The Pennsylvania State University

The Graduate School

ADDITIVELY MANUFACTURED COMPLIANT MECHANISM DESIGN AND

APPLICATION FOR SUAS CONTROL SURFACES

A Thesis in

Additive Manufacturing and Design

by

Thomas Jones

© 2021 Thomas Jones

Submitted in Partial Fulfillment

of the Requirements

for the Degree of

Master of Science

May 2021

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The thesis of Thomas Jones was reviewed and approved by the following:

Michael A Yukish

Head, Manufacturing Systems Division

Associate Professor of Aerospace Engineering

Thesis Co-Advisor

Simon W Miller

Assistant Research Professor

Affiliate Professor of Architectural Engineering

Thesis Co-Advisor

Timothy W Simpson

Paul Morrow Professor in Engineering Design and Manufacturing

Additive Manufacturing and Design Program Director

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ABSTRACT

A design study was conducted to bridge the gap between conventional hinged aileron and

compliant mechanism controlled morphing wing aileron designs for small unmanned aircraft

systems using additive manufacturing methods. Taking advantage of the rapid prototyping

capabilities of fused filament fabrication machines, several design options were created using

multiple materials. Final design selection was determined by part usability and by metrics of

reduced part count, weight, material use, assembly time, and other factors are enabled by additive

manufacturing. The final compliant design was printed, tested and compared to a printed,

conventional-style hinged aileron on a flying test-bed to look for aerodynamic benefits and to

prove functionality. Data suggests there is some aerodynamic benefit to be gained with the

compliant morphing wing aileron. Additional improvements include a reduction in total part

count and time to assemble with improvements in final component weight and material usage

possible using this design process. The use of additive manufacturing as a manufacturing process

enabled rapid prototyping of concepts, greatly accelerating the design process and resulting in a

novel design.

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TABLE OF CONTENTS

LIST OF FIGURES ................................................................................................................. vi

LIST OF TABLES ................................................................................................................... ix

ACKNOWLEDGEMENTS ..................................................................................................... x

Chapter 1 Introduction ............................................................................................................ 1

Additive Manufacturing ................................................................................................... 2 Small Unmanned Aircraft Systems .................................................................................. 3 Compliant Mechanisms .................................................................................................... 5

Chapter 2 Literature Review ................................................................................................... 7

Additively Manufactured Unmanned Aircraft ................................................................. 7 Morphing Wing Technology ............................................................................................ 13 AM with Morphing Wings ............................................................................................... 16

Chapter 3 Design Process ....................................................................................................... 20

Design Considerations and Limitations ........................................................................... 20 Aircraft Design ................................................................................................................. 22

Aircraft Structure...................................................................................................... 23 Conventional Aileron Design ................................................................................... 26 Component Selection ............................................................................................... 27

Initial Design Tests: TPU Arm Variants .......................................................................... 28 Secondary Design Tests: Compliant Shapes .................................................................... 31 Final Design Tests: Sliding Skin ...................................................................................... 36

Chapter 4 Testing and Analysis Methods ............................................................................... 44

Flight Campaign ............................................................................................................... 44 Experimental Data Collection .......................................................................................... 45 Analytical Data Collection ............................................................................................... 46 Additive Manufacturing Data .......................................................................................... 47

Chapter 5 Results .................................................................................................................... 48

Hinged Aileron ................................................................................................................. 48 Morphing Aileron ............................................................................................................ 50 Analytical and Experimental Models ............................................................................... 52 Assembly Comparison ..................................................................................................... 54 Print Time and Material Usage ........................................................................................ 55 Handling Qualities ........................................................................................................... 56

Chapter 6 Discussion and Conclusion .................................................................................... 57

v

References ................................................................................................................................ 59

Appendix A Weather Data ...................................................................................................... 64

Appendix B Supplementary Materials .................................................................................... 65

vi

LIST OF FIGURES

Figure 1: A schematic diagram of the FFF process (Gibson et al., 2014). .............................. 3

Figure 2: A typical fixed-wing aircraft control scheme (Flight Controls, n.d.). ...................... 5

Figure 3: The Southampton University Laser Sintered Aircraft (SULSA) in flight

(Paulson et al., 2015). ....................................................................................................... 8

Figure 4: The University of Virginia’s FFF printed sUAS (Easter et al., 2013). ..................... 9

Figure 5: The 3DAeroventures X-100 has a unique closed wing structure that is rarely

seen made in balsawood or foam construction methods (Haddad, n.d.). ......................... 12

Figure 6: The morphing wing system developed by Flexsys on their test aircraft (Kota et

al., 2016). ......................................................................................................................... 15

Figure 7: A schematic diagram of the morphing wing developed by Yokozeki et al.

(2014). .............................................................................................................................. 16

Figure 8: A schematic diagram of the FishBAC design (a) and images comparing the

deflections of the design (b) (Woods & Friswell, 2012). ................................................. 17

Figure 9: A schematic diagram of the compliant morphing wing developed by Fasel et al.

(2020). .............................................................................................................................. 19

Figure 10: A schematic diagram the Lulzbot Mini 2’s build volume (LulzBot Mini 2,

n.d.). ................................................................................................................................. 23

Figure 11: An earlier iteration of the aircraft with a weaker landing gear design. .................. 24

Figure 12: The final design of the aircraft in flight, notice the wingtip covers. ...................... 25

Figure 13: The conventionally hinged aileron design. ............................................................. 27

Figure 14: The first TPU arm design (top) and the second (bottom), notice the significant

stringing in the bottom design. This is undesirable due to added weight and

interference with motion. ................................................................................................. 28

Figure 15: The third TPU arm design (top, a) and the fourth (bottom, a). The unwanted

form of the fourth design during the inward deflection is demonstrated in (b). .............. 30

Figure 16: The CRAM test piece with PLA. ........................................................................... 31

Figure 17: A weight comparison of the Flex-16 pieces with the two modified Flex-16

pieces in PLA (a/c) and TPU (b/d). .................................................................................. 32

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Figure 18: The modified Flex-16 (a) and it rotated to about the maximum desired for an

aileron (b). ........................................................................................................................ 33

Figure 19: A profile of the planned TPU aileron segment undeflected (a) and deflected by

pulling on the lower arm and pushing with the top (b). ................................................... 33

Figure 20: A side view of the full length first TPU arm design. .............................................. 34

Figure 21: The first TPU arm design without (a) and with (b) the planned PLA leading

edge sheath. ...................................................................................................................... 35

Figure 22: A side view of the full length second TPU arm design. ......................................... 36

Figure 23: The second TPU arm design without (a) and with (b) the PLA leading edge

sheath. Note that this sheath would have been lengthened to cover the entire aileron

section. ............................................................................................................................. 36

Figure 24: The same surface-designed piece sliced in Simplify3D (a) and Cura (b). ............. 37

Figure 25: Three initial sliding skin designs showing the difference between the pieces

with no additional support and two different additional supports that connect to the

same locations. ................................................................................................................. 38

Figure 26: A CAD model of the first full-length sliding skin model, with two trailing

edge perimeters. ............................................................................................................... 40

Figure 27: A CAD model of the single-servo full-length sliding skin model. ......................... 40

Figure 28: A top-down view of both ailerons. ......................................................................... 42

Figure 29: The top surface of the CM morphing wing aileron. ............................................... 42

Figure 30: The CM morphing aileron in the up position (a) and down position (b). ............... 43

Figure 31: The final design of the aircraft with the morphing ailerons in flight...................... 43

Figure 32: An onboard view looking back at the left wing while inverted during a 360°

left roll with the morphing ailerons. ................................................................................. 45

Figure 33: A composite image of the morphing aileron at neutral, full up, and full down

positions. .......................................................................................................................... 46

Figure 34: The overall flight data for the hinge aileron. .......................................................... 49

Figure 35: The roll rate tests for the hinge aileron with average value lines for left and

right roll............................................................................................................................ 49

Figure 36: The overall flight data for the second morphing aileron roll rate flight. ................ 50

viii

Figure 37: The roll rate tests for the second morphing aileron flight with average value

lines for left and right roll. ............................................................................................... 51

Figure 38: Coefficient of lift versus the ratio of coefficient of drag for the morphing

aileron (a) and hinge aileron (b). ...................................................................................... 53

Figure 39: A plot showing the amount of deflection from neutral measured at each

aileron’s respective hinge point for a given control input percentage. ............................ 54

ix

LIST OF TABLES

Table 1: A comparison of AM technologies compared by Goh et al. (2017) for printed

sUAS and their advantages and disadvantages. ............................................................... 10

Table 2: A comparison of the same AM technologies compared by Goh et al. (2017) for

printed sUAS and their advantages and disadvantages for compliant mechanism

design. .............................................................................................................................. 11

Table 3: A table adapted from Li et al. (2018) showing the development of morphing

wing concepts over time. ................................................................................................. 14

Table 4: Components used and their purposes. ....................................................................... 27

Table 5: Print parameters used for the flown versions of both the conventional hinge

design as well as the CM. ................................................................................................. 41

Table 6: Derived from experimental data for the hinged aileron. ............................................ 51

Table 7: Derived from experimental data for the first morphing aileron flight. ..................... 51

Table 8: Derived from experimental data for the second morphing aileron flight. ................ 52

Table 9: Derived from experimental data for the full 360° rolls with the morphing

ailerons. ............................................................................................................................ 52

Table 10: Summary table of morphing versus hinged aileron performance. ........................... 52

Table 11: A total parts list for both aileron types. Since some equipment varies on the

components it comes with for a complete kit, parts for given items were noted but

not counted toward the total part count. A complete kit is counted. ................................ 55

Table 12: AM relevant parameters for the two aileron types as well as a comparison to

the flight-ready weight. .................................................................................................... 56

Table 13: Weather conditions for the hinge test flights. .......................................................... 64

Table 14: Weather conditions for the morphing aileron test flights at the start of the day. ..... 64

Table 15: Weather conditions for the morphing aileron test flights at the end of the day. ...... 64

x

ACKNOWLEDGEMENTS

I’d like to thank my advisors and colleagues that have provided extensive support and

assistance along the way, not only with this thesis but other projects and activities in and out of

school as well.

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Chapter 1

Introduction

This study aims to design and demonstrate a fully 3D printed compliant mechanism

morphing wing for the control of a 3D printed small unmanned aircraft, and culminated in a

successful flight test of such a compliant mechanism. At the time of writing, no other example of

such a mechanism that has been flown has been published. Examples in literature have either

been of ground-based studies or of flight-tested examples that were partially 3D printed, e.g., only

the internal structure with a non-printed skin.

This thesis first describes provides a brief overview on additive manufacturing, small

unmanned aerial systems, and compliant mechanisms. A survey of previous research is then

presented for state-of-the-art related fields that are combined in this effort to make a fully 3D

printed control surface for an unmanned aircraft, as well as an account of the incremental steps

taken to achieve this goal. This document will focus primarily on design for additive

manufacturing principles and demonstrate a potential application for the technology where it has

not yet been fully utilized. The design exploration is restricted to a material extrusion process due

to its low-cost and simplicity that makes it attainable for an average designer or builder.

Advanced aerodynamic analysis is outside of the scope of this document; however, the method

used will be briefly explained as it pertains to the flight test campaign evaluating the designs

presented. The experimental portion of this work compares the performance of both a

contemporary aileron with that of the designed compliant mechanism aileron on the same aircraft

platform. The results of the design as well as areas for future improvement are also discussed.

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Additive Manufacturing

Additive Manufacturing (AM) is a form of manufacturing that uses a layer-by-layer

buildup of material to create an object from a 3D digital model. Whereas traditional

manufacturing takes away material from a bulk shape (subtractive), AM adds material to a build

plate or pre-existing structure in a process commonly referred to as 3D printing. This form of

manufacturing can lead to the creation of more geometrically complex objects while producing

less wasted material. The AM process is also typically quicker and more cost effective for small

batch sizes since the production of dies or tooling is not required which makes it a very useful

tool for rapidly prototyping new designs.

There are seven different types of additive manufacturing processes that have been

standardized (ASTM Standard F2792, 2012). For the context of this study, a subprocess of

material extrusion, fused filament fabrication (FFF), is the process used (Figure 1). FFF works by

having a string of material, or filament, fed through a heater block where it is melted. This heat

pipe is typically attached to a gantry-like system that moves it around a build platform. Some

systems may have the build platform move instead. The melted material is fed from the heat pipe,

through a nozzle where it is extruded layer-by-layer using specialized machine instructions and

allowed to cool and solidify to create an object. Other material extrusion processes may use a

chemical reaction to cure and bond material layers by use of a curing agent, the air, or otherwise

(Gibson et al., 2014).

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Some popular FFF materials include polylactic acid (PLA), acrylonitrile butadiene

styrene (ABS), and polyethylene terephthalate glycol (PETG). Prices for 1kg of material range

from $15 up to over $60 for specialized material formulations.

In recent years, AM capable machines have come down in price considerably. This price

reduction in material and the printers has made the technology more accessible to individuals and

groups and has seen rapid growth in business and research opportunities as well as hobby-spaces

(Bourell, 2016).

Small Unmanned Aircraft Systems

A small Unmanned Aircraft System (sUAS) is the Federal Aviation Administration’s

(FAA) term for a remotely operated aerial vehicle, commonly referred to as an RC aircraft or

drone, that weighs under 55 pounds (AC 107-2A - Small Unmanned Aircraft System (Small UAS),

2021). sUAS vary in type and design greatly but most are categorized as either a rotorcraft or

Figure 1: A schematic diagram of the FFF process (Gibson et al., 2014).

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fixed-wing aircraft. A rotorcraft sUAS can take the form of a conventional helicopter or have

multiple rotating blades or propellers in a pattern around an airframe to generate lift, e.g., a

quadcopter. A fixed-wing aircraft sUAS is more akin to a conventional airplane where the wing is

rigidly attached to the aircraft body, or fuselage, and moved through the air to generate lift. This

study focuses on advancements for the latter.

Fixed-wing sUAS typically need at least four critical components to safely operate, a

receiver, power supply, propulsion, and flight control surfaces. The receiver connects wirelessly

to a ground-based controller that enables the operator to send commands to the aircraft. The

power supply, typically a battery, will power the aircraft’s motor and onboard electronics. For

sUAS using an internal combustion engine, the battery supplies power to the electronics while a

liquid fuel powers the engine. The propulsion method can be an electrically driven motor, internal

combustion engine, or for gliders, an upward air current.

The flight control surfaces for fixed-wing sUAS can come in many different

configurations, a typical configuration is ailerons on wings that control the rolling action,

elevator(s) on the horizontal tail(s) to control pitch, and rudder(s) on the vertical tail(s) to control

yaw. Most fixed-wing sUAS require at least two control surfaces, though some are able to use

less (e.g., a two-motor sUAS that uses differential and combined thrust to control direction and

pitch) while others add additional devices (e.g., flaps, slats, spoilers). A schematic of a common

fixed-wing aircraft layout is in Figure 2.

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As computer technology advanced and became more affordable, small autopilot systems

and other sensor devices have found their way into sUAS. Thanks to this, it is now possible to

capture real-time data, as well as obtain much more intricate sensor data from nearly any device

placed on board, making sUAS an even more valuable research tool for a variety of applications.

For example, Pixhawks are an open-source, low cost autopilot system with data-logging

capabilities (Open Source Autopilot for Drones, 2021).

Compliant Mechanisms

A compliant mechanism (CM) is a type of device that uses an input force or motion and

translates it to an outward force or motion through the shape of the design itself. This is

commonly achieved by having flexible members or joints within the device that allow its shape to

change in accordance to the input (Howell, 2013).

Figure 2: A typical fixed-wing aircraft control scheme (Flight Controls, n.d.).

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CMs have several advantages that make their study interesting. One such advantage is the

reduction of part count. Typical mechanisms have several pins, fasteners, hinges and other

components to transfer motion and force. Through clever design, compliant mechanisms are able

to replace hinges and connections with flexible joints while maintaining desired motion (Howell,

2013).

This decrease in part count, when coupled with 3D printing, can simplify the assembly

process. By having this simplicity, costs associated with producing the mechanism can be

reduced as well as operational costs as less lubrication is needed to reduce friction (Howell,

2013).

Brigham Young University’s (BYU) Compliant Mechanism Research Group (CMR) has

stated that CMs also allow for a more precise component movement. Since there has been a

reduction or elimination in fasteners, the locations that backlash can occur are also reduced or

eliminated. Backlash is a usually unwanted movement caused by having gaps in interconnecting

parts. The CMR also has shown that traditional mechanisms that have rubbing components will

eventually wear due to friction which can lead to deviations from the original designed motion

(Howell, 2013). Conversely, their research shows that because of how CMs work, they are

usually difficult to design. Many involve the use of non-linear equations to determine how they

will respond and that their motion can change based on material used. There are also concerns of

fatigue from the cyclical motions CMs undergo and the energy stored in certain states of

operation (Dirksen et al., 2013).

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Chapter 2

Literature Review

The chapter presents a brief overview of previous works done in AM sUAS literature,

morphing wing technologies, and the combination of AM and morphing wings. The focus is on

research performed in the sUAS and morphing wing spaces with highlights on factors that make

certain designs more or less feasible for hobby-grade, inexpensive, desktop FFF systems. The

review focuses primarily on printed sUAS made with the FFF system as this was the selected

print process for the study, though one produced with SLS is discussed.

Additively Manufactured Unmanned Aircraft

A “fully printed aircraft” in this thesis describes an aircraft where all aerodynamic

surfaces and the majority of internal structural members are printed. The design may use

additional unprinted members, but it does not rely on them entirely to fly.

The first fully 3D printed sUAS was developed in 2011 (P. Marks, 2011). This aircraft,

known as the SULSA, was made using the selective laser sintering (SLS) process with a nylon

material. Other than being the first 3D printed sUAS, the aircraft achieved a number of

impressive feats for a sUAS in general. It was able to be assembled within 10 minutes using a

toolless process, flew for roughly 30 minutes, and used no fasteners in assembly (Paulson et al.,

2015). An image of the plane is shown in Figure 3. While impressive, the technology used to

build the plane is not readily available to hobbyists or those without access to a SLS machine.

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One of the first sUAS to be produced using the FFF process was created by a team at the

University of Virginia (Easter et al., 2013). As the FFF process is more accessible to the general

public, this was an important step for printed sUAS. However, the aircraft (Figure 4) cannot make

the claim of being fully printed. Its printed parts are lightened with holes throughout the external

surfaces and the vehicle’s body is covered with a heat-shrunk film, similar to traditional

balsawood style models, to cover these holes. Easter also claims that the aircraft’s ABS frame

was said to withstand impact better than balsa designs and was more readily repairable due to

being able to print identical parts within hours. With the often-volatile environment that sUAS

operate in, impacts with obstacles and the ground are inevitable. An aircraft that is easier to repair

will ideally spend less time in the repair shop and more time performing the mission. Designers

later began creating models that were fully printed using FFF, apart from the necessary

electronics or additional structural members like carbon fiber spars or fasteners.

Figure 3: The Southampton University Laser Sintered Aircraft (SULSA) in flight (Paulson et al.,

2015).

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Goh et al. (2017) extensively documents the recent history of sUAS and also shows that

multiple other print processes have been used to create sUAS; however, many of these processes

have certain disadvantages that outweigh their potential advantages for a robust flying platform.

A table summarizing the advantages and disadvantages for printed sUAS as a whole are listed by

Goh and displayed in a modified form in Table 1. Additional advantages and disadvantages for

the same AM techniques but for compliant structures are discussed in Table 2. Observation of the

hobbyist 3D printed sUAS community shows that FFF is the leading method for production of

components, complete vehicles, and prototype vehicles with unconventional structures (Figure 5).

Figure 4: The University of Virginia’s FFF printed sUAS (Easter et al., 2013).

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Technology Advantages Disadvantages

Fused Filament

Fabrication (FFF, a

material extrusion

process)

• High strength material such as

Ultem is available

• ABS plastic has higher survival rate

compared to balsa during impacts

• Obvious stair stepping

effect in z-direction

Polyjet,

(A material jetting

technology)

• Ability to create functionally graded

parts with multi-material printing

• Ability to print fine features

• Good surface finish

• Insignificant stair stepping effect

• Slow recovery rate from

high loading condition

• Strength of AM material is

still inferior to that of the

biological bone

Stereolithography

(SLA, a

photopolymerization

process)

• Ability to print fine feature size

• Good surface finish

• Insignificant stair stepping effect

• Degradation of

photosensitive materials

leading to poor

performance under load

• Low tensile strength of

material

Selective Laser

Sintering (SLS, a

powder bed fusion

process)

• Ability to print parts with good

mechanical strength

• Large build area

• Relatively low cost

• Rough surface finish

Table 1: A comparison of AM technologies compared by Goh et al. (2017) for printed sUAS and

their advantages and disadvantages.

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Technology Advantages Disadvantages

Fused Filament

Fabrication (FFF, a

material extrusion

process)

• Wide variety of flexible filaments

• Relatively inexpensive process

• Options for multi-material

• Easy to start and stop process to

embed objects into the print

• Printed parts may have

“stringing” from flexible

material

• Bowden tube style designs

have a more difficult time

with flexible material

• Relatively large feature

size

Polyjet,

(A material jetting

technology)

• Fastest print process within a 5”

cube (Stereolithography vs. PolyJet:

Top 4 Differences: Stratasys Direct,

n.d.)

• Parts print fully cured

• Multi-material capable

• Can print materials with shore

hardness range of 20-90A in the

same build (Stereolithography vs.

PolyJet: Top 4 Differences:

Stratasys Direct, n.d.)

• UV cured materials can

degrade from light and

heat exposure over time

• Stratasys claims it isn’t

ideal for functional or

rugged protoyping/testing

(Stereolithography vs.

PolyJet: Top 4

Differences: Stratasys

Direct, n.d.)

Stereolithography

(SLA, a

photopolymerization

process)

• Can produce clear parts to observe

internal structures

• Fine feature size

• Flexible resins available

• Parts require additional

post processing to cure

resin, messy process

• Requires filled vat of

material, regardless of

material amount used on

part

• Single material prints

• Many parts not intended

for long term (Gibson et

al., 2014)

• Often toxic resin

Selective Laser

Sintering (SLS, a

powder bed fusion

process)

• No need for support material

(Gibson et al., 2014)

• Wide range of thermoplastics

available

• Build volume is relatively large and

multiple parts can be produced if

sized appropriately

• Requires personal

protection equipment

when dealing with

powdered material

• Parts require cooling time

(Gibson et al., 2014)

Table 2: A comparison of the same AM technologies compared by Goh et al. (2017) for printed

sUAS and their advantages and disadvantages for compliant mechanism design.

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Additively manufactured sUAS have since shown to have a wide versatility in research

applications. Projects range from project lifecycle studies (Miller et al., 2019), experimental

solutions, such as Ferraro et al.’s (2014) SLS printed sUAS fuel tank that was quickly designed

and iterated upon, and components with specific shapes that are constructed with more geometric

complexity compared to traditional building techniques like the SULSA’s elliptical wing (P.

Marks, 2011). Others have also taken advantage of being able to embed sensors and other

electronics in printed sUAS components to monitor the overall health of the vehicle that may

otherwise be undetectable to an operator on the ground (Stark et al., 2014). Many current designs

in research, commercial, and hobbyist spaces are able to take advantage of faster prototyping,

modularity in components, greater design intricacy that being fully printed allows compared to

conventional balsawood or foam building techniques.

Generally, weight reduction is paramount when designing aircraft to decrease the power

required to stay aloft and thus decrease fuel or battery consumption. AM sUAS are (typically)

Figure 5: The 3DAeroventures X-100 has a unique closed wing structure that is rarely seen made

in balsawood or foam construction methods (Haddad, n.d.).

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heavier than their traditionally constructed counterparts (e.g., density of balsa wood is 0.11-

0.14g/cm3, PLA is ~1.24 g/cm3), but have many other advances over traditional designs (e.g.,

geometric complexity, manufacturability) that some users may not want to sacrifice, though this

could change with material advancements.

Given that this material is heavier, designing for light-weight structures has even more

importance. For example, a traditional aircraft aileron has multiple components that must be

assembled to get a functional part. By reducing the amount of material per component, or even

the number of components altogether, weight savings can accumulate.

Morphing Wing Technology

Morphing wing technology has been around since the dawn of heavier-than-air aviation.

Early designers saw a way to mimic the control mechanism of birds with the lightweight fabric

and wood structures of the time (Barbarino et al., 2011). As aircraft became heavier and greater

wing loadings developed, having warping wings for control that were strong enough to support

the aircraft were not possible. The transition to hinged ailerons allowed designers to bulk up

wings for strength and higher loading while maintaining controllability. This has been the

standard ever since, with varying degrees of modification. Li et al. (2018) extensively documents

the progression of morphing wing technology and Table 3 clearly highlights the multi-decade gap

in morphing wing development. In some cases, morphing wings can take the form of a CM.

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Year Information Concept

1903 Wright Brothers’ flyer Twist morphing

1920 Parker variable-camber wing Variable camber

1979-1989 AFTI/F-111 MAW Variable sweep & camber

1995-1999 Smart Wing Program Phase I concepts Variable camber

1996-2001 Active aeroelastic wing Variable camber

1997-2001 Smart Wing Program Phase I concepts Variable camber

1999 Active hydrofoil Variable camber

1999 Finger concept by DLR Variable camber

2000 Belt-rib concept by DLR Variable camber

2000 FlexSys mission-adaptive compliant wing Variable camber

2003-2006 Lockheed Martin Z-wing concept Folding wing

2003-2006 NextGen aeronautics bat-wing concept Variable sweep

2003 SMA reconfigurable aerofoil Variable camber

2003 HECS wing Span morphing

2004 Multi-section variable-camber wing Variable camber

2004 Variable-gull-wing morphing aircraft Folding wing

2004 Virginia Polytechnic Institute and State

University telescoping-wing aircraft Span morphing

2005 Morphing inflatable wing Variable camber & twist

morphing

2006 Morphing HECS wing Span morphing

2007 Pneumatic telescoping wing Span morphing

2007 Supekar morphing wing Span morphing

2008 Antagonistic SMA-based morphing aerofoil Variable camber

2008 Bistable composite morphing-wing concepts Variable sweep

2008 Morphlet (morphing winglet) Folding wing

2009 Adaptive wing with SMA torsion actuators Variable camber

2010 Warp-controlled twist morphing wing Twist morphing

2011 Spa extending morphing wing Span morphing

2012 Multisegmented Folding Wing Folding wing

2012 SADE: seamless aeroelastic wing Variable camber

2013 Adaptive bending-twist coupling wing Twist morphing

2013 Bat-like morphing-wing Folding wing

2014 Compliant adaptive wing leading edge Variable camber

2015 Span-extending blade tip Span morphing

2015 Spanwise morphing trailing edge Variable camber

2016 GNATSpar wing Span morphing

2016 Twist morphing wing segments Twist morphing

2016 Morphing wing-tip Variable camber

2016 Compliant structures-based wing and

wingtip morphing devices Variable camber

2017 Feathered wing Folding wing

2017 Aquatic micro air vehicle Variable sweep

Table 3: A table adapted from Li et al. (2018) showing the development of morphing wing

concepts over time.

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Various design teams have been working to bring morphing wings back into the full-

scale aviation space. The company Flexsys has partnered with the Air Force Research Laboratory

(AFRL) to develop what they claim is the “first seamless, hinge-free shape morphing wing”

(FlexFoil, n.d.). As stated in Kota et al. (2009), one of the key drivers behind the technology is

reduced drag that leads to fuel savings. An image of the morphing wing system is in Figure 6.

Morphing wings could similarly by used for sUAS. Yokozeki et al. (2014) shows the

development and aerodynamic data for one such possible morphing wing design. This design

features a corrugated inner structure with a smooth outer skin and a wire that is tensioned to

deflect the airfoil (Figure 7). Their study compared a hinged control surface with their morphing

design and suggested improved performance in coefficient of lift. The design, however, is only

able to deflect in a single direction and is a bit complicated to assemble, with wires that need to

be a precise length and tension. The design was also not printed, but instead constructed of carbon

Figure 6: The morphing wing system developed by Flexsys on their test aircraft (Kota et al.,

2016).

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fiber reinforced plastics and plastic foam, though the generally monolithic form does lend well to

the FFF process as no support material would be needed on the mechanism portion of the design.

Montgomery et al. (2019) compared the aerodynamic results of morphing wings using

numerical and analytical approaches to look at the roll control of a morphing wing aircraft. Their

research shows that the analysis methods employed are “orders of magnitude faster than

computational fluid dynamics” (CFD). They repeatedly emphasize that the methods used are not

limited to the specific morphing aircraft observed in the study and can be used to analyze other

designs.

AFRL created their own morphing wing called the Variable Camber Compliant Wing

(VCCW) (Joo et al., 2015). The VCCW was designed to change the wings camber by controlling

both the leading and trailing edge of the wing using a single actuator. Different sizes of the wing

section were constructed and wind tunnel testing performed. C.R. Marks et al. (2015) further

explores the aerodynamics of the VCCW with a focus on airframe noise generation.

AM with Morphing Wings

There have been several advancements in recent years bringing morphing wings and

additively manufactured aircraft together. Woods & Friswell (2012) created what is known as the

FishBone Active Camber (FishBAC) concept. This design, built in a HP DesignJet that uses the

Figure 7: A schematic diagram of the morphing wing developed by Yokozeki et al. (2014).

17

FFF process, features a structure that mimics a fish’s spine and ribs with a trailing edge strip that

holds two tendons that are actuated near the leading edge (Figure 8). The tendons are put into

either tension or compression (one tendon opposite the other) and that pulls the solid piece in the

trailing edge, but because of the spine it deflects in a smooth, curved shape to the tensioned side.

The external skin of the FishBAC is an Elastomeric Matrix Composite (EMC), so the design is

not fully 3D printed. The design showed to have increases in the lift/drag ratio, similar to

Yokozeki et al. (2014), when compared to a regular hinged flap and standard airfoil. Moulton &

Hunsaker (2021) have also developed a slotted-skin printed morphing airfoil section that is made

with AM, they performed some preliminary analysis, but it has not yet been flight tested.

Acosta (2020) describes a later variant of the VCCW (discussed in previous section) and

the VCCW’s flight verification and mentions that the structure makes use of 3D printed carbon

fiber spars to connect the wing ribs. The VCCW model that was flown was tested on a sUAS and

able to deflect both up and down and functioned as full-span ailerons with three points of

(a) (b)

Figure 8: A schematic diagram of the FishBAC design (a) and images comparing the deflections

of the design (b) (Woods & Friswell, 2012).

18

deflection in each wing-half. Acosta (2020) shows that the morphing wing structure is functional

on an actual small-scale aircraft and that 3D printed components can be used, practically, in the

design.

Morphing wings are not limited to control surfaces. Vocke et al. (2011) created a span-

morphing wing with sUAS integration in mind. Their wing skin is made from an EMC while the

inside structure is a stereolithography (SLA) printed structure. With this structure, they were able

to produce a wing section that could increase wing area up to 100%. Vocke et al. (2011) also

makes note that further research would have to be conducted to see if their print method would be

viable for a sUAS, but serves as a good ground-testing piece.

Fasel et al. (2020) made a significant leap in printed morphing wing technology. They

designed and printed rib structures using continuous carbon fiber printing (an advancement of

FFF). These rib structures, however, each have their own servo that moves a rod that flexes the

trailing edge. The compliant nature of the structure then flexes in a desired curve. A schematic of

this design is in Figure 9. Several of these rib sections were assembled into a wing and covered

with a thermoplastic skin. The rest of the aircraft was similarly constructed with the tail also

having printed compliant structures. Fasel et al.’s (2020) greatest advancement is having brought

the aircraft out of the wind tunnels and flown it in real world conditions, being one of the first to

fly a sUAS where most of the control mechanisms are 3D printed. The design allowed them to

have a continuous morphing wing where several segments along its span could differentially

morph with a smooth transition from one to the next. This design, while advanced, does have the

drawback of being complicated to assemble, having many parts, and still cannot claim to be a

fully-3D printed morphing wing if that is a design goal.

19

In summary, the literature shows that much has been accomplished in the exploration of

morphing wing technologies as well as the development of AM produced sUAS with a start to

combining the two, however none of the fully-printed prototypes have a published test flight at

the time of writing.

Figure 9: A schematic diagram of the compliant morphing wing developed by Fasel et al. (2020).

20

Chapter 3

Design Process

The design process was conducted in a largely trial-and-error manner with relevant

research into other solutions in both additive and conventionally manufactured designs for

inspiration. The relatively quick turnaround time of additive manufacturing allowed for physical

prototyping of the design iterations, and is an example of how the technology has changed the

design process itself.

Design Considerations and Limitations

Three categories of design considerations were taken into account in the design process,

broken down into primary, secondary, and tertiary categories of importance. Primary

considerations dealt with material selection and print processes limitations. Secondary

considerations involved the overall mechanism shape and requirements to properly integrate it on

a sUAS testbed, discussed in the Aircraft Design section. Tertiary design considerations were

ease of assembly, material use, and print time; these are further discussed in the three Design Test

sections as they pertain to each part.

The first step taken was to select a print process to work within. ASTM F42 group has

developed standard terminology for seven different AM processes in F2792-12a. For the scope of

this study, only desktop applications that are simple to use and widely available to the general

public (via libraries, universities, makerspaces, personal use, etc.) were considered. This

immediately eliminates binder jetting, material jetting, powder bed fusion, and direct energy

deposition. Vat photopolymerization was eliminated due to the generally fragile nature of the

parts produced by this method. It was also eliminated because complex geometries were

21

anticipated and the parts produced by this process require ultraviolet curing. It is challenging to

properly cure the internal surfaces of thin, hollow parts and to do so properly would require

additional design considerations. Sheet lamination was also another process that was considered,

but it is one of the more wasteful AM processes, thus negating some of the reason to use AM for

a sUAS (Gibson et al., 2014). The process also can have difficulty with hollow parts in some

cases, which is a technique used by sUAS for weight savings (Mein, 2020). By far, the most

popular method among those making 3D printed sUAS appears to be FFF. Many designs have

become commercially and freely available that are designed for the FFF process (Šverko, 2019).

This process is also typically readily available in makerspace communities and as a desktop

machine. For these reasons, FFF was the printing process selected for this study.

Material selection was the next step. As FFF was the selected process, a thermoplastic

capable of being used on common FFF systems was needed. The most popular materials are PLA,

ABS, Ultem, PETG, and a foaming PLA variant. Goh et al. (2017) discusses the strengths and

weaknesses of some of these materials for sUAS in further depth. Notably for FFF materials, they

list ABS as the least dense material and polyphenylsulfone (PPSF) as the material with the

highest elastic modulus. TPUs of 90A, 95A, and 98A were added to this list for consideration for

the CM portion of the design because of their rubber-like flexibility and the potential use for such

a material. Rejected materials are summarized in the following list, with all prices as of March

2021 for a 1kg spool1:

• Ultem and PPSF - high cost, $250 and $225, respectively.

• ABS - concern of potentially harmful fumes released during print and

requirement of additional print enclosures for a more controlled “chamber”

temperature to prevent thermal warping, $30.

1 www.3dxtech.com

22

• PETG – denser than PLA and lower elastic modulus (Wang et al., 2017 and

Durgashyam et al., 2019), $32.

• Foaming PLA – weak when compared to regular PLA and TPU and tends to

deform on hot days, $55 for 750g (LW-PLA NATURAL, n.d.).

Multi-material printing was considered, but ultimately decided against due to the added

design complexity. Dual material extrusion is also a technology not as readily available to smaller

makerspaces or individuals due to the higher cost. PLA and TPU were ultimately selected as the

two materials of interest. PLA is one of the least expensive materials available with some spools

being available for less than $20 and TPU provides flexibility which was a consideration for this

project at a fairly low cost.

Aircraft Design

The requirements laid out for the aircraft design were driven by the considerations and

limitations of the CM. To make things simple, a symmetric airfoil was be selected so that the

final mechanism design could be mirrored about the chord line if needed for symmetric

movement. To ensure that there was plenty of room within the wing structure for mechanism

movement, a thicker airfoil was selected. The chord of the airfoil was limited to 200mm in length

to fit within the diagonal build plate area of a Lulzbot Mini 2. The maximum length of a single

aileron segment was set at 180mm to fit within the Mini 2’s Z-axis limits. Figure 10 shows a

diagram of the Mini 2. The final imposed requirement was the ability to readily swap the

outboard part of the wing between a conventional hinged aileron and the CM aileron. This feature

was implemented to make it easier to rapidly test both designs in the field. The remaining aircraft

dimensions were sized around the wing considerations.

23

An additional item to note on the design of AM sUAS is that the printed wing sections

are orientated such that the Z (vertical) axis of the FFF printer is in line with the span of the wing.

This orientation allows for a smooth wing outer mold line, however, with this orientation there is

greater chance of cracks or gaps appearing in the wing skin as layers separate. Care must be taken

to adequately design the wing segments so that the skin is not overstressed.

Aircraft Structure

The aircraft was designed to be as simple and easy to work on as possible (Figure 11).

The selected design minimized time spent designing the plane and gave more time to the design

Figure 10: A schematic diagram the Lulzbot Mini 2’s build volume (LulzBot Mini 2, n.d.).

24

of the CM. The fuselage of the aircraft is a square carbon tube extending from the nose to tail. It

has the motor and ESC mount, battery pod, wing, and tail surfaces screwed into it. The motor and

ESC mount, battery pod, landing gear, wing, tail, and electronics tray are all 3D printed out of

PLA. PLA was selected for these parts due to the earlier mentioned considerations in addition to

the ability to rapidly prototype and change various aspects of the plane as seen fit during testing.

The wing attaches to the fuselage tube via a central pod that tilts it up at a fixed 3° angle of

incidence. The tail pieces were basic flat plates with rounded leading and trailing edges with the

vertical stabilizer having a curve to its leading edge. These structures used a simple piano-hinge

style control surface attachment because they were not being tested. An early version of the

aircraft is in Figure 11.

The airfoil selected was a NACA 0018 due to its symmetry and thickness. The thickness

allowed for sufficient internal volume to fit servos, the wing spars, wiring, and the CM. The

symmetry was selected so as to have a mirrored design with symmetric movement. The wingspan

Figure 11: An earlier iteration of the aircraft with a weaker landing gear design.

25

of the aircraft is 1280mm with the center fuselage pod being 80mm wide. The wing is a constant

chord, rectangular planform wing with no twist or dihedral. A panel is fitted into the spars on

each wingtip to cover the open gap of the wing structure, seen in Figure 12. The panels also

notably gave improved yaw handling during initial prototype testing. The panels slide on and off

without tools and allow the outer wing section, containing the aileron, to be removed quickly and

easily. The ability to remove the aileron section easily was a key requirement to make the

experimental comparison process of the two aileron types quicker in the field, if needed. It was

also designed to make the ideation, print, build, time-to-fly cycle shorter.

The final design of the completed plane can be seen in Figure 12. The flying weight with

the hinge ailerons is 1923g while with the morphing ailerons the weight is 2000g. Aerodynamic

“cleanliness” was not a factor in the design of the central body since the ailerons were the area of

interest. Two pitot tubes were mounted in the wing equidistant from the aircraft centerline on

opposite wing halves. This was both for redundancy and so extra data would be collected in the

event that the prototype design caused significant yawing motion, which was not observed.

Figure 12: The final design of the aircraft in flight, notice the wingtip covers.

26

The aircraft underwent several modifications in its early design phases. AM promoted

this since components were easy to redesign and produce within a day if needed. During initial

testing, the landing gear broke several times as earlier versions were too weak. The tail originally

had a wheel for steerability on the ground, but after a few landings broke off. A skid was instead

printed and placed on the tail. It was designed to simply attach so that when it would slowly wear

over continual use, another could be quickly printed and replaced. Figure 12 also shows the

internal structure of the printed vertical stabilizer in silhouette.

Conventional Aileron Design

The conventional aileron design used as a control in this study was also 3D printed, but

the design was composed of multiple parts that required assembly (Figure 13). The inner structure

used the same strut pattern as the main wing. It is 180mm in spanwise length and maintains the

200mm chord of the wing. The aileron itself is a separate piece from the wing segment, connected

at three different hinge points. These hinge points are separately printed TPU tabs that are glued

into channels on both pieces. On the top side of the wing section there is a servo pocket with a

hole leading to the interior of the wing for the servo wire. A control horn is glued to the upper

surface of the aileron and a pushrod linkage is attached. A linkage is also attached to the servo

control horn and a carbon pushrod slotted through them and secured. Through flight testing, it

was determined that the servo pocket was too large and was causing airflow separation from the

wing. Flat pieces of scrap print material were glued over top with the edges sanded down. This

pocket, ideally, would be on the bottom wing surface, but the manner in which the aircraft rocked

while on the ground created concerns for the control horns striking the ground and detaching.

This extra material was not anticipated to affect the roll rate to a measurable degree.

27

Component Selection

The list of used components is in Table 4. The different servo used on the morphing wing

was due to an identical servo to the one used in the hinge aileron not being available at the time of

purchase.

Table 4: Components used and their purposes.

Brand Item Use

E-Flite Power 10 motor Propulsion

Aerostar 50A 2-6s ESC Motor speed controller

APC 11 x 5.5-inch propeller Propulsion

Holybro Power Module v3 Supplies power from battery to ESC and Pixhawk

3DR Pixhawk flight controller Flight controller, logs data and runs Ardupilot

software

3DR 915MHz telemetry radio Provides real-time telemetry data

3DR GPS Module Logs position data and assists with automated flight

modes

3DR Pitot Tube Gathers airspeed data

YEP 20A 2-12s SBEC Supplies 5V to servo rail of Pixhawk

FrSky X8R Control receiver

Corona CS919MG Control servo (hinge aileron, rudder, elevator)

Corona CS238MG Control servo (morphing aileron)

Zippy 3s 2200mAh Battery

Figure 13: The conventionally hinged aileron design.

28

Initial Design Tests: TPU Arm Variants

The first design explored was a short spanwise section of aileron to gain initial

impressions on the properties of the TPU used and to test the design creation process. Early tests

fairly quickly ruled out the use of TPUs with a shore hardness value lower than 98A for any

structural components or pieces requiring compression due to lack of stiffness; this also

eliminated the 90A and 95A TPUs. The first model shown in the top of Figure 14 takes the

profile of the airfoil with some basic structure to maintain the shape and arms attached to the

interior of the trailing edge. The thought with this design was that the servo arms would have a

push/pull motion on the TPU arms to deflect the trailing edge. At this time, cylindrical spars were

planned to be used in the aircraft wing and the design tried to leverage this to be able to rotate

around the spar. Manually pushing and pulling on the arms did produce a desirable movement of

the trailing edge, however it also caused the skin to wrinkle and the airfoil shape to twist in front

of the rear spar.

Figure 14: The first TPU arm design (top) and the second (bottom), notice the significant

stringing in the bottom design. This is undesirable due to added weight and interference with

motion.

29

The second design iteration (Figure 14 bottom) switched to square tube spars. This was to

prevent rotation of the wing due to aerodynamic forces. Due to the lessons learned from the first

variant, the second also saw two arms for a push/pull action but this time the arms ran mostly

independent to the intended servo position from the trailing edge. A thicker section rigidly

attached to the rear spar extends to the trailing edge to provide leverage and somewhat guide the

trailing edge along the intended path. This design ended up having relatively little movement and

wrinkling still occurred on the skin. It also showed, like version one, that the pushing motion was

unreliable with the TPU 98A.

Design versions three and four began exploring what would happen if the trailing edge

sections were printed in a way that biased them to spring outward. This action would mean that

they would naturally want to push against the oncoming airflow and would only need to be pulled

inward when a control deflection was not desired. This was partially inspired by a compliant

grasper device that pulled a central rod to bring together two gripper arms (Diepens 2015).

Version three, Figure 15 (top), experimented shown with a triangular cut pattern in the interior of

the arms to further promote a bending action. This worked well, however the rigidity to push

against the airflow was lost. Design four (Figure 15, bottom) had thicker arms to compensate for

this, but pulling them inward greatly distorted the airfoil outline. It is possible a combination of

thicknesses and key cuts may have produced desirable results; however, this was not explored.

All four designs were also printed in PLA as a comparison point, but they all broke when

subjected to the required deflections.

30

(a)

(b)

At this stage only 10mm long segments were being printed to save time and material,

allowing for rapid prototyping and quick evaluation of any potential concept. Focus was primarily

on the trailing edge so shortcuts were taken with the leading half of the parts by using standard

infill patterns with no custom design.

Figure 15: The third TPU arm design (top, a) and the fourth (bottom, a). The unwanted form of

the fourth design during the inward deflection is demonstrated in (b).

31

Secondary Design Tests: Compliant Shapes

The fifth attempt at a CM came in the form of the Compliant Rolling-contact Architected

Materials (CRAM) piece. The CRAM makes use of compliant rolling-contact joints (CRJ) to

have a single piece design that gets rolled or folded up and assembled into a mechanism with

desired movement (Shaw et al., 2018). To assess the feasibility of this design, a basic two-roller,

two-layer piece was printed in both PLA and TPU, shown in Figure 16. The PLA version was

initially very stiff after initial assembly, but soon broke after a few rolling motions. The TPU

version was much easier to assemble, however the bands connecting the two rollers also broke

quickly due to how thin it was required to be for proper movement. The design requires the use of

multiple fasteners which also increases overall weight and part count to undesirable levels.

Figure 16: The CRAM test piece with PLA.

32

The next design considered was a modified version of the Flex-16 (Figure 17, c and d)

system developed by Fowler et al. (2014). The original design is a monolithic flexure device that

can rotate 90° in both directions. The demonstration model was printed from titanium (Fowler et

al., 2014). To start, the same design was modeled and scaled appropriately to fit within the airfoil

section. This, however, proved to be too stiff for the given size if made from PLA and a modified

version needed to be designed. The modified Flex-16 featured less arms than the original and

allowed for movement. Roughly 0.3g was saved in the redesign with the PLA version (Figure

17c) and 0.2g with the TPU version (Figure 17d) for a 10mm tall section. With the Flex-16 TPU

version, a significant amount of stringing was present and the modified version eliminated most

of this without modifying printer parameters. The range of motion was not quite +/- 90° with the

PLA version, but roughly +/- 15-25° was achieved and is enough for an aileron. The TPU version

was able to reach the full 90°, but this is unnecessary. After just a handful of deflections, the PLA

version started experiencing fatigue in the corners where the arms meet the central circle. The

TPU version held up over several weeks of experiencing full deflections as a fidget toy.

(a) (b) (c) (d)

With this information, a full-length segment of the modified Flex-16 was printed using

TPU to see what forces would be required to deflect it for an aileron. Figure 18 demonstrates the

Figure 17: A weight comparison of the Flex-16 pieces with the two modified Flex-16 pieces in

PLA (a/c) and TPU (b/d).

33

deflection of a 10mm tall section. This 180mm segment proved to be too stiff for the size servo

being used so the decision to use it in limited sections of an aileron hinge was made.

(a) (b)

Development efforts then shifted to the creation of a full-length aileron section using the

modified Flex-16 hinge. The first version was a mostly monolithic shape that had an attachment

point to the rear wing-spar, the hinge connected to that, and the aileron on the other side. Coming

from the aileron at both ends were two sets of arms meant to attach to a servo for the push/pull

action. A 10mm tall section of this shape is in Figure 19.

(a) (b)

The hinge was placed in three 20mm long sections at each end of the aileron and in the

middle spanwise. This design was not optimized for AM as it was just meant to be an initial test.

Figure 18: The modified Flex-16 (a) and it rotated to about the maximum desired for an aileron

(b).

Figure 19: A profile of the planned TPU aileron segment undeflected (a) and deflected by pulling

on the lower arm and pushing with the top (b).

34

The print time ended up being 44 hours and 15 minutes using 111.5g of material neglecting the

supports. A matching PLA sheath was printed with this part to be the leading edge and servo

cover. It had a slightly inward biased skin to grip the TPU piece and minimize any gaps between

the two. While the desired motion was possible, weight was still a major concern with this piece

and so material reduction and manual optimization for printing was carried out. The part is shown

in Figure 20 and Figure 21.

Figure 20: A side view of the full length first TPU arm design.

35

(a) (b)

The new piece (Figure 22 and Figure 23) had triangular cuts made in the trailing edge

section to remove the bulk of material. Diagonal extensions were added to the hinge sections to

allow the printer to build on the overhang rather than use support material. The arms were slightly

shortened for a better fit. A test section at the trailing edge of the aileron was added as a type of

slot to hold the PLA outer skin. The modified TPU piece now took 31 hours and 6 minutes to

print and used 91.7g of material without supports. Very significant stringing occurred with this

part and would require more post-processing time if print parameters cannot be adjusted to

resolve this.

Figure 21: The first TPU arm design without (a) and with (b) the planned PLA leading edge

sheath.

36

(a) (b)

Final Design Tests: Sliding Skin

All prototypes until this point used Simplify3D2 for slicing. Designs had to be compatible

with the way this software processes slicing CAD models. As such, designs were made in a way

that allowed the software to make complete loops with its toolpath planning. If a design was

2 www.simplify3d.com

Figure 22: A side view of the full length second TPU arm design.

Figure 23: The second TPU arm design without (a) and with (b) the PLA leading edge sheath.

Note that this sheath would have been lengthened to cover the entire aileron section.

37

created using surface modeling, instead of solid body modeling, it would have to use complete

loops for Simplify3D to process it correctly. This limited some design options so the switch was

made to Cura3 which supports surface models natively. With Cura, surface designs could be made

without a completed loop and it handled single line extrusions properly. A comparison of the

same part sliced in Simplify3D and Cura is shown in Figure 24. The part has two free ends on the

outer loop that do not connect.

(a)

(b)

Experimentation with the PLA sheath from the full-length TPU variants, showed that a

thin wall of PLA may be stiff enough to withstand aerodynamic forces without requiring much

extra internal support. Realizing that the main challenge of previous designs was the skin

wrinkling, and with the switch to Cura, a new fully-PLA design was created (Figure 25). This

3 www.ultimaker.com/software/ultimaker-cura

Figure 24: The same surface-designed piece sliced in Simplify3D (a) and Cura (b).

38

design had a mostly standard structure for the leading-edge portion of the airfoil, but had the

trailing edge mostly empty with just the airfoil profile printed as a single perimeter. A separation

of this perimeter from the bottom skin of the airfoil was designed to avoid merging during the

print process. This inward deflected tab would be pushed or pulled by an internal servo to deflect

the remaining profile. An initial 10mm tall section was created, using a standard infill pattern for

the leading edge. The design worked as a test piece to demonstrate if the desired movement could

be achieved with the intended deflection. It was successful in this task, but the trailing edge was

determined to still be too flimsy.

More iterations of the test piece were created using one to five perimeters on the trailing

edge to add rigidity. These iterations also tested different potential methods of servo attachment

as well as other changes to ensure proper motion. Two pieces were made to test additional

supports leading from the servo attachment point to the location most susceptible to bending on

the trailing edge skin (Figure 25). These additional supports did take away from the desired

motion.

Figure 25: Three initial sliding skin designs showing the difference between the pieces with no

additional support and two different additional supports that connect to the same locations.

39

Full-length pieces were created for the pieces that had two and five perimeters on the

flexible trailing edge. These perimeters were designed on the surface-model side as opposed to

slicer side. The five-perimeter piece proved to be too stiff for the servos used so a graded-

perimeter piece was also made. This piece had the internal layers graded as they approached the

tip of the trailing edge to see if that would help maintain the proper shape and ease issues with

deflection. It was ultimately still too rigid. The two-perimeter piece showed promise as the

aileron deflected easily from the servo attachment point and the deflected aileron took a

considerable amount of force to deform out of the deflected position. It was decided to move

forward with this design.

A full surface-modeled design was created for the aileron wing section (Figure 26). This

would allow it to be lighter weight than the rest of the wing that had the internal structure

designed primarily with solid bodies. This does come at the cost of strength as there is one less

perimeter printed with the internal struts and each one is able to be a single extrusion width.

Strength in the structure is not as great of a concern in this part compared to the rest of the wing

due to the wing loading being less on the wingtip, nonetheless, the part was overdesigned to

ensure safety during testing. The same spacing in the structure pattern was used; different cutouts

from this pattern were taken compared to the conventional aileron. A single-perimeter aileron

version of the surface model was made to see if the two perimeters were actually necessary, but

the large, unsupported single perimeter aileron warped too much during the build process.

40

The piece used two servos at each end to fully support the aileron, both moving each end

of a rod that connected to the aileron skin. A ledge was designed into the part to glue the servos to

and the upper surface was given a curve to not impede the deflection of the aileron. With the

servos attached and hooked up, the piece performed nicely and gave the desired motion. To get a

more real-world sense of how much deflection would occur due to aerodynamic forces, the piece

was held out of a car window traveling at 50 miles-per-hour, slightly higher than the maximum

speed of the aircraft. The aileron did not appear to experience any significant adverse deflection

due to aerodynamic forces in either the up or down position as well as the neutral position and felt

very effective against the hand.

Figure 26: A CAD model of the first full-length sliding skin model, with two trailing edge

perimeters.

Figure 27: A CAD model of the single-servo full-length sliding skin model.

41

After determining the minimal effect of wind forces on the geometry, a single servo

variant was created in an attempt to lighten the overall piece further and keep part count low

(Figure 27 and Figure 29). This servo was placed in the middle and a slot was created in the servo

attachment bar for the servo arm. A thin metal rod was inserted in the rod and servo arm as an

attachment and held in place by glue. Note that additional support material was required to print

this slot, but it is roughly the same as that required for the two-servo variant. Two of these pieces

were printed for testing on the aircraft. Relevant printer parameters for this piece and the

conventional hinge are in Table 5 with an image comparing the two pieces in Figure 28.

Table 5: Print parameters used for the flown versions of both the conventional hinge design as well

as the CM.

Printing Parameter Value

Nozzle diameter 0.4mm

Layer height 0.2mm

Flow rate 102%

Extrusion width 0.42mm

Perimeter shells 1

Top/bottom layers 0

Hot end temperature 230°C

Build plate temperature 75°C for first layer, 70°C after

Fan speed 0%

Print speed 50mm/s, 42mm/s on walls

Travel speed 130mm/s

42

The final design met the requirement of fitting within the print volume of a Lulzbot Mini

2, used a single servo, minimal support material, and was a single piece that gave desired motion

(Figure 30). The design is seen in flight in Figure 31. With a design settled on, experimental

validation could begin.

Figure 28: A top-down view of both ailerons.

Figure 29: The top surface of the CM morphing wing aileron.

43

(a)

(b)

Figure 30: The CM morphing aileron in the up position (a) and down position (b).

Figure 31: The final design of the aircraft with the morphing ailerons in flight.

44

Chapter 4

Testing and Analysis Methods

The following section details the procedures taken to gather experimental, analytical, and

additive manufacturing data for the testing of the final design.

Flight Campaign

Each flight test campaign began with calibrating the aircraft accelerometers and compass.

When the aircraft was fully checked out and deemed ready for flight, weather conditions were

recorded (temperature, pressure altitude, winds, humidity, and air pressure). If the flight events

lasted an extended time, more weather was recorded partway through or at the end as well. Lists

of weather conditions for the data gathering flights are in the Appendix.

To compare overall effectiveness of the new design when compared to the original, the

parameter of interest for this study was the aircraft’s maximum sustained roll rate. The aircraft

performance was first evaluated with the conventionally-designed hinged aileron described in

Chapter 3. Earlier testing with this aileron was done to ensure functionality and that no additional

changes were required. After the aircraft was in a finalized state, flights were conducted to gather

the roll rate data for both the hinge ailerons and morphing ailerons. All flights were conducted

between 0 and 400 feet in altitude above the ground (AGL).

45

For the hinge ailerons, six total full-deflection events were conducted, rolling the aircraft

side to side. These rolls were made within limits of where the aircraft felt safe to operate as far as

handling (shy of approximately +/- 90° roll angle from wings-level). These rolls were made in

rapid succession from one extreme to the opposite.

For the morphing ailerons, six full-deflection roll tests were conducted in each direction

from level to the same limit as the hinge aileron. Two additional rolls were performed that did a

full 360° rotation in each direction.

Experimental Data Collection

With the Pixhawk flight controller, airspeed and altitude data were collected at 10Hz,

control output position data at 25Hz, and inertial measurement unit (IMU) data at 50Hz. For each

test, the data was imported into MATLAB and the relevant data extracted. Since the Pixhawk has

two different IMU data recordings (MPU-6000 and L3GD20 IMUs) and a recording from each of

Figure 32: An onboard view looking back at the left wing while inverted during a 360° left roll

with the morphing ailerons.

46

the two pitot tubes, the average of the pair was taken for each. Data was filtered using a second-

order Butterworth filter. Altitude data was not filtered as it was not required to get the roll data

and mostly serves as a visual when plotting. The mean and standard deviation for each roll

segment was found for airspeed and roll rate. Airspeed data was converted from indicated

airspeed to true airspeed using the observed temperature and pressure on the relevant day of

flight.

As the design of the morphing aileron seen in this document has no hinge point, a psudo-

hinge point was determined based on where two lines from the trailing edge, at maximum

deflections, are traced to corresponding symmetric points about the chord line intersect (Figure

33). Deflection angle and the aileron chord length are calculated from the point, giving a value of

82.8mm for the chord of the aileron. This value was compared to the actual hinge location on the

conventional aileron for deflection values.

Analytical Data Collection

Analysis was conducted with XFOIL to gain more aerodynamic data on the lift and drag

of the deflected morphing aileron in comparison with the hinge aileron. For the morphing

ailerons, a scanned profile of the aileron in deflected positions was used to generate the airfoil

coordinates. XFOIL’s built in hinge function was used for the hinge aileron.

Figure 33: A composite image of the morphing aileron at neutral, full up, and full down positions.

47

Additive Manufacturing Data

All additive manufacturing relevant analysis was done by comparing predicted material

usage and print time and actual material usage and print time for both of the two hinge and two

morphing ailerons. This data as well as part count and qualitative data, such as ease of assembly,

are discussed further in Chapter 5.

48

Chapter 5

Results

This chapter displays the processed flight data and details the results of comparing the

analytical and experimental models for the hinge and morphing ailerons. Key metrics considered

are roll rate and handling qualities, with the key results being that the morphing ailerons worked

well in both roll responsiveness and handling qualities. Additional aerodynamic factors and

differences in AM qualities are also discussed.

Hinged Aileron

The hinge aileron was first tested to provide baseline data for comparison. Figure 34

shows the commanded signal given to the aircraft, the roll rate, true airspeed, and altitude for the

test segment. Positive values for commanded signal and roll rate indicate a right roll. As shown in

the plot, airspeed fluctuated at times which corresponds with changes in altitude. This change in

airspeed does affect the roll rate to an extent, but the impact of rapid airspeed changes on roll is

damped due to the aircraft’s inertia. Once a commanded input was given, there was a brief rise

time while the aircraft’s roll rate accelerated before plateauing. The process repeats for each roll

direction. Roll rate to the left was higher than rolls to the right even though all deflections and

trim settings were equal. Figure 35 gives a larger view of the roll rate plot, with the yellow and

purple lines representing the average roll rate for each direction.

49

Figure 34: The overall flight data for the hinge aileron.

Figure 35: The roll rate tests for the hinge aileron with average value lines for left and right roll.

50

Morphing Aileron

The morphing aileron received similar treatment for data analysis. As mentioned in

Chapter 4, the test method was modified to return to level before the next roll test instead of

directly into the next roll. The “commanded signal” plot in Figure 36 shows this. In the roll rate

plot and Figure 37 the same short rise time and plateaus can be seen, though shorter with the

decreased time spent rolling. Average roll rates for each direction were more even than the hinged

aileron, however the left roll continued to be higher.

Figure 36: The overall flight data for the second morphing aileron roll rate flight.

51

The key takeaway for these plots is that a similar roll rate to the hinged aileron was

achieved with the morphing aileron. The data for each flight conducted is summarized

individually in Table 6 through Table 9 with all of the morphing aileron data combined in Table

10 and directly compared to the hinged aileron values from Table 6.

Figure 37: The roll rate tests for the second morphing aileron flight with average value lines for

left and right roll.

Table 6: Derived from experimental data for the hinged aileron.

Left Roll Right Roll

Average Roll Rate (deg/s) -122.88 88.91

Standard Deviation (deg/s) 12.52 14.64

Average True Airspeed (m/s) 25.50 21.80

Standard Deviation (m/s) 1.94 1.60

Table 7: Derived from experimental data for the first morphing aileron flight.

Left Roll Right Roll

Average Roll Rate (deg/s) -128.91 122.53

Standard Deviation (deg/s) 9.13 17.50

Average True Airspeed (m/s) 19.61 18.81

Standard Deviation (m/s) 1.16 4.96

52

Analytical and Experimental Models

A key metric of performance is the drag induced by aileron deflection as compared to the

lift generated. Through XFOIL analysis, a comparison of CL versus CD was developed. The plots

show comparable increases in drag with respect to lift for both configurations. Note that 100%

deflection with the morphing aileron corresponds to 70% on the hinge aileron to achieve the same

deflection angle from their respective hinge points.

Table 8: Derived from experimental data for the second morphing aileron flight.

Left Roll Right Roll

Average Roll Rate (deg/s) -101.74 97.47

Standard Deviation (deg/s) 20.09 12.60

Average True Airspeed (m/s) 16.00 13.86

Standard Deviation (m/s) 2.16 0.85

Table 9: Derived from experimental data for the full 360° rolls with the morphing ailerons.

Left Roll Right Roll

Average Roll Rate (deg/s) -124.44 121.64

Standard Deviation (deg/s) 13.60 26.58

Average True Airspeed (m/s) 19.38 19.46

Standard Deviation (m/s) 3.34 3.90

Table 10: Summary table of morphing versus hinged aileron performance.

Left Roll

(Morphing)

Left Roll

(Hinge)

Right Roll

(Morphing)

Right Roll

(Hinge)

Average Roll Rate (deg/s) -118.36 -122.88 113.88 88.91

Standard Deviation

(deg/s)

15.87 12.52 18.54 14.64

Average True Airspeed

(m/s)

16.97 25.50 16.07 21.80

Standard Deviation (m/s) 2.22 1.94 2.85 1.60

53

(a) (b)

Figure 39 shows the amount of deflection per given control input. It can be seen that the

hinge aileron follows a linear trend. The morphing aileron shows little extra input by the last

positive 50% input, about 1.33° more. The last negative 50% range saw 4.8° of deflection in

comparison. For the positive deflection, more skin surface is uncovered and does not have the

additional support of the underside tab, causing it to bow outward more instead of deflect at the

tip. The negative deflection is lessened due to the servo arm in this position causing the control

bar to move up and down more than in and out. The deflection angles were taken from the hinge

or psudo-hinge point and measured up or down from a neutral position.

Figure 38: Coefficient of lift versus the ratio of coefficient of drag for the morphing aileron (a)

and hinge aileron (b).

54

Assembly Comparison

Assembly for the hinged aileron is complex. There are many more components involved

that need to be assembled when compared to the morphing aileron. Initially, the hinge tabs need

to be glued into one of the two halves of the wing/aileron section. These tabs then need to aligned

with and inserted into the corresponding slots on the mating piece while the have glue on them.

This task can be tedious with the design used since sometimes the tabs do not go into the slot

immediately and require aligning while taking care to not accidentally glue them somewhere

undesired. Then, a servo needs to be glued in place, control linkages need to be attached to

control horns, a control rod needs to be cut to the required length and fit in place, and the aileron-

side control horn needs to be aligned properly with the servo arm and glued in position. A

component list for each aileron is in Table 11.

Two methods were created to assemble to morphing aileron. The first method involves a

metal rod slightly longer than half the aileron section’s length, slotting it through the connection

Figure 39: A plot showing the amount of deflection from neutral measured at each aileron’s

respective hinge point for a given control input percentage.

55

bar with the servo in the middle, and gluing it in place. The second method saves some weight by

using a shorter metal rod, but the print layers must be separated in the middle where the servo is

located to allow for manipulation of the two halves for proper placement before gluing the metal

rod and two skin halves back together. Both processes took less time than the conventional

aileron and saw a reduction in part count.

Once the servo control rod is attached, CA glue can be applied to the side of the servo

that will be in contact with the ledge and the connection bar with servo folded back under the

underside skin tab. The servo wire is then fed through one side of the wing section.

Print Time and Material Usage

The overall printing time and material usage of this morphing wing design was increased

in comparison to the hinge aileron. A table of the results is found below in Table 12. The raft is

excluded from the material use columns, but support is not. It can also be noted from this data

that the weight of components added to the hinge aileron is approximately 6.1g more than the

morphing aileron. The servo used for the morphing aileron weighs 22g and the servo for the

hinge aileron weighs 13g, model numbers are in Table 4. The reason for the dissimilar servos is

Table 11: A total parts list for both aileron types. Since some equipment varies on the components

it comes with for a complete kit, parts for given items were noted but not counted toward the total

part count. A complete kit is counted.

Part Hinged Aileron Morphing Aileron

Aileron/wing pieces 2 1

Servo (motor, screw, control horn) 1 1

Control horn 1 0

Pushrod 1 1

Control linkage (linkage, stopper, washer, hex nut) 2 0

Servo cover 1 0

Hinges 3 0

Total 11 3

56

that due to the availability and time between testing prototypes, new servos needed to be ordered

and an exact match could not be made. For the morphing wing servo, a higher torque servo was

selected to ensure safety with the new aileron type, similar to having the wing structure overbuilt

in this piece. Aerodynamic analysis was not affected by this decision.

Handling Qualities

The handling characteristics of the morphing aileron varied little from the hinge aileron,

and not adversely. Response was snappy and had little slop when making sharp movements. The

higher roll rate at lower airspeeds and increase in weight was not noticeable by feel. The

performance felt like a hinge aileron in both manual and automated flight modes.

Table 12: AM relevant parameters for the two aileron types as well as a comparison to the flight-

ready weight.

Aileron

Type

Estimated Print

Time (hr:min)

Actual Print

Time (hr:min)

Estimated

Material Use (g)

Material

Use (g)

Flying

Weight (g)

Hinge 7:33 8:45 72 65.03 85.31

Morphing 9:54 10:18 120 101.83 124.01

57

Chapter 6

Discussion and Conclusion

A morphing aileron design was designed, printed, and flown that shows promise for the

AM sUAS domain. Experimentally, it was shown that the morphing aileron can achieve higher

max roll rates than a printed hinge aileron at both a slower airspeed and lower deflection angle

from their respective hinge points. The morphing aileron showed strong controllability during

flight and was indistinguishable from a hinged aileron from an operator’s perspective. The

morphing aileron also was able to cut down part count from 11 to 3, but it also increased the

assembled weight. The assembly process, while different than what sUAS builders may be used

to, is simplified with the lower part count and one can make the conclusion that it can also take

less time because of this.

In its current form, the morphing aileron weighs more than the hinge aileron. Due in part

to a larger servo with higher torque being used as well as a more built up structure for safety

purposes. The total weight and material usage are, however, within 40g of the hinge aileron and

using the same servo would bring this within 30g. With additional refinements made to the

overall design of the morphing aileron it is reasonable to expect the weight can be lowered to that

of hinge aileron or less. Support material, while some consider an undesirable item in AM

produced parts, is used in the central channel that the servo control horn slots into. The amount

used is not enough to greatly increase the print time or material usage. With additional design

optimizations this could be removed while simplifying the assembly process further.

While this paper does not focus on aerodynamics, a brief analysis was conducted. The

XFOIL analysis of both airfoils at differing degrees of deflections indicate that a maximum

upward-deflected morphing aileron has slightly more lift and more drag than a hinge aileron at

the same angle of deflection. The morphing aileron also has a tighter range of lift than the

58

conventional aileron. Overall, the aerodynamics of the morphing aileron are similar to the hinge

aileron.

Future work can see a more robust aerodynamic analysis of the printed morphing aileron

and how it compares to its hinged counterpart. The model could be modified to take into account

different correction factors to use with the morphing aileron design. Different airfoils and sizes of

wing sections containing the aileron can also be used. As this is the first iteration of the morphing

aileron, it was not expected that it would fully improve upon the hinge aileron from an AM

perspective. Rather it set out to prove that by taking advantage of certain AM capabilities, such a

piece is possible and functional.

Future iterations of the morphing aileron could see an optimization of internal structure to

further reduce weight while maintaining required strength. A possible way to eliminate the need

for support structure could be to use a V-shaped channel instead of a rectangular one for the servo

control horn slot. This could also eliminate the need to split the skin layers to insert the smaller

control rod option. If the V-gap is large enough it could allow the rod to flex during insertion into

one of the two guide holes in the connection bar, something not possible with the current design.

In summary, this study highlighted a gap in morphing wing technology that had not been

filled yet. Through the use of AM technologies and leveraging the advantage of rapid

prototyping, many different iterations of a potential solution to the gap were trialed before finally

settling on a design. The design was tested experimentally and analytically to prove functionality

and get a glimpse into what aerodynamic performance would be like. The design shows promise

in several areas and with further iteration could surpass the current standard design for 3D printed

sUAS ailerons.

59

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64

Appendix A

Weather Data

Table 15: Weather conditions for the morphing aileron test flights at the end of the day.

Condition Value

Wind speed (knots) 3

Temperature (°C) -1

Pressure altitude (m) 494

Humidity (%) 78

Air Pressure (inHg) 30.27

Table 13: Weather conditions for the hinge test flights.

Condition Value

Wind speed (knots) 0-5

Temperature (°C) 24.1

Pressure altitude (m) 482

Humidity (%) 65.6

Air Pressure (inHg) 30.29

Table 14: Weather conditions for the morphing aileron test flights at the start of the day.

Condition Value

Wind speed (knots) 8

Temperature (°C) -2.5

Pressure altitude (m) 485

Humidity (%) 87

Air Pressure (inHg) 30.30

65

Appendix B

Supplementary Materials

• CM Aileron.stl

o Design file for morphing aileron that was tested and shown in Chapter 4

• CM Aileron Flights.mp4

o Video for morphing aileron test flights