Zephyr: A High Altitude Long Endurance Unmanned Air Vehicle ...

Zephyr:A High Altitude Long EnduranceUnmanned Air VehicleAnnabel RapinettA dissertation submitted to the Physics Departmentat the University of Surrey in partial fulfilment of thedegree of Master in Physics.Department of PhysicsUniversity of SurreyApril 2009i

AbstractZephyr is a lightweight UAV designed to fly at altitudes up to 70,000ft for months ata time, powered using solar panels and rechargeable batteries. In 2008 the aircraftflew for over 80hours, nearly trebling the current record for the flight duration of anunmanned air vehicle. It has gone through several stages in development an\dundergone acceptance testing at appropriate stages in the system development. Aftereach trial to monitor its performance, Zephyr has been amended and developedfurther, and should be completed by 2010.The current aircraft weighs less than 30kg, has a wingspan of 18m and is handlaunched. In order to fly using solar power it must be lightweight to reduce powerrequirements needed to keep the aircraft in the sky. The structure is made of carbonfibre as this is a light and strong material. Every component on board the aircraft isessential to enable flight and is often custom manufactured to ensure it is aslightweight as possible.This report covers the history and background of Zephyr, the technical aspects thatenable Zephyr to fly, the tasks that I undertook during my year, who Zephyr’scompetitors are and what can be done to ensure Zephyr has a successful future bothtechnically and commercially.ii

AcknowledgementsI’d like to thank the entire Zephyr team within QinetiQ, Farnborough for their helpand support throughout the year, with special thanks to my supervisors Mr DuncanFortune and Dr Daren Lock. I would also like to thank Professor Ben Murdin andProfessor Paddy Regan from the University of Surrey for making the placementpossible and for their continued support during the year.iii

List of AbbreviationsAbbreviationAoAADCa-SiCOTSDCeVFAIFCCFCSGCSGDTGPSHALEISALi-SNASANIOSHPCBUAVUCAVUSAUSDDefinitionAngle of AttackAnalogue-to-Digital ConverterAmorphous SiliconCommercial Off the ShelfDirect CurrentElectron VoltsFédération Aéronautique InternationaleFlight Critical ComputerFlight Control SystemGround Control SystemGround Data TerminalGlobal Positioning SystemHigh Altitude Long EnduranceInternational Standard AtmosphereLithium SulphurNational Aeronautics and Space AdministrationNational Institute for Occupational Safety and HealthPrinted Circuit BoardUnmanned Air VehicleUnmanned Combat Air VehiclesUnited States of AmericaUnited States Dollariv

ContentsChapter 1 - Introduction and BackgroundPage11.1 History of Zephyr Page 31.2 Environmental Issues Page 81.2.1 Air pressure and Densities Page 81.2.2 Temperature Page 91.2.3 Cosmic Rays Page 111.2.4 Mission Envelope Page 12Chapter 2 - Technical Data Page 142.1 Structure Page 142.2 Power Page 142.3 Propulsion Page 172.4 Avionics Page 182.5 Ground Control System Page 202.6 Payload Page 222.7 Safety Cut Down System Page 232.8 System Performance Page 25Chapter 3 - My Projects Page 263.1 Radiosonde Page 263.2 NIOSH Lifting Equation Page 293.2.1 Variables Page 313.3 Airspeed Sensor Page 353.3.1 Calibration Page 353.3.2 Accuracy Page 353.3.3 Built in Errors Page 363.3.4 Operation Page 363.3.5 Zener Diode Page 363.4 Payload Page 403.4.1 GPS Signal Page 41v

3.5 Angle of Attack Sensor Page 44Chapter 4 - Angle of Attack Experiments Page 474.1 Background Page 474.2 Experiment Page 494.3 Results Page 504.4 Conclusion Page 51Chapter 5 - Commercial Aspects Page 535.1 Uses of UAVs Page 535.1.1 Transport Page 535.1.2 Scientific Research Page 535.1.3 Warfare Page 545.1.4 Reconnaissance Page 555.1.5 Communication Page 555.2 Uses of Remote Sensing Page 565.2.1 Aerial Photography Page 575.2.2 Aerial Videoing Page 585.2.3 Infrared Page 585.2.4 Radar Page 595.3 Zephyr Competitors Page 625.4 Conclusion Page 655.5 Future Page 66Appendix 1 Page 67Appendix 2 Page 68Appendix 3 Page 69Appendix 4 Page 70Appendix 5 Page 71Bibliography Page 73vi

Chapter 1Introduction and BackgroundA HALE UAV is a high altitude long endurance unmanned air vehicle. As the namesuggests, it can fly at an altitude of over 30,000 feet, for as long and as far as thepower supply allows it. A UAV can either be controlled by a remote control, have apre-programmed auto pilot system, or use a telecommand system from a groundstation.There are many UAVs available, ranging from micro UAVs that are a few centimetreswide to large UAVs that are comparable in size to passenger planes, with their usesand applications ranging from precision bombing to monitoring hurricanes. There aretwo main categories that UAVs of all sizes fit into; those used in warfare and those forcommercial and civil applications. UAVs that are used in warfare are most commonlyfor reconnaissance (i.e. gaining battlefield information), combat (capable of droppingbombs and other weapons) and target and decoy (the opposition attack these thinkingthey are carrying the enemy, enabling the actual enemy to attack unsuspectingly). Thecivil and commercial applications have a much larger scope. The most commonapplications are for surveillance (i.e. monitoring an area for suspicious behaviour),remote sensing (taking images of the ground for mapping or environmentalmonitoring for example), communication, and scientific research (for example flyinginto a hurricane). UAVs are very useful as they can perform jobs that would be toorisky for a human to do, or that are simple, monotonous and long.To date, most UAVs use internal combustion or jet engines and are directly controlledvia remote control from the ground (as aforementioned). This means that not only will1

the UAV have to be regularly refuelled and will have to land in order to do so, theuser also has to be within a certain range in order control it. By having a UAV thatuses solar power it will not need to be refuelled and can theoretically fly for virtuallyunlimited periods of time, and by using an autopilot system it is possible for the userto be anywhere in the world.2

1.1 History of ZephyrZephyr is a lightweight, solar-electric, HALE UAV that is currently in itsdevelopment stages. The programme started in 2001 as a way of filming an attempt tobreak the world altitude record in a balloon and since then the development of theUAV has continued. The most recent version of the platform, Zephyr 6, flew for 82hours and 37 minutes in the summer of 2008. This nearly trebled the official worldrecord for the longest unmanned flight of 30 hours and 24 minutes (set by GlobalHawk in 2001).In 2001 development started with Zephyr 2, a proof of concept aircraft weighing lessthan 7kg that flew in both free and tethered modes (off the Clifton suspension bridgein Bristol) (see figure 1). After this successful initial research, the programme beganto develop and in 2002 a 12m long, 15kg UAV called Zephyr 3 was created. It wassupposed to fly tethered to a manned balloon attempting a world record height of132,000ft, but unfortunately the balloon had a technical problem and thus both theballoon and the aircraft never flew.After the balloon failed to launch it was decided that there was a possibility of theplatform being developed as a UAV for:Intelligence, Surveillance, Target Acquisition and Reconnaissance (ISTAR);Remote sensing / Earth Observation;Communications relay for ground forces;3

Coastguard patrol / fire spotting / disaster management.Figure 1. Proof of concept Zephyr 2Following on from this, the Zephyr 4 project was created in order to carry out criticaltechnology risk reduction and concept development work. Zephyr 4 also had a 12mwingspan, but weighed approximately 17kg, and was designed to be launched by theuse of a helium balloon (see figure 2). In February 2005, Zephyr 4 underwent a testflight in Woomera, South Australia, where it flew for one hour after being launched at30,000ft by the balloon. The primary objective of the test flight was to validate thefundamental characteristics of the HALE UAV concept; to see how much power isrequired to keep the aircraft flying and to what altitude the aircraft will descend toovernight, thus determining the endurance capability.Figure 2. Zephyr 4 ready to be launched4

The Zephyr 5 project continued with the risk reduction and development work andconsisted of two aircrafts, Zephyr 5-1 and Zephyr 5-2 (see figure 3). It was decidedthat the aircrafts should be hand launched from the ground rather than using a heliumballoon. The reasoning for using the more complicated balloon launch originally wasdue to concerns that Zephyr wouldn’t fly in the denser lower atmosphere. However itwas found that Zephyr was able to fly in the lower atmosphere although very slowlyand therefore the balloon launch was not needed. Zephyr is very susceptible to gustsin the lower atmosphere and that is why launch day has to be carefully chosen in orderfor there to be a perfect weather window.Both aircraft were identical apart from the power supplies, with Zephyr 5-1 using bothbattery and solar power (weighing 31kg) and Zephyr 5-2 being solely battery powered(25kg). Zephyr 5-2 used only primary non rechargeable batteries, so it could never flyfor more than one night.In December 2005 the two aircraft flew in New Mexico, USA, flying for 4 hours andsix hours respectively, demonstrating successful ground launch, ascent, cruise anddescent. In July 2006 the two aircrafts were flown again in USA with Zephyr 5-1reaching 36,000ft on an 18 hour flight including 7 hours overnight. Collectively thetwo aircrafts have a flying time of 35 hours.Figure 3. Zephyr 5-1 in flight5

Zephyr 6 (see figure 4) built upon the findings of the Zephyr 5 trials and moved awayfrom risk reduction and development and more towards capability demonstration. Theproject resulted in the first Zephyr platform to have a fully-functional and successfulpower system that can demonstrate solar power flying with in-flight battery recharging.Zephyr 6 has an 18 metre wingspan but weighs less than 30 kilograms as itis constructed out of ultra-light carbon-fibre. In July 2007 Zephyr 6 flew for 54 hoursin New Mexico at a peak altitude of over 58,000ft, thus flying in the targeted 50,000-60,000ft range and validating the thermal and performance models. In August 2008Zephyr 6 flew for over 87 hours in Yuma, Arizona, reaching an altitude of 60,000ftand surpassing the current world record for the longest unmanned flight. The currentrecord is 30 hours set by Global Hawk in 2001, but in order to have the official recorda member of the Fédération Aéronautique Internationale (FAI) must be presentthroughout the flight and during this flight there was no official members present [1].Figure 4. Zephyr 6 in flightCurrently Zephyr 7 (figure 5) is being developed which is designed to fly for monthson end at an altitude of 65,000ft during the day and over 45,000ft during the nightusing only solar and electric power. During the day amorphous silicon arrays are usedto generate solar power to fly Zephyr, and by night Zephyr flies using rechargeablelithium-sulphur batteries that are charged during the day using solar energy. It usesmany of the same technologies as Zephyr 6, housed in a larger (20m), more efficient6

airframe, with the main aim to improve the efficiency and reliability of Zephyr.The aircraft has a lightweight payload that can be used for security and surveillance,remote sensing and communications relay. As Zephyr can fly for long periods of timewithout needing to refuel, the payload can be used for long-term communication ormonitoring.A ground station allows the pilot to communicate with the plane. The aircraft is handlaunched and controlled using a remote control, and once it reaches a certain altitude,an autopilot system takes over. Meanwhile at the ground station, the crew and theautopilot systems constantly monitor the plane and the weather conditions to ensurethe plane is flying where intended. Co-ordinates can be set in order for the aircraft tofly to that particular point, but if something happens the pilot can take manual control.If, for some reason, the plane loses contact with the ground station there are a fewsafety measures in place. The plane will automatically fly towards a pre-programmedpoint and circle here while the crew try to re-establish contact or diagnose any fault.As an absolute last resort the aircraft can be ‘cut down’ and brought to the ground.This involves the plane accelerating downwards into a spiral before breaking intopieces and crashing into the ground. These measures are standard on an experimentalaircraft and have never been used in actual operations.7

Figure 5. A Zephyr 7 prototype flown successfully8

1.2 Environmental IssuesDuring the day Zephyr flies at its maximum height and during the night while it isrunning off batteries its altitude decreases. The reason for the decrease in altitude isbecause there is not enough charge in the batteries to drive the motors at the highrevolutions required to maintain altitude for the entire night. At lower altitudes theatmosphere is denser and less power is required to maintain flight, despite the higherresistance. This cycle happens daily for the whole duration of the flight, and can putpressure on the aircraft and its components. The low pressures, cold temperatures andcosmic rays all affect the components that are on board the aircraft.1.2.1 Air Pressure and DensityThe air pressure ranges from about 1000 millibar at sea level to 50 millibar at thehighest altitude, while the temperatures range from 40ºC (at sea level) to -80ºC duringthe night.At lower altitudes the air is thick and dense and as this is where our weather occurs sothere are many gusts of wind. As Zephyr is so light, it is very susceptible to gustswhich could knock it off course or cause the aircraft to break. In order to overcomethese winds, launch has to take place in perfect weather conditions in order tominimise risk of wind. This is one of the reasons the previous flight trials haveoccurred in the deserts of Australia and the USA as the desert provides still air andminimum turbulence.9

At higher altitudes the air density is lower which means there is less air to freeze andless water vapour to ice over components. It also means that there is less air resistanceand drag to impede the aircraft. However the propeller’s need air in order to move theplane forwards, so while less air means less resistance, it also means less propulsion.In order to keep the plane flying at the correct altitude and speed, the power needs tobe increased to overcome the lack of air.1.2.2 TemperatureThe troposphere, which reaches about 36,000 feet in altitude, contains approximately75% of the atmospheres mass and it is where the majority of our weather occurs. Thehigher the altitude the lower the temperature gets, at a rate of approximately 2ºC per1000ft. The coldest temperatures of -80ºC are found in an isothermal layer between36,000ft and 65,000ft, where the troposphere meets the stratosphere. Above 65,000ftthe temperature begins to rise despite the rising altitude. This is because of the ozonelayer which absorbs ultraviolet sunlight and emits it as heat. Zephyr 7 is set to flywithin the isothermal layer and will therefore be experiencing warm temperaturesduring the day, but very cold temperatures in the night. The cold temperatures and thedaily cycling over such a large range is the biggest problem that faces Zephyr.The cold temperatures mean that there is a high probability that critical componentssuch as sensors, motors and servos will malfunction. The cold temperatures can causeelectronics to fail and structures to become brittle. Metal components can alsocontract in the cold temperatures and if two metal objects need to remain in contact in10

order to function, they may not thus causing the sensor to break. The angle of attacksensor which operates by having a metal brush move over a resistive track gave faultyreading during the 2007 flights of Zephyr. This was because the lubricant in betweenthe track and the brush could not withstand the cold temperatures and froze, breakingthe contact between the two components.In order to minimise this problem these components must be insulated or heated tostop them from icing. Insulating each component adds extra weight onto the aircraftwhile heating each component uses power that could be used for flying the plane. Inorder to overcome this, either a lightweight insulator must be used, or heat that isdissipated from the components during their normal operation can be used to heatthem and keep them between the temperature limit constraints set by themanufacturer. In reality a mixture of both is used. Some components, for exampleelectronic circuit boards, create enough heat to insulate themselves while others donot and thus need to be insulated. The insulator that is currently used is a lightweightsilicon gel based substance about 6mm thick with a hard outer layer and a fibrouscentre. It is cut into the shape needed and then sealed around the edges.Even if the components are kept within the specified temperature range, the dailycycling can cause problems. When the temperature is high metal components willexpand and when the temperature is low, they will contract. Sensors that require twopieces of metal to be touching are most at risk, as the cycling will cause joints to beweakened. The metal can become brittle and break, or it can contract in such a waythat it is no longer in contact with the other piece of metal. This can cause incorrectresults or stop a component from working. Other than keeping the component11

insulated to limit the temperature range, not much can be done to prevent this.12

1.2.3 Cosmic RaysCosmic rays are energetic particles (90% of them are protons) that originate fromspace and enter our atmosphere. Due to their high energies (some have energies above1020eV), they can interact with interstellar matter and create secondary cosmic rays,or they can interact with molecules (mainly oxygen and nitrogen) in our atmosphereto create lighter particles. Both primary and secondary cosmic rays can alter the stateof elements in electronic circuits, causing transient problems like corrupted data inmemory or inaccurate behaviour of processors. As the intensity of cosmic radiationincreases with altitude, it causes problems for satellites and UAVs. The peak incosmic rays for both intensity and coincidences is at about 50,000ft (see graph 1).This is the altitude that Zephyr will be flying through when it is cycling so there is ahigh possibility that equipment could get corrupted. This could not only corrupt databut it could also create a critical fault. In the 100 hours of flying time that the Zephyraircrafts have had, not once have any of the onboard electronics been affected bycosmic rays.Graph 1. Graph to show variationof cosmic rays with altitude [2]13

1.2.4 Mission EnvelopeAs Zephyr is powered by solar energy it is essential that it is flown when there isenough solar power to power the aircraft. In the northern hemisphere this is during thethree months surrounding the summer solstice (May, June and July). As seen byGraph 2, the most solar energy is not at the equator, but it is in fact on the Tropic ofCancer at a latitude of 23° 26′ 22″. The closer you get to this latitude the more powerwill be available and the longer Zephyr will be able to fly. In London (51° 32’) theperiod of time when the solar power is large enough for Zephyr to fly is lower, andthus the mission length is shorter.Wm 2Graph 2. Graph to show how solar power varies depending on the time of the year andthe latitude [1]As aforementioned, another important condition is the current weather conditions. Itis important to try and begin Zephyr’s flight when there is a cloudless sky with low14

wind speeds. This is to ensure that the air is not turbulent and that there are no guststhat might blow Zephyr off course or more importantly cause it to stall. Launching atdaybreak tends to be the calmest part of the day (especially in the desert) and it alsoensures a full day of sunlight during which the batteries can be recharged and theaircraft can be flown using solar power.To summarise, in order to have a successful mission it is important to fly the aircraftduring the months with most solar power in order to have a long flight, and it isimportant to start the flight when the weather conditions are optimal.15

Chapter 2Technical Data2.1 StructureAs the aircraft operates using solar power, it needs to be as light as possible in orderto reduce the power needed to fly it, while still remaining strong and supporting theother components. The airframe structure is span loaded, meaning the weight isdistributed along the wings, and thus the wings are very large to support the weight.Each section of the wing supports itself and a bit more, so Zephyr must be carefullysupported when sitting on the ground. If it was picked up by the fuselage, the wingswould snap off. This is because the wings are designed to withstand upwards pressurefrom the air when in flight, and not the downward pull from gravity. Mostplanes/UAV’s aren’t like this and can sit on their point-loaded undercarriage. It ismade of ultra light carbon fibre and designed to carry a distributed avionics and powersystem.2.2 PowerOne of the most critical parts of the UAV are the batteries and the solar cells whichpower the plane. During the day an amorphous silicon (a-Si) solar array is used togenerate all the power needed to fly the aircraft at its maximum altitude, as well asfully recharge the batteries for the night flight. Amorphous silicon solar cells areknown as ‘thin film’ silicon cells as they use layers of a-Si only a few micrometersthick. As these cells are so thin, they use less than 1% of the silicon used in standard16

crystalline silicon cells and can be attached to practically any backing [3]. This meansthat they can be formed to fit applications and can be bent around materials, and arealso very lightweight. The solar arrays that are used are only 10% efficient (comparedto others available which are about 30% efficient), but they provide the best power tomass ratio which is important as mass is a limiting factor on Zephyr.The batteries are lithium sulphur (Li-S) produced by SION Power Inc, and are still intheir developmental stage with only three people in the world producing them. Due tothe low atomic mass of lithium (6.941) and the moderate mass of sulphur (32.065)[4], the batteries are relatively light. They also provide a much longer charge than themore commonly used lithium ion batteries; at a discharge current of 500mA it takesabout 4 hours to discharge compared to 2 hours for a lithium ion battery (see Graph3).Graph 3. Discharge profile of SION lithium sulphur cells with a discharge current of500mA [5]17

The Li-S batteries are not as efficient as lithium ion batteries, but they have thehighest power to weight ratio of all batteries in the current market and this is why theyhave been chosen.In order to keep the batteries at maximum efficiency they have to be heated to roughly20ºC. This is the temperature when they are most efficient, meaning that they can becharged to their maximum and most power can be extracted from them. If they werenot heated they would not be as efficient and no matter how fully they were charged,only some of the available power would be dischargeable during the night flight. Ithas been calculated that the power needed to heat the batteries to their efficienttemperature is much less than the power that would be wasted if they were not heated.As the batteries are developmental, they are very volatile. If they do self combust andcatch fire, the fire cannot be extinguished and they have to be left to burn themselvesout. Boxes have had to be created in order to store the batteries so if self combustionoccurs, the fire will be contained.Onboard Zephyr the batteries are recharged everyday which reduces their capacityand performance and thus the manufacturer is currently developing them further toensure that they will be able to last for a few months with enough power to fly theplane.In relative terms, the batteries are the heaviest part of the aircraft as they consist of576 cells split into strings, which are then grouped in parallel to create ‘super-packs’18

which are then insulated. In order to minimise weight and stabilise the aircraft, theyare span loaded thus distributed along the wing and are arranged into groups of superpacks.2.3 PropulsionIn the past the propulsion system used contained a DC motor driven by a speedcontroller that provided power to the propeller via a mechanical gearbox. Howeverduring the Zephyr 6 flights in 2007, there was a motor failure on the third day andanalysis of the motor and the gear box showed that the failure was due to a gearboxseizure (see figure 6).Since then a new propulsion system has been developed that still includes a motorcontrolled by speed controllers, but instead of a gearbox they provide power directlyto the propeller through the direct drive motor that is optimised for flight up to70,000ft. By removing the gearbox the efficiency is increased by reducing losses frommechanical drag, and it also increases the propulsion system reliability.Figure7. The original Zephyr 6-1 motor gearbox that failed on day 3. [1]19

The power settings are implemented via the avionics system from the ground and aredependent on the available power and minimum and maximum permitted powersettings. The minimum power setting is based on the minimum power needed tomaintain the desired flight overnight and to maintain the motor temperatures withinacceptable limits. The maximum power setting (and thus the associated altitude) isdetermined by available power during the night, and the torque limit during the day.2.4 AvionicsThe avionics system provides a telecommand and telemetry capability mounted on theavionics master board, providing command and control of all aspects of the aircraft.On the ground there is an antenna which has a 5-degree beam width and tracks theaircraft using either GPS or direct signal power tracking. The main component of thecommunication system used for telecommand (flight command uplink) and telemetrydata downlink is the air data terminal. This unit has three main functions including:Receiving telecommand data transmitted from the ground control system(GCS) to the aircraft and forwarding the signals to the mechanisms onboard,Receiving telemetry data from the sensors and sending them to the GCS,Implementing a safety mode if signal is lost for more than 18 seconds.Once in flight, Zephyr can be flown either manually or via an automatic FlightControl System (FCS) (autopilot). The avionics system on board Zephyr collects datafrom all air sensors, the GPS, and the inertial measurement unit which measures the20

motion of the aircraft including roll, pitch and yaw. The roll, pitch and yaw are theangles the aircraft is at in relation to the longitudinal, lateral and vertical axis, asshown in figure 7.Figure 7. Diagram to show the roll, pitch and yaw of an aircraft [6].The information from the avionics system is then sent to the FCS and is alsotransmitted as telemetry for manual monitoring. Using this information the flight ofthe aircraft can be monitored and thus the aircraft can be controlled. The controlactuators (any mechanical device used to control Zephyr) and the motors are bothcontrolled via the avionics system onboard the aircraft. Control is either manual fromthe ground control system or direct through the FCS when in autopilot mode (this istrue in the case of elevator and spoiler control).Launch and landing is performed under manual flight control and automatic flightcontrol is normally enabled at all other times. Automatic flight control isimplemented through angle of attack and roll stabilization, heading control, waypoint21

navigation via GPS, and ground track following waypoint navigation. All arecontrollable from the GCS through the transmission of new angle of attack/pitch/roll,heading or waypoint commands as appropriate. Rate of climb is always controlledmanually from the GCS via the motor power demand. The flight crew monitors theaircraft 24 hours a day and manual override of the automatic flight control system ispossible at anytime from the ground in case it needs to be used, but in practise theaircraft is usually flown by the autopilot.Forward looking pilot cameras onboard the aircraft are used to assist in take-off andlanding, to detect the horizon and cloud details during day and night. These camerashelp to assist the pilot in navigation and help to monitor the FCS performance.2.5 Ground Control SystemThe Ground Control System, GCS, consists of a group of ground antennas andportable cabins from which the aircraft is controlled. The primary systems found inthe cabins are a Ground Data Terminal (GDT), Flight Critical Computer (FCC) and asystem of computers operating via a Local Area Network for increased command,control and mission planning ability. The antenna systems, GDT and FCC are theonly flight critical components necessary for maintaining control of the aircraft andtherefore a completely separate duplicate system is running in case there is a failurewith one of the critical components. An uninterruptible power supply ensures that ifthere was a temporary loss of main electrical power to the GCS, it will not jeopardisethe mission.22

Within the portable cabins the crew control and monitor the flight of Zephyr throughthe following 6 crew positions;UAV Commander;Pilot;Flight Engineer;Navigator/Meteorology Analyst;GCS engineer;Payload operator.The UAV commander is a qualified and competent person who is in charge of theentire operation, including launch, flight and recovery. They may either be in directcontrol of the aircraft (i.e. undertaking the role of the pilot), working jointly with thepilot, or monitoring the state and progress of the air vehicle. In addition to being incharge of the aircraft, the UAV commander ensures that all range ground control andair traffic control instructions are followed promptly, and that Zephyr remains withinthe designated airspace.The pilot flies the aircraft when in manual mode via a conventional pilot control boxby sending information to the aircraft. The pilot uses telemetry data, a forwardlooking camera for horizon indication and a real-time moving map showing GPSposition to help maintain situational awareness both when the aircraft is beingmanually controlled and in autopilot.23

The flight engineer monitors the aircraft systems to ensure all the telemetry is beingcontinuously displayed and recorded. They keep the aircraft functioning and monitorpower usage and aircraft performance.The navigator/meteorology analyst monitors the aircraft position and path of theaircraft from the telemetry data, and monitors the current and future weatherconditions. If there is any risk of the aircraft flying outside a set boundary, thenavigator can command an immediate rapid descent.The GCS engineer monitors and maintains the ground control station to ensureeverything is working correctly. It is also their duty to ensure an adequate radiofrequency link is maintained with the aircraft at all times.The payload operator is the person who controls the camera on board the aircraft andis in charge of its operation.2.6 PayloadAlthough it’s not critical to the flight of Zephyr, it’s the payload that gives the aircrafta purpose. The payload on Zephyr is a camera and is controlled from the GCS fromwhich the commands are sent to the avionics on board which are connected to thepayload via the payload controller board, the PCB. The PCB provides the payloadwith a regulated power supply at the required voltage and ensures the correctcommand and control communications are transferred to the payload.24

Simple payloads instructions without complex command and control requirements,such as only powering on and off, can be controlled directly from the avionics systemwithout the need for a payload controller PCB.2.7 Safety Cut Down SystemThe Zephyr aircrafts are fitted with a safety and containment system to ensure theaircraft remains within its allocated airspace. This system has two main functions, thefirst of which is to keep it flying within its allocated air space and the second is to cutthe aircraft down in a safe and controlled manner in the event of a permanent failurein communications between the aircraft and the GCS. These two functions arecovered by the safety waypoint mode and the cut-down mode.The safety waypoint mode is triggered if the command link is lost for more than a predefinedtime interval; for manual flight this is 4 seconds and for automatic flight thisinterval is 2 minutes. Triggering the safety waypoint causes the aircraft to fly towardsthat point and then loiter until communication is re-established. The location of thesafety waypoint is reviewed and can be updated during the flight in order to reflectnew meteorological conditions or in the event of the allocated airspace being changedduring flight.The safety waypoint can also be triggered by entering an invalid waypoint demand orin the event of the main avionics processor being reset. The safety waypoint triggercan be cancelled by sending a valid waypoint or heading, or by returning the aircraft25

into manual flight mode.Cut-down is triggered if the command link is not re-established within a pre-definedtime interval after the safety waypoint mode is triggered, regardless of whether theaircraft has reached the safety waypoint or not. After the safety waypoint trigger, thetime interval before cut-down is triggered is 30 minutes; this is based on a softwarecountdown. There is also a cut-down in the event of a software failure in the safetysystem, and this is generated 45 minutes after the original safety waypoint trigger.The cut-down system can also be triggered by the following conditions:loss of master avionics for more than 5 seconds;loss of tail actuation command for more than 5 seconds;Or through manual commanded cut-down which can be performed at anytimefrom the GCS and will execute immediately.When cut-down is triggered, the following actions are applied; ½ down elevator and100% left rudder, leading to a spiralling pitch down manoeuvre and over-speed whichwill cause the wing to break at the centre. The wings and fuselage are tied togetherwith Spectra cord, and this combined with the aircraft wiring harness and solararray bonding keeps all the Zephyr structure together. The airflow over the brokenaircraft that is tied together will be largely separated, falling like a shuttlecock withthe wings and trailing tail. The flow will typically produce a separated dragcoefficient of 1.0 and the terminal rate of descent will be between 7 and 10 m/s. Thisensures a near-vertical dive which means that the location where the plane will hit theground is known, which is good for safety considerations.26

In the event that the wing does not break during cut-down, Zephyr will descend stablyin an inverted spiral at approximately 10m/s, and the aircraft will return to groundwithin close proximity to the cut-down point.2.8 System PerformanceThe limiting factor on Zephyr is the mass and thus the power. The aircraft needs to belight enough in order to be able to take off and remain in the sky using just solarpower. The heavier the aircraft becomes the more batteries will be needed which willadd to the weight of the aircraft until eventually the aircraft will not carry enoughbatteries to lift its own weight. As the weight is distributed evenly across the wings,the larger the aircraft becomes the stronger the structure will have to be in order tosupport its own weight. This will most probably be done by either adding more sparsor changing the material. Either of these will increase the weight so again furtherbatteries would need to be included and thus the weight increases further. It may bepossible to use another weave of carbon fibre that is stronger and lighter, but this willbe very expensive. If the structure is not changed, there is a possibility that theaircraft’s wings will snap under their own weight – as it is, the aircraft has to besupported carefully when on the ground to prevent this from happening.27

Chapter 3My Projects3.1 RadiosondeCurrently a program has been created that allows the user to enter the altitude theywant the aircraft to fly at and it automatically calculates the power needed to achievethis height. The program takes into account the air temperature, pressure, air speedand density amongst other things in order to calculate the power. While the otherthree variables are being measured on the aircraft continuously, the density values thatare used are from the International Standard Atmosphere (ISA) model which showshow a number of variables, including density, change over a range of altitudes. Thishowever is a general model and in reality the density varies depending on yourlocation, the air pressure and temperature, and the weather systems present.While on trial, NASA transmits radiosonde data to the team which gives accurate dataabout the weather conditions in the area at that time. A radiosonde is a type ofweather balloon that is radio controlled, which measures and transmits currentweather data. By comparing the standard data to the actual data it was possible for meto calculate the difference between the two. Knowing this information allows an errorto be incorporated into the program in order to give a more accurate density andtherefore a more accurate power estimate. Eventually the density will also bemeasured on the aircraft along with the other variables and there will not be a need forthe radiosonde data.28

Graph 4 shows radiosonde density data collected daily between 22nd July 2007 and31st July 2007 in White Sands, New Mexico as well as the standard density predictedby the ISA model.Graph 4 showing the variation between radiosonde and ISA density dataAs you can see, the densities from the radiosondes are all almost identical to eachother (there is some variation near ground level but as Zephyr will not be flying at thisaltitude during a mission it does not matter). It is also clear to see that the ISA datadiffers from the radiosonde data quite significantly. At the altitudes where Zephyr islikely to fly (between 45,000ft and 65,000ft), the difference between the real andpredicted densities is the greatest; about 50g/m 3 at 50,000ft, with an average of about20g/m 3 .29

I then calculated how many bits would be required to send density data from Zephyr(if there was a sensor on board) in order for it to be more accurate than the ISAestimates. If the density was measured on board the aircraft it would have to betransmitted at a resolution of half the average error in order to be more accurate thanthe ISA. If the average error is 20g/m 3 , the density needs to be transmitted at aresolution of 10g/m 3 , across the range of densities from 0 to 1100g/m 3 .This means that a total of 11000 numbers (1100 x 10) needs to be sent.If 2 n = 11000 (where n = number of bits)Thenn ln2 = ln11000Son = 13.4 bitsAs it is 13.4 bits are required, 16 bit telemetry is needed (bits are in multiples of 8) tosend more accurate density data from the aircraft than the ISA model currentlyprovides.30

3.2 NIOSH Lifting EquationThe hand launch of Zephyr must be achieved without causing harm to the body ordegrading user performance for the rest of the mission. According to the US NationalInstitute for Occupational Safety and Health (NIOSH) it is highly fatiguing to usemaximum force and maximum body strength repeatedly or for a significant period oftime, so this should be avoided. It is better to use the maximum strength that isconsistent with long-term safety and ability.Individual joint and muscle strength are of little use as launching the UAV uses morethan one joint simultaneously – both the legs, back and arms are all required in liftingand launching. During launch the team all lift Zephyr to above shoulder height, andwhile holding it they run at full speed before letting go of the aircraft. The start oflaunch (as the team is lifting and carrying Zephyr to its starting position) is shown infigure 7.Figure 7. Picture showing Zephyr being lifted at the start of launch31

The NIOSH have devised an equation that estimates the maximum load that the top90% of men (and the top 75% of women) can lift in accordance to long-term safety.The NIOSH lifting equation takes into account the horizontal distance between theuser and the object being carried, the height above ground from which the lift starts,the height moved during the lift, the twisting required by the user, the frequency ofthe action and the quality of the grip.Each variable has its own ‘multiplier’ which is dependent on the actual value and thenread from the NIOSH lifting tables. The product of all the multipliers together with alifting constant is calculated, producing an estimate for the maximum load the usershould lift.The NIOSH lifting equation is given below, where L is the maximum lifting weight.L = hm1 . vm2 . dm3 . tm4 . fm5 . gm6 . cThe six variables and their meanings are shown in table 1.Variable Description MultiplierH (cm)V (cm)Horizontal distance betweenspine and handsVertical distance fromground to object at start oflifthm1vm232

Variable Description MultiplierD (cm)T (degrees)FDistance the load travelsduring the liftAngle of load relative tousers normal axisFrequency of lifts per minutefor n hoursdm3tm4fm5G Effectiveness of grip gm6C Load Constant (23kg) CTable 1. Table showing the variables, meanings and their corresponding multipliers.By finding the numerical values of the variables I was then able to calculate themaximum mass a human can lift.3.2.1 VariablesH = 35cmFor hand launching Zephyr, the horizontal distance from the spine to the hands is thelength of the upper arm as both the shoulder and the elbow will be bent at 90° with thelower arm parallel to the body. On average the length of the upper arm is 1/5 theheight of the user. If the height of the user is an average 175cm (see below) then theupper arm is 35cm.33

V = 90cmThe UAV is on a trestle that is just under a metre in height. This is the startingposition and the height from which the users will lift from.D = 90cmThe average height of a man in the UK is 175.2cm according to the Health Survey forEngland 2007 [7]. As men are launching the UAV from a height of 90cm to justabove head height, the vertical distance travelled is 90cm.T = 0°The UAV is facing in the direction of travel so the user does not need to twist to liftthe object.F = 1 a dayThe UAV will only be lifted once a day and carried for no more than a minute,however by looking at the lifting tables a lift lasting up to one hour will not alter theresult.G = GoodThe hands wrap around the aircraft comfortably as there are small launch handlesbuilt especially for this purpose.C = 23kgC is the load constant within the equation and it is always equal to 23kg.34

Using the given values, the corresponding multipliers can be read from the NIOSHlifting tables. Table 2 shows a summary of the variables and their multipliers.Variable Actual Value Multiplier ValueH 35cm hm1 1.00V 90cm vm2 0.96D 90cm dm3 0.87T 0° tm4 1.00F 1 a day fm5 1.00G Good gm6 1.00C 23kg C 23kgTable 2. Summary table of variables and their multipliersBy substituting the values into the NIOSH equation, the maximum weight that can belifted per person can easily be calculated:L = hm1 . vm2 . dm3 . tm4 . fm5 . gm6 . cL = 1.00 . 0.96 . 0.87 . 1.00 . 1.00 . 1.00 . 23L = 19.2 kgThe NIOSH lifting equation shows that in order to maintain user safety and long termperformance, no person should be asked to carry more than 19.2kg. This means that atleast two people are needed to lift Zephyr currently as it weighs around 30kg.As Zephyr is span loaded, two people are needed at either end of the wings in order to35

ear the most weight, and a third is needed in the centre to hold up the tip which onlyweighs a few kilograms.If Zephyr ever weighs more than 40kg, either an extra person can be used to lift andlaunch it, or by positioning Zephyr slightly lower, between 70 and 80 cm themaximum weight increases slightly to 19.8kg each.With this in mind, in order for Zephyr to be lifted without degrading user performanceand taking into consideration long-term safety, it should be designed so thatindividual members of the launch team are not required to lift more than 20kg, even ifthis means that more people are required to launch the aircraft.36

3.3 Airspeed SensorThe current airspeed sensor on board Zephyr is the Honeywell DC001NDR4 SiliconPressure Sensor. As its name suggests, the airspeed sensor measures the velocity ofthe air that Zephyr is flying through and thus gives the actual velocity of Zephyr,taking into account air currents. It is important to measure the airspeed as Zephyr canonly fly to 11ms -1 before it can’t withstand the strain and breaks. The current sensorhowever, has been found to give faulty readings when in flight. Although there is noexact reason to why this happens there are a few possibilities.3.3.1 CalibrationThe airspeed sensor is not calibrated and therefore it does not take into account thechange in air flow around the wing of the aircraft. When the air hits the front of theaircraft some of it is forced under the aircraft while some is forced above the aircraft.The air that is forced above the aircraft increases in velocity as it passes over Zephyr.This means that the airspeed taken above the aircraft will only measure the localairspeed and thus give a reading higher than the true airspeed of the aircraft.3.3.2 AccuracyThe sensor can measure the airspeed to an accuracy of 0.25ms -1 . This might beaccurate enough when travelling at high speeds, but Zephyr’s maximum velocity isonly 9.5 ms -1 . This means that the measurements taken by the airspeed sensor may be37

not accurate enough and when plotting a graph to examine average velocities, theerror may give distorted speeds.3.3.3 Built in ErrorsThe sensor has a maximum built in error of 3.0% which can contribute to theproblem. As the manufacturer states that the error could be between 1% and 3% itis hard to account for this error.3.3.4 OperationThe sensor has only been tested to a -25ºC operating temperature so it is not knownwhether it will withstand, and then correctly measure, the airspeed at high altitudeswhere it is colder. Evidence suggests that although it still manages to take a reading, itis not accurate and gives an airspeed value that is much faster than reality.3.3.5 Zener DiodeThe voltage supply to the sensor on the aircraft is 12V, and the maximum outputvoltage is 4.5V. The sensor is connected through a Zener diode into an analogue-todigitalconverter (ADC) on a printed circuit board (PCB), as seen in figure 8.38

12VSensorPCBZenerDiodeFigure 8. Diagram to show how the sensor is connected to the PCBThe output voltage depends on the air pressure, with the maximum voltage being4.5V when the air pressure is 0 and thus the airspeed is 0. The PCB to which theoutput voltage is transmitted can only receive a maximum of 3.3V otherwise it willburn out and short circuit. It is important to protect the PCB as it is vital to the flightof Zephyr as it connects many of the other critical components. In order to protect thePCB a resistor and Zener network is used. A Zener diode is a type of semiconductingdiode that becomes conducting when a certain voltage is applied to it. This is neededto protect the PCB in case it receives the maximum 4.5V or if the sensor fails and thewhole 12V is transmitted.When the Zener diode receives a voltage of 3.3V it becomes conducting and drawsthe current in order to prevent it reaching the PCB. If it receives any more than 3.3V,it puts the extra voltage on a resistor on either side of the diode. If, for example, thesensor did fail and transmit the whole 12V, the diode would take 3.3V and the resistorwould take the remaining 8.7V. This will prevent the voltage reaching the PCB andshort circuiting it. If a voltage reaches the Zener diode that is less than 3.3V the Zenerallows it to pass straight through to the PCB.39

Therefore max pressure: p = ½ x 1.225 x (9.52) 2p = 55.51 Pap = 0.56 mbarThis means a sensor should be found that has a range between 0 mbar and 0.56 mbar.Once I had calculated the pressure ranges I could find a sensor (All Sensors Inc,model DS0032, see Appendix 1) that had a corresponding range of 0 – 0.6 mbar [8].Its voltage output range is also 4.5V (when the pressure/airspeed is 0), and 0V (whenthe pressure/airspeed is at its maximum, i.e. 0.6mbar).The average airspeed that Zephyr flies at is 6ms -1 , which corresponds to a pressure of3.5mbar. The corresponding voltage at this pressure is less than 2V and therefore theZener diode should not draw any current, thus eliminating the error.As the range is much smaller than the previous sensor, and much closer to the speedsof Zephyr, the output voltage will always be towards 0V as the velocity will be at theupper end of the range towards 10ms -1 . The previous sensor had such a large rangethat even at Zephyr’s maximum speed it was still in the lower end of the range andthus the higher end of the voltage.41

3.4 PayloadThe payload is the carrying capacity of an aircraft, for example cameras, weapons, orpeople. On Zephyr the payload is a camera which is used for aerial observation of theearth. The camera onboard the aircraft is a commercial off the shelf (COTS) system,meaning that it has been manufactured outside of QinetiQ and it is commerciallyavailable for anyone to use. It has been manufactured by Cloud Cap Technology Inc,and one of the mean reasons Zephyr is using it is because it lightweight (Appendix 2).It measures 12.7cm x 11.2cm x 17.8cm and weighs only 900 grams [9]. Zephyr uses acamera in order to show potential customers what is achievable with a cameraonboard Zephyr, but they are free to use any camera they like as long as it weighs lessthan 2kg and is compatible with the electronics.This camera had not been used or tested so it was important to assess how it worked,if it was easy to use and if it is compatible with Zephyr.The camera has its own software and user interface in order to control the camera.First it had to be checked that there was sufficient bandwidth between the inbuiltcamera software and the aircraft. The bandwidth available between the GCS and theaircraft is only 7 frames a second, but it was found that the camera transmitted at arate of 2 frames a second meaning there was more than sufficient bandwidthavailable.42

Once I found that there was sufficient bandwidth and thus the camera (and itsinterface) could be used with Zephyr, I examined the software to find the camera’scapability. I found that in addition to the standard functions of zooming, rotating andfocusing, the camera could also track moving objects and follow their movementsalong the ground. The camera could also stay aligned with a particular point ofinterest on the ground if the object that is to be monitored is stationary. It could alsomove and focus on any particular object in its view, as well stream live video of whatit is viewing. It can stabilise itself in case Zephyr goes through any turbulence and bycounteracting this movement it can produce a non smeared picture.These are all advantages that Zephyr has over a satellite, including the fact that asatellite can only observe the ground when it falls under its orbit whereas Zephyr canobserve any area for any length of time and track any moving object.3.4.1 GPS SignalThe other main function of the software is that if the camera has a GPS signal thesoftware can use this to plot the camera’s position on a map. This means that whilethe camera streams live video of the earth below, the map shows the place names ofwhat the camera is seeing. In order for this to be tested to see how accurately itworked, a GPS antenna had to be connected to the camera.In order for a GPS antenna to work properly, it should be grounded/earthed. GPStravels at a speed of 1GHz, so by using the speed of light it is possible to calculate thewavelength:43

where λ is the wavelength, c is the speed of light and f is the frequency.Substituting the values gives:λ = 29.98cmIn order for the GPS to work correctly, the maximum distance between the groundingplate (or the earth) and the antenna needs to be less than a tenth of the wavelength:Normally the antenna is touching the ground but in case it is not and it is flying on aplane / Zephyr, the distance between the antenna and the grounding plate must be lessthan 3cm.If the grounding plate is not a solid piece of material but has gaps in it, for examples amesh, the distance between the gaps must also not be more than a tenth of awavelength, thus the gaps cannot be bigger than 3cm.Finally the antenna should be situated in the centre of the grounding plate, and thelength of the grounding plate should be of a similar length to the wavelength, i.e.30cm (see figure 9).44

GPS antennaGroundingPlateλ/10 (3cm)λ (30cm)Figure 9. Diagram to show the setup of a GPS antenna in order for it to functionproperlyWith this in mind I found that the best place to put my antenna in order for the camerato have a GPS signal was hanging out of the window (I was on the second floor). Thewindow had a metal mesh that could act as a grounding plate.45

3.5 Angle of Attack SensorThe angle of attack (AoA) is the angle between the relative wind and the longitudinalaxis of the plane, see figure 10.LongitudinalaxisWinddirectionAngle of AttackFigure 10. Diagram to show the angle of attack.The angle of attack is important as it helps to generate the lift. When the wind hits thefront of the plane some of the air flows under the plane while some flows over theplane. As the wind flows over the top of the plane its velocity increases and thus thepressure decreases. This creates an uneven pressure distribution around the plane andforces the aircraft upwards towards the low pressure. The greater the angle the largerthe lift coefficient and the more the aircraft rises.At the critical angle of attack the air no longer flows smoothly over the wing and thiscauses a dramatic drop in lift. The air becomes turbulent meaning that it no longerlifts the aircraft. This causes the aircraft to rapidly accelerate downwards entering aspin which can cause it to break. This is not desirable, so the angle of attack has to becarefully monitored during flight to ensure the aircraft stays within the limits.46

There is a sensor (see Appendix 3) on Zephyr that measures the AoA, however ittends to have some error in its readings. The most likely reason for this is the lowtemperature causing the mechanism to fail as it is only rated by the manufacturer to -40°C. The sensor functions through a potentiometer which consists of a wind vaneconnected to a metal brush that wipes along a circular resistive track, outputting avoltage [10]. It is likely that in the cold temperatures (down to -80ºC), either thelubricant that allows the brush to move freezes, or the lubricant on the potentiometerfreezes thus breaking the contact between the track and the brush.As knowing the angle of attack is so important, new sensors had to be investigated inthe hope that they would be more accurate and reliable. These included the originalsensor but with a modified brush and lubricant which would allow the sensor to workat colder temperatures, and a non-contact sensor (in the way of magnetic or optical)which would remove the issue of parts needing to be in constant contact.After considerable research I found that the optical sensor (figure 11), whichconsisted of a glass disc that allowed light to shine through, was too delicate forZephyr’s environment.Figure 11. Diagram toshow the optical encodersetup [11]47

The magnetic sensor seemed much more feasible as it did not have any delicate parts(figure 12). The magnetic sensor uses the Hall Effect by having a vane attached to onemagnet and as it is moved by the wind it causes the magnet to move and thus the HallVoltage to change. Although this had clear advantages as it did not have any criticalparts that had to be in contact (unlike the potentiometer), it was not compatible withthe electronics onboard Zephyr. It was decided that although a magnetic sensor was agood idea in the long run, it was not feasible to change the electronics on the aircraftat the present time.Figure 12. Diagram to show the magnetic sensor. The wind vane would be attached tothe brown magnet which would be placed inside the silver pod [12]Other magnetic sensors were investigated, but they were all too big and too heavy forZephyr. A sensor has to be small so it can be placed on the aircraft without affectingthe aerodynamics, and light not to add significant weight onto Zephyr.There was another possible solution which involved using the original sensor butheating it in order to keep it within its certified temperatures. It was decided however48

that due to the limited power available onboard Zephyr from the solar panels, thesensor should be modified and not heated.I finally decided that it was best to use the existing sensor but to modify it in order toprevent it failing at cold temperatures. After many discussions with themanufacturers, SpaceAge Control Inc, it was decided to remove the lubricant betweenthe track and the brush, and change the lubricant on the bearings within the brush tolow temperature lubricant. This would prevent the vane from freezing and not movingand it was hoped that this would solve the problem. Once the new improved vane wasdelivered it had to be tested in a thermal chamber to ensure that it would work at thecold temperatures.49

Chapter 4Angle of Attack Experiment4.1 BackgroundThe existing angle of attack sensor used on Zephyr was found to give erroneousreadings during flight. The sensor was then tested in a thermal chamber in order to seeif the problems would appear and once again the sensor was found to give incorrectreadings.At cold temperatures ‘spiking’ was evident when there was briefly no contact betweenthe resistive track and the scraper, thus giving a voltage spike of 0V. This was alsoseen clearly on an oscilloscope. Although the exact cause of this was not known, itwas predicted that the lubricant on the potentiometer element was beginning to freezeand this was causing the break in electrical contact.Two vanes were tested in the chamber in order to show that it wasn’t just a fault dueto manufacturing error that was causing the problem and that it was due to thetemperatures. Both of these sensors failed at varying temperatures – the first onespiked at -83C and the second at -74C and -70C. This showed that there is somemanufacturing variation and therefore testing the new sensor to -75C would not beadequate enough and a much larger range was needed to ensure the new sensorwouldn’t fail at the colder temperatures.In order to try and overcome the spiking, SpaceAge Control Inc modified their AoAsensor by removing the lubricant on the potentiometer element and replacing the50

lubricant in the bearings with low-temperature lubricant. It was claimed that theimproved vane would be able to withstand temperatures down to -80C and pressuresdown to 50 millibar. The harshest environment that the sensor will experience duringflight is 50 mbar and around -75C. This means that the new sensor should functionproperly at all times.4.2 ExperimentI decided to split the test in two to confirm that the modified sensor operated correctly andmore accurately at colder temperatures than the original. The first test was identical to theoriginal performed on the old sensor and this went through five cycles between 40C to -75C. By keeping these two tests identical, it meant that it was easy to directly compare theperformance of the modified sensor to the old sensor without introducing and other variables.The second test was performed to temperatures down to -100C in order to give confidencethat if the sensor did work it was due to the modifications made and thus the sensor will workcorrectly during a flight.The sensor was placed into the thermal vacuum chamber with thermocouples attachedto it. This was in order to monitor the actual temperature of the sensor and not thetemperature of the whole chamber. The pressure inside the chamber was set to50mbar meaning that there was not a lot of air for convection to happen quickly. Thismeant that when the temperature was changed it took a long time for the sensor toreach the set temperature and hence the need for the thermocouples to monitor thesensor’s temperature.51

The sensor was powered using an external power supply and was controlled by aservo actuator. The servo was connected to the vane and could be controlledexternally to make the vane move and replicate the movements it would face onboardZephyr. The movement of the vane caused the brush to move along the resistive trackwithin the potentiometer casing thus changing the voltage output which could bemonitored outside the chamber. An oscilloscope was also connected to the sensor toenable and voltage to be seen on screen as well as on the voltmeter. This allowed anyvoltage spikes to be seen easily rather than having to be read from the voltmeter(Appendix 4).4.3 ResultsBoth tests were completed without any problems. The voltages recorded were asexpected and there was no spiking evident in either test at any temperatures. Theprevious sensor lost contact at -83C, -74C and 70C but the modified sensor keptcontact at all times.Although the testing was supposed to be conducted at 50 mbar, there was a problemwith the chamber meaning that it could not get to a low temperature at such a lowpressure. It was decided that the temperature was the most important variable and thatincreasing the pressure by 50 mbar to 100 mbar would not affect the results greatly.The first test which was conducted in order to compare results with the previoussensor was mostly completed at 50 mbar (see Appendix 5).52

After the tests were completed successfully, I decided to test the sensor further byleaving it in the chamber overnight at -100C with the servo constantly moving thevane. This would enable us to find if the sensor would be able to endure constantmovement at such a cold temperature. The sensor would never have conditions thisharsh on the aircraft so if it managed to survive the overnight test it would giveconfidence that it would definitely work onboard the aircraft.After 15 hours at -100°C, the vane was still working and still outputting the correctvoltages. It was then switched off to see if the potentiometer would freeze when not inuse (the original sensor lost contact due to the freezing of the lubricant) and thenswitched on again to see if it would lose contact, but again it performed perfectly.Once all the tests had been completed and the sensor being tested was removed fromthe chamber, the sensor was examined to see if it had suffered any wear. It was notedthat there was no damage to any part of the equipment, and even though there was nolubricant between the brush and the track, there was no damage to the resistive track.4.4 ConclusionAfter testing the modified angle of attack sensor, it was concluded that it performedbetter than the original sensor and does not lose contact (which would have caused thevoltage to spike to 0V). With that in mind, it was recommended that the modifiedsensor should be used instead of the original angle of attack sensor onboard Zephyr.53

It has to be noted however, that only one modified sensor was tested and this is notenough to definitely say that it performs better at colder temperatures compared to theoriginal - it might just be luck that this sensor performed correctly. It is possible to saythough, that we are very confident that the modified sensor does not lose contactdown to -100C. I recommended that further modified sensors should be tested tocheck for any manufacturing variations that affect low temperature performance.Both sensors are of a contact design however, and although the modified one doesperform well at cold temperatures it is possible it could lose contact for reasons otherthan icing, for example the brush might deform in the cold temperatures. Theexpected lifetime of the modified sensor is 50 million cycles minimum which is muchmore than it will ever have to endure on Zephyr, but ultimately it is recommended thata non contact sensor should be used, for example a magnetic sensor.54

Chapter 5Commercial Aspects5.1 Uses of UAVsUAV’s can be used for a number of purposes. Zephyr is being used for its cameracapabilities but many UAVs are created that do not have onboard cameras and have atotally different use.5.1.1 TransportUAVs can be used to transport goods from one place to another. This is mostcommonly done during warfare (for example transporting medical supplies oremergency equipment). Most payloads are stored within the airframe in an internalbay in order to protect what it is carrying, but for many unmanned helicopters externalpayloads can be fastened to the bottom. For a UAV which has fixed wings thepayload can be attached to the airframe, but the aerodynamics and weight imbalanceneed to be accounted for. In these situations it is normally easier to have anaerodynamic pod pre-attached to the UAV in which the payload can be stored.5.1.2 Scientific ResearchUAVs are capable of flying to areas that are too dangerous or out of reach forhumans. Many UAVs have been used for scientific research, for example studyinghow organisms such as bacteria survive at 60,000ft. It would not be possible for ahuman to fly to these altitudes and ‘open the window’ to allow the bacteria to55

experience the atmospheric conditions, but onboard a UAV this would be possible.A further, very important use of UAVs for scientific research is in monitoringhurricanes, in which it would be too dangerous to send a human. The NationalOceanic and Atmospheric Administration (NOAA) use UAVs to monitor and gatherdata about a hurricane [13]. They use a UAV created by Aerosonde Pty Ltd, whichcan fly into a hurricane and send near real time data to the National Hurricane Centerin Florida. It can measure atmospheric pressure and data like any other ‘hurricanehunter’ is able to, but it provides measurements much closer to the sea water thanpreviously captured.5.1.3 WarfareA UAV can be used in warfare to carry and fire missiles at a target, with a greatdegree of autonomy. It would be too dangerous for soldiers to fly on planes and shoottargets in case they got shot down and killed/captured. UAVs used for this reason areknown as unmanned combat air vehicles (UCAV), and are currently being developedto operate autonomously without human control.The Predator is a UCAV that is currently used by the United States Air Force,amongst other nations, to help in combat. It is armed with two missiles that have arange of 8km and weigh around 50kg [14]. Armed Predators were first used in late2001 for targeted assassinations in Afghanistan, from bases in Pakistan andUzbekistan. Since then Predators have been used widely in warfare, including attacksin Pakistan from Predators based in Afghanistan [15]. On November 3 rd 2002, a56

Predator was used to kill suspected al-Qaida terrorists in Yemen [16]. This was thefirst time a Predator attack was conducted outside the country where a war is takingplace.5.1.4 ReconnaissanceUAVs can also be used by the military for reconnaissance, which is the exploration ofthe battlefield to gain information. It would be too dangerous to send a plane withpeople over a war zone in case the plane gets shot down and the passengers get killedor become hostages. It is much safer to send a UAV over the battlefield to take realtimeimagery or video footage of the land in order for the army to gain intelligenceabout where the other army is stationed and what they are planning. UAVs are alsomuch quieter and fly at higher altitudes than planes so they may not get spotted by theopposing army.It is also possible to use a UAV as a target and decoy. The aim is that the opposingarmy see the UAV in the sky and think that it is a plane carrying personnel from theother army. They then try and attack the UAV in order to bring it down and while theground and aerial forces are concentrating on the UAV, the other army can attackwhen least expected.5.1.5 CommunicationMostly commonly satellites are used for communication, for example the SkyNetgeostationary satellite used by the British Military [17]. Not only are satellites57

5.2.1 Aerial PhotographyThe most common type of remote sensing is aerial photography; taking photographsof the earth. UAVs have an advantage over satellites as they can be sent as often as isneeded to any area, compared to a satellite that can only take imagery when it isorbiting above that particular spot. Zephyr will be mostly used for aerial photographyas this has so many practical applications.Aerial photography can be used to create a map of land, for example OrdinanceSurvey who display the contours of the land as well as roads on their maps. GoogleEarth also display aerial photography, but some of its data is out of date as some ofthe images were taken a few years ago. By using a UAV, recent images could beproduced for a fraction of the cost and in a much quicker time and more accuratelythan the satellites they currently use. Aerial photography can also be used to takedetailed pictures of cities in order to decide where it is possible to build new houses orto monitor their expansion.By using UAVs and aerial photography it is possible to monitor environmentalproblems. It would be possible to take regular pictures of the ice caps to see the rate atwhich they are melting, as well as taking images of the rainforest to monitor itsdepletion. On a national scale it would be possible to take detailed images of thecoastline to see the erosion of land and what towns are at risk.On top of the physical and human applications, aerial photography can be used duringnatural disasters. They can take imagery of areas that have been hit by an earthquake59

or hurricane for example, to see the damage and find an accessible route to deliverfood and aid. During forest fires UAVs could be used to see how the fires arespreading or if they are dying down.5.2.2 Aerial VideoingAerial videoing is just like aerial imaging but instead of still images it produces eitherlive or recorded videos. The benefit of video compared to images is that you can seeexactly what is happening in real-time. This is especially useful for security, forexample monitoring an event such as the Olympics to ensure there are no terroristattacks. This is a lot like CCTV but as it is from the sky a large area can be monitoredat any given time.Live video can also be used to track an object from the sky. In a police chase forexample, currently helicopters are used to track a vehicle while ground police chasethe car along the road. If a UAV had a camera capable of tracking this could be usedinstead as it is much cheaper, doesn’t need to be refuelled, and doesn’t waste policetime.5.2.3 InfraredAnother type of camera that can be used on a UAV is a forward looking infraredcamera. Unlike normal pictures that are taken using visible light, infrared camerastake images using infrared radiation which has a wavelength between 700nm and100nm [23]. These images show the heat radiating from the object at a given time,60

meaning that these cameras can be used at night (unlike a visible light camera). Notonly can they be used during the night, but they can help with weather forecasting.Using an infrared camera it is possible to look at the temperature of the air and theocean and thus monitor the shape, size, strength and direction of hurricanes. Thisenables forward planning of emergency services and of evacuations which take a longtime to organise.As well as tracking hurricanes, using infrared cameras to look at sea temperatures canhelp monitor and track ocean currents especially El Nino and La Nina. It can also helpto predict long term weather forecasts and even give a 3D map of the climate.5.2.4 RadarRadar imaging has many uses. It can create maps showing the rainfall, monitorvolcanic activity (see figure 14) and it can measure how tall buildings are. Radar canalso be used to find mineral deposits within the ground and to monitor water usageespecially where there are shortages, like in south USA.Figure 14. A radar image of the Miyakejima volcano in Japan [18]61

A beam of electromagnetic waves are sent, normally in pulses, from the UAV towardsthe ground which then get reflected off the earth back to the UAV. By knowing thewavelength of the beam and the time between sending and receiving the beam, thedistance can be calculated.The Sahara desert lies on top of a network of ancient lakes and rivers, some of whichstill contain water. Radar can be used to find the existing water as the waves gothrough the sand but reflect off the water. Figure 15 shows a radar image of theEastern Sahara desert and it is clear to see the many rivers below the surface. Oncethe water is found it is possible to build wells and provide water to the people in andaround the desert. This would then help to prevent famine as it would not matter ifthere was a drought if there was a large supply of water available. In Libya this hasalready happened. A man made river has been created carrying water fromunderground aquifers that can be used for drinking and irrigation [19].Figure 15. Radar image of the Sahara Desert showing the underground rivers [18]62

Infoterra have a project called FarmStar that use normal, infrared and radar imaging tohelp farmers. They look at the height, biomass, nitrogen and chlorophyll levels in thesoil that the farmer uses. Using this information they determine both how much andwhat type of fertiliser the farmer should use to have the biggest yield. If the farmeruses too much fertiliser or the wrong type then the crops can die, which not only isvery costly but in poorer countries it might be vital to their survival. On the otherhand not enough fertiliser could mean the crops don’t grow. Using FarmStar not onlyreduces the amount of fertiliser the farmer will use to the optimal level and thus savehim money, but it will also help him have a larger output of crop and thus make moremoney. In one example, FarmStar helped increase output by ½ a tonne per hectare ona farm in France [20].63

5.3 Zephyr CompetitorsThe two nearest competitors to the Zephyr UAV are the Global Hawk (manufacturedby Northrop Grumman) [21] and the Global Observer (designed by AeroVironment)[22]. The Global Hawk is already being used by the US Air Force for surveillance inboth Afghanistan and Iraq, and was also used to monitor the Californian wildfires inOctober 2007. The Global Observer is currently being developed and should be inproduction by 2011. Table 3 shows the specifications of the Global Hawk, GlobalObserver and Zephyr.Global Hawk Global Observer ZephyrMaximum Altitude 65,000 ft 65,000 ft 65,000 ftMaximum Payload 2000 lbs 400lbs 5 lbsPower SourceTurbofan engineLiquidhydrogenSolar-electric(jet engine)internal combustionEndurance 35 hours 7 days 3 monthsTable 3. Table showing the main specifications of Zephyr and its competitorsThe Global Hawk which has a huge capability for a payload can only fly for 35 hoursbefore having to refuel. For some applications this may not be a hindrance, but it rules64

it out for long endurance missions. Furthermore, at a cost of $35 million USD they arevery expensive as they are still very new. During the Iraq War seven Global Hawkswere used with two lost due to technical failures, although the manufacturers claimedthat this was due to the lack of trained staff and spare parts. It does however have afailure rate per hour flown of over 100 times that of the F-16 fighters also used in thewar.As it is on the market first it automatically has an advantage as it is a market leader inunmanned surveillance and reconnaissance missions and it can carry such a largepayload. On the other hand, not only is the price of one Hawk so large, but with therising cost of fuel and the publicity about global warming, the public may not behappy with their government spending taxes on these fairly unreliable UAVs.The Global Observer is still in its development stage like Zephyr and also uses anenvironmentally friendly method of flight. It is being designed as a communicationrelay and remote sensing UAV, however with its short endurance it would need to berefuelled weekly if it was to be used as a longer term telecommunications platform.Although it has a much larger payload capacity than Zephyr, its endurance isconsiderably smaller and thus is a limiting factor. It is much easier to replace thepayload with a much lighter one than it is to lengthen the time a hydrogen poweredUAV can stay up in the air for.Zephyr has the advantage against its nearest competitors as it can fly for a few monthswithout needing to be refuelled. It is cheap to buy (in comparison to its competitors)and cheap to run, so that even if one did was involved in an accident the cost of65

eplacement would be minimal in comparison to the Global Hawk. It plans to do thesame job as the other two UAVs – reconnaissance, surveillance, remote sensing andcommunication relay, but is able to do the job for longer and for less money. The factthat it is so small also means it is very difficult to spot on radar so if it was requiredfor reconnaissance missions it could do the job undetected. See figures 16 to 18 forimages of the three UAVs in flight.Figure 16. The Northrop Grumman Global HawkFigure 17. The AeroVironment Global Observer66

Figure 18. QinetiQ’s Zephyr5.4 ConclusionHALE UAVs powered using sustainable sources are a thing of the future. The currentHALE UAVs require refuelling every few days and are very expensive to buy and torun. The nearest competitor to Zephyr is the Global Observer, however while it canonly fly for a week before needing to be replenished with liquid hydrogen, Zephyr canfly for months on end. It is still in its research and development stage with prototypesbeing built and tested, but it has however broken current records and will only exceedthem in the future.In order for it to be able to fly for as long as possible, it needs to be lightweight so lesspower is used to keep it in the sky. The aircraft is made of carbon fibre and all thecomponents are made and altered to keep the mass down.All the components on the aircraft are important to keep it in flight, however it is thebatteries and the solar panels that are the most critical. They use state of the artscience and are still being developed for maximum power, which can only mean astime goes on and more tests are being carried out they will get better.The only part of the aircraft that isn’t critical is the payload, but this is needed in orderto give the platform a use. There is no point having an aircraft that exceeds all othersin its category but cannot actually be used for anything. Whether the use iscommunications or imaging, in a few years time once Zephyr is completed, othercompetitors products will be look like they are from the dark ages.67

5.5 FutureZephyr is currently way ahead of anyone else in building a sustainable long enduranceunmanned air vehicle. Eventually however, competitors will catch up and Zephyr willhave to improve as technology advances in order to be better than the rest. In order todo this, the Zephyr team should try and find a way of increasing the payload limitabove 2kg. At the moment it is hard to develop a camera that is powerful enough toview the earth closely and weigh less than 2kg. A larger payload would allow othermanufacturers to place better and heavier cameras on board, or even allow for morethan one, for example a visible and infrared camera.A larger payload would also allow Zephyr to be used for more than just imaging, forexample it could have more instruments to measure atmosphere data or even to flythrough hurricanes.Zephyr can currently be controlled from anywhere in the world, but it currently takesat least five people and a large area with multiple computer screens to do so. Forfuture development it would be better if the aircraft could be controlled fromsomething much smaller, for example a laptop, and only require one person. Thiswould mean that a lot of the current functions that people perform would have tobecome automated. This would be a benefit as if Zephyr was used by the police forceto track a vehicle for example, it would be possible for a single operator in a policecar to monitor and control Zephyr following the tracked vehicle at the same time. Thiswould save time and money as the information from Zephyr could go directly to68

where it is needed rather than having to be relayed via third person or a control centre.Appendix 1The new All Sensors airspeed sensor.69

Appendix 2Information about the CloudCap camera.70

Appendix 3Schematic diagram of the current angle of attack sensor manufactured by SpaceAgeControl Inc71

Appendix 4Diagram of the experimental setup of the angle of attack experiment:ServoAngle of attack vaneThermocouplesThe experimental procedure:1. Place the sensor in the vacuum chamber, check that it works, seal the chamber2. Set the chamber to the desired temperature and the desired pressure3. Once both variables have been reached, allow the chamber to stabilise forthirty minutes4. Use the servo motor to move the vane. Starting at 150 counts move the vaneclockwise to 110 counts and counter clockwise to 190 counts5. Record the voltage, actual temperature and pressure at the three vane positions6. Return the vane to its starting position of 150 counts and set the chamber tothe new temperature7. Repeat steps three to six for all given temperatures in test 1.8. Once the test has been completed, test the sensor again but at the coldertemperatures of test 2.72

Appendix 5The results table for the first angle of attack experiment, comparing the new sensor tothe old:Desired SensorTemperature (C)Actual SensorTemperature (C)Voltage at110 countsVoltage at150 countsVoltage at190 counts0 -2.6 0.713 1.344 2.493 45.3-40 -40.2 0.721 1.333 2.486 44.3-75 -74.9 0.716 1.330 2.470 45.2-40 -39.4 0.725 1.411 2.481 44.80 0.7 0.718 1.463 2.495 45.1+40 40.1 0.710 1.404 2.489 44.40 -0.5 0.716 1.403 2.500 44.9-40 -39.3 0.720 1.407 2.494 45.3-75 -74.3 0.724 1.356 2.487 45.0-40 -40 0.717 1.401 2.487 102.40 0.9 0.720 1.412 2.471 200+40 40.3 0.711 1.402 2.476 199.40 0.0 0.0 1.323 2.427 45.2-40 -40.3 0.736 1.429 2.460 107-75 -75.1 0.731 1.419 2.456 105.3-40 -39.2 0.723 1.419 2.441 69.90 -0.1 0.727 1.406 2.460 62.9+40 40.0 0.719 1.408 2.469 67.90 0.2 0.715 1.399 2.484 64.4-40 -40.0 0.727 1.416 2.484 70.4-75 -75.0 0.725 1.417 2.471 44.5-40 -42.1 0.734 1.427 2.503 45.70 0.3 0.724 1.412 2.499 43.8+40 39.4 0.714 1.397 2.484 44.7ChamberPressure (mbar)73

The table of results for the second angle of attack experiment at much coldertemperatures:Desired SensorTemperature (C)Actual SensorTemperature (C)Voltage at110 countsVoltage at150 countsVoltage at190 countsChamberPressure (mbar)-40 -42.0 0.722 1.419 2.499 45.0-50 -51.1 0.735 1.421 2.499 45.1-60 -60.7 0.736 1.424 2.470 46.0-70 -70.1 0.735 1.422 2.472 46.3-80 -80.1 0.735 1.425 2.468 51.8-90 -89.8 0.735 1.422 2.473 56.7-100 -99.2 0.737 1.422 2.482 70.5-90 -90.1 0.737 1.421 2.439 90.1-80 -80.2 0.736 1.421 2.473 78.9-70 -70.3 0.733 1.417 2.446 80.7-60 -60.1 0.734 1.416 2.447 79.1-50 -49.8 0.736 1.418 2.446 80.2-40 -39.8 0.736 1.411 2.438 78.9-50 -50.6 0.732 1.414 2.459 79.9-60 -59.8 0.733 1.409 2.452 79.7-70 -70.2 0.729 1.409 2.435 79.8-80 -79.3 0.737 1.409 2.449 85.1-90 -89.7 0.727 1.409 2.442 46.3-100 -99.5 0.728 1.408 2.440 52.6-90 -89.9 0.734 1.409 2.448 89.1-80 -80.1 0.732 1.412 2.449 80.4-70 -70.2 0.729 1.418 2.447 80.4-60 -60.1 0.739 1.419 2.462 79.9-50 -49.6 0.738 1.417 2.460 79.7-40 -39.9 0.736 1.415 2.452 80.074

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