Remote Control Hovering Ornithopter

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					                       Remote Control Hovering Ornithopter

                                                 Christopher N. Deal1
                                      Mechanical Engineering Undergraduate
                                    University of Arkansas, Fayetteville, AR 72701

                                 Academic Advisor: Dr. Po-Hao Adam Huang2, Ph.D.
                                     Mechanical Engineering Assistant Professor
                                   University of Arkansas, Fayetteville, AR 72701


               The concept of an ornithopter (flapping-winged aircraft) to fly in a similar manner to
           that of a hummingbird or bumble-bee has yet to be successfully implemented into an
           effective design. The design proposed in this paper utilizes a simple proposal while taking
           into account that much is left to be considered. Basically, the design has been conceptualized
           around the idea of a passive pendulum, similar to the way a bumblebee uses its thorax.
           Normally, hover in an ornithopter would require active control in multiple degrees of
           freedom to provide stable flight. This design is simplified with hovering being the primary
           flight pattern when the pendulum is “passively hanging”. This will cause the stroke plane,
           or the path of the wings, to be horizontal and maintain vertical vectors for both lift and
           thrust. By utilizing the pendulum, this will create a mechanism to actuate from and
           therefore change the plane of the stroke, initiating forward, backward, and side-to-side
           flight. Design considerations described in this report include: actuation, wing design, and
           basic mechanical system design. Also, the limitations of this design are theorized in this
           report along with proposed changes to solve these problems. Through this design, a basic
           aircraft can be created to begin mimicking the flight of a hovering bird or insect.

                                                    Nomenclature
Φ            =   total stroke amplitude
Θ            =   maximum stroke deviation
α            =   mid-stroke angle of attack
τo           =   flip start
∆τ           =   flip duration
J            =   advance ratio

                                                     I. Introduction

M      illions of different types of insects, bats, and birds all fly with flapping wings, so it is therefore logical that
       scientist, biologist, and aerodynamicist would be interested in learning more about how to create an artificial
flapping winged flyer [1]. This form of aircraft is generally known as an ornithopter (from the Greek ornithos
“bird” and pteron “wing”) [2]. Most hobbyist build ornithopters for the shear enjoyment of imitating a bird-like or
insect-like artificial aircraft, but there has been a recent interest to study ornithopters for practical applications.
Ornithopters utilize the flapping wings for lift and thrust, which increases energy efficiency by minimizing flight
components. Many practical application involving ornithopters would require it to transition to hovering mode
under non-quiescent conditions. So far, none have achieved the ability to transition even in quiescent environments.
Although, a few artificial machines have been made with the ability to hover, including the Mentor, built at the
University of Toronto Institute of Aerospace Studies (UTIAS) [3], and the Delfly, built by the Delft University of
Technology Aerospace Department [4]. Both aircraft are very limited by their size. Also, by observing video of the
Delfly, it tends to be more of a slow hovering flight similar to how a fixed wing aircraft would “hover” with its nose
pointed to the sky.

1
    Undergraduate Student, Mechanical Engineering Department, University of Arkansas, AIAA Member
2
    Assistant Professor, Mechanical Engineering Department, University of Arkansas, AIAA Life Member


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                                  American Institute of Aeronautics and Astronautics
  The aerodynamic properties of ornithopters are very complex because steady-state aerodynamics do not accurately
account for the required forces [5]. Instead, unsteady aerodynamic modeling is required to understand the leading
edge vortices (LEVs), which create the majority of the forces on flapping wings [6]. By effectively manipulating
these aerodynamic forces to provide lift, propulsion, stability, and control, an ornithopter can be made to imitate an
insect or hummingbird like flight. To be able to fully imitate this type of flyer, would mean to have a way of
actively controlling each individual wing’s amplitude and angle of attack along with several other means of control.
Because of this difficulty, a design was created to make this type of hovering control by another means. This paper
proposes a design to passively control an ornithopter to mimic a hummingbird type flight.


                                          II. Practical Applications
        Applications typically include the use of ornithopters in the form of micro air vehicles (MAVs) for gathering
surveillance information such as audio, video, or photo on targets of interests. Key requirements for an ornithopter
in such missions would be its ability to hover with high maneuverability and be extremely power-efficient.
Although small helicopters on the order of a few grams have been demonstrated to be able to hover, it does not
possess the ability to maneuver in gust conditions nor transition from
horizontal flight. These capabilities can be achieved by small
ornithopters given the ability of natural flapping flyers to master the
acrobatics of flight. For example, Nathan Chronister has built a 3.8
gram ornithopter (Fig. 1) which has a complete RC system weighing
less than a gram [7].            Another possible requirement for
reconnaissance would be for the ability of the ornithopter to perch on
tree limbs, fences, etc. In any case, the ability for the ornithopter to
hover in gust conditiosn is key for most applications, but no current
artificial flyer has this practical ability, especially in the sense of      Figure 1: Nathan Chronister’s 3.8
MAVs.                                                                        gram ornithopter [7]



                                    III. Aerodynamics of Ornithopters
       The aerodynamics in this report are described for                              FD
the concept of hovering flight. The diagram in Fig. 2
shows the stroke of an ornithopter during hovering flight
                                                                                                                   Fnet
[10]. The high number of parameters affecting the stroke
pattern gives way to a very complicated system. The net
force Fnet is a combination of the forces caused by the
                                                                                      FU
complete cycle of the downstroke force FD and upstroke
force FU which balances the weight of the body for
hover.
       This net force, or lift, created by flapping wings is
                                                       (a)
due to the leading edge vortex (LEV) created by the
plunging and pitching of the wing, similar to the effect of
                                                               Figure 2: Schematic diagram of the six parameters
the LEV on a delta wing [9]. These similarities can be
                                                               that were varied in the experiments. This cartoon
seen in a comparison of the two in Fig. 3. The LEV is
                                                               represents a 2-D projection of a 3-D kinematic
far more pronounced in the photograph of the delta wing
                                                               pattern [8]. Personal revisions are in orange.
because of steady state nature of fixed wings.
Theoretically it seems that flapping wings should stall
because of the incredible high angle of attack during the
flapping motion. But, the intense LEV provides the high
suction lift at the high angles of attack, thus preventing
the wings from stalling. Similar to the LEV on delta
wing, instead of flow separation, the vortex sticks to the
wing traveling along the span and results in the desired
maximum lift coefficient at a high angle-of-attack [1].
This accounts for the high lift of a flapping wing. In
                                                               Figure 3: Similarities in the LEV of a Flapping
addition to the stroke path, the ability for the flapping
                                                               Wing (a) [5] and Delta Wing (b) [10]

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                                 American Institute of Aeronautics and Astronautics
wings to deform aeroelastically greatly affects the aerodynamic properties. Bats can change the degree of tension in
the spars of its wings to change the wing camber and therefore affect the aerodynamic loading.

                                  IV. Basic Mechanical System Design
        Most artificial ornithopter designs consider the forward flight to be primary. As mentioned before, only a
few ornithopters come close to having the ability to hover. Most aerodynamic research on hovering insects should
lead to designers focusing on the hovering aspect as primary, then finding ways to create thrust in the fore, aft, and
lateral directions. The hovering stroke plane can be stabilized by having the body hang below the wing base to
benefit from a type of passive pendulum stability [1]. This is similar to how most insects fly and can be seen in Fig.
4 with a horizontal stroke plane while the center of mass is close to the junction of the thorax and abdomen. In
hover, the downstroke and upstroke are symmetrical.
        The pitch and roll can be induced by adjusting the stroke plane. This effectively changes the direction of the
net force and creates a component of thrust. The advance ratio J (defined as the ratio of vehicle velocity versus wing
flapping velocity) will increase as the thrust component becomes stronger. This can be seen in Fig. 5. Typically, an
insect will increase the amplitude or angle of attack of the downstroke. This will increasingly dominate the force
balance on the downstroke to unbalance the forces in the horizontal direction and cause forward pitch, and therefore
forward thrust.




         Figure 4: Insect wing flapping                              Figure 5: The wingtip paths for an
         motion [11]                                                 insect at different flight speeds [1].


       Instead of pitching the stroke plane by means of wing control, it can be changed by simply adjusted the stroke
plane by means of actuation. This causes problems because actuation can not be completed in mid-air. This is
where the pendulum stability comes into factor. Excluding wind conditions, a pendulum hanging under the stroke
plane can provide a semi-stable surface to actuate from. The basic design is shown in Fig. 6.

                   Lift                                                             Lift
                                              Stroke Plane
                                                                                                Thrust

      Actuators                     Universal Joint



      Pendulum



                                    Figure 6: Basic Hovering Ornithopter Design




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                                American Institute of Aeronautics and Astronautics
A. Actuation
   1) Shape Memory Alloy (SMA) (Muscle Wires)
           These wires are high strain, low stress devises that will shorten when the activation temperature is
       reached. Since they do not push, they can be arranged in a pull-pull configuration or used in conjunction
       with a spring for the restoring force.

            The biggest limitation for using these as actuators is that they are either “on” or “off”. This will not
         give the necessary control unless the amount of deflection can be varied. This would have to be done by
         controlling the temperature of the muscle wires by applying a specific power. This would prove to be
         highly difficult because of convection by the varying air flow and the unknown activation temperature
         range of the material.

   2)    Magnetic Servos
            Small servos at this scale have not been researched to much of an extent so there has not been much
         determined about how they would compare to using SMA’s.

             They are much larger in size and could cause the actuation system to be too heavy and bulky. Some
         servos from airmidimicros.com for planes up to 20 grams were found to be 7 x 8 x 4 mm and 0.4 grams.

B. Wing Design
    Several factors have to be taken into account in the design of the wing. These factors include: size, flapping rate
(frequency), stiffness, and mono or bi wings.

   1.   Size / Frequency / Mass
           The size of the wing highly depends upon the mass that
has to be supported by the wings. A graph created by C. P.
Ellington (Fig. 7) can be used to determine the size of the wings
according to the projected frequency and mass. For example: a wing
that beats at 30 Hz and supports a 10 g load would have to be
approximately 75 mm in length. The actual area of the wing is
determined by proportionality.

   2.    Stiffness
         The stiffness of the wing has the largest effect on the design
because it controls the angle of attack and the flip duration. The
stiffness can be adjusted in the spanwise and chordwise manner by
changing the material used for the wing membrane and the way the
spars are made. The spars can be adjusted by the material being
chosen, thickness, number, and separation. Some example layouts             Figure 7: Wing Frequency vs.
are shown if Fig. 8.                                                        Support Mass vs. Wing Length. [1]




        Figure 8: Example Wing Layouts


   3.    Mono or bi wings
           The choice of mono or bi wings primarily depends on how powerful the motor is and required stability.
A single set of wings (mono) will cause the body to oscillate because of the shifting forces caused by the
downstroke and upstroke. If the wings are larger because frequency is limited, then forces on each stroke would be
larger and last for a longer period of time. This could easily cause problems in this design because the effective lift
and thrust component need to be effectively stable. A bi wing configuration (similar to a dragonfly) would cancel
out this oscillation effect. These configurations are modeled in Fig. 9.

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                                 American Institute of Aeronautics and Astronautics
    Figure 9: a) Bi-Wing Design                b) Mono-Wing Design


C. Flapping Mechanism
    The flapping mechanism is a simple cam system using a small electric motor and gear. This creates an
oscillating motion for the wings and can be seen in Fig. 10. A prototype of this mechanism was created using a
rapid prototyping machine and worked despite a few minor problems. A redesign is in the works using more
precision parts and a more powerful motor. This prototype can be seen in Fig. 11.




         Figure 10: Cam                                        Figure 11: Ornithopter Prototype
         Mechanism for Flapping


                                                  V. Conclusion
    The design and testing of the passive pendulum ornithopter is currently under-going at the University of
Arkansas. The prototype flapping mechanism has been built with 3D ABS plastic printer and is currently being
tested on a miniature 6-component force balance. Tests cases representing hovering and horizontal flights will be
performed outside and inside a 1’x1’ low speed wind tunnel, respectively. Preliminary results will be presented at
the conference.

                                                  Acknowledgments
   The Authors would like to thank the AIAA Region IV for providing the travel support to present this paper at the
AIAA Region VI student conference in San Jose.

                                                      References
1
  Ellington CP, (1999), “The Novel Aerodynamics of Insect Flight: Application to Micro-Air Vehicles,” The Journal of
      Experimental Biology, Vol. 202, pp. 3439-48.
2
  “Ornithopter,” Website Reference, http://en.wikipedia.org/wiki/Ornithopter
3
  “Mentor the Hovering Ornithopter,” Website Reference,
      http://www.andrew.cmu.edu/user/deberhar/Mentor.html
4
  “DelFly,” Website Reference, http://www.delfly.nl/?lang=en
5
  Ho S, Nassef H, Pornsinsirirak N, Tai Y, Ho C, (2003), “Unsteady Aerodynamics and Flow Control for Flapping Wing Flyers,”
      Progress in Aerospace Sciences, Vol. 39, pp. 635-681.
6
  Ramamurti R, Sandberg WC, (2002), “A Three-Dimensional Computational Study of the Aerodynamic Mechanisms of Insect
      Flight,” The Journal of Experimental Biology, Vol. 205, pp. 1507-18.

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                                  American Institute of Aeronautics and Astronautics
7
  “The Ornithopter Zone,” Website Reference, http://www.ornithopter.org/
8
  Sane SP, Dickinson MH, (2001), “The Control of Flight Force by a Flapping Wing: Lift and Drag Production,” The Journal of
       Experimental Biology, Vol. 204, pp. 2607-26.
9
  Zbikowski R, (2002), “On Aerodynamic Modelling of an Insect-like Flapping wing in Hover for Micro Air Vehicles,”
       Philosophical Transactions: Mathematical, Physical and Engineering Sciences, Vol. 360, No.1791, Biomimetics:
       Technology Transfer from Biology to Engineering, pp. 273-290.
10
   http://www.nasaexplores.com/show2_articlea.php?id=03-080
11
   “Micro-Aerial Vehicle (MAV) Research : Designing An Ornithopter for Urban Search and Rescue,” Website Reference,
       http://www.eng.unsw.edu.au/current/scholar/tr0506/results/sys/text.htm




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                                  American Institute of Aeronautics and Astronautics

				
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