Portable Military Quadcopter

Northrop Grumman Advisors: Michael Kariya, Michelle Kao Vanderbilt University School of Engineering Team: Claire Benjamin, Alex Browne, Michael Burkard, Zachary Korman, Tyler Smolen

Table of Contents

1. Introduction and Background 2 a. Problem Statement 2 b. Design Constraints 2

2. Design Choices and Justification 2 a. Why Quadcopter? 2 b. Competing Products 4 c. Justification for Portable Military Quadcopter Design 6

3. Final Design Improvements 6 a. Frame 6

i. Materials Selection 7 ii. Composite Quad Arms 8 iii. Body Panels 11 iv. Payload Adapters 12 v. Landing Legs 13

b. Parachute Implementation 16 c. Flight Modes 21

4. Testing and Analysis 26 a. Propeller Lift Testing 26 b. Frame/Load Testing 28 c. Battery Testing 32 d. Flight Testing 33 e. Parachute Testing 33

5. Bill of Materials 38 6. Budget 40

a. Cost Total 41 b. Hour Total 41

7. Conclusion 42 a. Design Evolution 42 b. Strengths of Portable Military Quadcopter Design 45 c. Recommendations 46 d. Implementation Plan 48

8. Appendix 49 a. Table of Contents 49

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1. Introduction and Background: Preventing Communication Breakdown

During field operations, military personnel use radio devices to communicate when they need assistance. However, mountainous terrain or urban structures can sometimes block these radio signals, resulting in unheard distress calls. Northrop Grumman, a global aerospace and military defense contractor, challenged our VUSE design team to formulate a solution to this communication breakdown. Problem Statement

Create a device that enables communication regardless of landscape by carrying a relay to a height sufficient for line­of­sight communication. Design Constraints

The device must be… ­ Capable of lifting a payload of no less than 1 kg ­ Man­portable ­ Capable of maintaining its position in space within a 10 meter radius error margin ­ Reusable and cost no more than $500 ­ Capable of altitude adjustment ­ Capable of position control ­ Able to trigger a backup parachute in the case of system failure ­ Relatively quiet

Note: No official lifetime constraint was provided. The necessary lifetime will be a function of how high the quadcopter must go and how long the communication takes. Reasonable order of magnitude estimate: 5­10 minutes

2. Design Choices

Why Quadcopter?

After considering various altitude­gaining methods, a quadcopter proved to be the most effective device to use. It satisfied all the initial design constraints: it has the ability to maintain its position in space, even in relatively strong winds, to the precision of the onboard GPS unit (which is within the given 10m radius sphere), and it can reach and maintain altitudes in the ranges necessary. It can be designed to fit the requirement of “man­portable” as well as to carry the necessary payload. With the implementation of a rechargeable battery, the device is reusable, and with the use of GPS waypoints and semi­autonomous control, the device can easily be controlled. A quadcopter is also conducive to the creation of a custom frame that can fold into a compact storage position small enough to fit into a standard military­issue packpack.

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The other major alternatives we had been considering were a balloon, a rocket, and a balloon­quadcopter hybrid. The balloon idea was eliminated first, as it would have involved floating a platform with the communication software under a helium balloon. This design would require the operator to carry a bulky helium tank, and the balloon would need to be large in order to remain buoyant in the atmosphere with its 1 kg payload. Initial estimates using a high­tech balloon with helium yielded balloon diameters exceeding 2 ft, which would likely be visible in the sky given its associated volume. Most importantly, a balloon would not be able to maintain its position within the given error margin. With the balloon tethered, only the distance to the user would be fixed; any wind at all would cause drastic and unfixable deviation from the desired location, both in horizontal position and altitude depending on the wind’s force compared to the buoyancy.

The second vetoed alternative was a rocket­payload system in which the communication relay hardware would slowly descend via parachute after the rocket reached a maximum design height. This alternative was eliminated as a rocket has little active control after launch and would therefore be at the whim of cross­winds. It would be loud and visible, and the exhaust could be seen by the naked eye or by an infrared camera. It would not be able to maintain its location within the error margin, as the whole system would be slowly falling the entire time, unable to maintain its altitude. The rocket itself would be relatively heavy and difficult for a user to transport, and its reusability would be low; not only would it be difficult to recover the rocket body and communication software after landing (possibly in unscouted and/or dangerous territory), but even if it were recovered, it would need to be refueled and repackaged after each use.

The third eliminated alternative was a balloon­quadcopter hybrid in which the buoyancy of a balloon would provide most of the lift to hover, while a low power quadcopter would provide the remaining lift to rise and hover. By using this design, the lifetime could be extended by a large factor if the motors were not supplying the majority of the lifting power. Additionally, if the system’s power was cut off, the terminal velocity of the system would be very low due to the balloon’s buoyancy and drag. Furthermore, the ability to maintain position in space would be similar to that of a stand­alone quadcopter, and it would be quieter, with less noise from propeller speed. The disadvantages, however, still outweighed the benefits of the stand­alone quadcopter. The problem of carrying around a helium tank remained, and adding a balloon to a quadcopter would increase the complexity of the system and the number of possible failure points to an unacceptable level; whereas, quadcopters are known to work reliably and effectively. Construction vs. Modification

Following the selection of a quadcopter, a decision had to be made between modifying an existing quadcopter and building one from scratch. We decided to build our own quadcopter based on existing ones known to be able to support the target payload and attain the appropriate

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altitude. Building allowed for the implementation of motors, propellers, and other parts of the exact desired specifications, as well as the creation of a unique, foldable frame for storage. Additionally, this choice allowed us to test different parts in certain areas of the design to provide justification for the selection of those parts.

Competing Products

As is commonly known, the military has increasingly used unmanned technology to allow more complex operations to be performed without putting a human in harm’s way. Despite this trend, no portable, individually­deployed product is currently used for the specific purpose delineated in our given problem statement. However, quadcopters similar to our design are commonly used for surveillance and reconnaissance purposes when a small, quiet device is required that has the option to remain still in the air as well as maneuver around or above potential obstacles. These quadcopters are also capable of being fitted with custom payloads.

1. Aeryon Scout™

The Aeryon Scout™ is produced, along with the larger and more robust Aeryon

SkyRanger™, by Aeryon Labs Inc. and features some incredible feats of design. From its data sheet, it has an operational range of 3 km or up to 25 minutes, and can fly up to 450m above the ground. It weighs, without a payload, 1.4 kg and is made from predominantly polycarbonate. It is fully GPS waypoint programmable and features a smart touchpad interface as a transmitter, which also displays real­time camera data. The camera may be swapped out for a custom

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payload. The arms of the quadcopter pop out for storage and the battery pops out for quick replacement. The more robust model, the SkyRanger™, folds downward into a compact design that fits into a custom backpack. The plates underneath the propellers and on the side of the main body are actually fins meant to regulate internal temperatures. Although these quadcopters feature characteristics that resemble our chosen design, ours is likely an order of magnitude (if not two) less expensive, is lighter, and likely can support a heavier payload relative to its own unencumbered weight. Sources: http://www.dtwc.com/sites/default/files/datasheet/Datron_Unmanned­Solutions.pdf

https://vimeo.com/58547229

2. XAircraft X650 Pro

The XAircraft X650 Pro is another quadcopter that has potentially been used for military purposes. It has a flight time of 20 minutes and a maximum weight (when encumbered with a maximum payload) of 3 kg. Its arms fold horizontally into a slightly more portable position for

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storage such that they look like an “H” when viewed from the top. This quadcopter as shown can be fitted with a Hero™ camera, and is priced at roughly double the cost of our design. Sources: http://www.xaircraft.com/products/x650­pro­2/ http://download.xaircraft.com/manuals/XAircraft%20X650%20Pro%20User%20 Justifications for Portable Military Quadcopter Design Over Competitors

The quadcopters discussed above represent two of the three categories of quadcopters. The first is the stunningly capable and stunningly expensive military­grade quadcopter, the second is the moderately priced but well­engineered quadcopter. The category not represented above is the cheap and cheaply manufactured quadcopter for short duration consumer use. Our design is a member of the second category due to various features such as folding arms for portability and storage, quick battery exchange, and payload­ready lift characteristics, but is unique in that it does not cost nearly as much as quadcopters with similar performance characteristics. On the basis of reduced cost, our design is the most useful for Northrop Grumman and for the U.S. Military in this specific application.

3. Final Design Improvements Frame

As is clearly highlighted in figure 1, there have been many improvements made to the frame for the final prototype of the Portable Military Quadcopter design from previous iterations.

Figure 1: Final Prototype of the Quadcopter

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Materials Selection

Leading up to this final prototype, a comprehensive materials study was performed to obtain strength­to­weight ratio data (see figure 2) for various potential materials. After some further analysis and discussion, medium density fiberboard (MDF) was removed from the study. Not only was it the weakest material observed in this study by a significant margin, it also has a poor reputation for both moisture resistance and cracking. These negative aspects were proved in more recent testing of the flight modes, when upon hard landing, the quad arms had a tendency to allow very little deformation before cracking completely through. Therefore,, although MDF is inexpensive, it was not suitable for this application.

Figure 2: Strength­to­Weight Ratio of Various Materials

The next study involved strength­to­weight­to­cost ratio data (see figure 3). This study

was standardized between materials by using the same vendor’s pricings for similar geometries (in this case, the price of a 6x6x1/8” sample of the material). Although this method has some flaws, it was determined to be a good representation of overall strength and cost trends.

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Figure 3: Strength­to­Weight­to­Cost Ratio Data for Various Materials (note the logarithmic axis)

As seen in figure 3, Nylon (6/6), extruded Acrylic, and 6061 T6 Aluminum have the best

strength characteristics for their costs, with Aluminum winning by a slight margin. After some discussion of how to implement these newly­studied materials, it was determined that the best way to implement the aluminum was in very thin sheets for the quadcopter’s arms (see next section for assembly and fabrication details). Unfortunately, there was not a practical way to implement the same composition tactics on the body of the quadcopter. In the end, the choice between using the Nylon and the Acrylic was dictated by which was more readily available and which was more able to be cut using a laser cutter. After some research, it was determined that Acrylic laser cuts well, while Nylon has the tendency to melt badly around the laser­cut edges. Increasing the ease of the choice, ⅛” Acrylic was a supplied item, available for immediate use.

Later design choices led to the use of aluminum in a few other frame considerations. Aluminum was the primary material used for both the payload attachment connections and for the landing legs. Composite Quad Arms

In order to maximize the load­supporting characteristics of the sheet aluminum for the quad arm design, we decided to implement a composite arm comprising of two aluminum sheets sandwiching a rigid, closed­cell polystyrene foam (see figure 4). (Pink polystyrene foam is the most readily available color.) With this setup, during flight, the whole assembly would act as a homogenous piece of material, bending like a cantilevered beam. This type of deformation would allow the top sheet of aluminum to be near­purely in compression and would allow the

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bottom sheet to be near­purely in tension (assuming the force is directed upwards as lift from the motors).

Figure 4: Initial Composite Arm Idea

After failure testing this concept, (see testing section), it was noted that bolting down

components tightly over the foam caused rapid and extensive deformation. In order to retain the lightness of the foam and still allow for tightly bolting down, for example, the motor bracket to the arm, it was decided that acrylic spacers could be placed at both ends of the arm. It was also determined that it was neither practical nor necessary to machine the irregular shape into the aluminum where the motor bracket mounts (on the right in the above figure). From these considerations, the model shown in figure 5 became the final model for the composite arm (see Appendix for detailed, dimensioned drawings of the composite arm’s individual components).

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Figure 5: Final Composite Arm Model

Fabrication of four of these arms required the individual cutting and then adhesion of

acrylic spacers, aluminum sheets, and foam, The Acrylic spacers were first laser cut with center holes which were drilled out to the correct diameter. Three thicknesses of the ¼” Acrylic were then glued with Acrylic glue and stacked into groups of three to make a ¾” spacer. From there, since neither the foam nor the aluminum could safely be cut by the laser available for quick use, a model of the outermost rectangular dimensions of the aluminum was laser cut from MDF as well as a two copies of a similar model for the foam insert. The first model was used to trace out lines on a large sheet of 0.016” aluminum, selected because it is the thinnest available sheet before it becomes foil and easily tears with regular use. These lines were then used with a hydraulic press to cut out the models (eight in total).

From there, the eight nearly identical aluminum models were stacked, Acrylic spacers were lined up on top of them as they would be in the final product, and the whole assembly was tightly clamped. The laser cut Acrylic was used to locate the holes that needed to be drilled into the aluminum. After the holes were drilled through all eight models at once (to ensure each one had the same geometry), the aluminum components were bolted tightly together and taken to a belt sander to achieve the final, desired shape.

The two MDF models of the foam sample were sanded to have a smooth edge, and then a slightly oversized piece of polystyrene foam was placed between them. The smoothed MDF provided an accurate and precise form for a hot wire foam cutter to follow. After some careful cutting, four nearly­identical samples were produced in this way.

Finally, strong adhesive was applied to all surfaces while the correctly sized bolts and pins held them in alignment. In this way, days of preparation and fabrication produced four identical, tightly­toleranced composite arms (see figure 1).

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Because the polystyrene foam performed very well in compression but not well in tension, the entire composite arm was later tightly wrapped in electrical tape. The tape took the tensile loading and allowed the foam to remain predominantly in compression. Body Panels

The main body panels which house most of the electronics were laser cut from a sheet of ⅛” thick Acrylic. The basic geometry has not changed significantly from the previous iterations as that geometry has proven to be successful in safely fastening the low power and high power electronics, as well as in being light but sturdy (see figure 6). For the Finite Element Analysis (FEA) of the Acrylic body, see the Testing section of this report, and for detailed, dimensioned engineering drawings, see the Appendix.

Figure 6: Model of the ⅛” Acrylic Body with Spacers Instead of Arms

After some quick drilling and preparation after laser cutting, the body was ready to have

components attached (see figure 7).

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Figure 7: Body Panels Shown with Pre­Fabricated Arms

Payload Adapters

One important, but anaddressed, topic throughout the design, building, and testing of the prototypes was exactly how the unspecified payload would attach to the quadcopter. Up until this final prototype, testing was always conducted using sturdy rope to tie on a payload hung underneath the prototype. Finally, however, a method of attachment was decided upon involving an aluminum L­bracket and a bent aluminum sheet clevis U­bracket (see figure 8).

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Figure 8: Payload and adapter to quadcopter body

Though not incredibly robust, this design was quick and easy to manufacture and suited

our purposes. The design necessitates that the payload has the correct U­bracket adapters, but it was decided that using this type of interchangeable design allows for easy adaptation to any payload size. Since the exact hardware used to solve the problem given in the problem statement would need to be a product designed specifically for this task, it was important not to design a payload bay that could potentially be too small in one dimension or another. This design is also beneficial because this way, any payload can be attached regardless of size (within the limitations of the quadcopter’s lift and landing leg length), whether it be communication hardware or a surveillance camera of sorts. Landing Legs

The design and fabrication of the landing legs was driven primarily by applying continual changes to an original, unspecific idea, undergoing extensive modification to fit what was feasible and practical. The idea started out as shown in figure 9. The legs could be hinged about a mounting point on the body such that the landing forces would be transmitted up to the

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quadcopter through where the arms bolt to the body, which is the strongest point on the design. The hinges would be high friction so the arms would either stay deployed or stay folded away, and there would be a hard stop to ensure the legs would provide actual support for landing procedures. In this way, the landing legs could fold up to a very compact position for storage, promoting the portability of the design.

Figure 9: First Idea for Landing Leg Fabrication

This idea was tested first. Readily available galvanized steel wire was used as the pin,

and aluminum sheet metal, with a thickness of 0.032” for better strength characteristics, was used as the leg and the inboard attachment plate by bending a small section of it around the steel wire. Upon clamping down the portion of the aluminum bent over the wire to promote a friction hinge, it was quickly determined that not only was the idea was not viable, but also that the quadcopter’s vibration would likely cause the legs to sag down to vertical, rendering the entire setup prone to collapse.

The second iteration of the idea involved a similar concept, but instead of a high friction hinge, a spring­loaded pin joint would provide the resistance to sag. The same basic concept of bending the aluminum around a pin was used; however, room was left in the center for a light­duty torsional spring. Instead of the steel wire acting as the pin, a size 10 bolt was used instead, since it better fit the center diameter of the torsional spring and therefore kept the twisting forces from interfering with the action of the spring (see figure 10).

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Figure 10: Landing Leg Hinge

This idea, when implemented properly, provided a surprisingly effective spring­loaded

hinge. However, when the setup was mounted to the body, it became clear that the hard stop mechanism was not going to be able to function as well as designed. It was decided, instead, to use a double­threaded string on the other side of the hinge to act as a hard stop for tension. This way, the resting angle of the deployed legs could be adjusted easily by changing the string length before the final knot was tied.

This redesign worked well, and as long as the angle of the deployed legs was close enough to vertical, but not so close that it would risk collapse, the stings don’t experience very much force from supporting the assembly. Upon attempting to mount the design, it was found that there was nowhere feasible on the body itself to mount the landing legs that would allow them to fold up neatly without interfering with other parts of the assembly. Luckily, the strength of the new composite arms was much higher than the old MDF arms, so the design team felt safe

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allowing the arms to support both the weight of the quadcopter in flight and the weight of the quadcopter while grounded. This also allowed for the legs to fold up against the arms in a way that was convenient and compact, and did not add anything to the overall folded length, width, and height. The legs were held closed with medium strength hook­and­loop fastener to avoid unwanted deployment while stored (see figure 11).

Figure 11: Final design and placement of the landing leg showing the hook­and­loop

Parachute Implementation

True to the given design constraint, our quadcopter is equipped with a parachute that can be deployed automatically and manually as a safety mechanism in case of system failure. The parachute is folded in a plastic container rested on an elevated platform above the flight controller. It is held down by a rubber band attached on each side by a solenoid pin and servo motor.

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Figure 12: Close­up of Parachute Deployment Mechanism

Automatic Deployment (Solenoid):

The solenoid is powered directly by the battery. When the battery is on, the solenoid acts

as a magnet, preventing the pin from releasing. If the battery dies, the solenoid is turned off, releasing the pin, and allowing the parachute to unfold. The solenoid only draws 2.92 W, as

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opposed to the 250 W drawn from each of the four motors, and thus, its effect on battery life is minimal.

Figure 13: Solenoid Mechanism for Automatic Deployment

Figure 14: Solenoid connection to battery

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Figure 15: Solenoid Piston removed

Manual Deployment (Servo):

The servo motor holds the other side of the rubber band down. Turning a knob on the

transmitter rotates the servo motor head up, releasing the rubber band and thus the parachute. This would be used if a part other than the battery malfunctions.

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Figure 16: Location of Servo Motor for Manual Deployment on Transmitter

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Figure 17: Servo head rotates up releasing the rubber band that is holding the parachute.

Testing of the parachute mechanism and deployment during flight will be discussed in the Testing and Analysis section. Flight Modes

The quadcopter is equipped with three flight modes: Manual, Loiter, and Auto. These modes can be controlled by a three position switch on the transmitter controller.

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Figure 18: Location of Flight Mode Switch on Transmitter

The quadcopter was tested on multiple days, but we were unable to make the flight

modes work. When switching from manual control to Loiter or Auto flight mode, the quadcopter would behave erratically and take off suddenly in a random direction. Initially, we thought the GPS was faulty. However, after configuring the Radio Telemetry, we could see on the phone that the GPS was accurate. By using other methods to troubleshoot, we came to the conclusion that the magnetic field generated by the ESC/Power Distribution Board was interfering with the onboard flight controller compass. Therefore, we added a thin aluminum plate under the flight controller as a shield to AC­generated magnetic fields and then tried flight testing again. This time, both Loiter and Auto flight modes worked well on the first try.

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Manual This flight mode is used to control quadcopter manually using the transmitter controller

joysticks. This mode is mainly used for manual takeoff and landing.

Loiter

In Loiter mode, the quadcopter maintains its altitude and position in space. Once the quadcopter reaches the desired altitude, the user would switch to Loiter mode while the quadcopter is sending its radio transmission through the communication device.

Auto

When switched to Auto mode, the quadcopter will fly along a preprogrammed path comprised of GPS coordinates and flight headings. The quadcopter can be commanded to start from a home position, fly to a specified height and location, and return after a set time. This flight mode almost completely eliminates the necessity of good piloting skills.

Figure 19: Picture of the GPS Waypoint setup at one of the Vanderbilt fields. In this setup, the

quadcopter will fly to the home position and then to Waypoint 1.

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Radio Telemetry After flying a certain distance, the quadcopter becomes difficult to see. Thus, radio

telemetry was implemented. Using radio telemetry, the GPS location and altitude can be tracked on a phone.

Figure 20: Setup on phone showing GPS and altitude settings

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Figure 21: Radio Telemetry USB connection to cell phone. Receives flight data from the

Onboard Radio Telemetry device.

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Figure 22: The onboard radio telemetry device sends GPS and altitude data to the Radio

Telemetry Receiver connected to the cell phone.

4. Testing and Analysis Propeller Lift Testing

For lift testing, the motor and propeller were fastened to the end of an aluminum extruded bar, which was vertically attached to a wooden block and the motor using slightly modified aluminum L­brackets originally meant to attach two aluminum extruded bars perpendicularly. The ESC, the battery, the battery wattmeter and voltage analyzer, and the receiver were placed on top of the wooden block. We obtained a scale, attached the test assembly to the platform of the scale, and then attached the scale securely to a workbench. We tested by turning on the system and slowly increasing the throttle to speed up the motor using the transmitter. After each small throttle increase, we used the wattmeter and voltage analyzer to read and record the current the motor was drawing and used the scale to record the resulting lift, in grams. We continued this process, collecting data until the motor was at full throttle.

The results of our lift testing showed a positive correlation between the current that the motor was drawing and the lift of the assembly, and the 12” diameter, 6” pitch propellers had the greatest lift:current ratio (see figure 23 below).

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Figure 23: Results of lift testing (for full data sets, see Appendix). Note: for

example, for the first data set, “12” denotes propeller diameter and “6” denotes pitch angle

We also explored carbon fiber propellers to test if they might be more efficient

than plastic propellers. However, the carbon fiber propellers showed no significant improvement in efficiency, as seen in figure 24 below.

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Figure 24: Carbon Fiber and Plastic Propeller Comparison

Frame/Load Testing

The testing of the composite aluminum­polystyrene­foam arm went successfully. As the aluminum used was almost as thin as can be purchased without risking immediate tearing by hand, the results were not expected to match simulation data. However, the aluminum performed better than expected. See figure 25 for a Solidworks FEA simulation that was performed on the composite arm under flight loading maximum conditions. As shown, the maximum Von Mises stress experienced in the aluminum sheet is about 15 MPa (megaPascals), where the yield strength of that composition and heat treatment of aluminum typically has a mean value of 275 MPa. So, assuming the assumptions made in simulation, this arm would perform far better than the MDF arm and likely far better than the alternatives. Unfortunately, not all the assumptions made by the simulation were valid. Along with the great load­bearing characteristics of the arm in simulation, it was also estimated to have a mass of 18 grams, nearly half of the mass of the MDF quad arms.

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Figure 25: Composite quad arm simulation under ideal flight conditions

After simulation, the arm was assembled. In order to adhere the “plys”, a 5 minute epoxy

was mixed and spread on the aluminum arms before firmly sticking them to the foam. After a day of curing, the arm was ready to test. At first, the same protocol was followed from when the first quad arms were tested: it was bolted down and sandwiched between two stronger pieces of plywood. Unfortunately, the foam rapidly gave way upon tightening the bolts. It was decided that this design could only be feasible if a small, more rigid component was placed at the base (and at the motor mounting point when flight testing occurs) to facilitate the ability to tighten the assembly down. After this modification, the arm was ready to test.

A bolted­in plywood insert was used to maintain rigidity at the mounting point, and the insert was then clamped down firmly to a table. Small notches were cut into the aluminum on the sides at the loading point so a string could be hung, with a basket containing known weights suspended on the other end (see figure 26). Under the basket, after 4 inches of room to allow deformation, was a stool to keep the weights from falling and potentially cracking. This was the same testing protocol used for previous failure tests.

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Figure 26: Failure Test Assembly for Composite Arm

Weights were then incrementally added to the basket, and the system was given time to

deform. We quickly noted that though this arm was lighter than the tested MDF arms, it deformed less at the same weights and very quickly surpassed the MDF arms in terms of maximum supported load. Finally, the arm failed after supporting 5.3 kg (see Figure 27), which not only is nearly double the weight supported by the MDF arm, but nearly double the entire weight of the quadcopter with payload!

Analysis of the quad arm yields interesting results. As seen in the same figure below, the arm failed through thin “column” buckling of the bottom aluminum ply in compression. This is where the simulation could not simulate properly. Since column buckling typically occurs at a slight, potentially imperceptible inhomogeneity in the material, the simulation cannot simulate the actual situation well enough to account for it. The real reason the arm failed, however, was the foam. Ideally, if the foam were harder and more rigid, buckling failure would be postponed to an even higher applied load, if not negated completely.

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Figure 27: Failure Mode of the Composite Arm

As it stands, even though the arm failed in an easily remedied way, it supported a far

greater load than would ever be experienced (with the exception of a crash) in common usage. Even so, it was recommended that a thin layer of crushed fiberglass and resin be applied to the sides. This would only add a gram or two to the weight of the arm, but significantly increase the rigidity of the arm, providing a better connection between the top and bottom aluminum sheets, and possibly delaying buckling failure in a way similar to how using a more rigid foam would. It was decided, however, that due to the overwhelming success of the arm as it was, it was not necessary to make major alterations other than in increase in width and a smoother foam cut.

The overwhelming success of the failure test of the composite arm lessened the importance of spending time to test other materials in a similar manner. Since the 6061 T6 aluminum had one of the best strength­to­weight ratios as well as the best strength­to­weight­to­cost ratio, we deemed it unnecessary to test other quad arm concepts through a similar process. As stated in the improvements section, ⅛” thick Acrylic was used for the main body panels. Below in figure 28, the deformation of the body under loading conditions is shown. After simulation, it was determined that operating conditions allow for a

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yield­stress­based factor of safety of above 10, and the main design constraint was deformation. The thickness shown, with deformation very exaggerated, causes under 2 mm of maximum deflection. This value was determined to be very reasonable and the body was then laser cut (see improvements section).

Figure 28: ⅛” Acrylic Body Concept Simulation

Battery Testing

To test the life of our Lithium­Polymer battery when the quadcopter must support a payload, we performed flight testing with a 1.1 kg payload. This mass exceeds the maximum weight required to be carried by the device. We recorded the initial amount of charge in the battery, then began flight testing. We began timing when the payload left the ground and stopped timing when the payload landed again. The charge used during transient periods of flight was considered negligible. The quadcopter was flown twice, once for 120 seconds then again for 60 seconds. Each time, the amount of charge left in the battery was recorded. As seen in Table 1, the battery had 35% charge drop after the initial 120 second flight and 15% drop after the second (60 second) flight.

Time (seconds) Charge Usage (% of overall charge)

120 35%

60 15%

Table 1: Initial Battery Testing Results

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For our purposes, the quadcopter only needs to remain in the air long enough to reach the desired altitude, allow the individual to broadcast a brief message, and return to the ground. Because of these stipulations, our estimated maximum flight length of about five minutes (without draining the battery to dangerous levels) can be considered successful.

Time (seconds) Charge Usage (% of overall charge)

100 40%

Table 2: Final Battery Testing Trial

For a final trial with the most recent frame, battery testing using the same protocol was performed on the encumbered quadcopter (see table 2). With all the additions to the final prototype, the battery life came out to be, using linear interpolation, approximately 4 minutes, which is on the low end of what is considered reasonable for this application. Flight Testing Stability as well as the functionality of the Loiter and Auto flight modes were tested in a large, open field. The quadcopter showed greatly improved stability with no notable oscillation after tuning the PID parameters. In order to test the loiter mode, the quadcopter was piloted to a comfortable location and altitude, then switched to the flight mode. After tweaking PID values and shielding the flight controller from magnetic interference, the loiter mode worked exactly as intended. The quadcopter did not waver or drift by any significant amount once it was switched to loiter mode. To test the auto mode, a simple flight plan was created which directed the quadcopter to start from a home position and fly to a specified waypoint. During testing, the quadcopter was able to successfully complete this procedure. Parachute Testing

The weight of our entire quadcopter, including the payload is about 2.6 kg. Our goal was to have the quadcopter fall no faster than 5 m/s. Using the drag coefficient calculator we calculated the theoretical size of our parachute.

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Figure 29: Weight of Parachute with 1 kg payload (black box)

Drag Force = 0.5 x density x velocity^2 x drag coefficient x area

Drag Force = Weight = mg = 2.6 kg x 9.8 m/s^2 = 25.48 N

25.48 N = 0.5 x 1.2 kg/m^3 x (5 m/s)^2 x 1.2 x area

Area = 1.42 m^2 = PI*r^2 r = 0.67 m

d = 1.34 m = 39 in We decided that we would buy a parachute bigger than 39 in and scale it down if necessary. Looking online parachutes cost around $20 and more. To save money, we decided to buy an umbrella, cut the nylon, and use it to make our own parachute. We decided to buy a 60 in diameter umbrella.

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We tested the umbrella by dropping a surrogate parachute frame with weights added to so that it weighed 2.6 kg. We first dropped it without the parachute and timed it to determine the height.

Figure 30: Dropping the frame without the parachute to determine the height We then dropped the frame with the parachute and recorded its descent. Afterwards we watched the video and found the time when the parachute was fully deployed (open), and from there timed the descent of the parachute.

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Figure 31: Picture of parachute drop. D2 is the point at which the parachute was fully deployed.

The time was measured starting at d2 (via replaying the video) and the d2 distance was calculated by measuring the ratio of the d2 and d1 red lines on the picture.

On average, the velocity of was 4.56 m/s from three trials (4.57 m/s, 4.32 m/s, 4.78 m/s). We did a total of 12 trials but the other trials were ones where the frame either excessively hit the brick wall during descent or got caught in trees. Plugging the data back into our theoretical equations and calculating Cd:

Diameter = 55 in Note: although the we bought a 60 in umbrella, some of the material needed to be cut in order to make the parachute. Upon measurement it was roughly 55 inches in diameter, or 1.4 m.

Area = PI(d/2)^2 = PI(1.4/2)^2 = 1.54 m^2 25.48 N = 0.5 x 1.2 kg/m^3 x (4.56 m/s)^2 x Cd x 1.54 m^2

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Cd = 1.33, which is pretty close to the theoretical drag coefficient of 1.2 The difference is most likely due to sources of error as well as drag from the frame body itself.

To test the parachute release deployment, another surrogate frame was constructed and the parachute attached to it. We dropped the frame, releasing the parachute as soon as we dropped it. We tested dropping it both flat and at a 45 degree angle. In both cases the parachute released properly without getting caught in the propellers.

Figure 32: Parachute testing frame

Figure 33: Picture of before and after release of parachute

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Figure 34: Picture of parachute deployed

As seen in the pictures above, the folded parachute shoots out a good distance from the

quadcopter when released. This is due to the parachute having a natural spring force from being folded tightly. Although the parachute did not hit the propellers during testing, for our final design we further elevated the parachute to ensure the propellers do not damage the parachute upon deployment.

4. Bill of Materials

Item Price Supplier Quantity Total

USB Adapter 3.99 Amazon 1 3.99

Telemetry 25.99 Amazon 1 25.99

Motor 15.42 Hobby King 4 61.68

Power Distribution Board 3.70 Hobby King 1 3.70

Prop Drive 1.80 Hobby King 4 7.20

GPS 19.98 Hobby King 1 19.98

Electronic Speed Controller 8.95 Hobby King 4 35.8

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APM board 67.99 Hobby King 1 67.99

Transmitter and Receiver 59.99 Hobby King 1 59.99

Battery 33.92 Hobby King 1 33.92

XT60 Connector 0.81 Hobby King 1 0.81

Solenoid 5.27 Amazon 1 5.27

Parachute 7.44 Amazon 1 7.44

Servo 3.98 Amazon 1 3.98

Propellers 1.08 Hobby King 4 4.32

Tupperware container 1.00 CVS 1 1.00

Acrylic 12x12x1/8” 8.63 McMaster­Carr 2 17.26

Aluminum 24x24x0.016” 29.59 McMaster­Carr 1 29.59

Aluminum 12x12x0.032” 12.99 McMaster­Carr 1 12.99

Polystyrene Foam 12x12x1” 19.19 McMaster­Carr 1 19.19

4­40 Bolts, 1” length 5.40 McMaster­Carr 1 5.40

4­40 Bolts, ⅜” length 8.07 McMaster­Carr 1 8.07

6­32 Bolts, 1.25” length 5.00 McMaster­Carr 1 5.00

10­24 Bolts, 1.5” length 11.66 McMaster­Carr 1 11.66

4­40 Locknuts 2.74 McMaster­Carr 1 2.74

6­32 Locknuts 2.67 McMaster­Carr 1 2.67

10­24 Nuts 2.74 McMaster­Carr 1 2.74

5/32” Dia. Quick­Release Pins 1.80 McMaster­Carr 4 7.20

Total 492.23

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5. Budget Monetary Cost

Item Price Supplier Quantity Total

USB Adapter 3.99 Amazon 1 3.99

Telemetry 25.99 Amazon 1 25.99

Motor 15.42 Hobby King 6 92.52

Power Distribution Board 3.70 Hobby King 2 7.4

Prop Drive 1.80 Hobby King 4 7.20

GPS 19.98 Hobby King 1 19.98

Electronic Speed Controller 8.95 Hobby King 6 53.7

APM board 67.99 Hobby King 1 67.99

Transmitter and Receiver 59.99 Hobby King 1 59.99

Battery 33.92 Hobby King 2 67.84

XT60 Connector 0.81 Hobby King 1 0.81

Solenoid 5.27 Amazon 1 5.27

Parachute 7.44 Amazon 1 7.44

Servo 3.98 Amazon 1 3.98

Propellers (Wide Variety) N/A Hobby King N/A 80

Tupperware container 1.00 CVS 1 1.00

Acrylic 12x12x1/8” 8.63 McMaster­Carr 2 17.26

Aluminum 24x24x0.016” 29.59 McMaster­Carr 1 29.59

Aluminum 12x12x0.032” 12.99 McMaster­Carr 1 12.99

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Polystyrene Foam 12x12x1” 19.19 McMaster­Carr 1 19.19

4­40 Bolts, 1” length 5.40 McMaster­Carr 1 5.40

4­40 Bolts, ⅜” length 8.07 McMaster­Carr 1 8.07

6­32 Bolts, 1.25” length 5.00 McMaster­Carr 1 5.00

10­24 Bolts, 1.5” length 11.66 McMaster­Carr 1 11.66

4­40 Locknuts 2.74 McMaster­Carr 1 2.74

6­32 Locknuts 2.67 McMaster­Carr 1 2.67

10­24 Nuts 2.74 McMaster­Carr 1 2.74

5/32” Dia. Quick­Release Pins 1.80 McMaster­Carr 4 7.20

Battery Charger 32 Hobby King 1 32

Current/Voltage Reader 19.70 Hobby King 1 19.70

Postal Scale 12 Office Depot 1 12

Payload box 8 Office Depot 1 8

Total 701.31 Time Cost

Week of: Hours Sum for Prototypes (hours) Sum for Prototypes ($)

9­21­14 27

9­28­14 32

10­5­14 37.5 Prototype 1: 96.5 ~ $375

10­19­14 61 Prototype 2: 61 ~ $15

10­26­14 to 12­6­14 90 Prototype 3: 90 ~ $10

1­4­15 9

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1­11­15 and 1­18­15 30.5

1­25­15 and 2­1­15 61 Prototype 4: 100.5 ~ $100

2­5­15 to 2­12­15 18

2­16­15 to 3­12­15 67

3­17­15 33

3­26­15 59 Prototype 5: 127 ~ $100

Through 4­9­15 103 Prototype 6: 153 ~ $100

Total 628 628 ~ $700

Total labor cost (at $8.00/hr)

$5024

6. Conclusion

Design Evolution Evolution of Frame:

Figure 35: One arm frame Figure 36: First full frame

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Figure 37: Second frame Figure 38: Final frame Overall the final frame is significantly lighter and stronger than the original design. Evolution of Propeller Testing:

Figure 39: Testing using mechanical scale Figure 40: Testing using digital scale Evolution of Parachute Device:

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Figure 41: Initial parachute design

Figure 42: Added parachute container and hook attachment

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Figure 43: Used a smaller servo and repositioned the solenoid

Strengths of Portable Military Quadcopter Design

With our given design constraints, we can say that our quadcopter design successfully performs to Northrop Grumman’s specifications. Compared to similar, pre­built quadcopters with comparable payload lifting, ours is significantly cheaper. Compared to similarly priced quadcopters, none have the payload capabilities necessary to lift a communication device for long enough. Testing shows that our quadcopter can easily lift a 1 kg payload and safely support it for about five minutes, giving it more than enough time to reach necessary altitude and transmit a radio signal. Another benefit of our design is that the battery is accessible and held in place by velcro so a dead battery can be easily removed and replaced, compared to other designs which require significant time to disassemble the body, replace the battery, and reassemble it. The payload itself is attached to the quadcopter frame only by clips on the bottom, meaning that different sized payloads can be accommodated, provided they can be attached with the same clips. Additionally our design makes use of programmable GPS waypoints, allowing a soldier to move it to a specified point without the need for manual flight. This is helpful under conditions of low visibility. The quadcopter itself weighs about 1.6 kg, and when folded measures about 14” x 14”, making it light enough and small enough to easily be handled by a person.

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Recommendations Planning flight modes through phone

Although our quadcopter has the theoretical capability of being able to plan flight modes and switch between flight modes, we did not have enough time to verify this from testing. Also, a mount needs to be made to attach the phone to the flight controller Battery Testing

Although we did some battery testing, extensive battery testing needs to be done for more accurate flight planning. Additionally, a way to monitor the battery would be ideal. Weather­proofing

Making the quadcopter relatively weatherproof to light rain would be beneficial. This can be done potentially by using an ultra­hydrophobic spray. Propellers

When the arms of the quadcopter arms are folded, the propellers stick out, which would make the quadcopter difficult to store (Figure 35). Finding a quick and easy way to remove and install the propellers would be needed to remedy this, as now the propeller nuts must be fully unscrewed and the propellers stored separately for storage. Batteries

If the payload is to be the full 1 kg, the lifetime of the quadcopter would be greatly improved with a further investment of $80 to $100 into a higher capacity battery. As of now the fully encumbered lifetime is at the low end of what was deemed reasonable (~ 5 minutes).

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Figure 44: Quadcopter Folded

Landing Legs

Although the current legs work, the hinge joints could be done in a more robust and durable way, and a better hard stop could be implemented. Also the addition of a rubber cap where the legs make contact with the ground would be a good addition. Payload Connection

Although the current payload adapters serve their function, it would be idea to have a more quickly connecting adapter to facilitate quick deployment (especially when so many other components of the design were designed with deployment speed and portability in mind).

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Implementation Plan

After handing the quadcopter off to Northrop Grumman, extensive battery testing should be the first step taken. The battery life of the quadcopter should be measured while hovering at different payloads. This will help determine flight times based on different scenarios such as how high and fast the quadcopter needs to go, and the actual weight of the carried payload. The next steps after this would be to add a battery monitor and to make it weatherproof (by spraying it with an ultra­hydrophobic spray). Other possible steps would be to upgrade the payload attachment to allow for faster payload attachment using clips instead of bolts and nuts, and to upgrade the landing legs to make them more robust, more durable, and less unstable. Additionally, finding a way to quickly to attach and detach the propellers would be helpful, possibly by using one­piece clip­on propellers instead of the current ones which require fully unscrewing the propeller nut and fully tightening before flight. Finally, a small extra investment in a higher capacity battery can vastly improve the lifetime of the fully encumbered quadcopter, and is definitely a recommended investment.

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7. Appendix

Table of Contents

1. Engineering Drawings of Relevant Parts 50 a. Top Panel 50 b. Bottom Panel 51 c. Quad Arm 52 d. Outboard Spacers 53 e. Inboard Spacers 54 f. Foam Insert 55

2. Full Lift Testing Data 56 a. Lift vs. Current Tables 56 b. Trendlines 57

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Engineering Drawings of Relevant Parts:

50

51

52

53

54

55

Full Lift Testing Data

Table 1: Lift and Current for Various Size Propellers

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Table 2: Trend Line Equations for Propellers

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