cling flip' mechanism

The University of Arizona Micro Ornithopters

D. Silin*, B. Malladi

*, and S. Shkarayev

†

The University of Arizona, Tucson, AZ, 85721

The research and development project outlined in the paper addresses the

aerodynamic design of flapping-wing micro air vehicles (also called

ornithopters). Rigorous wind tunnel testing was conducted over the wide range

of geometric, elastic, and kinematic parameters of flapping wings. Specifically,

effects of a wing’s bending stiffness on the generated thrust force and power

required were investigated at different flapping frequencies. The lift and drag

forces on flapping wings were studied with the stroke plane angle varied from

horizontal to vertical. The major result of this study is that no abrupt stall was

found for the full range of angles at all test speeds. Three ornithopters were

designed utilizing the results of aerodynamics studies. The smallest ornithopter

has a 15 cm wingspan, weighs only 9 grams, and has a flight endurance of 3

min. The 20-cm ornithopter with a thrust-to-weight ratio in excess of 1.2 is

capable of hovering, as well as of sustained steady flight. The 53-cm ornithopter

is equipped with an autopilot, and several fully autonomous flights have been

performed to date.

I. Introduction

Because of their small size, micro air vehicles are often considered for applications ranging

from military to scientific, and their versatility allows them to perform in conditions that might

otherwise endanger human life. Their reconnaissance capabilities were the driving factor for the

first generation of micro air vehicles (MAVs). These developments concerned fixed wing

MAVs.

Flapping-wing micro air vehicles generate lift and thrust for forward motion using their

flapping wings, emulating birds and insects. However, just mimicking the flight of birds and

insects is insufficient in designing flapping-wing vehicles. Here is how this viewpoint was

elucidated by Mueller and DeLaurier:1 “The primary motivation for studying animal flight is to

explain the physics for a creature that is known to fly… An ornithopter designer, in contrast, is

trying to develop a flying aircraft, and its ability to achieve this is no given fact.” And

conversely, a successful micro air vehicle design provides a verifiable physical model of flight

in nature.

Aerovironment pioneered the designing of radio-controlled micro ornithopters called

Microbats.2 The most successful vehicle of this type has a half-ellipse wing planform with a 20-

cm wingspan flapping at 22 Hz. The project proved to be challenging because of the limited

knowledge on unsteady aerodynamics of flexible flapping wings of this small size.

* Graduate Student, Department of Aerospace and Mechanical Engineering, University of Arizona, 1130

N. Mountain, Tucson AZ, 85721. † Associate Professor, Department of Aerospace and Mechanical Engineering, University of Arizona, 1130

N. Mountain, Tucson AZ, 85721, Senior Member of AIAA.


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DeLaurier and his group developed a 35-cm radio-controlled ornithopter capable of

hovering.3 The kinematics of the 4 wings (X-wing) mimic the “cling-flip” mechanism employed

by some insects and birds. This mechanism balances flapping forces and decreases vibrations.

Hovering flights in excess of one minute were achieved with a flapping frequency of 28 Hz. It

was noted that transition to forward flight remains a problem, but that it may be overcame by an

intelligent flight stabilization system.

Ellington4 summarized the flight kinematics of insects, which could be useful for prospective

insect-sized MAV designs. The motion of the flapping wing is described with respect to the

flapping plane, also called a stroke plane. Insects have been observed to perform gentle

maneuvers by tilting the stroke plane of their wings, just like a helicopter. Lateral direction

changes can be accomplished by a roll of the stroke plane (often by increasing flapping

amplitude and/or angle of attack of the outside wing). Angle of attack changes also initiate low-

speed acceleration. For slower flight and hovering, the body hangs below the wing bases, and

the insect benefits from a passive pendulum-like stability.

An ornithopter competition was added to the 8th International MAV Competition in Tucson,

Arizona in 2004. This competition involves building the smallest radio-controlled ornithopter

that can fly the most laps around a pylon course in 2 min. The pylons were spaced 40 feet apart

and the ornithopters flew either an elliptical course around them or a figure-8 through them. The

University of Arizona (UA) won the 2004,5 2005,

6 and 2006

7 competitions with 28-cm, 20-cm,

and 15-cm ornithopters, respectively.

For the 2006 US-European MAV technology demonstration, we are planning to present the

three ornithopter designs shown in Fig. 1. The smallest known ornithopter has a 15 cm

wingspan, weighs only 9 grams and has flight endurance of 3 min. Birds and insects are

unstable flyers, yet they have evolved very sophisticated methods of maneuvering capabilities,

including vertical takeoff, landing and hovering.8,9

The 20-cm ornithopter with a thrust-to-

weight ratio in excess of 1.2 is capable of hovering, as well as of sustained steady flight.

Our team has extensive experience in the successful integration of an autopilot into fixed-

wing micro air vehicles. We have developed two fully autonomous air vehicles, Dragonfly and

Zagi, that have demonstrated the ability to complete practical surveillance missions.10

In the

present project, the 53-cm UA ornithopter was outfitted with a Paparazzi autopilot11

for fully

autonomous operation, and several fully autonomous flights have been performed to date.

a) 15-cm, smallest b) 20-cm, hovering


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c) 53-cm, autonomous

Fig. 1. UA ornithopters.

II. Wind Tunnel Measurements on Flapping Wings

Studies to date on the aerodynamics of flapping flight, although beneficial to an

understanding of the subject, have not taken into account all the details that are necessary to

obtain a complete and thorough understanding (and more accurate representation) of the true

aeromechanics of flapping flight. For the same reasons, no design methods for flapping wings

are readily available and, therefore, the first phase of the present project was focused on the

aerodynamics of flapping wings currently used for UA ornithopters. Wind tunnel measurements

were performed on flapping wings to complete the following tasks:

− the investigation of the effect of a wing’s bending stiffness on the thrust force and power

required at different frequencies;

− the determination of the lift and drag forces with the stroke plane angle varying from

horizontal to vertical.

Testing was performed using the UA Low Speed Wind Tunnel (Fig. 2a). The test section is

3 4× ft and has a velocity range from 2 to 50 m/s. The flow is laminarized in a settling chamber

to less than 0.3% turbulence in the axial direction.


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a) Model in the wind tunnel

b) Model schematics

Fig. 2. Wind tunnel testing of flapping wings.


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The force balance is capable of accurate measurements of lift and drag. Force measurements

are made using precision strain gages. Data from these strain gages are logged using two

National Instruments SCXI-1321 terminal blocks in a low-noise SCXI-1000 chassis capable of

sampling at 330,000 Hz.

The flapping wing model consists of a mounting rib, wing, and flapping transmission

(Fig. 2b). The wing is a half-ellipse planform with a 250 mm wingspan and 70 mm root chord,

resulting in a 13,700 mm2 wing area and aspect ratio of 4.56.

The wing structure consists of a membrane bonded to the front and radial spars with rubber

cement. The membrane is 0.015-mm Mylar. Front and radial spars are pultruded carbon rods. In

the basic flapping wing model (model A), carbon rods are used for the spars, T315-412

of

diameter 0.8 mm for front spars, and T305-412

of diameter 0.5 mm for radial spars. Two radial

spars are placed in each wing, as shown in Fig. 2b. The total weight of the wing is 1.1 g. The

front spar flapping motion sweeps through 72° of travel with a 19° dihedral offset (Fig. 2b).

Since the main goal of this testing series was to obtain wing-only aerodynamic data, the

flapping-wing models were supported as close to the trailing edge as possible with an

aerodynamically clean mount system. An aluminum mount was constructed with minimum

frontal area and a smooth aerodynamic leading edge. The mount was reinforced by patches

along the mid-chord from the leading to the trailing edges. These parts also functioned as the

mounting points to the wind tunnel pylon.

In order to measure the flapping frequency of the wings, an experimental setup was equipped

with a built-in optical tachometer. The tachometer consists of a CP-36 photodiode and ECG-

3038 phototransistor connected to the data acquisition board.

Fig. 3. Thrust vs flapping frequency.


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Fig. 4. Thrust-to-power vs flapping frequency.

Wing design trade-offs are their weight and stiffness. Since the center of pressure and the

shear center do not typically coincide in the actual wings of insects and birds, a resultant torque

causes the wing to twist. Hence, the one very key element in the production of the lift and thrust

by flapping wings lies in the flexibility or stiffness of the wing structure, and not only in the

flapping motion. Stiffer wings allow for higher maximum flapping frequencies and, therefore,

higher thrust force. However, having a more rigid wing increases the wing structure weight.

Increased weight requires more energy to induce flapping. Is there an optimum?

In order to investigate the effects of wing stiffness on the developed thrust and required

power, in addition to the base wing (model A), described above, two more wing models were

built – models B and C, with increased and decreased stiffness, respectively. The increase of the

stiffness in model B was achieved by attaching an additional 0.5-mm-diameter carbon rod along

the half wingspan adjacent to the root of the wing. The stiffness in model C was decreased by

sanding the front spar to make it tapered, with diameters of 0.8 mm at the root and 0.35 mm at

the tip of the wing. The wing tip deflections were measured on all three models under the static

load of 15 g applied to the wing tip and were found to be 27 mm, 19 mm, and 35 mm for

models A, B, and C, respectively.

All three models were tested on the wind tunnel balance without airflow. The thrust force

was measured for flapping frequencies up to 22 Hz, and the results are presented in Fig. 3. The

stiffer wing model B shows the highest thrust, followed by the most flexible model C. In order

to explain this effect further, experimental studies on wing shapes and air pressure distributions

are needed. Also, the electric power input was recorded and the thrust-to-power ratio is

presented in Fig. 4. Model A is 1.5-2 times more power effective than models C and B.


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Fig. 5. Variation of the lift force with the stroke plane angle and velocity.

Fig. 6. Variation of the drag force with the stroke plane angle and velocity.

The stroke plane angle is the angle between the free stream velocity vector and stroke plane

formed by a motion of the flapping wing tips (Fig. 2a). Birds have developed flight methods

that allow them to demonstrate sophisticated maneuvering, as well as vertical takeoff, landing,

and hovering. During transitioning between maneuvers and from horizontal flight to vertical

and back, high stroke plane angles, sp

α , often occur. Wind gusts can also generate high sp

α .

The flapping-wing model A was tested for a range of stroke plane angles from the horizontal

( 0sp

α = ° ) to the vertical ( 90sp

α = ° ), and velocities of 2.7, 5.4, and 8.1 m/s. The results of these

tests are presented for the stroke-averaged lift force (Fig. 5) and for the stroke-averaged drag

force (Fig. 6).

The lift curves are typically one of the first things to look at when designing a micro air

vehicle. For comparison purposes, the lift force was measured at 0sp

α = ° at zero flow speed


8

and was found to be 0.035 N. Note the significant lift at 0sp

α = ° resulting from the dihedral

introduced into the flapping wings. At small sp

α , the data are more scattered due to the small

magnitude of the measured forces, and the lift forces at non-zero flow speed are significantly

(more than 2 times) greater than the one at zero speed.

In terms of trends, it appears that as the velocity increases, the slopes of the lift curves and

the lift force increase. The lift force for the velocity of 8.1 m/sec at 30sp

α = ° is 1.77 times

greater than that for 2.7 m/sec.

The lift force plots demonstrate nonlinear behavior at moderate angles and maximum lift

forces of 0.34 N, 0.42 N, and 0.56 N are reached at 72 , 58 ,sp

α = ° ° and 53° , respectively. After

reaching maxima, lift forces slowly decrease, with the stroke plane angle approaching

90sp

α = ° . It is seen from Fig. 5 that flapping wings do not show the typical, abrupt stall seen

with fixed wings. This important result warrants further studies on the physics of flow and

associated aerodynamic forces and moments on flapping wings at high sp

α .

Similar to a fixed wing, the drag force plots for flapping wings in Fig. 6 are concave. In

these plots, the negative drag actually means a pointing-forward thrust force, and a zero value

means that thrust and drag are balanced. The balance is achieved at 56 , 37 ,sp

α = ° ° and 27° at

velocities of 2.7, 5.4, and 8.1 m/s, respectively.

III. Ornithopter Design

Designs of successful flapping-wing micro air vehicles are presented here. A complete CAD

rendering of a typical UA ornithopter design is presented in Fig. 7, and the mass breakdown of

three vehicles are presented in Table 1.

Fig. 7. SolidworksTM rendering of a typical ornithopter design.


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Table 1. Mass breakdown of UA ornithopters.

Mass, g Components

15 cm 20 cm 53 cm

Wing 0.3 0.4 10.8

Fuselage & Tail 1.8 4.5 23.6

Motor 2.9 12 12.4

Battery 2.4 10 18.9

Speed Controller 0.4 1 7.5

RC Receiver 0.4 1 2.4

Servos 0.8 3.5 7

Autopilot board - - 16.9

Infrared sensors board 6.6

Total 9 32.4 106.1

The battery and motor account for 55% of the total weight of the ornithopters, making the

powertrain the heaviest system in the aircraft—it remains the biggest design issue. The

transmission is powered by an electric motor. The following motors are used: Super Slick

3.3 Ohm and Micro DC5-2.4 coreless electric motors for the 15-cm and 20-cm ornithopters,

respectively, and brushless motor BA-BL1230 for the 53-cm ornithopter. The motor is regulated

by a speed controller and radio receiver. Energy is provided by lithium polymer batteries.

The motor drives a crankshaft mechanism through a reduction gearbox. The crankshaft

initiates a flapping motion of the front spar by the action of cranks connected to push-rods. The

15-cm and 53-cm ornithopters had their motors aligned with the longitudinal axis of the vehicle.

Designs with a streamlined longitudinal motor arrangement proved difficult to trim for level

flight due to the powerful rolling moment produced by the motor torque. The motor for the 20-

cm vehicle was mounted in the transverse direction. For this design, the torque reaction

produces a positive (nose-up) pitching moment, improving stability and controls. The cranks are

counter-rotating in the 15-cm and rotating in the same direction in the 20-cm and 53-cm

vehicles.

The fuselage frame was made from balsa wood and carbon rods. The frame and components

are protected from the impact of landing by a EPP foam nose. The wing structure consists of the

carbon rod frame and a membrane. The wing of 15-cm vehicle has a front spar made of 0.5 mm

diameter carbon rod. The 20-cm ornithopter utilizes 0.8 mm main spar and 0.5 mm root section

reinforcement for high output thrust needed for hovering. The wing of 53-cm ornithopter has a

spar combined of 2 1× mm carbon tube at the root and 1.2 mm diameter carbon rod, tapered to

0.8 mm diameter at the wing tip. The following materials were used for the membrane: nylon of

73.7 g/m2 density, Mylar of 16.8 g/m

2 density, and Mylar of 7.0 g/m

2 density for the 53-cm, 20-

cm, and 15-cm ornithopters, respectively. The 15-cm and 20-cm aircraft use conventional tail

with rudder and elevator, while the 53-cm vehicle has V-tail with elevons.

The design goal for the 15-cm ornithopter was to demonstrate that current progress in

miniature radio-control components and electric power trains allows the creation of a flapping-

wing MAV within the sub-15-cm linear scale.


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The uniqueness of the 20-cm ornithopter is in its high agility and, specifically, hovering

capability. The first time this ornithopter was presented was in 2004 at the 4th European MAV

competition in France. It was able to takeoff from the ground and hover for a few seconds. The

flight time was limited to around 50 seconds due to the low battery capacity. Further

optimization of flapping wings allowed the creation of 30-33 g ornithopters that generate 37-

40 g of thrust.

The main design feature of the 53-cm autonomous ornithopter is, of course, the autopilot.

However, it resulted in about a 20% weight increase. Therefore, the design goal in this project

was aimed at reducing the weight of the structure. The frame and single crank transmission

from a commercial R/C ornithopter kit13

were used in the construction. The fuselage frame was

lightened and a new crank with a shorter arm was machined. To increase the lifetime of the

structure, all sleeve bearings were replaced with ball bearings. Although not the lightest

available solution, experience has shown that Falcon servos offer more consistent control than

other types of lightweight actuators. A lighter, but 30% larger, tail was installed for more

effective control and stability at low speed.

IV. Autopilot Integration into the 53-cm Ornithopter

Utilizing previous experience in autopilot integration,10,14

a Paparazzi autopilot11

was

integrated into the 53-cm UA ornithopter in the present project. Specially for this project, a thin

0.8 mm PCB with no connectors was manufactured. The ready-to-install autopilot board has

dimensions of 7 31.5 63.5× × mm and 16.9 gram of weight.

The Paparazzi autopilot and ground station are a set of software and hardware assets,

flexible enough to work with various types of flying vehicles. The system uses microprocessors,

a GPS unit, and an infrared sensor board to determine the current attitude of the vehicle. A

receiver and transmitter provide communication between the vehicle and the ground station.

The autopilot software controls all avionic devices and algorithms for guidance, navigation, and

control.

The flight control software consists of several modules—configuration files, flight plan,

map, autopilot, and GPS tools—and two major parts—the autopilot software on-board the

airplane and the ground station software. In flight, the autopilot sends telemetry data back to the

ground station. Currently, the telemetry data include: GPS-based data; speed, altitude, and climb

rate of the airplane; attitude of the aircraft provided by infrared sensors; autopilot status data;

and position of the control surfaces. These data play a major role in performance analyses

during the flight tests and adjustments of gains.

Longitudinal control of the ornithopter is accomplished by proportional control for altitude

hold, with an inner pitch attitude loop. Similarly, lateral-directional control is accomplished by

an outer heading hold loop and the inner bank angle control loop, both using proportional

control.


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Fig. 8. Autopilot hardware integration in the 53-cm ornithopter.

The autopilot board was installed on the top of the frame right behind the wing (Fig. 8). In

order to protect the board from vibrations, it was installed using two T-shaped mounts with soft

rubber foam pads. All the radio control components are located right under the board in the

frame cutouts, thus the weight of the wiring is also minimized.

V. Flight Testing

Flight testing is an important step in bringing together the results of the wind tunnel testing,

design concept, and construction techniques. One of the main problems with all ornithopters is

their sensitivity to the center of gravity position. Affecting the static margin, minute changes in

the center of gravity may result in loss of the aircraft. During initial flight testing, the pitching

moment in level flight was monitored and the installation angle on the horizontal tail was

adjusted as needed.

The first flight tests of the 15-cm ornithopter revealed following problems: low climb rate

and inefficient pitch control. The reason for the low climb rate was found to be the too-forward

location of the center of gravity, which was initially set based on the previous 20-cm designs.

But those ornithopters had a flapping transmission with a transverse crankshaft orientation that

delivered positive pitching moment as the throttle increase. The present design has two counter-

rotating cranks oriented along the symmetry axis and is not as sensitive to the throttle. After

adjusting the center of gravity position, the ornithopter flies well: it is able to withstand up to a

3 m/sec wind, can do a series of sharp turns while maintaining altitude, and has a flight time at

moderate throttle in excess of 3 min. At its demonstration,7 it made 7½ figure-8s around two

pylons 40-feet apart, in 2 min.

Flight tests of the 20-cm vehicle show that a high thrust-to-weight ratio of about 1.2 is not

the only condition for continuous hovering. Another important feature is the effectiveness of the

control system during hovering. The current control system includes a horizontal tail with an

area equal to about 50% of the area of the wing and a vertical tail located at the bottom of the

horizontal tail. Such a two-channel control system allows effective pitch control at any speed

and angle of attack with the help of the elevator deflecting air flow behind the flapping wings.

The issue of roll and yaw coupling is specific to the current vertical tail configuration. In order

to improve this situation, further studies will be focusing on the development of a tail with three

control channels.

A 53-cm ornithopter has performed a number of flights with endurance more than 3 min

without a payload. It has also demonstrated an ability to climb and perform basic maneuvers

with 28 g of payload.


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With the autopilot installed, the ornithopter is used only for the flights in horizontal plane

with a constant altitude. Waypoint navigation and altitude control algorithms previously used

for fixed wing airplanes are utilized in the autopilot installed on the ornithopter.

VI. Future Studies

The successful completion of this project provides us with a unique opportunity and a new

approach to in-flight study the aerodynamics of a flapping-wing apparatus. Our future research

and development efforts will focus also on increasing the robustness of the flight control system

and widening the flight envelope. Enhanced control laws will be developed to satisfy the needs

for flying very aggressively. The nonlinear nature of the ornithopter system suggests a control

design based on dynamic inversion.

Acknowledgments

This project was sponsored by the College of Engineering at the University of Arizona. The

authors also would like to thank the other members of the Micro Air Vehicle Project at the

University of Arizona for their contributions to this work: Roman Krashanitsa and Deva

Coopamah. Photographs for this paper are courtesy of Ed Stiles, to whom we are very grateful.

References

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Fixed and Flapping Wing Aerodynamics for Micro Air Vehicle Applications, edited by T. J.

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Vehicle Competition, Tucson, Arizona, 10 p., April 2004.

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Shkarayev, S., “Wind Tunnel Testing and Design of Fixed and Flapping Wing Micro Air

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and Shkarayev, S., “Simulation Wind Tunnel Testing, and Design of Fixed and Flapping Wing

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Competition, May 2006, Provo, Utah.


13

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10

Krashanitsa, R., Platanitis, G., Silin, D., and Shkarayev, S., “Autopilot Integration into Micro

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11Drouin, A., and Brisset, P., “PaparaDzIY: do-it-yourself UAV,” 4

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Meeting, Toulouse, France, Sept. 15-17, 2004.

12

Material Data Sheets, CST - The Composites Store, Inc., Tehachapi, California, 2006.

13

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14

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cling flip' mechanism

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