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SUBMITTED BY

Daniel Chabolla, Sean Gowen, Aimee Kim, Michael Lo, Wenbin Zhao

May 7, 2013

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TABLE OF CONTENTS

PROJECT OVERVIEW...................................................................................................... 3

OVERALL DESIGN........................................................................................................... 4

TESTING/PROTOTYPING RESULTS............................................................................... 8

PROPOSED IMPROVEMENTS/LESSONS LEARNED..................................................... 12

REQUIREMENTS COMPLIANCE...................................................................................... 14

COST.................................................................................................................................. 15

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PROJECT OVERVIEW

An ornithopter is a flying machine which uses flapping wings for propulsion. Due to the closeresemblance to insects and birds in both their physical and flight characteristics, ornithopters have beendeveloped for clandestine surveillance as well as to study the aerodynamics of flapping wings. At therequest of Dr.Howard Hu, the team has decided to design and build the smallest ornithopter possible fromthe ground up. Dr.Hu is interested in the study and manipulation of wing tip vortices which only havesignificant relevance at small scales. Furthermore, Dr. Hu would like an ornithopter for recreationalpurposes.

According to the requirements of the team’s customer, the ornithopter should be a maximum of 10inches in wingspan and be able to operate at normal building altitudes of the University of Pennsylvania atstandard room temperature and pressure. Additionally, the ornithopter should be able to produce levelflight for at least one minute.

An additional goal which the team set for itself was to have the ornithopter feature at least rudderand throttle control. These control features were not required by the customer.

While there are a few ornithopters on the market at 10 inch wingspan, Dr. Hu wanted a freelycustomizable model that he could easily change in his lab using the tools available at UPenn.

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OVERALL DESIGN

FLAPPING MECHANISM:

The team focused on constructing a 2-winged ornithopter. Flapping is achieved through the drivetrainillustrated below:

Two acrylic plates (4), held together by a 0.09” diameter carbon fiber rod, house the DC motor (6). Themotor used in the model is a 30:1 High Power Brushed DC Gearmotor purchased from Pololu. The carbonfiber extends beyond the gearbox of the motor where it interfaces with the flapping mechanism bracket(5). The wing actuator component (1) pivots about front plane of the flapping mechanism bracket (5).The wing actuator component (1) interfaces with the connecting rod (2) which in turn interfaces with the

motor coupler component (3). The motor coupler component (3) is directly attached to the D-shaft of theDC motor. Two Lithium-Polymer cells, capable of providing a nominal 3.7V each (for a series total of7.4V), provide the onboard power necessary to operate the DC motor.

When a voltage is applied between the two terminals of the DC motor, the shaft of the motor spins. This inturn causes the motor coupler component to rotate. As the motor coupler rotates the end of theconnecting rod interfacing with the wing actuator components moves in a vertical fashion. This motion inturn causes the wing actuator to pivot about a point which induces the flapping motion that results in flight.

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MOTOR SELECTION

Typically two types of motors are utilized to operate an ornithopter: DC pager motors andbrushless DC motors. Regardless of motor choice, a gearbox is required and the team did not havesufficient equipment for fabricating the gearbox. Pre-fabricated gearboxes available online did not featurethe necessary gear ratios or were too heavy. Therefore, the team decided to work with Pololu High PowerGearmotor, which featured integrated, lightweight gearboxes at the appropriate gear ratios. Currentlythere is no ornithopter in existence that makes use of brushed DC motors.

ELECTRONICS AND CONTROLLER:

A microcontroller onboard a hand-held controller communicates via the nRF24LE1 wireless module toissue commands to the on-board ornithopter microcontroller. The packet structure features a start packetfollowed by two 8-bit numbers signifying the voltage to be supplied to the motor and electromagneticactuator. The user is able to control the control number values by manipulating two joysticks whichproduce a varying voltage that is sampled by the microcontroller on the hand-held.

On- board the ornithopter is a wireless receiver which receives signals from the user’s handheldtransmitter. The packets received are read by the on-board microcontroller. The microcontrollerdetermines how much voltage to supply to the motor and actuator and regulates these voltages throughmeans of pulse width modulation (PWM) signals that is essentially buffered by a half-bridge. The rate atwhich the wings flap is dictated by the rate at which the motor shaft rotates, which is proportional to thevoltage applied to the motor.

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CONTROL SURFACES:

It is important to note that the only control surface on the ornithopter is a rudder. The rudder wasfabricated from 1/ 16’’ balsa wood and actuation was achieved using an electromagnetic coil. The positionand area of the control surface was determined through test flights. Elevation control was achieved byaltering the flap rate of the ornithopter, which it turn altered thrust produced and moment generated. Afaster flapping rate would result in climb while a smaller flapping rate would result in the descending of theornithopter.

An image of rudder design with implemented magnetic actuator follows:

WINGS:

Fixed to the wing actuator components were carbon fiber rods of 0.025’’ diameter. The rods functioned asthe main structural member for the wing. Tissue paper wings were fabricated using a wing template to

achieve consistency in fabrication, and were attached to the carbon fiber rods using adhesive. Tissuepaper material for wing fabrication was attractive because of its low weight and its deformation properties,which resulted in larger thrust generation. To improve lift performance carbon fiber struts were added towing to stiffen the material.

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TAIL :

Unlike the rudder, the tail was set in a fixed position. It was fabricated from three carbon fiber rods,laminate, and balsa wood. As with the wings, the carbon fiber rods provided structural members on whichthe laminate rested. The three carbon fiber rods interfaced with a balsa assembly that is presented in theimage below. The balsa assembly interfaced with the 0.09’’ carbon fiber rod (main body rod). Trimming of

the ornithopter was achieved by varying the angle of the middle balsa brackets of the tail angle adjuster.This angle was changed before each flight test to find an optimal angle.

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TESTING/PROTOTYPING RESULTS BEFORE MIDREPORT:Previous Tests:

The team completed multiple flight tests in the lobby of the Levine building. Precautions were takenso that when the ornithopter landed, no parts were broken. This was done by having team members catchthe ornithopter. Many ornithopter components were laser cut multiple times in order to ensure that quickfixes could be made during the testing process in the case of any breakages.

The team’s initial test of was of the original acrylic model with a single laminate wing without avertical stabilizer. In the video (Test 1) the ornithopter flew a couple feet then drastically veered to the rightand fell. Around 5-7 attempts were made and all had the same conclusion, a vertical stabilizer was

needed so the ornithopter flew straight and the weight needed to be reduced.

Test 1: http://www.youtube.com/watch?v=Hkihjml_GW4

The second flight test (Test 2) consisted of the same model as the previous test but included avertical stabilizer fabricated from laminate and 2 thin carbon fiber rods as support and parts were madefrom 1/16” acrylic. The results of this test were significantly better. The ornithopter flew straighter andfurther than in the first field test. In order to get this model to fly further and more level the team workedtoward reducing the weight as well as increasing the force produced by the wings.

Test 2: http://www.youtube.com/watch?v=1KRnKT1msB4

The third flight test (Test 3) addressed the ornithopter weight problem by attempting to use adouble layer of tissue. This change was made on the test 2 model. Although the weight was slightlyreduced, the tissue paper did not produce enough lift. This prototype failed to provide stability whichcaused the ornithopter to drift off in different directions.

Test 3: http://www.youtube.com/watch?v=2slzwaJ55AA

The fourth test (Test 4) addressed the lack of force issue by attempting to use a double layer oflaminate in hopes of creating a stronger thrust and greater stability. Multiple tests were run and it wasconcluded that double layered laminate wings was the least effective material of all the materials the teamtested. The videos of the flights showed definitely lack of force and stability. Even with the verticalstabilizer the ornithopter did not fly straight. The material was too stiff and did not allow enough motion tocreate thrust upward and downward.

Test 4: http://www.youtube.com/watch?v=YXiYeBI1loI

http://www.youtube.com/watch?v=Hkihjml_GW4http://www.youtube.com/watch?v=Hkihjml_GW4http://www.youtube.com/watch?v=Hkihjml_GW4http://www.youtube.com/watch?v=1KRnKT1msB4http://www.youtube.com/watch?v=1KRnKT1msB4http://www.youtube.com/watch?v=1KRnKT1msB4http://www.youtube.com/watch?v=2slzwaJ55AAhttp://www.youtube.com/watch?v=2slzwaJ55AAhttp://www.youtube.com/watch?v=2slzwaJ55AAhttp://www.youtube.com/watch?v=YXiYeBI1loIhttp://www.youtube.com/watch?v=YXiYeBI1loIhttp://www.youtube.com/watch?v=YXiYeBI1loIhttp://www.youtube.com/watch?v=YXiYeBI1loIhttp://www.youtube.com/watch?v=2slzwaJ55AAhttp://www.youtube.com/watch?v=1KRnKT1msB4http://www.youtube.com/watch?v=Hkihjml_GW4

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AFTER MIDREPORT:

The test stand, shown in the photo below, provided the means to compare the performance ofdifferent wing materials and designs in thrust generation. The test stand was composed of an L-shapedbeam pivoting about an aluminum rod inserted through the trapezoidal stand.

The initial design was to mount the ornithopter directly on top of the L-beam with the bottom of theL-beam resting on a scale. The idea was to determine exact thrust by measuring the reaction force on thescale caused by the thrust generated —because the L-beam would be static the thrust-moment and scale-moment would be equal. However, a consistent reading on the scale could not be achieved due to thevibration caused by the flapping wings.

To resolve this problem, the team decoupled the measuring system from the flapping system. Asshown in the figure, the ornithopter was attached to a fixed stand next to the L-beam. A plate of laminatesheet was attached to the top of the L-beam to catch the wind produced from the flapping. As the thrusthit the laminate cover, it created a moment on the L-beam, pushing the bottom of the L-beam onto thescale. Multiple trials were conducted in order to ensure consistent readings. Although this method couldonly measure relative thrust, the results were repeatable.

Once the team was confident in the effectiveness of the test stand, it was used to determine the mostdesirable set of parameters for thrust generation, specifically the flapping range, wing material and wingdesign. Wings of different materials were flapped at the same voltage and wings made of tissue paper

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produced the most thrust of the materials tested, as shown in the figure below. This result differed fromthe previous conclusion that laminate was the best material for wings. Slow motion video showed that the

tissue paper deformed much more fluidly than the laminate which may have contributed to its efficiency inthrust generation. Flights tests showed, however, that a wing composed of plain tissue paper was not rigid enough to

generate sufficient lift force at the applicable velocities since it featured too much “give”. It was clear thatreinforcement was needed to provide the rigidity for the tissue paper. Thin carbon fiber rods were anobvious choice since they were relatively strong and light. The team experimented with variousarrangements of carbon fiber rods on the wing. The wing design as shown previously produced the mostthrust while still capable of sustaining level flight. Test stand results showed that carbon fiber stiffeners didnot significantly vary the thrust characteristics of the flapping wing.

The team also experimented with various lengths of the motor coupler to determine the best flappingrange. As indicated in the figure below, the thrust output is nearly linearly proportional to the length of themotor coupler. However, a motor coupler which was too long created instability in the flight of theornithopter by making the average wing position anhedral. The team chose the longest motor couplerwhich maintained a dihedral configuration.

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The optimal components found from these tests were incorporated into the final design. As evidentin flight Test 5, the ornithopter exhibited high lift potential. Afterwards, the team introduced the on-boardelectronics for controlled flight. The added 12 grams from the electronics proved too heavy for this

prototype. To resolve this issue, the team increased the wingspan to 18”. At this size, thrust control wasachieved. However, yaw control required a more powerful actuator than the one the team possessed.Level flight with on-board electronics can be viewed in the Test 6 video linked below.

Test 5: http://www.youtube.com/watch?v=n16VXJRbaMM&feature=youtu.be

Test 6: http://www.youtube.com/watch?v=JPMnAx2E6xg&feature=youtu.be

http://www.youtube.com/watch?v=n16VXJRbaMM&feature=youtu.behttp://www.youtube.com/watch?v=n16VXJRbaMM&feature=youtu.behttp://www.youtube.com/watch?v=n16VXJRbaMM&feature=youtu.behttp://www.youtube.com/watch?v=JPMnAx2E6xg&feature=youtu.behttp://www.youtube.com/watch?v=JPMnAx2E6xg&feature=youtu.behttp://www.youtube.com/watch?v=JPMnAx2E6xg&feature=youtu.behttp://www.youtube.com/watch?v=JPMnAx2E6xg&feature=youtu.behttp://www.youtube.com/watch?v=n16VXJRbaMM&feature=youtu.be

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PROPOSED IMPROVEMENTS/LESSONSLEARNED

Problems - Wingspan and Lack of Literature Currently, there is no definitive literature on thrust generation from flapping wings and none of the

faculty has had any experience studying ornithopters. This resulted in a lot of guess and check work aswell as time spent in iterative development based on empirical results. Simple aerodynamic wing andpropellor theory was used to guide progress but never matched up adequately to test results.

A task in which lack of theory proved especially problematic was motor selection. During most ofthe project, the group was in the dark as to which motor would be most appropriate for the ornithopter anddesigning flapping mechanism around different motors was time consuming. Only through extensivetesting did the team finally arrive upon the Pololu Micro Gearmotor, which is not typically used to poweraircraft, as the final selection.

Once the team designed a prototype that produced adequate thrust, the 10 inch wingspanrequirement was next to cause problems for the team. At such a scale, careful machining andmanufacturing is required as even small discrepancies in each wing can cause huge differences in flightcharacteristics and stability.

Lessons Learned - Parallel Path Approach, Setting Feasible Goals, and Lead-Time Management The team had most success approaching the project on parallel paths--with one team focusing on

robust design and the other on weight reduction. It seemed that when one team encountered difficulty, the

other would find success. The most difficult part of this process, however, is knowing when to reconveneand work together. The process thrives, to some degree, on competition to drive individual paths, yetexcessive competition and stubbornness can cause a delay or failure in the reintegration process.

The team originally set out to make a hovering ornithopter with the knowledge that it took Aerovironment 4.5 years with full time engineers and incredible amounts of funding to accomplish such afeat. The group questioned all the current methods of building ornithopters and attempted to innovate onevery front. Many of these ideas had to be dropped due to lack of time or simply because they were notfeasible and, in the end, the group had to rush to design a functional ornithopter. Also, it should be notedthat a previous senior design group that had tried to make an uncontrolled ornithopter and failed to makeit fly at all, which should have been an indication to the current team exactly how difficult it is to fly withflapping wings at all.

The group also learned the importance of factoring in lead-time for purchasing parts. This lessonhas significant importance moving forward because we will most likely deal with purchasing procedures inour future careers. These procedures, such as submitting purchase orders for approval, will add lead timeto part delivery. This will affect the scheduling for deliverables on future projects.

Future Improvements/Changes Given greater funds, the team would have been able to purchase micro-machined gearboxes

allowing the use of light, high-efficiency coreless 6-7mm motors or even high-efficiency, expensive

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Maxxon motors. Alternatively, had there been more time, the team could have produced a high efficiency,light gearbox out of thin acrylic, bushings, and low module gears. Using these alternate gearboxes would

have allowed for greater thrust generation and more robust flight.With more time the team would have been able to professionally make the finalized PCB’s, whichwould allow for more robust mechanical interface between the electronics and main frame--a stepnecessary to stabilize the ornithopter.

The team would have liked to design a protective structure to encapsulate the flapping mechanism,preventing various components from breaking when the ornithopter impacts the ground. This would savetremendous amount of time spent laser cutting parts, disassembling the broken ornithopter andreassembling. This could drastically reduce up to an hour in downtime between test flights. Using ABSinstead of acrylic may also have made the flapping mechanism less brittle and prone to breakage.

The team would have had professionals manufacture the wings as the team had clearly reachedthe fullest potential of hand manufacturing. Professional manufacturing would most likely stabilize theornithopter as the lift on either wing would be nearly identical and the dihedral configuration would be ableto compensate for the minute amount of roll instability remaining.

Finally, the team would have begun sourcing carbon fiber rods from BP Hobbies in New Jerseyrather than DragonPlate in California, which would allow for more rapid prototyping because of a two-week reduction in lead-time.

Overall, the team believes that, with a couple more weeks, it could have fixed the instabilityproblem simply by using more careful and time-consuming manufacturing techniques and methods.

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REQUIREMENT COMPLIANCE Customer Requirements: Three functional requirements were set forth by Dr. Howard Hu at the opening of the project: 1) Maximum 10” wingspan 2) Minimum 1 minute flight time 3) The ornithopter should operate at normal Penn building altitudes

The team designed a model with a 10 inch wingspan that achieved level flight for about 50 feet (asseen in our testing/prototyping video). The lithium-polymer batteries that were purchased did allow formore than a minute of continuous flapping. This is known because the 160mAh batteries would be able toprovide 6 minutes of power at the 1.6A stall (max) current draw of the system, and the motor drew abouthalf that amount during flight.

Team Goals: 1) Yaw Control 2) Thrust Control

The team set the two personal goals stated above. Yaw control was achieved by using anelectromagnetic actuator attached to the rudder. Although the team could not demonstrate the ruddercontrol in the final test due to broken leads, the group tested the functionality of the electromagnetic

actuator prior to the breakage. The team was able to manipulate the rudder using the wireless controller.

Thrust control was achieved by PWM signals generated by a microcontroller. During the finalpresentation and in test videos, the team was able to demonstrate how the thrust could be varied usingthe joystick on the wireless controller.

Unfortunately the ornithopter faced issues with stability mainly due to limits of manufacturingaccuracy. Should the project have continued, the group would have worked on improving manufacturingtechniques or, more likely, had wings made by professional manufacturing companies.

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COSTSCompany Description Amount

Radical RC 34756 Pinion Gears $12.00

Dragonplate 15915 Carbon Fiber $45.50

Ebay - ZHANGZIXING2008 Universal Joints $43.96

McMaster Carr PO 291727 M2 screws, aluminum rods $14.84

Reliance Precision Bearing Spacers $92.50

Technobots Worm Gears $28.15

Dragon Plate Carbon Fiber Carbon Fiber $21.15

VXB Bearings Bearings $55.93

Robotshop C8209726 1:30 Pager Motor $26.90

Robotshop 1676-7268 1:30 Pager Motor $24.67

McMaster Carr PO 2938115 Aluminum Rods, $61.50

BP Hobbies.com 467070-13 Carbon Fiber $32.77

Dragon Plate 16985 Carbon Fiber $24.81

Digikey Force Sensors $124.06

Amazon E-flite Motors $194.53

DragonPlate Carbon Fiber $23.45

Amazon.com Magnetic Actuator $19.78

Amazon.com E-Flite Batteries $24.20

Tower Hobbies Balsa Sheets $46.78

Solarbotics 1:30 Pager Motor $61.53

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Mouser Molex Headers $22.39

Amazon Fog Liquid $9.53

Amazon Fog Machine $31.79

Amazon Magnetic Actuator $38.56

Pololu Polulu Motors $44.85

BP Hobbies Pager Motors $34.58

BP Hobbies Pager Motors, Carbon Fiber $42.93

BP Hobbies Pager Motors, Carbon Fiber $44.98

Digi-Key Atmega32U4 $34.75

Digi-Key Resistors, Capacitors $23.46

Digi-Key Voltage Regulator, Motor Drivers $43.64

Digi-Key Atmega32U4 $37.01

Digi-Key PCB Material $9.60

Digi-Key Joysticks, Wireless Chips $39.43

Total $1,436.51

Max budget: $1500.