FINAL ENGINEERING DEFINITION REPORT Group: 2R Supervisor: Andres Marcos

Students: Eliott Wertheimer Robert Dibble Noe Bhandari Louis Ireland

Simon Wilkinson James Gilchrist Hugo Hunt Matthias Wüstenhagen

Contents 1. SUMMARY ............................................................................................................................................. 4

2. INTRODUCTION ..................................................................................................................................... 4

3. CONCEPT SELECTION AND JUSTIFICATION ............................................................................................ 5

3.1. Concept .............................................................................................................................................. 5

3.2. Underslung vs Integrated payload ..................................................................................................... 6

3.3. Rotors ................................................................................................................................................. 6

3.4. Rotor Hub ........................................................................................................................................... 6

3.5. Tail Rotor ............................................................................................................................................ 7

3.6. Propulsion ........................................................................................................................................... 7

3.7. Back Up Power Supply ........................................................................................................................ 8

3.8. Undercarriage Selection ..................................................................................................................... 8

4. MISSIONS ............................................................................................................................................... 8

4.1. Cargo .................................................................................................................................................. 8

4.2. Firefighting ......................................................................................................................................... 9

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4.3. Surveillance ...................................................................................................................................... 10

5. FINAL DESIGN ...................................................................................................................................... 11

5.1. Rotor Blades ..................................................................................................................................... 11

5.2. Rotor Hub ......................................................................................................................................... 11

5.3. Material Selection ............................................................................................................................ 12

5.4. Landing Gear..................................................................................................................................... 12

5.5. Engines and Gearbox ........................................................................................................................ 13

5.6. Electric Tail ....................................................................................................................................... 13

5.7. Weights and stability ........................................................................................................................ 14

5.8. Avionics & Electrical Systems ........................................................................................................... 14

6. TECHNOLOGY LEVELS & RISK ANALYSIS .............................................................................................. 14

6.1. Electric Tail Rotor ............................................................................................................................. 14

6.2. Low Altitude Hover ........................................................................................................................... 15

6.3. Fire Fighting Water Tank .................................................................................................................. 15

6.4. Lidar System ..................................................................................................................................... 15

6.5. Sense and Avoid System ................................................................................................................... 15

6.6. Use of COTS Equipment ................................................................................................................... 15

6.7. Fuel Leakage ..................................................................................................................................... 16

6.8. Composite Tail Boom ....................................................................................................................... 16

6.9. Rotor Hub Flexbeam ......................................................................................................................... 16

7. ECONOMIC ANALYSIS & COMPETITIVE ANALYSIS ............................................................................... 17

7.1. Mistral unit cost................................................................................................................................ 17

7.2. Fuel Costs ......................................................................................................................................... 18

7.3. Maintenance Costs ........................................................................................................................... 18

7.4. Competitive analysis......................................................................................................................... 18

8. CRITICAL ANALYSIS OF DESIGN ............................................................................................................ 19

9. CRITICAL ANALYSIS OF WAY OF WORKING ......................................................................................... 21

10. CONCLUSION ................................................................................................................................... 22

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ACRONYMS

ADS-B Automatic Dependent Surveillance Broadcast CG Centre of Gravity COTS Commercial Off the Shelf EO Electro Optical FLIR Forward Looking Infrared GPS Global Positioning System LIDAR Light Detection And Ranging MAUM Maximum All Up Mass MTOW Maximum Take-off Weight NDT Non Destructive Testing NOTAR No Tail Rotor NPAS National Police Air Service OEI One Engine Inoperable PA Personal Address PEEK Polyether Ether Ketone SFC Specific fuel Consumption SSD Solid State Drive TBO Time Between Overhaul TCAS Traffic Collision and Avoidance TRL Technology Readiness Level UAS Unmanned Aerial System UAV Unmanned Aerial Vehicle

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1. SUMMARY This report contains the final design of the Mistral, a rotary UAS for civil aerospace applications. The Mistral is designed as a multipurpose helicopter to fulfil three types of missions: cargo transportation, firefighting and surveillance. To perform cargo transportation, the UAV is capable of carrying a 500kg payload in the form of an LD2 container over 460km in under 120 minutes whilst flying at an altitude of 1000ft. For the firefighting mission, the UAV can autonomously approach a suitable water source, pump 500L of water in 13.2 seconds into a semi rigid water tank integrated into the payload bay and deliver it onto a fire repeatedly for a total of 90 minutes. Finally, the UAV is capable of performing surveillance missions conducting 9 periods of loiter and hover for 10 minutes each at 500ft, totalling 3 hours on station. The Mistral stands out in its capacity to switch from any mission to surveillance at any given time as the basic surveillance equipment is fully integrated. This also offers the asset of providing assistance during landing and doubling up as a fire monitoring device during firefighting missions. Furthermore, the extra available space in the cargo bay can house additional fuel tanks to increase the duration of the mission. The avionics are tailored to fulfil the surveillance mission whilst providing assistance with the transportation and firefighting missions and to conform to the CAP 722 UAV guidance. A full sense and avoid system is defined along with a system for close range obstacle avoidance. The Mistral which designed to be fully autonomous is also capable of being remotely piloted at the ground control station. The Mistral which designed to be fully autonomous is also capable of being remotely piloted at the ground control station. Two identical Rolls-Royce M250-C20B engines are used to power the main rotor along with two generators for the ancillaries and tail rotor. This will provide redundancy in case of engine failure allowing the UAV to meet the requirement specification of being able to sustain straight and level flight covering a minimum distance of 3km. The aircraft uses an electric tail engine drawing power from the two generators hence emphasizing the need for a redundant system. This removes the need for a complex mechanical drive system and tail rotor gearbox, often prone to failure, as well as removing the need for substantial tail structure to support it. To provide the tail boom’s required stiffness and fatigue strength against inherent vibrations, carbon fibre with thermoplastic PEEK resin was chosen. The thermoplastic PEEK facilitates the manufacturing process by heat bonding the components together hence reducing the parts count saving costs and time.

2. INTRODUCTION A UAV offers several advantages over a manned aircraft; primarily it is their ability to operate in dangerous environments but they are also ideally suited to repetitive missions due to their amenability to autonomy. A demand for a multi-role UAS specifically designed for three dull, dirty and dangerous missions is thus anticipated and this provides the motivation for this design. Currently UAVs exist predominantly in a military capacity, or as personal hobby aircrafts. Whilst their applicability and potential benefit to the civil sector has been identified there has been limited progress due to strict regulations which are difficult for a UAS to meet and test. This report shall detail the high level group design decisions and provide a summary of the overall design. This report is divided into sections discussing the high level design decisions, final design, missions, risks, economics followed by a critical analysis of the product and the process which led to it.

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3. CONCEPT SELECTION AND JUSTIFICATION

3.1. Concept In this section the main configuration for Mistral is described. Table 1 gives an overview of the main configurations considered, which are detailed next.

Table 1: Qualitative comparison of potential configurations

Configuration Advantages Disadvantages

Penny Farthing -Established technology -Cheaper maintenance

-Tail rotor power consumption and failures

Coaxial -No power wasted on tail rotor -Reduced noise -Symmetrical lift

-Hub size and complexity -Increased weight, drag and maintenance costs

Tandem -No power wasted on tail rotor -CG variation tolerance

-Heavy and complex due to power system redundancy

Syncopter -No tail rotor -No lift asymmetry

-Hub and transmission complexity -Reduced forward speed performance

Quadcopter -Commonly used in small UAVs -Power system complexity

Compound Lift -Improved performance in Forward flight

-Reduced hover performance

Based on the qualitative characteristics presented in Table 1 a trade-off study was conducted taking into account the Mistral’s requirements and mission definitions leading to the down selection of the Coaxial, Tandem and Penny farthing configurations. Consequently using actuator disk theory, assuming the lower rotor in the vena contracta of the upper one for the coaxial configuration and no overlap for the Tandem, power estimates were produced for a range of forward speeds. Figure 1 shows that within our range of forward speeds, the penny farthing would consume significantly less power. Hence, by also considering its maturity and lower drag, the penny farthing configuration was eventually selected.

Figure 1: Quantitative comparison of configurations

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3.2. Underslung vs Integrated payload Another preliminary design decision was whether to have an underslung or integrated payload carrying configuration, be it cargo or water. The configurations were compared by looking at their relative performance, safety, public acceptability, cost, upgradability and related legislation. Ultimately the decision for an integrated payload bay was based on CAP 462 that prohibits flying over built-up areas with an underslung load. It could be argued that there are many ground based alternatives to fighting fires and delivering cargo in built up areas and that therefore a UAV would only be used for firefighting and cargo delivery in remote non-urban locations. This would make choosing an underslung configuration a viable solution. However the specification explicitly states “This UAV must operate in both urban and rural locations” so this requirement would have had to been relaxed for this configuration to be selected. Considering the advantages and disadvantages of the underslung and integrated configurations, aside from the ability to operate over urban environments, did not give an obvious result. Therefore this restriction to our operational capacity would not be compensated for by an advantage in some other area. As most other firefighting helicopters use underslung configurations the integrated payload and the associated ability to operate over urban environments provides the Mistral with a unique selling point.

3.3. Rotors The solidity of the rotors was minimised in order to minimise profile drag whilst ensuring that the blades had an aspect ratio that led to manufacturable blades. The number of main rotor blades was selected to be as high as possible to minimise the vibrations of the main rotor but it was not possible to have a 5 bladed rotor with appropriate aspect ratio blades, therefore a 4 bladed configuration was chosen.

3.4. Rotor Hub The main concept selection required for the rotor hub was the hub type, this was a decision between articulated, hingeless and bearingless hubs as depicted in Figure 2 and compared in Table 2. In consideration of the requirements relevant to the rotor hub, low weight and high maintainability were determined to be critical design drivers; a bearingless design was therefore chosen.

Figure 2: Comparison of different types of hub configuration

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Table 2: Quantitative comparison of different hub types

Advantages Disadavantages

Articulated Simpler analysis Large number of moving parts - high maintenance and weight

Hingeless No hinges - fewer parts - less weight and maintenance

Pitch bearing - difficult to maintain adds weight

Bearingless No moving parts - further weight and maintenance savings

Complex analysis required- adds risk

3.5. Tail Rotor Conventional tail rotors are the most common due to their aerodynamic efficiency and simplicity. Ducted (Fenestron) tail rotors install the rotor in the tail fin giving the benefit of reduced tip losses; no fin blockage and reduced noise but at a reduced efficiency. As the rotor is embedded, the tips are shielded which reduces the chance of being damaged from hard landings or bird strikes and also reduces the risk of injuring personnel on the ground. NOTAR uses the Coanda effect and a pilot controlled nozzle at the tip along with pressurised air to create counter-torque. This configuration has no exposed rotating parts making it even safer from damage than a ducted rotor; safer for personnel on ground; and quieter. Whilst Ducted and NOTAR configurations have their benefits, with no stringent noise requirement and minimal exposure to untrained personnel on the ground these benefits are not worth the inefficiencies they incur. Therefore a conventional exposed tail rotor configuration was chosen for the Mistral. If required, the centre of gravity envelope can be enlarged using a canted tail rotor at the cost of increasing the complexity of the flight control systems to overcome the coupling of the yaw and pitch motions.

3.6. Propulsion The various concepts and technologies considered as the primary source of propulsion for the Mistral in our preliminary research include gas turbines, reciprocating, rotary, diesel and Hybrid electric engines as well as tip jets, Fuel Cells and Nuclear Fission. The last three were discarded early in the design process due to their high profile drag and limited technology readiness levels. The chosen option is the gas turbine, the justification for which, and the process by which it was chosen, is outlined below. The first option considered is gas turbines, in a turboshaft format. These are very popular in small rotorcraft due to their high power to weight ratio, reliability, maturity, and efficiency. Moreover, there is relatively little maintenance required per flying hour, and they have a small frontal area thus minimising drag. Drawbacks include the emissions and the need for a large transmission due to the high shaft output speed. Reciprocating and rotary internal combustion engines were considered, due to their simplicity and low cost, however their reliability, efficiency, and power to weight ratio were not sufficient for it to be a viable option. Similarly, diesel internal combustion engines are too heavy for this application, despite the advantage of reduced emissions compared with reciprocating and rotary engines, and excellent SFC. Electric Propulsion seemed a viable candidate for the design, which shows much promise for the future, in conjunction with a diesel or gas turbine generator. The implementation of a hybrid system offers even

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greater potential, since the powerplant can operate at optimum speed, with excess energy stored, and increased energy demand drawing power from the storage source. However Rolls Royce do not believe that such an aircraft will be seen in service until 2035 (1).

3.7. Back Up Power Supply In the instance of single engine failure, the rotorcraft must be able to travel a minimum of 3km and land safely. Different back-up power supply configurations were considered to satisfy this requirement. The first being the use of a large gas turbine in conjunction with a smaller gas turbine, the smaller engine being powerful enough to reach a distance of 3km at maximum contingency power for 2.5 minutes. However, discrepancies in maintenance intervals and spare parts would incur a maintenance cost penalty. Supercapacitors are energy storage devices offering very high energy, power densities and rapid discharge that can be used to supply power in the event of engine failure (2). In addition, they have superior low temperature operation, and can sustain up to a million cycles (3). However, their lack of maturity is such that their weight exceeds that of the previously discussed options. A more developed technology is proven as part of KERS technology in Formula 1. It requires not only energy storage in the form of batteries but a generator to convert kinetic into chemical energy, and a motor to reverse the process when extra power is desired (4). Such a system to power the Mistral would weigh in excess of 240kg, making it a non-viable option. The chosen option is the use of two identical turboshafts, each engine alone is sufficiently powerful to propel the aircraft 3kms away operating at maximum contingency power for a short time and providing equally in normal operation. The caveat is that due to the operation of the remaining engine beyond its design envelope it may require overhaul, however since this is only an emergency scenario, it is deemed an acceptable drawback. The two turboshafts would have the same TBO and spares, thus reducing the maintenance costs.

3.8. Undercarriage Selection Skids offer the best overall capabilities in terms of weight, cost, maintenance and operational flexibility (5). Indeed, the large surface area offered accepts most kinds of terrains. Furthermore, the skids can be retrofitted with accessories such as floating pods or skis to allow even greater flexibility. Their low weight will enable higher carrying capabilities. It does however require external ground handling wheels, but these are widely available at most airfields. Wheels do offer the advantage of a softer landing but it is not a major requirement as there is no crew. Also, wheels often amplify ground resonance due to the tyre flexibility and oleo struts. Finally, sufficient damping can be obtained from the skids’ flexibility to protect the structure in case of hard landing.

4. MISSIONS The missions and the ways in which they drove the final design are outlined in the following sections.

4.1. Cargo The Mistral UAV is designed to perform cargo transportation over long distances. To do so, it has to be capable of transporting a standard LD2 unit load device over 460 kilometres in under 120 minutes. This mission shall be operated at 4000ft cruise altitude.

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These requirements dictate a large portion of the design decisions taken across the aircraft from the airframe sizing to the position of the various components for stability. In order to reduce CG variation the payload is integrated in the airframe directly below the main rotor. Several configurations exist to do so, one possibility is through the back of the aircraft as demonstrated by the Lockheed C-130. The disadvantages of that configuration relates to the tail rotor clearance hazard for the ground crew as well as the limited available manoeuvring space for the LD2 loader. Alternatively, the payload can be loaded from an opening Nose trap, as seen on the AIRBUS A300-600ST. Although offering enough clearance, this would require a heavy hinge mechanism, as well as complicated wiring due to the avionics situated in the nose of the aircraft. Finally, a conventional side door was chosen as it offers a minimal amount of obstruction during loading. Skid clearance for the loader has been taken into consideration in the design process to ensure that it is able to reach the payload bay without obstructions. The door is hinged on the vertical main load paths. This was chosen over a trap door hinged on the top beam as it does not obstruct the main rotor downwash when opened, and hence avoids additional stresses to the door hinges. This will allow for a quicker turnaround and swapping between missions as the rotors may then remain in motion whilst accessing the payload bay. The cargo bay has been fitted with rolling wheels on the floor to efficiently slide the payload in and out. To secure it in position, a trade off study between electrically actuated clamps, manually actuated clamps and a hybrid system was made. A manual clamping system is chosen as it reduces the downtime risk and the empty weight of the aircraft. Rubber fittings are placed on the clamps to diminish the vibration transmission between the airframe and the payload, hence increasing fatigue life of the surrounding components. The airframe must withstand manoeuvring accelerations of -0.5 to 3.5 g which proved challenging when sizing the floor beams. Similarly, the size of the payload was challenging as it dictates the large pitch of the airframe’s main bulkheads. Furthermore, the variable LD2 centre of gravity (±25% fore and aft of the geometric centre) was considered in the stability analysis to ensure that the aircraft remains efficiently balanced for any configuration. Finally, the position of the payload, directly under the rotor shaft, improves stability by shifting the CG closer to the main rotor when loaded in its centre.

4.2. Firefighting Two main firefighting systems currently exist in rotorcrafts, integrated water tanks and Bambi buckets. The Mistral will be equipped with a 500L collapsible water tank accompanied by a hose and pump. First of all, the choice of designing a cargo bay to contain an LD2 container implies that a volume of approximately 3.9 m3 will be available within the aircraft when not undertaking cargo missions, almost 8 times more than necessary to house a 500L tank. Housing the water tank and its equipment within the aircraft allows the Mistral to maintain its aerodynamic performance whereas the use of an external Bambi bucket would significantly increase drag and therefore reduce speed. Similarly, being able to constrain the position of the significant water mass necessary for this mission enhances the flight handling qualities of the helicopter by keeping the CG within defined limits without the use of additional complex control systems and manoeuvres, particularly in high winds. Due to the use of fire retarding chemicals, Bambi buckets present risks of pollution when dropped into water sources whereas the use of an integrated water tank allows to contain these retardants separately and preserve the environment. In addition, CAP 46 over urban areas can become a real issue with underslung, a significant drawback regarding the Mistral requirements.

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One of the key design drives is future proofing the aircraft for 30 to 60 between 30 - 60 years of life. Due to the increasing population density in cities, urban firefighting is becoming a larger problem, especially in the case of a high-rise building catching fire, it can prove extraordinarily difficult to extinguish. For this reason, it was important to ensure that we are able to operate over urban areas. The use of an integrated tank also allows the current system to be upgraded, if the customer desires it, with a directional hose. This would allow the Mistral to be flown up close to the skyscraper and deliver fire extinguishant directly onto the fire. The Mistral is also equipped with a LIDAR system which allows the helicopter to safely fly close to buildings and thus reduce the power requirements of the hose. As mentioned previously the firefighting system will be based on a 15kg, 500L collapsible water tank, in order to reduce sloshing when mounted with appropriate constraints and allow quick turn-around time thanks to its lightweight and easy storage. Water is pumped into the tank through a 0.102m diameter hose running through the floor of the Mistral. The hose is preliminarily filled with a small amount of water to allow for the water to be pumped. A default length of 1.85m from the bottom of the fuselage to the hose end was estimated by establishing a hose length to rotor diameter ratio from systems already in use. However, as the hose is winched in and out the aircraft, the customer has the possibility to extend its length. The pump could not reasonably be sized to survive an OEI when hovering to fill the water tank, which would require a hose length superior to 140ft. The 44kg Kawak HOHRP uses 3.5kW and was selected for its ability to fill the 500L water tank in 13.2s, therefore reducing the necessary time below a safe altitude. The water is then released from the tank through a vane in the floor. Finally, the tank can easily be replaced or extended to allow for higher capacity within the power limits of the aircraft. Another key feature of the Mistral is that during every mission it is equipped with a state of the art FLIR camera system. This allows the aircraft to provide integral information about the fire to firefighters on the ground. Through the use of the infrared camera and GPS data, the Mistral can provide aerial maps of fire affected areas, showing detailed information such as temperature and direction of movement that could prove crucial. It also has the ability to fly and see through thick smoke that would render a manned helicopter inoperable useless. The Mistral is also equipped with a PA system which can be used to warn residents of fire hazards in the areas out of the firemen’s reach and to also direct ground crew and civilians.

4.3. Surveillance The requirements from AgustaWestland state that the aircraft must travel to a surveillance site 27.78km from base at a pressure altitude of 3500ft. It shall operate at 500ft above ground level spending equal amounts of time loitering at best endurance speed and surveilling in hover. The mission requirements state that the aircraft should be fitted with “state of the art surveillance” equipment but as no specific guidelines were provided, the NPAS were contacted, identifying the key design drivers as a quick launch time and long endurance. Infrared and electro-optical cameras were also identified as the most useful mediums to be able to observe but that a wide field of view was required to identify areas of interest before using higher resolution cameras with a narrower field of view.

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Based on this information, the gyroscopically stabilised FLIR system which incorporates electro-optical cameras as well as an InfraRed camera is to be used for the surveillance mission, the benefits of which are outlined below:

No need to spend time between missions fitting equipment No extra mass has to be taken on board Can change from a firefighting or cargo mission to surveillance missions mid operation Can operate surveillance missions in parallel with others e.g. use the FLIR to monitor fires and the

PA to direct ground personnel The data will be transferred via a downlink if within an allowable range or stored on a SSD if outside the downlink range and transferred once back within range. Further investigation must be made into the legislative requirements for broadcasting sensitive data such as encrypted information. If any specialist surveillance equipment is required at any point in the future, these could be accommodated in the payload bay with the lenses protruding through the hole in the floor. For example if covert surveillance is required, then a camera suited to operating at altitude can be fitted and the mission can be completed at a higher altitude reducing the aircraft observability from the ground. An extra fuel tank could also be installed in the payload bay with minimal alterations to the fuel system.

5. FINAL DESIGN This section of the report shall detail the final design of each of the major aspects of the Mistral.

5.1. Rotor Blades The rotor blades are made out of a carbon fibre composite with [±45, 02]S layup, a woven glass outer layer and a Nomex core. The blade shall also feature a Titanium erosion shield and copper conductive mesh. The main rotor blade has the OA212 profile at the root, SC1094R8 at the mid-section and the OA206 at the tip. The tail rotor blade has the SC1095 profile throughout its length. The geometries of the blades are summarised in Table 3:

Table 3: Summary of blade geometries

Main Rotor Tail Rotor

Number of Blades 4 2

Length (m) 4.86 0.901

Chord (m) 0.317 0.161

Angular Velocity (RPM) 412 2227

Linear Tip Twist (degrees) -9 -8

5.2. Rotor Hub The effective flapping hinge stiffness and offset was determined to be 7,500Nm and 0.12R respectively, for lead lag they were 100,000Nm and 0.09R. The flexbeam was chosen to be made out of unidirectional carbon fibre and has a constant rectangular cross section 68mm in width and 19mm in height. A flexbeam-enclosing torque structure made of carbon fibre transfers torque from the pitch control mechanism to the root of the blade. The pitch control mechanism is a standard swash plate configuration to improve

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maintainability. An elastomeric lag damper built into the torque structure was used to suppress ground resonance, the critical damping ratio required is 3.04% and this requires damping of 4270 Nm/rad2. The shaft and shaft head are made out of a single machined piece of titanium alloy TIVAl4V. The shaft is a solid circular section with an 18.8mm radius. Attachments between the flexbeam and the shaft head and the blade are bolted to allow easy disassembly and maintenance.

Figure 3: Diagram of rotor hub

5.3. Material Selection Table 4 outlines the material selection throughout the airframe.

Table 4: Summary of materials used for different components

Component Material Selection Summary

Main Load Path Aluminium 7075-T6

Engine & Gearbox Cowling E-glass Nomex Honeycomb with Phenolic Resin

Gearbox Struts Aluminium 7075-T6 struts - AN Steel Pin

Tail Boom High strength carbon fiber with Thermoplastic AS4 PEEK

Tail fin surface Carbon Nomex Sandwich Panels

Fuselage Skin Carbon Nomex Sandwich Panels

Nose Kevlar 49 with EX-1522 Epoxy Resin

Under Side Aluminium 6061 Sheet Metal

Skids Aluminium 7075-T6

5.4. Landing Gear The skids are made out of high fatigue strength aluminium 7075-T6 combined with replaceable steel wear plates fitted on the surface in contact with the ground. A rectangular cross section for the main load

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carrying members was chosen. For aerodynamic purposes, a glass fibre curved cover shall be located on the frontal part of the skids.

5.5. Engines and Gearbox As outlined in the preliminary concept selection section, the Mistral has two turboshaft engines. The two M250-C20B engines sit in parallel above the payload bay, fore of the rotor shaft. The M250-C20B outputs 313kW of maximum continuous power at 6016 RPM and only weighs 71.7kg. The Mistral satisfies the OEI landing requirements, and can sustain forward flight at low altitudes at a range of speeds with 95% MAUM. The Main Rotor Gearbox, shown in Figure 4, consists of three main stages: Collector, Bevel, and Epicyclic. The reduction ratios and respective speeds are outlined in Table 4 above. The collector stage comprises of 3 different sized helical gears on parallel shafts to combine the power from the two engines. The spiral bevel turns through an angle of 93 degrees to provide the required shaft angle of 1.8 degrees. The epicyclic stage is planetary, with 3 planets, and provides the majority of the reduction in order to minimise the mass of the previous stages. The design incorporates passive sprag and actuated clutches, and a shear neck on the output shaft in order to allow continued operation, or autorotation, in the instance of engine or gearbox failure.

Figure 4: Schematic of gearbox

Table 5: Summary of key MRGB characteristics

Stage Reduction Ratio Input Speed (RPM)

Coupling 1.21 6016

Bevel 1.83 4992

Epicyclic 6.60 2731

Total 14.54 413.8

5.6. Electric Tail The tail rotor is powered by an electrical drive system. A 40kw ‘Yuneec Power Drive 40’ brushless motor, powered by the main generators and dual redundancy cabling, drives the tail rotor. As it is already in

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operation in the aviation industry it shall not require D160 certification and it also outputs at a low RPM to reduce the requirements of a gearbox.

5.7. Weights and stability The tail rotor was placed 200mm above the main rotor to limit bank angle in hover. A main rotor shaft angle of 1.8° was chosen to shift the Mistral’s pitch attitude in the cargo configuration at MTOW as close to zero degree as possible. The horizontal and vertical stabilisers were sized to improve stability and offload the tail rotor during forward flight. The vertical stabiliser is equipped with an electric actuator to vary its incidence. The horizontal stabilizer is placed starboard of the tail boom at the same longitudinal station as the tail rotor and vertical fin.

Table 6: Summary of stabiliser geometries

Vertical Stabiliser Horizontal Stabiliser

Span (m) 1.524 0.991

Chord tip (m) 0.428 0.244

Chord root (m) 1.067 0.244

Incidence (degrees) 6 (variable) -12

5.8. Avionics & Electrical Systems A conventional TCAS and ADS-B components integrated in a Honeywell CAS 100 system create the sense and avoid system. To detect non-cooperative airspace users, the Mistral uses a system of 2 x 3 electro optical camera in conjunction with kalman filters and image processing hardware. The aircraft uses a Cobham SATCOM system for command control with a SVP HDT-02 microwave downlink to relay HD footage back to the ground control station. The Mistral has a STAR Safire 380-HDc Forward Looking Infrared (FLIR) camera system integrated into the airframe, which is used across all missions. The electrical system uses two identical 40kW to generate AC power and three transformer rectifiers to convert this to DC power which all of the avionics systems use. The Mistral also includes a LIDAR system for obstacle avoidance.

6. TECHNOLOGY LEVELS & RISK ANALYSIS Each novel technology or high risk decision had to be analysed. The benefits as well as the likelihood and risk of each technology or decision is summarised below.

6.1. Electric Tail Rotor One of the most novel technologies incorporated into the Mistral UAV design is the electrical tail rotor. It is an immature technology, especially for manned helicopters for which there are no electric drive tail rotors in service. Progress is being made in incorporating electric tail rotors into new designs, as well as retrofitting them to existing ones (6) (7). It is estimated to have a TRL of 7. The key benefit of the electric drive system is the potential for increased reliability over its mechanical counterpart, therefore is inherently a risk mitigation in itself. However in order to minimise the risk of tail rotor failure it must be implemented with precautions. The Mistral features dual redundancy in both the generators, and the cabling supplying the motor. Furthermore, the electric motor used, the Yuneec Power Drive 40, is designed for use in aviation therefore its reliability and safety reflect those same traits that the Mistral strives for.

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6.2. Low Altitude Hover When one engine becomes inoperative, the Mistral is expected to drop by 140ft before being able to maintain altitude. This is a particular problem in the case of the firefighting mission when picking up water as the pump could not be reasonably sized for a 150ft hose. Therefore the water pump design was focused on filling speed rather than security as at this level of proximity, the water could not be avoided in case of engine failure. Buoyancy aids on the skids could save the equipment or eventually allow for a one engine water takeoff. This will be considered later in the design process.

6.3. Fire Fighting Water Tank A considerable problem with carrying a water tank with such a high weight with respect to the one of the aircraft when filled, is the CG and inertia variations sloshing produced. It is to alleviate this problem that a collapsible water tank was chosen, so that through appropriate constraints the volume of the water when shrinking can still be kept within a controlled space.

6.4. Lidar System LIDAR has been chosen as an obstacle sense and avoid system for use predominantly in urban areas. The main sense and avoid system will not be effective for small, thin objects such as pylons or cables which must be detected and avoided as being common place in built-up urban environments. LIDAR is currently at TRL 6 with only a few systems currently making use of it which leads to a lack of assurance about its ability to operate successfully in all conditions. As such, the system may not be fully utilised and be subject to excessive maintenance which could lead to unnecessary downtime and costs. During the first 3-5 years of the aircraft's service life, when flying in close proximity to obstacles, the pilot at the ground control station must approve critical actions, as a precaution to a LIDAR malfunction. Once the LIDAR system has proved fully effective, the pilot’s approval will be gradually faded out and the mission will transition to fully autonomous.

6.5. Sense and Avoid System Sense and Avoid is one of the most crucial systems on any UAV, especially in a civil landscape due to stringent requirements that mandates UAVs to have an equal level of safety as a manned equivalent. The TCAS and ADS-B system used by the Mistral to sense and avoid cooperative aircraft is a well proven technology that is being used extensively throughout the civil aerospace. The Electro-Optical (EO) system defined is still yet to be fully proven on a commercial level. There have been some successful examples of military UAVs that have implemented an EO system, however military systems providers, such as Northrop Grumman rarely provided detailed information on prototype equipment. An early example of a EO prototype sense and avoid system is demonstrated by (8). The risk is exacerbated by the average altitude of the Mistral’s missions, as defined in the AW specification. These missions are mostly conducted between 500 and 1000ft, which is likely to be populated with non-cooperative airspace users that the system needs to detect. It is expected that EO sense and avoid will be successfully integrated onto commercial UAVs within the next 5 years. The use of Electro-Optical (EO) sensors are used to sense the non-cooperative airspace users is assigned a TRL of 7.

6.6. Use of COTS Equipment Whilst there are large benefits in the use of COTS such as their improved reliability and reduced cost, they can carry certification problems. Whilst the equipment will be provided with recommended operating

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conditions, these cannot always be guaranteed and therefore it is not sufficient to just choose equipment with recommended operating conditions that cover the range of expected conditions. Every item of equipment will be subject to different environmental conditions, such as the vibration profiles which can vary dramatically depending on if the equipment in located in the avionics bay in the front of the aircraft or located in the tail boom. Extensive DO160 tests must be carried out on any COTS equipment not currently used on helicopters, which can be very expensive and time consuming. To mitigate this risk, the environmental conditions and whether DO160 testing had been carried were considered when selecting Avionics equipment. Any equipment that didn’t meet the environmental conditions as noted in the AW specification such as temperature were dismissed. It wasn't possible however to do this for every condition as allowable conditions such as vibration profiles were not available and often datasheets provide only limited information.

6.7. Fuel Leakage A fuel leakage could cause harm to the environment and therefore has to be prevented as much as possible. Especially during the firefighting mission, when the aircraft operates close to water reservoirs or drops loads of water, the danger of dispersing fuel is enlarged. As well as the environmental impacts, a fuel leakage could lead to the corrosion of equipment within the aircraft, or in some remote circumstances, if heated by the engine or environment (particularly during a firefighting mission) could ignite. To reduce the risk of damage, the fuel tanks sit on an aluminium honeycomb sandwich panel design to linearly crush in the event of a 3.5g crash landing. The panel can be replaced easily as it was sized from an off the shelf (9). Furthermore, the placement of the fuel tanks within the main structural bulkheads, is key as this area is the least prone to deformation, reducing further the risk of a punctured fuel tank. Further examinations have to be applied on the fuel tank materials and crash cases, in order to classify the risk level.

6.8. Composite Tail Boom The tail boom is a critical part of the aircraft’s load path as it is essential in holding the tail rotor in place to counteract the main rotor’s torque. The use of carbon fibre although offering attractive characteristics such as strength and fatigue resistance, is susceptible to brittle failure due to out of plane loading, notches or laminate defects. As such, a further analysis should assume existing microscopic cracks to size the laminate accordingly. Furthermore to reduce the risk of failure, defects need to be identified before becoming critical. Several NDT methods exist to do so, such as expert coin tapping methods or using ultrasonics. But the critical part remains in the manufacturing process whereby thorough monitoring will be required to ensure consistency and hence quality of the parts being manufactured.

6.9. Rotor Hub Flexbeam The harsh loading environment to which the flexbeam is exposed makes quantifying its fatigue strength under these conditions difficult. Most data does not consider such a large number of loading cycles and on top of this the effect of combined bending, axial and shear stresses is difficult to determine. Fatigue tests simulating the loading of the flex beam would need to be conducted and given the high number of loading cycles required, could take a significant amount of time. Ultimately it may be necessary to replace the flexbeam more often than the given requirement of 10,000 hours, however this would not be too costly due to its simple design.

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7. ECONOMIC ANALYSIS & COMPETITIVE ANALYSIS

7.1. Mistral unit cost In order to estimate the unit price of the Mistral, current metrics for UAVs based on the price in dollars per empty weight in (10) were used to obtain a unit cost of $3.294M. In order to validate this result the obtained value was compared to the civilian single engined Boeing AH-6 UAV.

Table 7: Comparison of the Mistral with a similar aircraft

UAV Empty weight MTOW Max cruise speed Unit cost

Mistral 996 kg 1705 kg 260 km/h $3.294M

Boeing AH-6 722 kg 1610 kg 250 km/h $2M

As Table 7 outlines, its weight and maximum cruise speed are slightly lower than the Mistral’s but the reduction in price is significant. Using the same metric for the AH-6, a unit cost of $2.387M was obtained, although the extensive use of composite, state of the art avionics and the twin engine configuration of the Mistral justify its higher price, this result outlines the fact that it remains a conservative estimate. A statistical model was created by plotting the unit cost against the all up mass of several twin engines helicopters specializing in cargo, surveillance or firefighting. From this, a linear trend was fitted to obtain an estimate of current twin engines rotorcraft unit cost for the AUM of the Mistral.

Figure 5: Current twin engine rotorcraft unit cost trendline. Figure 7 allowed to estimate a unit cost of $2 540 433 for an unmanned equivalent of the Mistral. This shows that although the estimated unit cost of the Mistral of $3.294M is a conservative estimate, it is considerably higher than similar manned aircrafts currently on the market.

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7.2. Fuel Costs By estimating the fuel consumption per hour for the different missions as well as obtaining the current kerosene JP-4 price per litre (1.007$/l) and a 30 years forecast (1.458$/l) , the Mistral Fuel costs for each mission were obtained and are summarised in Table 8:

Table 8: Summary of fuel consumption and costs for the Mistral

Mission Surveillance Firefighting Cargo

Fuel consumption (L/hr) 81.86 81.31 107.5

Fuel Costs today ($/hr) 82.43 81.88 108.3

Fuel Costs in 30 years ($/hr) 120.1 119.3 157.9

7.3. Maintenance Costs The same aircrafts as for unit costs were used to obtain a linear trend of their operating costs per hour (excluding fuel) against AUM.

Figure 6: Current twin engine rotorcraft operating cost trendline From Figure 6, the operating costs excluding fuel of a manned equivalent of the Mistral was estimated to be 695.4 $/hr. It is difficult at the current stage of UAS technologies to estimate accurate operating costs, however from different sources and particularly military data from the US Air Force, it can be inferred that unmanned air vehicles are generally cheaper to operate than their manned counterparts (11).

7.4. Competitive analysis Using the fuel consumptions from Table 8, it is possible to compare the fuel costs per hour of the Mistral to other aircrafts of similar weight and capabilities. From the maximum cruise speed, fuel economy and capacity of the aircrafts used in the statistical model, their fuel consumption per hour was obtained against all up mass. A linear trendline was fitted after being compared to an exponential one in order to obtain the most conservative estimate of fuel consumption for twin-engines currently available on the market, with the same AUM as the Mistral.

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Figure 7: Current twin engine rotorcraft fuel consumption trendline. Figure 7 allowed to estimate a fuel consumption for an equivalent manned aircraft to the Mistral of 127 L/hr. This value is significantly higher than the specialist estimates, as seen in Table 8, for the three missions, showing that the use of the Mistral, either for firefighting, cargo transport or surveillance will significantly reduce fuel costs compared to similar aircrafts on the markets. The competitor’s calculations also do not take into account the additional costs caused by the drag of an underslung payload during cargo or firefighting missions making the Mistral even more competitive than the drawn comparison. Hence, likely cheaper maintenance and fuel costs than similar manned aircrafts can compensate for the Mistral’s higher initial cost over time. In addition to all the advantages conferred by the use of an unmanned system such as the reduced risk to human life, human errors mitigation, increased payload capabilities and in the case of the Mistral the possibility of increasing fuel capacity and therefore range and endurance; its multi-role nature will provide customers a significant strategic advantage. Single-role entities such as firefighters will be able to take advantage of its high performances in firefighting and lease it when out of the fire season to reduce annual costs (12).

8. CRITICAL ANALYSIS OF DESIGN Due to the hostile environment in which firefighting takes place, it is extremely beneficial to be able to operate remotely, removing the chance for loss of human life in the event of a failure. It also improves the design as no weight and power need be dedicated to life support systems and the airframe does not have to be designed to withstand such high crash loads. From an economics point of view, the design costs will rise in comparison to a conventional helicopter as there is not much experience with large-scale rotary wing UAVs and the amount of avionics is increased, in order to replace the missing pilots. The operating costs will be reduced as no crew is needed. However, there will still be people monitoring the helicopter from the ground.

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The absence of a cabin affects the design of the fuselage and shifts the centre of gravity rearward; to mitigate this the avionics were placed in the nose of the aircraft. For a better stabilisation during flight, the two fuel tanks were placed fore and aft of the payload. The fuel will be drawn from the tanks at equal rate hence retaining the longitudinal CG position. Future proofing was an important part of the design as it governs the ability of the aircraft to be improved in order to meet the future client’s needs. One main consideration was to create a highly modular layout. Parts should be interchangeable in order to increase the efficiency, the capacity, the lifetime and even more, if required or new technologies were available. Therefore the design was strictly separated into different sections e.g. payload bay, avionics bay, fuel tanks and so on. The main focus lay on the fulfilment of the functionality, performance and safety requirements. Then further optimisations to reduce size, maintenance and costs were considered. Aspects like noise and emissions were not further optimised as long as they fulfilled international guidelines. Considering the flight performance the hardest conditions were hover, take-off, forward flight and a single engine failure. The higher the altitude, temperature or all-up-mass the greater the required power to sustain hover. The largest amount of power would be during the firefighting mission at the end of the first water pick-up. As the hovering is out of ground effect as well, this is the key design driver. The power available at that stage is enough to fulfil the requirement. Regarding take-off, four different mission start points have to be considered - for the payload delivery, the firefighting, the surveillance mission and a second surveillance mission, which was defined by the specifications to depart at 9,000ft pressure altitude, but with a shortened mission length. At the given conditions the UAV was largely capable of making a category A take-off. In order to evaluate the forward flight performance, a closer look had to be taken with respect to the highest speeds occurring during flight. The maximum speed, that has to be achievable is 140kts at 5,000ft pressure altitude. As the ambient conditions have a great influence on the performance, the highest velocity of the surveillance mission starting at 9,000ft height had to be examined to verify whether it lies within the flight envelope or not. With 135kts the speed is close to the maximum required speed, but at a higher altitude. By evaluating the flight performance through the helicopter envelope it was found that the key design drivers could be met. Hover needs a lot of power. One engine is not enough to cope the power requirement during that flight condition. This is why a single engine failure forces the aircraft to change its flight condition to one that does not require as much power. The UAV must not descend more than 300ft with 95% of the MTOW. A transition to forward flight has to be performed. Calculations on the engine failure case in hover showed, that the helicopter drops 140ft before it regains height at best endurance speed again. One case where this cannot be met would be when hovering close to the ground or water for the firefighting mission. In order to rescue the aircraft from loss or damage further study would be necessary. The final UAV of team Mistral met 92.3% of the given requirements. 4.8% of the requirements need to be verified more extensively before they can be assessed as met or not. This includes a general maintenance interval of at least 200 flight hours, a time between overhaul for all mechanical parts of 1000 flights hours or more or that the overall vertical vibration level shall be less than or equal to 0.1g.

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Only 2.9% of the requirements could not be met by the final design, which are: 1. The UAS should be free of any flight instabilities.

● The rotorcraft exhibits an unstable phugoid mode in hover which is not uncommon and considering the calculated cyclic period would be easily damped through an adequate control system.

2. The UAS shall be capable of operating in ice. ● This requirement was mistakenly thought to be relaxed and would be implemented in

later design stages. 3. The UAS shall cope safely with a single powertrain failure at the most critical point in the missions.

● In case of an engine failure in hover the helicopter will drop 140 ft, before altitude control can be regained. When picking-up water during the firefighting mission the hover would occur a few meters above the water reservoir. An engine failure at this stage will lead to a crash into the water. Buoyancy aids could help to solve this issue otherwise this requirement needs to be relaxed.

9. CRITICAL ANALYSIS OF WAY OF WORKING Working together as a group is challenging, therefore different methods were applied in order to enable an effective way of working. The main areas affected were communication, workload distribution and data management. Facebook was initially used as the main means of communication outside of meetings due to its convenience and availability to all team members. After the PDR, the method for communication was reviewed and Slack was employed to replace Facebook as internet based means of communication. It allowed to keep topics separate and team members to decide their own level of involvement and notification in the discussions of specific topics as well as allowing for individual direct messaging. Slack also provided a platform for file sharing, integrated well with our data management service as well as separating social and work related information. In the early stages of the project, meetings were long and inefficient due to members’ inexperience in group projects and lack of leadership. To address this, a chairperson was appointed for each meeting whose responsibility was to direct the discussion towards topics of relevance. Once technical roles were allocated, all decisions were made between the technical specialists that they affected and the systems and integration engineer. Regarding important decisions, the team leader and chief engineer were tasked to solve decision based conflicts. As the missions did not fall under any technical role, a mission leader was assigned to each of them. They took a role similar to that of the team leader, ensuring that all mission related decisions were made appropriately according to the technical roles they affected and that the requirements for that mission were met. Google drive proved to be an effective data management system allowing team members to work on documents simultaneously, improving productivity. Once the content was completed, the documents were transferred to Microsoft Word to allow a single member to finalise the formatting. Older versions and duplicates were archived in case they needed to be referred to in the future. An N2 diagram was used to streamline the flow of information during design iterations as well as inform individual team members of what values needed to be output. Despite having this in place alongside a

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single central document where all values required throughout the N2 from all disciplines were stored, determining an initial estimate still proved difficult due to the team’s lack of engineering experience. Once the first iteration was made, subsequent ones became far simpler and quicker. A knowledge matrix was created to allow specialists to confer with other team members that had sufficient knowledge or interest in the same area to validate difficult decisions. The requirements document was created at the start of the project based on the specifications provided by Agusta Westland and other regulatory authorities. This document was constantly updated throughout the design process to ensure that it was being driven by these requirements. This allowed for brief regular assessments to be made and the unfulfilled requirements to be satisfied. Despite all the tools used, it was not possible to prepare for all the unprecedented difficulties encountered. At a late stage, at which the design was considered to be frozen, the engine performance was found to be insufficient at high altitudes. A new engine had to be found to meet the power requirements which resulted in an unexpected design iteration having to be performed.

10. CONCLUSION The final design of the Mistral encompasses a multi-role UAS able to fulfil the firefighting, cargo transportation and surveillance missions. It fulfils 92.3% of its requirements, 4.8% still need to be verified but 2.9% were not met. Exploring those which were not met, the absence of flight instabilities could not be satisfied as the phugoid is unstable in hover. This type of unstable motion is common and is typically stabilised by the control system. No anti-icing systems were considered during this phase of the design, but would have to be considered for the final design in order to satisfy the all-weather operation requirement. Finally, as it is necessary to be within a few meters of a water source when filling the water tanks, a single powertrain failure would lead to an unavoidable ditching of the helicopter although the implementation of buoyancy devices could alleviate the risks of equipment loss when landing in water. The novel nature of this type of rotorcraft makes it difficult to accurately estimate its costs. However, using statistical data and diverse verification methods the Mistral unit cost, although conservative, was predicted to be quite high compared to similar manned helicopters available on the market and presenting the same capabilities. However, the multiple advantages offered by a UAV added to its competitive fuel consumption and operating costs can compensate for this higher initial price and attract both specialised and multi-role customers. Novel technologies were incorporated into the design in order to optimise performance and remain at the forefront of technology. The tail is powered by an electric motor decreasing failure rates compared to a mechanical equivalent. However, as it is an immature technology, tests and further optimization of the system will be necessary. An electro-optical system is used at the front of the aircraft to spot and monitor other aircrafts in the vicinity of the Mistral. TCAS is a proven technology, extensively used in commercial aviation, however, the electro optical system remains to be fully implemented on a commercial scale but market indicators conclude it could be by 2020. LIDAR is used to sense and avoid nearby obstacles such as power cables and high rise buildings. The airframe was designed using a significant proportion of composites for their improved specific strength and fatigue resistance properties. However this implies that additional thorough analysis will need to be conducted to size them for their specific properties and create the adequate manufacturing processes.

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Such a design task posed different challenges and difficulties that were treated through the use of different tools. ‘Slack’, a multimedia communication platform, was used to contain conversations within their relevant topics and notify the relevant technical member directly. A google drive folder structure allowed the organisation of the multitude of documents relevant to the different sections of this projects as well as the simultaneous editing of documents. N2 and knowledge matrices were generated to keep track of the required outputs from the different specialists as well as their impact on others, and aid the design process for each individual by facilitating the access to help or advice in their specific areas. For important design decisions that often conflicted with several specialist areas, a technical panel consisting of the Chief Engineer and Systems Integrator were used to make a final decision. As in any design exercise, difficulties arose such as the late reselection of the power subsystem and the modification of the hub flapping and lead lag stiffnesses which in each case impacted several other design areas and required additional iterations. Strong team dynamics and understanding of each other’s difficulties enabled convergence to the presented final design of the Mistral UAV, a well performing and novel multi-role UAV, at the forefront of the technological field.

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BIBLIOGRAPHY

1. Rolls Royce - Present and Future Challenges. Parker, R. 2014. IMechE Lecture. 2. A. Schneuly, R. Gallay. Properties and applications of super capacitors. garmanage. [Online] http://www.garmanage.com. 3. TECATE GROUP. Back-up power for military applications. [Online] http://www.tecategroup.com/ultracapacitors-supercapacitors/military-applications.php. 4. MARELLI, MAGNETI. KERS. [Online] http://www.magnetimarelli.com/business_areas/motorsport/technological-excellences/kers. 5. Design and Structural Analysis of Landing Gear. Naresh Kumar, J. Abdul Schukur, K. Sriker. s.l. : Inpressco, 2014, Vol. International journal of current technology. 6. LaunchPoint Technologies. Helicopter Electric Tail Rotor. [Online] http://www.launchpnt.com/portfolio/transportation/helicopter-electric-tail-rotor/. 7. Navy Small Business Innovation research. Helicopter Electric Tail Rotor Drive. [Online] http://www.navysbir.com/13_1/171.htm. 8. James Utt, John McCalmont and Mike Deschenes. Development of a sense and avoid system. s.l. : AIAA Infotech at Aerospace, 2005. 9. S. Mukherjee, A. Chawla, D. Kumar, T. Nakatani and M. Ueno. Predicition of crushing behaviour of honeycomb structures. New Delhi : Indian Institute of Technology, 2002. 10. Vallerdi, Ricardo. Cost metrics for unmanned aerial vehicles. Cambridge, MA : Massachussets Institute of Technology. 11. Mailey, Chris. Are UAS More Cost Effective than Manned Flights ? AUVSI. [Online] http://www.auvsi.org/HamptonRoads/blogs/chris-mailey/2013/10/24/are-uas-more-cost-effective-than-manned-flights. 12. A. L. Westerling, A. Gershunov. Climate and Wildfire in the Western United States. [Online] 05 2003. http://journals.ametsoc.org/doi/pdf/10.1175/BAMS-84-5-595.