Micro Air Vehicle: comparison and proposed implementation (report)

Micro Air Vehicle Component Comparison and Proposed Military Reconnaissance Design Implementation

Prepared for Dr. C.I. Chang, Director Army Research Office Research Triangle Park, NC

By Daniel Dell Alex Macleod Yaron Mordfin

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Micro Air Vehicle Component Comparison and Proposed Military Reconnaissance Design Implementation Table of Contents Acknowledgements ....................................................................................................................... 2 1. Preface........................................................................................................................................ 3 2. Engine Design and Propulsion................................................................................................. 4 2.1 Internal Combustion Engine ................................................................................................. 6 2.2 Electric Motors...................................................................................................................... 9 2.3 Alternative Means of Power ............................................................................................... 11 2.4 Recommended Propulsion System ..................................................................................... 13 3. Wing Design............................................................................................................................. 14 3.1 Reynolds Number Considerations ...................................................................................... 15 3.2 Fixed Wing.......................................................................................................................... 16 3.3 Morphing Wing................................................................................................................... 17 3.4 Flapping Wing .................................................................................................................... 19 3.5 Recommended Wing Design .............................................................................................. 20 4. MAV Control System ............................................................................................................. 22 4.1 Control Unit ........................................................................................................................ 23 4.1.1 Multiple Component Unit ............................................................................................ 23 4.1.2 Backpack Unit.............................................................................................................. 24 4.2 Navigation System .............................................................................................................. 25 4.2.1 Vision-Based Navigation ............................................................................................. 25 4.2.2 GPS-Based Navigation ................................................................................................ 27 4.3 Recommended Reconnaissance System ............................................................................. 29 4.3.1 Control Unit ................................................................................................................. 29 4.3.2 Navigation System ....................................................................................................... 29 5. Conclusion ............................................................................................................................... 31 APPENDIX A: Additional Wing Design Figures..................................................................... 33 APPENDIX B: Additional Control System Figures ................................................................ 34 Glossary ....................................................................................................................................... 35 Bibliography ................................................................................................................................ 37 Author Qualifications ................................................................................................................. 40

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Acknowledgements We would like to extend our thanks to all who have contributed in any way to the success of this project: Dr. Walter Knorr - Professional Writer Dr. Theodore Mordfin – Usability Tester Dennis Williamson – Editor Jung Nam – Technical Reviewer We would also like to acknowledge each other. Thanks for all of your help and support, Daniel Dell Alex Macleod Yaron Mordfin

December 2, 2004

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1. Preface The idea of Micro Air Vehicles (MAVs) has been around for about seven years. MAVs are tiny airplanes, no larger than 6 inches in any dimension, which can be flown from a remote location and used for various reconnaissance and detection applications. New technological advancements in the miniaturization of everything from motors to sensors have fueled much interest in this topic. Since its conception, the idea of a Micro Air Vehicle specifically designed for use on the battlefield has interested The Department of Defense. Today, military operations have gone from large scale, open field battles to close quarter urban combat. Because of this, real time information about enemy positions and troop movement is important to a mission’s success. A battlefield commander will use a MAV for reconnaissance to get this needed information. Small enough to carry in his backpack, the commander will be able to call it into action at any time, fly it around enemy positions, and see what it sees. As the technology matures, MAVs will even be able to fly inside buildings to locate hostages, ammunition dumps, or command centers. With the use of miniature sensors, MAVs will also be able to detect any biological or chemical agents released by the enemy in an area and alert incoming soldiers of the dangers ahead. A MAV will be able to land on enemy vehicles and electronically tag them for destruction by artillery or air strikes. The MAV’s small size will allow it to accomplish all of these mission objectives with little chance of being seen, giving the soldiers a highly superior advantage over their enemy. In order to make MAVs a reality, research is underway to understand small scale flight since its characteristics are significantly different than those of large scale flight.

Many

prototypes have been built using a variety of engine, wing, and control unit designs. The

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following sections compare the most viable designs for these components and propose a design for implementation in reconnaissance applications.

2. Engine Design and Propulsion Micro air vehicles are capable of completing many tasks, none of which are possible without an adequate propulsion system. The propulsion system is the heart of every MAV. It is what powers the craft and enables it to fly. Because of the small size of MAVs, the propulsion system poses a unique challenge. It must be light-weight and provide the MAV with enough power to move as required for extended periods of time. "Propulsion is definitely the long pole in the tent," said Richard Foch, head of the vehicle research section in the off-board countermeasures branch of the Naval Research Laboratory's Tactical Electronic Warfare Division.

"These systems require a method to generate enough aerodynamic thrust in an

extremely efficient manner,” he exclaimed [1]. Foch brings up a good point. To produce enough power at a sufficiently light weight, the propulsion system must be highly efficient. Power output and weight are the two most important factors of a MAV propulsion system. On average, a MAV using a six inch propeller needs around 2.5 watts of power to fly and at least double that to turn and climb [1]. While five watts might not seem like much power, given the size requirement it is challenging to create an efficient means of producing these five watts. Weight also plays a significant role. Since the whole weight of the MAV is measured in grams, the addition of a single gram could decrease the endurance of the flight by 30 seconds [2]. Currently, propulsion systems comprise up to sixty percent of the total mass of a MAV with structure in second place at only seventeen percent [2]. Clearly, lighter is better.

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There are three main ingredients to every propulsion system, a source of thrust, a driving medium and an energy source [3]. Sources of thrust include propellers or helicopter rotors. The driving medium consists of two main categories: internal combustion engines and electric motors. There are other types still in developmental stages that may be considered in the future, like micro turbines and fuel cells. Finally, sources of energy include fuel, battery and possibly solar power. This document will focus on propeller driven MAVs because they seem to be the most feasible and reliable option at this time. Propeller design and size are important parameters and play a large role in propulsion, especially in thrust generated. Size is a huge factor when designing a propeller for a MAV. Five centimeter diameter propellers seem to be the minimum size that is practical to produce enough thrust. So far, a propeller of this diameter spinning at 25,000 RPM has achieved an efficiency of fifty percent [1]. While larger propellers may prove to be more efficient, larger motors with more torque are needed to turn these, thus increasing the weight. Gear reduction is another possibility that allows smaller motors to turn larger propellers, however there is still a weight gain and some loss due to friction between the gears. At this time, most propellers used in MAV design are production model airplane propellers or modified model airplane propellers [2]. These propellers provide a strong reliable platform but do not achieve the high efficiencies needed for light weight, long endurance MAVs. In order to optimize propeller efficiency, computer modeling and optimization techniques will be utilized to produce the best propeller for a reconnaissance MAV design.

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2.1 Internal Combustion Engine The internal combustion engines used in MAV applications are two stroke engines. These engines do not have any valves, which simplifies their design and minimizes their weight [4]. Because of this design, two stroke engines have the ability to pack almost twice the power of a similar sized four-stroke engine and have a much larger power to weight ratio. This is what makes them desirable for MAV applications [4]. As with any two stroke engine, special oil must be mixed with the fuel to lubricate the bearings and pistons of the engine because a separate oil reservoir is not used as in a four stroke engine. Internal combustion engines used in MAV applications are directly adapted from those of model airplanes. These model airplane engines offer a good solution for now because they provide large amounts of power at low cost and are readily available [5]. While only about five percent efficient, they use high energy fuels to produce sufficient amounts of power [1]. Most commonly these engines are single cylinder motors which use a glow plug ignition system. Electrical energy is supplied to the glow plug and it reaches high enough temperatures to ignite the fuel. Once the engine is running, the electrical power can be removed from the glow plug and it stays hot enough to ignite the fuel as long as the engine continues running. Current research shows that power output increases with increasing engine size [5]. One specific model airplane engine that has been used successfully in MAV applications is the Cox hobbies Tee Dee .010 (see Fig. 2.1). This engine is the world’s smallest production engine and weighs in at just under a half an ounce while operating at up to 30,000 RPM [6]. This engine does not come with a throttle valve from the factory which greatly lessens control over the engine. However, Micro-Flight offers custom proportional exhaust throttle valves for the Tee Dee .010 [7]. With the addition of this valve the motor can run at any speed from idle to

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full throttle. A successful MAV was built by students and professors at Worcester Polytechnic Institute using this same engine and throttle valve.

Figure 2.1 Cox Tee Dee Engine Specifications.

Adapted from Design of a Micro Air Vehicle

[7].

As with any other internal combustion engine, fuel is needed to power the engine. Because of the poor efficiencies of the motors at smaller scales (usually only around five percent), a high energy fuel must be used to provide sufficient power. Model aircraft engines use a mixture of methanol, nitro methane (CH3NO2) and castor or synthetic oil for lubrication [8]. Increasing the nitro methane percentage yields more power. Most fuels used range from ten to forty percent nitro methane [8]. This blend of alcohol and nitro methane creates a potent and powerful mixture to fuel the engine.

Storage of this fuel is important because it is very

flammable, ignites readily, and burns very hot. In MAV applications the fuel tank must safely store enough fuel for the desired run time without weighing too much. The tank should be placed away from the engine exhaust and in such a way so that the center of gravity of the craft is not altered as fuel is consumed. Internal combustion engines used in MAV applications have one strong advantage: the large amounts of power generated in such a small package. Compared to a similar sized electric motor, the internal combustion motor can create much more power. In addition to this, fuels

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used to power these engines are very high in energy giving the internal combustion engine a high energy density. Sufficient power is necessary for a MAV to perform its tasks, so the large amount of power generated by these motors leads to increased maneuverability. Since model aircraft engines are already in production, engines are low in cost and available in high volumes. Another advantage is long lifespan. These engines are capable of fairly long life spans and can be refueled and sent back out into the field on another mission. While internal combustion engines offer the advantage of increased power, they also have disadvantages.

The two largest disadvantages of these engines are noise and heat

generation. Internal combustion engines generate a large amount of noise and heat, both of which can be easily detected by current military technologies. Using this type of motor severely limits the MAV’s stealth abilities [9]. MAVs which use internal combustion engines can easily be heard by ground troops and destroyed. Because of this, reconnaissance missions would be limited. In addition to noise and heat generation, engines this small can be unreliable and finicky especially in extreme temperature environments [9]. To operate the throttle of the engine, a servo, receiver, and transmitter are needed as well. Servos and receivers do come in small packages, but because the motor cannot power these electronics, a battery of some type is needed. This adds more weight to the MAV in addition to the fuel needed to power the engine. The exhaust generated from these engines makes them poorly suited for interior use. Furthermore, the fuel only has a shelf life of about a year and if spilled, poses an environmental and safety risk. Internal combustion engines at this size are not efficient at all. Most run at around five percent efficiency and compensate for this low efficiency by using high energy fuels. Efficiency is everything in a MAV. More efficient means of producing power means increased weight

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savings and increased power, which translates into longer run times and more maneuverability. Small engines such as these do not produce a consistent amount of power all the time. Factors such as altitude and temperature can produce fluctuations in power output, which could cause under powering of the vehicle causing it to become unstable or crash.

2.2 Electric Motors Electric motors have been used in everything from toys to cars and have been around for many years. They work off of the simple principle of magnetism and electromagnets. An electromagnet resides in the center of the motor and draws power from the battery. Surrounding this electromagnet is a permanent magnet that produces a constant magnetic field. By applying power to the electromagnet, the opposite poles attract each other. As the opposite poles are drawn to one another, the field of the electromagnet is switched, making the opposite poles far away again [10]. It is the switching of the electromagnetic field at the correct time that creates the rotating shaft power of an electric motor. Two types of electric motors are small enough for MAV use, brushless motors and motors with brushes. Motors with brushes were designed first. The brushes bring the electrical power to the electromagnet as it is spinning. They are usually pieces of metal or carbon that make contact with the electromagnet [10]. Electric motors were improved when new Neodymium Iron Boron magnets were introduced giving motors more power, and by the 1990’s, brushless motors were widely available [11]. These motors boast higher efficiencies and reliabilities with lower weights [11]. Currently, brushless Neodymium Iron Boron magnet motors, as seen in Fig. 2.2, are running at near 90-percent efficiencies [1]. This makes brushless motors an excellent choice for MAV

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applications. A speed controller must be used with an electric motor if maximum speed is not desired at all times. These speed controllers reduce the voltage and current going into the motor, thus reducing the speed and power output. A speed controller is similar to the servo used for controlling the throttle valve of an internal combustion engine, but speed controllers do this electronically.

Figure 2.2 Micro Brushless Motors.

Adapted from Modern electric propulsion systems for small MAV’s [11].

A power source is needed to power the electromagnet in all electric motors. For MAV applications batteries are used for this purpose. There are many types of batteries and trade offs for each type. Power density and energy density are two factors used to characterize battery types. Power density is the power per unit area that the battery produces and energy density is the amount of energy stored in system per unit volume. Nickel-Cadmium (NiCd) and NickelMetal-Hydride (NiMH) batteries have high power densities but low energy densities [2]. These types of batteries are rechargeable and can be charged and discharged numerous times. Lithium batteries are usually not rechargeable; however they have high energy densities and low power densities [2].

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Ideally for a MAV application, the battery should have a high power and high energy density. At this time this so called “ultimate” battery does not exist. Most MAVs that have been successful use the Lithium battery because it offers longer run times than any other type [1]. Lithium batteries can be packaged very small and lightweight making them more appealing to MAV usage. Electric motors have many advantages over internal combustion engines when used in MAV applications. First, electric motors are quiet and do not make much, if any audible noise. This makes them better than internal combustion engines for stealthy missions. Electric motors are now able to run at near 90 percent efficiency while providing a reliable, constant power output [1]. With further advances in battery technology, longer run times and more efficient storage methods will become available. New battery technology may even integrate the battery as part of the actual structure of the MAV itself [12]. Most MAVs that have been successful use electric motors and battery systems because of their high efficiencies and reliabilities. The major drawback to electric motors is their current lack of power compared to internal combustion engines. While the internal combustion engines may be highly inefficient and noisy they do produce more power than an electric motor of a similar size. MAVs rely on power to propel them through the air so enough power must be supplied for the MAV to even get off the ground. Just like the internal combustion engine, as time progresses more advances in brushless motors and battery technologies will influence electric motor systems of the future.

2.3 Alternative Means of Power Electric motors and internal combustion engines are not the only means of producing suitable power for a MAV, but at this time they are the only available ones. Other means of

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producing power include fuel cells, micoturbines, solar power, and thermal electric generation [1]. Fuel cells are not yet miniaturized enough for MAV use but in the near future a fuel cell the size of a centimeter tall playing card could provide enough power for a MAV with only air used for fuel [1]. Miniature Impulse turbines are being researched as an option as well. These gas driven turbines would be coupled to a generator which would produce electrical energy to power the MAV [13]. Figure 2.3 shows the size of the impeller compared to a dime. At this time these turbines are in the working stages but are not yet practical because of the high pressures needed to store the driving medium.

Figure 2.3 Impeller of Miniature Turbine.

Adapted from Design Fabrication, and Testing, of a Miniature Impulse Turbine Driven by Compressed Gas [13].

Solar power is another option. While it has been shown that state of the art solar panels can produce enough power to propel a MAV, specific sun conditions must exist to generate enough power [14]. At this time solar power could be used to charge batteries on the ground but would add too much weight for flight. Finally, thermoelectric-generators (TEGs) could be added to MAVs that use internal combustion engines. Thermoelectric-generators convert heat energy Military Sensitive

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into electrical energy without any moving parts [15]. This small amount of electrical power generated using a TEG could be used to power electronics included on the MAV. While these forms of power are not yet developed enough for MAV use, within the next five years one of these technologies could become a reliable power source for micro air vehicles.

2.4 Recommended Propulsion System For a reconnaissance MAV the use of an electric motor/battery propulsion system with an optimized propeller is recommended. The electric motor was chosen for its reliability and efficiency.

Since the MAV should be stealthy so enemies can not detect it, an internal

combustion engine is out of the question.

The motor type used should be a brushless

Neodymium Iron Boron magnet motor which has an efficiency of nearly 90 percent. The battery should be a non rechargeable lithium battery because it will allow the MAV an average run time of approximately 30 minutes. While the electric motor does not produce as much power as the internal combustion engine, the benefits of the quiet, efficient electric motor outweigh the lack of power. The propeller should be a minimum size of three inches but not larger than five inches. Calculations will be run to optimize propeller efficiency to achieve the most effective and efficient motor/propeller combination. MAV propulsion is all about efficiency and lightweight. The proposed propulsion system demonstrates both of these.

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3. Wing Design Of the different types of wing designs available for use in Micro Air Vehicles, the three that show the most promise are the fixed wing, the morphing wing, and the flapping wing. Similar to the wings used in modern day aircraft, the fixed wing design implements a rigid wing that cannot be easily deformed. This type of wing can be used in either the regular fuselage/wing configuration or the flying wing configuration. Like today’s passenger planes, the fuselage/wing is the most common configuration used where two wings are connected to either side of the center fuselage. The flying wing configuration combines the wings and fuselage into one large wing shaped aircraft. The most widely recognized flying wing aircraft is the United States Military B2 Bomber. Morphing wings are usually incorporated in a flying wing configuration where the wing is made out of a material that allows it to deform during flight. Wing stiffness is controlled by the type of material chosen and the number and configuration of its reinforcing structural members. An alternative to stationary wing flight is flapping wing flight where two wings rhythmically beat up and down similar to a bird’s flight mechanism. The flapping wing design usually incorporates this type of flight in a fuselage/wing configuration. Due to the small scale of MAVs, different flow characteristics dominate their flight as compared to conventional aircraft. To understand these differences and the role they play, consideration of low Reynolds numbers associated with small scale flow must be taken into account. It is the characteristic of this small scale flow that will determine the best design for reconnaissance applications.

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3.1 Reynolds Number Considerations The Reynolds number is defined as “the ratio of inertia force on an element of fluid to the viscous force on [the] element” [16]. This ratio is used to determine whether the flow being studied is laminar (smooth, layered flow) or turbulent (chaotic, highly fluctuating flow) [16]. At low Reynolds numbers, the flow is laminar, while at high Reynolds numbers the flow is turbulent. Characteristic of MAV flight is low airspeeds, which results in low Reynolds numbers and initially laminar flow. At low Reynolds numbers, the laminar flow over the wings separates and then reattaches downstream causing a transition to turbulent flow. Due to its nature, the turbulent flow induces a high amount of drag relative to the amount of lift being produced. Small fluctuations in the magnitude of drag can lead to highly unsteady effects produced on the wings making it difficult to maintain stable flight. Viscous effects (impeding forces applied on the vehicle by its resistance to flow) on the MAV’s surface also become important at low Reynolds numbers since the boundary layer (a thin layer of fluid, inside which friction forces play a large role [16]) around the vehicle is large compared to the effect of turbulence [17]. Wing gusts produced by any unsteadiness in the ambient wind must also be considered since the magnitude of these gusts makes up a larger percentage of the average airspeed than in large scale flight [18]. In conventional aircraft, these gusts can usually be neglected since the aircraft’s average airspeeds are far greater than the magnitude of wind gusts encountered during flight. Most importantly, at low Reynolds numbers characteristic of MAV flight, the ratio of lift to drag decreases, thus reducing the overall performance of the vehicle. Therefore, to be considered a good design, the wing must be able to effectively reduce the flow separation over itself, thereby minimizing the amount of drag and unsteady effects experienced by the vehicle.

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3.2 Fixed Wing Out of the three types of wings being considered, the characteristics of fixed wing flight are understood the best. However, the negative effects of low Reynolds number flight have the biggest impact on fixed wing MAVs. The best way to minimize these effects and maximize the lift to drag ratio is to use a cambered, flying wing configuration which can effectively reduce the amount of flow separation during flight. The downside to using this fixed wing design is that its effectiveness and lift to drag ratio are greatly reduced as its angle of attack (“the angle between the upstream flow and the axis of the object” [16]) increases. This is caused by the increased amount of flow separation that occurs at higher angles of attack [17].

Therefore, at that

operating condition, the power needed to maintain stable flight goes up. If wind gusts become strong enough during flight, they could tip the MAV into a high angle of attack causing it to stall. If the engine cannot produce enough power to stabilize the vehicle, the MAV will spiral out of control. For average wind gust conditions, the unsteady effects on the vehicle are too chaotic for even a skilled pilot to control. Therefore, a fixed wing MAV requires an effective control system that can automatically adjust for these unsteady effects, leaving the option open for manual pilot control [18]. The effectiveness of the flying wing configuration is greatly reduced as its thickness increases. Therefore, using the wing itself as the fuselage to house any payload is not a viable option. Furthermore, attaching the fuselage directly to the underside of the wing also reduces the wing’s effectiveness. A possible alternative is to raise the wing above the fuselage shown in Fig 3.1 [7].

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Figure 3.1 Fixed wing MAV with raised wing. Adapted from Design of a Micro Air Vehicle (MAV) [7].

This minimizes the blockage caused by the fuselage which maximizes the effectiveness of the wing. By using a fixed wing design of this type, the negative effects of low Reynolds number flight can be reduced to acceptable levels. However, since the fixed wing design is so sensitive to fluctuations in its operating environment, it may not be the most suitable for close quarter, tight maneuvering applications.

3.3 Morphing Wing The morphing wing design offers improved stability over the fixed wing design. However, less information is available about its flight characteristics.

A flying wing

configuration employing the use of a morphing wing performs better than one using a fixed wing. Due to its flexibility, the morphing wing can dynamically cause a natural change in its camber during flight to adapt to varying degrees of unsteady effects. This dynamic change limits the flow separation and reduces drag, allowing the wing to operate at higher angles of attack than

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its conventional fixed wing counterpart. Furthermore, the reduction of separation at high angles of attack produces considerably better lift to drag ratios as shown in Fig 3.2 [17].

(a)

(b)

Figure 3.2 (a) L/D, lift to drag ratio, fluctuation as a function of time for a fixed wing. (b) L/D, lift to drag ratio, fluctuation as a function of time for a morphing wing. Notice that the morphing wing has a higher average L/D value. Adapted from Flapping and Flexible Wings for Biological and Micro Air Vehicles [17].

The result is that morphing wing MAVs are capable of achieving higher angles of attack without stalling. This makes them better candidates for close quarter flight. Morphing wing designs are also capable of maintaining excellent flight stability due to their natural dynamic deformation. Ifju [18], with his design shown in Appendix A, Fig. A, showed that remotely piloted morphing wing MAVs are possible without the use of an automated stability control system. The MAV was naturally stable enough to maintain a steady, real time image on the monitor. Using a morphing wing in place of a fixed wing in a flying wing configuration also allows the fuselage to be attached directly underneath the wing. Ifju [18] attached a streamlined fuselage to the underside of the wing, as can be seen in Appendix A, Fig.

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A(b) through Fig. A(d). The fuselage was able to hold the camera used for remote piloting without any problems. Due to its ability to perform at higher angles of attack, the use of a morphing wing provides better performance than its fixed wing counterpart and is the better choice for close quarter, tight maneuvering applications.

3.4 Flapping Wing The flapping wing design offers more maneuverability than both the fixed wing and morphing wings designs. However, the understanding of flapping wing aerodynamics is still in its infancy. The flapping wing design uses two wings in a fuselage/wing configuration that oscillate up and down in a sinusoidal pattern. Unlike the previous designs, this design uses the wings as the propulsion mechanism as well as the lift mechanism. The wing oscillations can be separated into two distinct parts: the down stroke and the up stroke. The down stroke produces lift and thrust, and is the most effective part of the flapping cycle [17]. There is a tradeoff between the amount of lift and the amount of thrust generated. This can be controlled by the angle of attack of the wings. As the angle of attack increases, more thrust is produced at the cost of lift. In order to effectively control the thrust and lift, the wing must be deformable so that it can twist into the appropriate position for the desired conditions. Similar to morphing wings, flexible wing flappers perform far better than fixed wing flappers because the wing deformation allows them to naturally adapt to wind gusts to provide stabilization. With high enough flapping frequencies and active wing deformation by an automated control system, the flapper could achieve stable hover. This would give a flapping wing MAV superior close quarter maneuvering ability. However, this technology may not be available for many more years during which flapping wing research will mature.

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A simple flapping wing MAV, called the Microbat, was developed by Keennon and Grasmeyer [19]. The Microbat was built in a fuselage/wing configuration with an added tail to provide stability as shown in Fig 3.3.

Figure 3.3 Side view of the Microbat MAV. Adapted from Development of the Black Widow and Microbat MAVs and a Vision of the Future of MAV Design [19].

While the Microbat did achieve stable flight, it is difficult to compare it to the previous design types because it did not incorporate an automatic or manual control system and was not designed to carry a payload.

Although flapping wing MAVs will have superior handling and

maneuverability in close quarters, the technology is too young to consider viable.

3.5 Recommended Wing Design Three different wing designs were considered for their potential use in a Micro Air Vehicle. First, the fixed wing design was considered because much is known about fixed wing flight. Next, the morphing wing design was considered due to its previous record of successful, stable flight.

Finally, while not much is known about it, the flapping wing design was

considered for its excellent potential in close quarter, tight maneuvering applications. While the

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flapping wing would be ideal for reconnaissance, the infancy of the technology and the lack of information about its aerodynamics make it an unsuitable candidate for near future implementation. With its advantage over fixed wing MAVs, the morphing wing design looks most promising. By combining its natural wing deformation ability with an automatic stability control system, a morphing wing MAV will be able to overcome unsteady flow effects. This will provide the user with superior maneuverability and recovery capability in moderately close quarter reconnaissance environments, as well as in open air flight. Finally, a morphing wing MAV is stable enough to allow for manual piloting via a visual display or it can fly autonomously via GPS and other auto-navigation technologies. Therefore, the morphing wing design is the best choice for a reconnaissance Micro Air Vehicle.

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4. MAV Control System Maneuverability and controllability are very important considerations in the design of a MAV for reconnaissance use. The pilot controlling the MAV from the ground must be able to use the MAV to perform the desired reconnaissance mission, while at the same time remaining unnoticed. While it is true that certain other aspects of the design, such as the type of wing and engine, help keep the MAV unnoticed, even with stealthy engine and wing choices, a lack of sufficient control of the MAV will result in a failed mission. The MAV’s very small size and light weight make it difficult to achieve this sufficient control. Wind speed and other atmospheric conditions remain at full scale, creating unstable conditions for such a small aircraft [20]. Its low Reynolds Number, which was described in detail in Section 3, also has a major impact on the ability to control the MAV [21]. These conditions make controlling the MAV a task for only very experienced pilots [20].

This is undesirable since most reconnaissance

missions that require the use of a MAV are spontaneous and there is not always an experienced MAV pilot available. The MAV device must be able to be piloted by an ordinary soldier [22]. If the MAV is so hard to control that it is impossible for it to successfully complete a mission, then it is not worth the money to invest in this type of system for reconnaissance. The purpose of this document is to propose a design for the MAV that is worth the money. Other considerations must also be taken into account when designing a suitable control system like the system configuration and the type of input control by the pilot. The following section will evaluate the different types of control systems currently available for MAVs and suggest a plan for a control system design that could be commanded by a common soldier and suitable for a reconnaissance mission.

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4.1 Control Unit Currently, there has not been much research in designing the actual control unit for the MAV device, at least not one that could be easily used in a military reconnaissance operation. The bulky systems used today have many large components that greatly reduce their practicality for use in a reconnaissance mission. A new unit in the research stages is solely contained inside a backpack and has the potential to benefit the MAV’s use in a reconnaissance mission.

4.1.1 Multiple Component Unit The systems of today usually consist of a laptop or desktop computer, a joystick or controller, a receiver, and a decoding device if it is needed, depending on the type of reconnaissance. If the MAV is detecting chemicals or tagging for smart bombs, the decoder relays that information to the computer. The computer is the main information storage device; it sends the RF signal from the pilot and receives the information from the video receiver. The pilot uses the computer screen, which shows exactly what the camera on the MAV sees, to visually control the flight path using the joystick, much like a video game. In the case of GPSbased flight, the pilot would not so much rely on the camera picture; he would instead input his destination into the computer and then simply view the video afterward. GPS-based flight will be discussed more thoroughly later. The receiver simply collects the video information that the MAV device sends back to the computer as shown by the schematic in Figure 4.1 [21]. This unit also requires electricity as well as a suitable shelter to house the entire layout.

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Figure 4.1 MAV Control Unit. Adapted from Control System: Block Diagram [21].

4.1.2 Backpack Unit Due to the large nature of this current system, a few institutions, mainly under DARPA’s supervision, have begun research on a control system that conveniently fits inside a soldier’s backpack. This requires no additional control stations that must be moved around with the soldiers, and also allows for easy and fast deployment at any time. The whole unit is one piece and is powered by a battery that is charged by a small 500 Watt gas turbine. This is very beneficial because the soldier can carry a days worth of fuel for the charger in his backpack along with the system and the entire pack only weighs about 10 pounds [23]. This system, however, seems more geared toward GPS-based navigation than vision-based. The system is so small that the video view would be very hard to see making the MAV hard to control. Since the GPS-based navigation system has not yet been fully developed, this backpack system is not fully functional either. However, the benefits that this system can provide when the research and development is completed will allow for much easier use of a MAV device in a reconnaissance situation.

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4.2 Navigation System Navigation is also an important issue in the control of the MAV. There are two main types of navigation in the research process at this point in time. The most widely used type of navigation is vision-based. With this system, the pilot controls the MAV using the actual video transmitted back from the MAV. The other type of navigation is GPS-based. In this case, the pilot controls the MAV by inputting GPS coordinates.

4.2.1 Vision-Based Navigation Vision-based control of the MAV is currently the most widely used type of navigation system. This is mainly because it is the easiest type of navigation to implement. The video image transmitted from the on board camera of the MAV is received by the computer and displayed on the screen. A joystick is used by the pilot to control the MAV based on what he actually sees on the screen, much like a video game flight simulator [24].

This form of

navigation on its own is very unreliable because the images from the cameras tend to lack depth and peripheral vision. In an enclosed area, such as a building, this lack of perception could mean the difference between hitting a wall and successfully completing the mission. Maneuvering the MAV in open areas is almost as difficult as maneuvering it inside a building, especially at night. The MAV is so small and lightweight that the slightest wind can force the MAV into an undesired movement [22]. The pilot’s only way of keeping the plane under control is therefore to overcompensate based only on what he can see on the computer screen. For an amateur pilot, or for instance a soldier using the MAV, this compensation could be a very difficult task.

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Recently the University of Florida has been researching a way to improve the visionbased navigation system of the MAV.

Their new system is called the Horizon-detection

algorithm. The algorithm works to help the pilot adjust the MAV’s two most important degrees of freedom: pitch angle and bank angle, measured using the camera mounted to the MAV. The algorithm projects a straight line onto the camera image representing the horizon line. The computer determines this line by distinguishing between sky pixels and ground pixels. Since sky pixels are usually blue or grey colored, and ground pixels are darker greens, browns, or black, the computer calculates where on the screen there is an average change between sky and ground pixels and then projects a straight line onto the screen. Using this line, the computer can calculate the MAV’s two angles. Depending on what those values are, the computer can then tell the MAV what to do in order to compensate for an unbalance [25]. Figure 4.2 shows examples of Horizon-detection.

Figure 4.2 Horizon-Detection Examples. Adapted from Horizon-Detection Examples [21].

After the Horizon-detection algorithm was tested, the results showed that the algorithm significantly helps to compensate the MAV bank and pitch angles. The graphs in Appendix B compare the self-stabilized flight using the Horizon-detection algorithm to the normal vision-

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based piloting. The center line of the graph is completely stable, and the further the graph moves from the center line, the more unstable the MAV becomes. The results show that the Horizondetection algorithm greatly increases the stability of the MAV. There are some downfalls to this system, however. The first is that the algorithm does not work at night because ground and sky pixels on the camera image appear virtually the same. Another is that the pilot must still control the two hardest aspects of flight without the help of the system, take off and landing [25]. .

4.2.2 GPS-Based Navigation The new technology for navigation of MAVs is GPS-based. This type of navigation is much easier to use, but requires a bit more research before implementation. In the GPS system, the pilot inputs the various minor destination locations, called waypoints, into the computer using specific latitude, longitude, and altitude coordinates until the final destination is reached. The pilot will then input what task the MAV will perform when it reaches its destination, for example, taking video. Currently, research institutions such as the University of Florida are working to develop the GPS-based navigation system so that eventually the MAVs will fly autonomously (without human intervention) [26]. The GPS-based navigation system integrates a small on-board computer into the MAV’s design. This computer constantly records data about the current state of the MAV. Two small sensors added to the design send this information to the computer. The GPS sensor collects the location data from the satellite, and the 3D positioning sensor detects potential obstacles ahead that may block the fight path and atmospheric data such as wind speed and direction using ultra wideband radar (radar comprised of short pulse emissions) [27].

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the data to calculate the best path to reach the next waypoint. Since the aircraft must remain stable at all times to avoid being noticed or destroyed, the 3D positioning sensor must relay data to the on-board computer very quickly. A speed of 30Hz was determined to be optimal for maintaining stable flight.

A wireless data transceiver communicates between the MAV

computer and the computer on the control unit. No external antenna is required because the system is integrated with an internal ceramic patch antenna. This allows a lower wind resistance factor. The on-board computer is an Atmel-based computer with an 8-bit processor, 128kb of Flash memory, and 4kb of SRAM [26]. The computer can be seen below in Fig. 4.3.

Figure 4.3 On-Board GPS Computer. Adapted from Figure 3 MAV Atmel-based Computer – MAV 128 [26].

The GPS-based navigation system has the potential to be very beneficial as a method to autonomously control the MAV while at the same time maintaining stability. In reconnaissance missions the pilot may want to see what is going on all around instead of simply inputting the waypoints and waiting until the mission is over [28]. Some very important information along the way could be bypassed. Therefore, further research is needed to integrate the vision-based system with the GPS-based system in order to develop a MAV that works exclusively and flawlessly for a reconnaissance mission.

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4.3 Recommended Reconnaissance System An ultimate design for the control system of a reconnaissance MAV has been developed that is superior to currently available systems. The design was based on weighing the pros and cons of both the control unit and the navigation system in order to determine a MAV design that is best suited for a reconnaissance mission.

4.3.1 Control Unit The control unit is mainly comprised of the backpack unit. Since it will be integrated with the GPS-based navigation and vision-based navigation systems, discussed in section 4.2.2, the backpack system will work best. A soldier will be able to carry the whole system in his backpack and deploy the MAV at any given time when certain reconnaissance information is needed. The only addition to the backpack system is a laptop computer which will connect to the system and provide a constant video feed to the pilot. The addition of the laptop does not affect the size of the system very much, since it only weighs a few pounds and can be put into the backpack with the rest of the system when the mission is completed. This control unit will be the most effective for use in a reconnaissance system with integrated vision and GPS-based navigation.

4.3.2 Navigation System As mentioned in section 4.3.1, the control system for the reconnaissance MAV will integrate the vision and GPS-based navigation systems. This is the most practical for this type of mission since a video feed is desired at all times [29]. The GPS-based system will work as described. The pilot will input waypoints, and the onboard computer will work with the sensor

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to keep the MAV constantly stabilized and always searching for the next waypoint. The addition of the vision-based system will greatly increase the reconnaissance potential of this type of MAV. With the constant video feed, the pilot will always be able to see what is going on around the MAV, and in case there is something that the sensors do not pick up, he can record what he has seen, or even take control of the MAV. The laptop will be equipped with the horizondetection system and the pilot, if needed, will be able to take control of the MAV. This is essential if the MAV gets into an area where the sensors may not be able to find a way out, such as a corner inside of a building. When the pilot takes the controls, the vision based navigation system will kick in and override the GPS system. However, not all of the GPS system will be shut down. Only the GPS sensor that forces the MAV to constantly search for the next waypoint will cease. The 3D positioning sensor will continue to accept data from the surrounding area and stabilize the MAV. Therefore, the pilot is only concerned with the direction of flight. He can control forward, left or right, and up or down. The 3D positioning sensor will take care of the rest of the flight. This way, an experienced pilot will not be necessary and the user can still receive optimum performance from the MAV for a reconnaissance mission.

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5. Conclusion The various components of a MAV for use in a reconnaissance mission discussed were the engine, the wings, and the control system. After assessing the various options, a suggested design for each part of the system was presented. The design will now be proposed as a whole in order to request funding for research, development, and implementation of this design for military reconnaissance applications. As mentioned in Section 2, the engine chosen is an electric motor/battery propulsion system with an optimized propeller, because of its reliability and efficiency. The motor is a brushless Neodymium Iron Boron magnet motor using a non rechargeable lithium battery to allow the MAV an average run time of approximately 30 minutes. The propeller will be a minimum size of three inches but not larger than five inches. The wing design chosen in Section 3 is the morphing wing. This will provide the user with superior maneuverability and recovery capability in moderately close quarter reconnaissance environments, as well as in open air flight. The morphing wing MAV is stable enough to allow for manual piloting via a visual display or can be flown autonomously via GPS, which leads to the final aspect of the design. The proposed control system is an integration of the vision-based and GPS-based navigation systems compacted into the backpack control unit. This will allow for the best stability and ease of control, but will also allow the pilot to remain in contact with the MAV, which is necessary for a reconnaissance mission. After further research, this design will yield a highly effective reconnaissance Micro Air Vehicle for use in battlefield environments. Investments in this design will not only further

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advance the knowledge of small scale flight, but will also prove to be indispensable to military commanders.

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APPENDIX A: Additional Wing Design Figures Figure A

(a)

(b)

(c)

(d)

Figure A Morphing wing MAV shown at different angles. (a) Top down view. (b) Underside view. (c) Front View. (d) Side View. Adapted from Flexible-Wing-Based Micro Air Vehicles [18].

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APPENDIX B: Additional Control System Figures Figure B

Figure B Human Piloted vs. Self Stabilized Horizon-Detection Flight. Adapted from Human-piloted vs. Self-Stabilized Flight [27].

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Glossary Angle of Attack - “The angle between the upstream flow and the axis of the object” [16]. Autonomous – A way to control the MAV without any human intervention other than inputting the destination and objectives Boundary Layer - A thin layer of fluid, inside which friction forces play a large role. Electromagnet – A magnet made of a coil of wire around an iron core. When current flows through the coil the iron core becomes magnetic. Energy density - the amount of energy stored in system per unit volume. Fixed Wing – A stiff, non-deformable wing. Flapping Wing – A pair of wings that oscillate up and down in a sinusoidal pattern, like a bird’s wings. Flying Wing Configuration – Airplane design which combines the wings and fuselage into one large wing shaped aircraft, like the US Military B2 Bomber. Fuselage/Wing Configuration – Airplane design where the two wings are connected to either side of the center fuselage, like a passenger plane. Glow plug – Takes the place of a spark plug in a standard engine and ignites the fuel by means of high temperatures. GPS-based navigation – A type of navigation system for the MAV that uses GPS coordinates to determine the destination for the reconnaissance mission objective Laminar Flow – Smooth, or layered fluid flow. Morphing Wing – A flexible wing that can deform during flight. Power density - power per unit area. Propulsion – A driving or propelling force Thermoelectric-generator (TEG) – A means of converting heat energy to electrical energy without any moving parts. Turbulent Flow – Chaotic or highly fluctuating fluid flow.

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Two stroke engine – A valve-less internal combustion engine which produces more power and weighs less than a conventional four stroke engine. Ultra-Wideband Radar - type of radar used on MAVs that emits short pulses to receive information on certain movements and atmospheric conditions going on around the MAV Viscous Effects - Impeding forces applied on the vehicle by its resistance to flow. Vision-based navigation – A type of navigation system for the MAV where the pilot uses a computer screen display to control the MAV. This system is much like a flight simulator video game. Waypoint – A checkpoint in GPS-based navigation that is used to guide the MAV to its final destination where the mission objective will be performed.

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[12] Thomas, J., Qidwai, M., Matic, P., Everett, R., Gozdz, A., Keennon, M., and Grasmeyer, J., 2002, “Multifunctional Structure-Plus-Power Concepts,” Proceedings, 43rd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, Denver CO., AIAA-2002-1239. [13] Holt, D., and Kozak, J., 2004, “Design Fabrication, and Testing, of a Miniature Impulse Turbine Driven by Compressed Gas,” Proceedings, 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Fort Lauderdale, FL., AIAA-2004-3750. [14] Roberts, C., Vaughan, M., and Bowman Dr. W., “Development of a Solar Powered Micro Air Vehicle,” Proceedings, 40th Aerospace Sciences Meeting and Exhibit, Reno, NV., AIAA-2002-0703. [15] Fleming, J., Ng, W., and Ghamaty, S., 2004, “Thermoelectric-Based Power System for Unmanned-Air-Vehicle/Microair-Vehicle Applications,” Journal of Aircraft, 41, pp. 674-676. [16] Munson, Bruce R., Young, Donald F., and Okiishi, Theodore H., 2002, Fundamentals of Fluid Mechanics, John Wiley and Sons, New York, New York. [17] Shyy, Wei, Berg, Mats, and Ljungqvist, Daniel, 1999, “Flapping and Flexible Wings for Biological and Micro Air Vehicles,” Progress in Aerospace Sciences, 35, pp 455-505. [18] Ifju, P.G., Jenkins, D.A., Ettinger, S., Lian, Y., and Shyy, W., 2002, “Flexible-Wing Based Micro Air Vehicles,” AIAA Paper 2002-0705, 40th AIAA Aerospace Sciences Meeting and Exhibit, Reno, Nevada.

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[23] Creative Energy Concepts, 2003, “Defense Outlook,” (Internet), (cited 22 November 2004), available from World Wide Web: . [24] Jenkins, David A., Ifju, Peter G., Abdulrahim, Mujahid, and Olipra, Scott, 2001, "Assessment of Controllability of Micro Air Vehicles," Proc. Sixteenth Int. Conference on Unmanned Air Vehicle Systems, Bristol, United Kingdom. [25] Ettinger, Scott M., Nechbya, Michael C., Ifju, Peter G., and Waszak, Martin, 2002, "Vision-Guided Flight Stability and Control for Micro Air Vehicles," Proc. IEEE Int. Conf. on Intelligent Robots and Systems, 3, 2134-40. [26] Grzywna, Jason W., Plew, Jason, Nechyba, and M.C., Ifju, P.G., 2003, "Enabling Autonomous MAV Flight," University of Florida, Gainesville, FL. [27] Fontana, Robert J., Richley, Edward A., Marzullo, Anthony J., Beard, Lance C., Mulloy, Robert W.T., and Knight, E.J., 2002, "An Ultra Wideband Radar for Micro Air Vehicle Applications," Proc. IEEE Conference on Ultra Wideband Systems and Technologies, Baltimore, MD, May 2002. [28] Lawlor, Maryann, 2003, “Miniaturization, Networking Pervade Future Unmanned Systems,” Signal Magazine. [29] Webb, T., Prazenica, R., Kurdila, A., and Lind, R., 2004, "Vision-Based State Estimation for Autonomous Micro Air Vehicles," AIAA-2004-5349, AIAA Guidance, Navigation, and Control Conference and Exhibit, Providence, Rhode Island.

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Author Qualifications Daniel Dell is a senior Mechanical Engineering student at the University of Maryland. He is very familiar with internal combustion engines including two stroke glow engines found in MAV applications. His experience and education provides a sufficient background for MAV propulsion systems.

Alex Macleod is a Biological Resources Engineering student at the University of Maryland. His courses in electronics and some of his mechanical engineering electives have allowed him to understand the aspects of the electronic devices. He used this knowledge and adapted it to researching a suitable control system for the MAV, and proposed a design for the best system for use in a reconnaissance MAV.

Yaron Mordfin is a Mechanical Engineering student at the University of Maryland. Through his course in fluid mechanics this semester, he became proficient in the fundamentals of fluid flow over bodies. He was able to use this knowledge to research the various wing designs currently being tested for use in Micro Air Vehicles. Thus, he was able to present an argument based on existing technology for the most promising wing design.

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