigure 6.5: Autopilot configuration file (e)47

Chapter 6: FlightGear Autopilot and Flight PathFigure 6.6: Autopilot configuration file (f)48

Chapter 6: FlightGear Autopilot and Flight PathFigure 6.7: Autopilot configuration file (g)49

Chapter 6: FlightGear Autopilot and Flight PathAccording to Olson (2004), the best start to constructing the autopilot configurationfile for the modelled aircraft is by copying the autopilot configuration file from anexisting, similar aircraft, and tuning the autopilot parameters to adapt to the modelledaircraft. The most basic method of tuning autopilot parameters is the trial and errormethod. In this method the proportional gain, integral time, and derivative time areadjusted until the performance is acceptable. (Please refer to the reference for moredetails about tuning methods and tips).Fortunately, in the case of the Cessna 182 RC, the Rascal 110 RC autopilotconfiguration file was very effective, and few refinements were made. However, thereare some parts in the file are not documented clearly, and could not be understood.6.3 Constructing the UAV Flight Path (Route)FlightGear is used for navigation fixed waypoints such as airports and navigation aidssuch as radio stations. The fixed points are determined by latitude and longitude.FlightGear uses a database created by Robin A. Peel, in which a unique ID and itslatitude and longitude coordinates identify each waypoint. When a waypoint is enteredin the aircraft route during the simulation time, FlightGear checks this database to seeif it is a valid fixed point or not. This data is stored in the compressed file calledfix.dat, which can be found in the directory FG_ROOT\ FlightGear\data\Navaids.Because FlightGear is used to replicate the real navigation system around the world, itwas clear that none of the waypoints stored in the FlightGear fix database weresuitable for the Cessna 182 RC UAV. Consequently, a way of calculating the UAVflight route had to be found.6.7.1 Calculating the Cessna 182 RC routeSince the UAV is a small aircraft and has a limited range, it was logical that the flightroute should be short. The free software map Google Earth provides a very helpfulway to calculate the waypoints of the flight route.An area around Sheffield University was chosen to calculate the flight path byidentifying a number of waypoints and draw a line to connect them. For eachwaypoint, the latitude and longitude values were recorded (they should be in degrees)and assigned a unique ID (for instance UAV01, UAV02, etc). An example is shown inFigure 6.8.50

Chapter 6: FlightGear Autopilot and Flight PathFigure 6.8: Cessna 182 RC flight path6.7.2 Deploying the Calculated Waypoints with FlightGear DatabaseIn order to deploy the calculated waypoints with FlightGear, the fix.dat database had tobe altered by adding those waypoints with their IDs. This can be done simply byextracting the fix.dat file, opening it with TextPad and adding the waypoints, andfinally deleting the old compressed fix.dat file and recompressing the modified fix.dat.Figure 6.9: Modifying the FlightGear fix databaseThis chapter indicated that tuning the FlightGear autopilot to adapt with the modelledUAV does not require a specialist control engineer, because it is a generic and can bereused with similar aircrafts. It also indicated that in order to construct the flight path,the FlightGear navigation system must be altered, which could be done becauseFlightGear is an open source project.51

Chapter 7: Running and Testing the SimulationChapter 7 : Running and Testing the SimulationTo run the modelled Cessna 182 RC UAV simulation FlightGear flight simulatorversion 0.9.10 should be installed and configured properly.( Please refer to theFlightGear documentation on how to install and configure FlightGear). The projectwas done using Windows operating system.The modelled UAV main folder (Rc-Cessna-182) with its contents should be placed inthe directory:FG_ROOT\FlightGear\data\Aircraft\Rc-Cessna-182For instance:C:\Program Files\FlightGear\data\Aircraft\Rc-Cessna-1827.1 Starting FlightGear Flight SimulatorThe FlightGear can be started from command line, from the desktop short cut shown inFigure 7.1, or from the programmes list. The processes described here use the desktopshort cut.Figure 7.1: FlightGear desktop short cut7.2 Selecting the Cessna 182 RCThe first window in FlightGear wizard allows the selection of an aircraft. The Cessna182 RC can be selected by clicking on the UAV name (c182(R/C)), and then clickingnext as shown in Figure 7.2.52

Chapter 7: Running and Testing the SimulationFigure 7.2: Selecting Cessna 182 RC in FlightGear wizard7.3 Selecting the Take off LocationThe next window after Select aircraft window is Select a location window. In thiswindow the airport is selected. It does not matter which airport is selected, because thelocation of the Cessna 182 RC will be defined by latitude and longitude later. Figure7.3 shows the Select a location window.Figure 7.3: Selecting a take off location in FlightGear wizard53

Chapter 7: Running and Testing the Simulation7.4 Setting the Simulation ParametersThe third window in the FlightGear wizard is the settings window. From this window,the parameters of the simulation can be specified. The main settings window is shownin Figure 7.4.Figure 7.4: Main settings window in FlightGear wizardImportant settings should be done from the advanced settings button. These settingsare:7.4.1 Setting the Flight Dynamic ModelThis can be done from the advanced settings window by clicking on the Flight Modelon the left window as shown in Figure 7.5 , and making sure that FDM setting is jsp (JSBSim model ), which is the default FlightGear setting.54

Chapter 7: Running and Testing the SimulationFigure 7.5: Setting Flight Dynamic Model in FlightGear wizard7.4.2 Setting Latitude and Longitude Coordinates of the Take off PointThis can be done from the advance settings window by clicking on initial Position onthe left window as shown in Figure 7.6, and specifying the coordinates in degrees. Thispoint is the first point in the calculated flight route.Figure 7.6: Settings of the Cessna 182 RC initial position in FlightGear wizard55

Chapter 7: Running and Testing the Simulation7.5 Running the SimulationThe simulation can be run from the main settings window by clicking on the Runbutton. Please refer to the FlightGear user manual on how to use the keyboard ormouse to control the aircraft, and to see the aircraft from different views. Figure 7.7shows the Cessna 182 RC on the ground before take off.Figure 7.7: Cessna 182 RC before take off7.5.1 Settings for the AutopilotThe autopilot settings can be specified from the autopilot settings window shown inFigure 7.8.Figure 7.8: Autopilot settings window56

Chapter 7: Running and Testing the Simulation7.5.2 Specifying the WaypointsWaypoints can be specified before or during the flight from the add waypoint window,as shown Figure 7.9. It is also possible to remove any waypoint during the flight.Figure 7.9: Add waypoints window7.6 Testing the SimulationAfter setting the autopilot and adding the waypoints to the flight path, an autonomousflight simulation was ready to test. The autonomous simulation was performed withvery satisfactory performance. For instance, autopilot can control the UAV to ascendto the specified altitude and hold it, as shown in Figure 7.10 and Figure 7.11 byactivating the altitude hold in the autopilot settings window.Figure 7.10: Cessna 182 RC in hold altitude flight57

Chapter 7: Running and Testing the SimulationFigure 7.11: Side view for Cessna 182 RC altitude hold flightIt is also possible to hold a constant speed, zero bank angle, pitch angle, angle ofattack. To force the UAV to follow the selected waypoints, the true heading should beactivated in the autopilot settings window. Figure 7.12 shows the Cessna 182following the waypoints listed in the top left of the screen.Figure 7.12: Cessna 182 RC in True heading flight58

Chapter 8: Conclusion and Future WorkChapter 8 : Conclusion and Future WorkUnmanned aerial vehicles (UAVs) can replace manned vehicles in many applications,either military or civilian, due to the possibility of reaching dangerous and inaccessibleareas without exposing pilots to any risks. The UAVs can be remotely piloted byground operator or autonomously by an onboard autopilot. Some UAVs have internalspace to carry payload such as sensors and cameras. Although UAVs have beendevolved mainly in military laboratories to perform military operations, there isgrowing interest in the developing process from academic researchers and hobbyists toextend their use to other applications in civilian operations such as searching formissing people, aerial photography, monitoring of sea pollution and traffic control.Flight Dynamic Models and Flight Simulators are used in the development process ofthe UAV to test its design and control systems, since it reduces the time and the risksof damaging the UAV in the real flight.In any Flight Dynamic Model, the coefficients of lift, drag, and side forces and roll,pitch, and yaw moments must be estimated in order to calculate the forces andmoments acting on the UAV.The combination of FlightGear Flight Simulator and JSBSim Flight Dynamic model,which are open source projects written in the C++ programming language and used inthis project, provided a solid base for building the simulation environment.To model the Cessna 182 RC UAV (or any aircraft) in JSBSim Flight Dynamic Modeland simulate it with FlightGear flight simulator, essential configuration files must beconstructed. These files include main Cessna 182 configuration, engine configuration,propeller configuration, electric system configuration, and 3D model configuration, foran autonomous flight autopilot configuration file is required. All these files are tiedtogether in the top level configuration file.The real challenge was to construct an accurate model for the Cessna 182 RC due tothe need to estimate its parameters as accurately as possible. Some of these parameterswere measured physically from the Cessna 182 RC UAV, and others were generatedby the free web application Aeromatic v8.0. However, not all parameters could begenerated by Aeromatic, therefore, the similar UAV Rascal 110 RC was used toestimate the missing parameters, and to refine some parameters. The constructedconfiguration files format should be the format of JSBSim version 2.0 in order to workwith the current version of FlightGear and JSBSim.In order to visualise the JSBSim model output of the Cessna 182 RC UAV withFlightGear, a 3D graphical model was constructed by using the commercial 3Dmodelling software AC3D. The 3D graphical model of the real Cessna 182, which isavailable to download from the FlightGear web page, was the starting point forconstructing a 3D graphical model of the Cessna 182 RC UAV. The model was resizedto fit the UAV dimensions, and a quite tedious process was performed to allocatethe 3D model within FlightGear.The autopilot configuration file was constructed with the help of the Rascal 110 RCautopilot configuration file. The tuning process involved changing the autopilotconstants until an acceptable performance was achieved.The flight path needed to be followed by the autopilot was constructed from waypointscalculated over a selected area by using the free software map, Google Earth. Then thecalculated waypoints were deployed in the FlightGear navigation system.By combining all the files and arranging them in specific folders within FlightGearflight simulator, the simulation was performed perfectly, and the modelled Cessna 182RC UAV was flown normally by an operator, and autonomously by its autopilot. The59

Chapter 8: Conclusion and Future Workautopilot has the capability to perform many tasks such as altitude, velocity hold, bankangle, pitch angle, and true heading hold.Although many challenges were faced at the beginning of the project due to the needto investigate different approaches, and the lack of clear documentation for FlightGearflight simulator, the aims of the project were met, and the project deliverablesspecified in section 1.2 were all delivered.The outcomes of the project were a complete JSBSim flight dynamic model and a 3Dgraphical model for the Cessna 182 RC UAV, an effective FlightGear autopilot. Anautonomous Flight was performed.This project also opens the doors to future work in performing Hardware-in-the-loop(HITL) simulation, in which the physical system, for instance onboard autopilot, to betested is fooled into thinking that it is working with real inputs and outputs. This canbe done by connecting the autopilot hardware to FlightGear flight simulator andconfiguring it to control the Cessna 182 RC UAV instead of FlightGear autopilot.60

ReferencesReferencesAnderson, D.F. & Eberhardt, S. (2001), Understanding Flight, McGraw-Hill.Berndt, J.S. (2000), JSBSim The Open Source Flight Dynamics Model in C++viewed 16/08/ 2006 .Berndt, J.S. (2005), A Journal for the Creation and Refinement of a JSBSimAircraft Flight Model, viewed 18/08/ 2006.Cameron, K. (1995), Unmanned Aerial Vehicle Technology, Report NumberDSTO-GD-0044, Aeronautical and Maritime Research Laboratory,Melbourne, Australia.Houghton, E.L. & Carpenter, P.W. (2003), Aerodynamics for EngineeringStudents, Fifth ed, Butterworth-Heinemann.Megginson, D. (2002), Mini-HOWTO: 3D Aircraft Models in FlightGear, viewed19/08/ 2006 .Olson, C.L. (2004), FlightGear Autopilot: Theory, Configuration, and Tuningviewed 18/08/ 2006 .Zyskowski, M.K. (2003), 'AIRCRAFT SIMULATION TECHNIQUES USED INLOW-COST, COMMERCIAL SOFTWARE', paper presented to theAIAA Modelling and Simulation Technologies, Austin, Texas, 11-14August 2003.(WWW1), 'Flight Simulator', viewed 16 August 2006 ,.(WWW2), AeroSim BlocksetUnmanned Dynamics, viewed 16/08/ 2006 .(WWW3), Introduction to FlightGear, viewed 17/08/ 2006.(WWW4), FlightGear Features, viewed 17/08/ 2006.(WWW5), Microsoft Flight Simulator 2004, viewed 17/08/ 2006.(WWW6), Aeromatic, viewed 19/08/ 2006.(WWW7), AC3D, viewed 20/08/ 2006 .61