Microbat: A Palm-Sized Electrically Powered Ornithopter

T. Nick Pornsin-Sirirak, Yu-Chong Tai , Chih-Ming Ho*, Matt Keennon**

Electrical Engineering, California Institute of Technology, Pasadena, CA 91125, USA

*Mechanical and Aerospace Engineering, University of California, Los Angeles, CA 90095, USA

** MAV Program, AeroVironment Inc., Simi Valley, CA 93063. USA

Abstract:

This paper reports the successful development of “Microbat,” the first electrically powered palm-sized

ornithopter. This first prototype was flown for 9 seconds in October 1998. It was powered by two 1-farad super

capacitors. Due to the rapid discharge of the capacitor power source, the flight duration was limited. To achieve a

longer flight, a rechargeable battery as a power source is preferred. The second prototype houses a small 3-gram

rechargeable Ni-Cad battery. The best flight performance for this prototype lasted 22 seconds. The latest and

current prototype is radio-controlled and is capable of turning left or right, pitching up or down. It weighs

approximately 12.5 grams. So far, the best flight duration achieved is 42 seconds. The paper also discusses the

study of flapping-wing flight in the wind tunnel using wings developed by MEMS technology. This enables a better

understanding the key elements in developing efficient wings to achieve aerodynamic advantage in flapping-wing

flight.

Keywords: Microbat, MEMS wings, flapping-wing flight, ornithopter, unsteady-state aerodynamics

1. Introduction:

Flapping-wing flight has undoubtedly been a sophisticated realm of flight and has intrigued humanity for

hundreds of years. To present days, it still remains an under-explored realm of flight. Early attempts at flapping-

wing flight relied on designs mainly based on natural flying creatures, such as, birds, bats and insects. These

biological creatures have mastered the low Reynolds number, unsteady flows that is unmatched by any man-made

systems [1-3].

The development of flapping-wing flight can be dated back to the early days when Leonardo da Vinci

designed his flapping-wing device around 1500 A.D. [4]. The first flapping-wing aircraft that had flown

successfully was in 1870 when Gustave Trouve’s ornithopter, powered by an internal combustion engine using

gunpowder, flew 70 meters in a demonstration to the French Academy of Sciences. Simple ornithopters powered by

rubber bands were soon developed. It has taken several hundred years of development in flapping-wing flight.

Today there are competitions for indoor small ornithopters that are as light as paper to heavy outdoor ornithopters

powered by gas combustion engines that have wingspan as large as 2 meters. All are fascinating to design and build

because of the endless possible variations. However, the challenge remains for the race to be the first to design a

small (palm-sized) and lightweight (<15 grams), electrically powered ornithopter. This micro-air-vehicle (MAV) is

light and has the size of a small bird (Figure 1). This has proven to be more challenging because of the limitation of

our knowledge of aerodynamics of flapping-wing flight for ornithopters of this size. In addition, the search for a

suitable power source that can sustain a long flight proves to be difficult as well. In this paper, the first successful

development of such vehicle called “Microbat” is discussed, ranging from wing and transmission designs to flight

performance of various prototypes developed thus far in the last 3 years.

2. Wing Design and Fabrication:

Given the size and weight limitation of “Microbat,” it is important that every component of the overall

system is light and minimal in weight, especially the wings. The conventional way to construct the wings is to build

the wing spars and membranes from light yet very strong materials. We first built the model wings (non MEMS-

based wings) using carbon fiber rods with 750-? m diameter as wingframes. Thin mylar film or thin paper was glued

to the carbon rods as wing membranes as seen in Figure 2. This method of making wings is cumbersome and there

are several disadvantages: 1) glue adds weight and wings become too heavy; 2) identical sets of wings are difficult

to achieve unless molds are made for each fabrication; 3) it is costly, time-consuming, and has a slow turn-around

time. This method cannot accommodate effectively and efficiently the study of the design variable changes.

For many reasons we claim the new MEMS wing technology is necessary because MEMS wings enable

systematic research in terms of repeatability, size control, weight minimization, mass production, and fast turn-

around time. Moreover, complicated structures, such as dragonfly, butterfly, and beetle wings can be easily

fabricated using photolithography technology. We have experimented with various materials for wingframe

structures. For this project, we have chosen titanium-alloy metal (Ti-6Al-4V) for several reasons [5]. It is light and

strong and can be easily tapered to vary the thickness of wingspars. Because titanium-alloy is ductile, it also can be

bent to create wing camber to improve performance. In addition, the etching process of titanium-alloy can be

conducted at room temperature and yields a reasonable etching rate. For wing membranes, we selected parylene-C

[6-8]. There are several advantages of using parylene-C as wing membrane: 1) it can be deposited directly onto

titanium-alloy at any desired thickness; 2) its adhesion to titanium-alloy is excellent; 3) parylene film is light and

strong, and can withstand high flapping frequency of more than 30 hertz without tearing; and 4) parylene-C is

deposited at room temperature and yields a conformal coating. Thus, step corners can be uniformly coated. Figure

3 shows various fabricated titanium-alloy MEMS wings ranging from insect to simple spar wings. The fabrication

process of titanium-alloy MEMS wing samples is shown in Figure 4. First, the dry film resist was laminated on both

sides of the titanium-alloy substrate. The front-side resist was patterned and the substrate was etched in a mixed

solution of 5% HF and 2% HNO3. Then the resist was stripped and the new dry film resist was laminated on the

backside. Next, parylene-C was deposited. Afterwards, the dry film resist was stripped, leaving clear membranes of

parylene-C. Finally, to strengthen the wing membranes, the second layer of parylene-C was deposited. This new

wing design results in a 40% wing area reduction compared to the non-MEMS wings and yet still outperformed

them in terms of lift and thrust productions.

3. Transmission Design

A lightweight, low-friction transmission mechanism was built to convert the rotary motion of the driving

motor into the flapping motion of the wings. Four transmission designs were considered and shown in Figure 5.

Based on simplicity, minimal weight, and flapping symmetry, only the design of transmission C was implemented

and built as shown in Figure 6. This design restricts the flapping motion in a plane perpendicular to the motor shaft.

A small DC motor with gearbox ratio of 22:1 was used to drive the transmission. The power of 1.5 watts can be

used to drive this motor. At this power, with no wing attached, the transmission can flap up to 42 Hz continuously

for a few minutes without overheating the motor. The wings were then mounted on the transmission system and

several flapping tests were performed. They could withstand more than 30 Hz of flapping. Neither breaking nor

tearing of wing membrane was observed.

4. Wind Tunnel Test Results:

A high quality low-speed wind tunnel with velocity uniformity of 0.5% and speeds from 1 m/s to 10 m/s

was constructed. The wind tunnel has a 30x30x60 cm test section with a 4:1 contraction. Force measurements were

taken using low capacity 2-D force loadcells. This test setup is shown in Figure 7. The unsteady-state aerodynamic

performance of natural insect wings, non-MEMS wings, and MEMS wings were studied. It is found that the

spanwise stiffness is an important factor in lift production in flapping flight. For the same size of wings, the wings

with rigid leading edges produce larger lift coefficients compared to those with flexible leading edges as illustrated

in Figure 8. It is also found that flapping frequency is an important factor by observing the advance ratio, J, and lift

relationship. The advance ratio is the ratio of the flight speed to the speed of the wingtip. Typically, unsteady-state

flight has an advance ratio less than 1, which means that the flyers flap their wings very fast. Natural fliers such as

bumblebee, black fly and fruit fly have an advance ratio in free flight of 0.66, 0.50 and 0.33, respectively [9]. At

this low advance ratio regime, generation of vortices can be observed. For large advance ratios (>1), no vortices

were created and the flow was always attached (quasi-steady flow.) However, as the advance ratio was reduced

below unity (unsteady flow), the dynamic stall vortices appeared regardless of the chord size of the wing. The

diameters of the dynamic stall vortices were observed to be 3-4 cm near the midspan region. At the start of the

downstroke, the flow stagnates at the leading edge of the wing. The stagnation line progressively moves to the

upper surface of the wing, thereby by forming a leading edge vortex. This vortex continues to grow and attains its

maximum size at about the middle of the downstroke. It is finally shed at the start of the upstroke. This process is

illustrated with flow visualization aid in Figure 9.

The wind tunnel test results also show that nature-mimic MEMS wings with complicated structure

performed poorly compared to the real wings. This is because the real natural wings are much lighter and more

rigid. They also have 3-D shapes. Therefore, instead of trying to mimic the natural wings, the focus was shifted to

design and fabricate simpler wings that could generate enough lift and thrust to fly the prototypes. These wings are

compared and listed in Table 1. Lift and thrust coefficients resulted from the wind tunnel test are shown in Figure

10 and the input power required to flap these wings is shown in Figure 11. The current MEMS wing type D

(CIT7x3S20) with rigid leading edge shows the best result in terms of lift, thrust, and power required among the

rest. It only requires one watt of power to flap at 30 hertz while other wings would have required higher power

inputs.

For the first time the effect of the inboard and outboard region in relation to thrust and lift generations is

also identified. This effect is shown in Figure 12. Two wings were compared where the inboard region of one wing

was arbitrarily removed. Since the wing speed varies along the span, the strength of the dynamic stall vortex will

also vary and thus the lift. The rotational speed of the wings is higher towards the tips and leads to stronger amounts

of vorticity in the outboard region of the wings. Therefore, it can be expected that the bulk of the lift be produced in

the outboard region of the wings. Removal of the inboard region did not affect the lift coefficient as seen in Figure

12 because the vorticity outboard was not affected. However, the thrust production was influenced. The thrust

performance of the wing without the inboard region deteriorates when compared to that of the wing with the inboard

region, which is yet another indication of the dependence of thrust on the vortex shedding.

5. Microbat Prototype Vehicles and Its Flight Performance

5.1 Super Capacitor-Powered Ornithopter

The first Microbat prototype was built as a super capacitor-powered electric motor free-flight ornithopter,

shown in Figure 13 a). This prototype weighs only 7.5 grams. The mass summary is shown in Table 2. The system

is composed of a light electric motor, a transmission system, two 1-farad super capacitors, wings, a carbon-fiber-rod

fuselage, tail stabilizers, and component covers to reduce drag. On the bench test, the flapping duration was less

than a minute before having to recharge the capacitors. The first successful free flight was achieved with the longest

duration of 9 seconds. This marked the first milestone in electrically powered flapping-wing flight. The time,

however, is much shorter compared to the Ni-Cad battery’s discharge time. Therefore, the battery power is

preferred.

5.2 Battery-Powered Ornithopter

Because the goal is to use battery to provide a longer power source, the next and improved ornithopter is

battery-powered electric motor free flight with dc-to-dc converter. This prototype is shown in Figure 13 b). As the

size of the flyers decreases, finding a powerful, yet light, power source has become one of the most difficult

challenges. The lightest rechargeable battery available found “off-the-shelf” in the market is Sanyo Ni-Cad N-50. It

weighs about 3.5 grams. The casing was trimmed as thin as possible to reduce the weight to 3 grams. Since the

battery produces only 1 volt nominally and the drive motor requires 4 to 6 volts, the dc-to-dc voltage converter was

custom-built to step up the voltage required to operate the electric motor. It weighs only 2 grams. The voltage

output is adjustable and can be set before each test flight. The advantage of the converter, single Ni-Cad cell

propulsion system is that it is lightweight and can fully takes advantage of the specific power and energy of the 50-

mAhr Ni-Cad cell. It appears that a higher quantity of smaller batteries to achieve the same performance for weight

cannot be used due to weight limit. The operating endurance of the entire system varied between almost 4 minutes

at 4 volts and approximately 50 seconds at 6 volts on the bench test. Figure 14 shows the discharge time for various

output voltage settings of the converter. As the battery continues to discharge to a certain length of time, the voltage

starts to drop. This will shorten the flapping frequency. As a result, the lift generation will deteriorate and the flight

time will decrease.

This battery-powered version of Microbat has a wingspan of 6 inches and weighs about 10.5 grams. The

mass summary is listed in Table 3. The fuselage length is designed to be longer than that of the last prototype to

achieve a better flight stability. For the same wingspan, the best flight achieved has a duration of 18 seconds,

doubled the last prototype’s flight duration. With the wingspan increases to 9 inches, the best flight duration

achieved was 22 seconds.

5.3 Battery-Powered Ornithopter with Radio Control System

The latest Microbat prototype, shown in Figure 13 c), has the wingspan of 9 inches. The aircraft weighs

approximately 12.5 grams. It features airframe that was redesigned to be lighter, stronger, and more aerodynamic.

The structure is made up of carbon rods for the body, and plastic foam sheet for the tail. The body is covered with

very thin mylar film to reduce drag. It also includes a larger motor, the improved gearbox design, power converter

and the new radio control system. In the spring of 2000, the motor and gearbox configuration were improved to

increase the power and efficiency of the motor, and decrease the mass of the gearbox. The mass of the gearbox was

reduced from 1.8 g to 0.9 g, and it was machined from aluminum instead of plastic. The aircraft power source is a

single Sanyo 50-mAhr Ni-Cad cell, the same as in the previous prototype. A custom power converter was used to

boost the nominal 1.1 volts to 5 volts for the propulsion motor and radio control system. A simple remote control

system was implemented to allow a pilot to control the throttle, elevator, and rudder on the Microbat prototype. The

wireless radio link is based on the RF Monolithics (RFM) ASH receivers and amplitude modulation transmitters.

These are small multi-chip-modules, which are lightweight and require few additional components to operate. The

radio receiver operates at 916 MHz and is a simple direct conversion type using on/off keying which is similar to

amplitude modulation. Using the RFM parts for the radio link, a digital protocol was then designed to encode and

decode direction commands. Since the RFM radio link is low power and prone to spurious noise over the desired

signal, digital command encoding was necessary to prevent erroneous controls. The digital processing was done

using a Microchip Technology brand 8-bit microcontroller, commonly called a PIC chip. This component has

sufficient processing power, with low power consumption, and small size and weight.

The elevator and rudder are controlled using muscle-wire actuators in a pull-pull arrangement. The

actuators are a new lightweight design. The installed mass of each actuator is approximately 0.1 grams. Muscle

wires are shape-memory-alloy (SMA) wires that have the property of shrinking in length when heated, and

expanding when cooled. The actuators are built into tail surfaces, both the rudder and elevator. A voltage is applied

to a wire; it heats, then shrinks, and then pulls on the control flap (rudder or elevator). Actuating the wire on the

opposite side executes the opposite control. The SMA wire also has sufficient electrical resistance to allow the wire

itself to act as an electrical resistive heater at reasonable voltages and currents. The voltage used is typically 3 to 6

volts and the current ranges from 30 to 90 milliamps.

The first flight test of this prototype was very successful. On December 7th, 2000, the ornithopter flew for

42 seconds on its first flight. The aircraft flew under radio control, where the pilot could control left and right turns,

pitching angle, and motor on/off. During this flight right and left turns were commanded and aircraft responded

appropriately. The flight duration was mainly limited by the power system and vehicle’s weight.

6. Future Consideration

For a MAV at this size, the weight constraint may have almost reached its limit for the current design. To

fly longer, further reduction of weight may be required. A new design may have to be considered. It perhaps

require upgrading to an even bigger motor with better efficiency, a new high-efficiency DC-to-DC converter, larger

wings to gain more lift and thrust, or a larger battery to achieve a longer life. This also means a higher weight

budget to accommodate all these changes. Nevertheless, the key component is the power source and it will remain

an important issue to the next. Unless the future technology can provide a lighter yet more powerful rechargeable

battery, it will be a challenging task to significantly improve the flight performance of the current ornithopter of this

size.

7. Conclusion

We have presented an overall development of Microbat, the first small-sized, electrically powered

ornithopter, including the development of novel titanium-alloy wingframe and the wind tunnel test results. We

believe that only MEMS technology can easily and systematically accommodate these many variable changes with a

fast turn-around time. These wings have been tested under cyclic conditions to assess long-term reliability in a wind

tunnel. Three main types of Microbat prototypes were built and test-flown: super capacitor-powered, battery-

powered, and battery-powered with radio control system. The best free flight duration of 9 and 22 seconds were

achieved by super capacitor-powered and battery-powered ornithopters, respectively. With radio control system, the

flight duration was significantly longer at 42 seconds.

Flapping-wing is one of the most fascinating yet least understanding realms of flight. Early attempts were

frustrated by the complexity and power requirements. Although flapping flight is often seen as a throwback to the

early days of aviation, it actually requires sophisticated technology and will not realize its full potential until well

into the future. We have nature's fine examples to show what is possible and to look forward to. Although nature

has over millions of years of evolution head start, what we were able to achieve today is considered a reasonably

good start.

8. Acknowledgements

This work is supported under DARPA/TTO MAV program DABT63-98-C-0005. The authors would like

to thank Dr. Hany Nassef and Steve Ho from UCLA for the wind tunnel testing, and Joel Grasmeyer at the MAV

program, AeroVironment, Inc., for his assistance in the system integration and flight test.

References:

[1] M. Okamoto, K. Yasuda, A. Azuma, “Aerodynamic Characteristics of the Wings and Body of a Dragonfly,”

Journal of Experimental Biology, vol 199, 1996, pp. 281-294.

[2] M. Sato, A. Azuma, “The Flight Performance of a Damselfly Ceriagrion Melanuram Selys,” Journal of

Experimental Biology, vol 200, 1997, pp. 1765-1779.

[3] M. F. M. Osborne, “Aerodynamic of Flapping Flight with Application to Insects,” Journal of Experimental

Biology, vol 28, 1951, pp. 221-245.

[4] J.B. Anders, “ Biomimetic Flow Control,” Fluids 2000, Denver, CO, June 19-22, 2000.

[5] American Society for Metals, “Metal Handbook,” 9th edition, vol 3, 1980, pp. 388-391.

[6] X.Q. Wang, A. Desai, Y.C. Tai, L. Licklider, T. D. Lee, “ Polymer-based Electrospray Chips for Mass

Spectrometry,” MEMS 1999, Florida, USA, Jan. 17-21, 1999, pp. 523-528.

[7] J.T.C. Yeh, K.R. Grebe, “Patterning of Poly-Para-Xylylenes by Reactive Ion Etching,” Journal of Vacuum

Science & Technology A, vol 1, issue 2, 1983, pp. 604-608.

[8] Parylene Conformal Coatings Specifications and Properties, Specialty Coating Systems, Inc., 5707 West

Minnesota Street, Indianapolis, IN 46241, Tel: (800) 356-8260, Fax: (317) 240-2073.

[9] S. Vogel, “Life in Moving Fluids,” 2nd edition., Princeton University Press, Princeton, 1994.

10

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110

210

310

-1

1

101

10 2

Hummingbirds

PasseriformSphingids

HoveringDiptera

Win

g le

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(cm

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Weight(g)

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Figure 1: Size of natural flyers

A

A’Ti-alloy

Dry film resist

Parylene

1) Laminate dry film resist

2) Pattern dry film resist

3) Etch Ti-alloy

4) Strip and relaminate resist;deposit parylene

5) Strip dry filmresist

6) Deposit backside parylene

Figure 4: Fabrication process oftitanium-alloy MEMS wings

a) Carbon fiber wings with mylar

b) 3-D carbon fiber wings with paper

Figure 2: Non MEMS-based wings

Figure 5: Various transmission designs

Transmission A

Wing SparWing angle ?Fulcrum arm

lLinkage

a Crank Arm

Transmission D

a

ll

Transmission C

lla

x x

Transmission B

a

x x

a) simple spar

c) butterfly

b) dragonfly

d) beetle

e) bat

f) CIT7x3S20

Figure 3: Titanium-alloy MEMS

7 cm

Figure 6: Fabricated transmission C

a) Low-speed wind tunnel b) 2-D force gauge

Figure 7: Low-speed wind tunnel test setup

Figure 9: Leading edge vortexseparation during downstroke

0 0.5 1 1.5 2 2.5 3 3.5 40

0.2

0.4

0.6

0.8

11.2

1.4

1.6

1.8

Quasi-Steady

Unsteady

CL

Advance Ratio, J = U/(2? fb)

SpanwiseRigid

SpanwiseFlexible

0.1 2 3 4 5 6 7 8910-1

-0.8-0.6-0.4-0.2

00.2

0.4

0.6

0.8

Advance Ratio, J = U/(2? fb)

CL

Quasi-Steady

Unsteady

1

a) titanium-alloy bat 1 wings

b) Cicada wings

Figure 8: Spanwise stiffness effect

0 0.5 1 1.5 2 2.5 30

0.5

1

1.5

2

2.5

3

Advance Ratio, J = U/(2? fb)

Lif

t Coe

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, CL

0.5 1 1. 2 2.5 3-0.050

0.00.0.10.0.25

0.0.30.0.4

Thr

ust C

oeff

icie

nt, C

T

Advance Ratio, J = U/(2? fb)

A: Carbon rod + mylar

B: Carbon rod + paper

C: CIT7x3S10

D: CIT7x3S20

Figure 10: Lift and trust coefficientsof various types of wings

0 5 10 1 20 2 3 30

0.2

0.4

0.6

0.8

1

1.2

1.4

Flapping Frequency, f [Hz]

Inpu

t Pow

er, P

[W

]

Figure 11: Input power

3.5

4.0

4.5

5.0

5.5

6.0

0 50 100 150 200 250

6.0 V5.7 V5.5 V5.0 V4.5 V4.0 V

Out

put V

olta

ge (V

)

Discharge time (sec)

Figure 14: Continuous discharge time for various outputvoltage settings of the DC-to-DC converter

0 1 2 3 4 5 6 7 8 9 1000.511.522.533.544.5

Advance Ratio, J = U/(2? fb)

Lift

Coe

ffic

ient

, CL

0 1 2 3 4 5 6 7 8 9-1.8-1.6-1.4-1.2-1-0.8-0.6-0.4-0.200.2

Thru

st C

oeff

icie

nt, C

T

Advance Ratio, J = U/(2? fb)

Figure 12: Effect of the inboard region

Without inboard regionWith inboard region

a) super capacitor-powered prototype b) battery-powered prototype

Figure 13: Microbat Prototypes

c) battery-powered prototypewith a radio control system

Table 1: Properties of Various Wing Designs

Wing types A B C DWeight (each), mg 220 220 150 150Frame material C C T

iTiMembrane material my pap Par par

Angle of diagonal spa, ? 45? n/a 10? 20?Planform LxW, cm 7x5 7x3 7x3 7x3

C = carbon fiber rod; Ti = titanium-alloy;myl = mylar; pap = paper; par = parylene-C;L = spanwise; W =chordwise

Components Weight (g)Wings 0.4Motor & transmission system 2.8Super capacitors 2.0Fuselage, tail, switch, misc. 2.3Total 7.5

Table 2: Mass Summary of a Super Capacitor-PoweredMicrobat

Components Weight (g)Wings 0.4Motor & transmission 3Battery 3.0DC-to-DC converter 1.9Fuselage, tail, switch, misc. 2.2Total 10.5

Table 3: Mass Summary of a Battery-Powered Microbat

Components Weight (g)Wings 0.5Motor & transmission 2.8Battery 3.0DC-to-DC converter 1.8Radio-control components 2.1Fuselage, tail, switch, misc. 2.3Total 12.5

Table 4: Mass Summary of a Battery-Powered Microbatwith a Radio Control System