Investigation into Dragonfly Wing Structure and Composite Fabrication

Investigation into Dragonfly Wing Structure and Composite
Fabrication
Wesley Ross McLendon
under
Dr. John D. Whitcomb
Texas A&M University, College Station, TX
Among natural flight systems, dragonflies possess a plethora of unique flight
capabilities including hovering and flight in multiple directions.
Because
dragonflies do not actively change the shape of their wings to achieve such
maneuvers, the passive bending resulting from the wing structure is the primary
quality of the wing which allows for the dragonfly’s unique flight abilities. This
research focuses on understanding this structure as well as developing novel new
methods for small-scale composite systems fabrication, both of which are necessary
to eventually facilitate the creation of biomimetic ornithopter systems patterned
after dragonflies.
I. Introduction
The potentials presented by
micro air vehicles are exciting. A small
vehicle with the flight capabilities of a
dragonfly could find a variety of uses,
including military reconnaissance,
search and rescue, and other applications
which could benefit from sensing
technology being deployed into a small,
cramped area. While technology such as
this is a long way off, the first step to
achieving it is gaining an understanding
of the biological dragonfly flying system
and developing methods for mimicking
it structurally. One of the most critical
structural components needed to be
understood is the structure of the wing.
The wing is, in essence, a smart structure
because the natural deformation it
undergoes during flight is the quality of
the wing that allows it to produce lift.
Therefore, understanding the structure of
the wing is necessary to replicate
dragonfly flight with manmade
structures. As can be seen from the
pictures in this paper, the wings of
dragonflies are very complex structures
consisting of a network of various sized
veins, and to exactly copy this structure
would be not just impractical, but nearly
impossible. Therefore, the most likely
way to be able to replicate this structure
is to identify the overall structural trends
in the dragonfly wing and design a
simpler structure with similar properties.
Composites offer much potential as a
material which could be used to fabricate
such a structure due to their light weight.
Therefore, a useful parallel investigation
to an examination of dragonfly wing
structure is to try various composite layup methods in order to begin developing
ideas about what sorts of methods for
composite fabrication hold the most
promise for building very small scale
wing-like structures. This combination
of research lays a foundation for future,
more in depth research to be conducted
to more specifically identify and
characterize wing structure as well as
further investigations into very smallscale composite fabrications.
II. Wing Structure
The wing of a dragonfly can be
broken into a variety of basic structures.
The overall two types of structure
present are the veins and the membrane.
Both consist of cuticle which is
composed of the material chitin1. The
veins provide the primary structural
support for the wings. As their name
suggests, the veins are hollow and carry
hemolymph which serves to prevent the
cuticle of the wing from becoming
brittle1. The membrane is the primary
aerodynamic structure of the wings. It is
a very thin structure, with a thickness of
only 2 to 3 m2. Because it is such a
thin structure, the membrane is thought
to carry only tensile loading in the
wings, while buckling under the slightest
compressive stress2. Quantitative
analyses of parts of various insect wings
have yielded a variety of different
Young’s Modulus values. These are
often around the value of 1-5 GPa, but
some tests on wing components have
yielded values in the range of 15 GPa.
This wide variety of values is the result
Basal Wing Section
costa
triangle/ subcosta
supertriangle
(levers trailing edge
downward)
of inconsistent testing methods, samples
from different species of insect, and
samples from different parts of the wing
on the same insect3,4,6. It has been
proposed by some that the wing’s
flexural stiffness varies along the wing
span, and this model has been shown to
have results which closely match actual
wings’ behavior5.
The primary overall structural
property of wings is span wise stiffness
and chord wise flexibility. The leading
edge of the wing is comprised of a very
stiff structure with three dimensional
relief in order to provide high rigidity to
the span of the wing1,2,6. This causes the
flexural stiffness along the span to be 12 orders of magnitude greater than along
the chord5.It is obvious that this quality
contributes greatly to the wing’s
aerodynamic properties. There are a
number of key structures in the wing,
shown below in figure 1, which
contribute to the manner in which the
wing bends in flight and therefore help
to facilitate the wing’s aerodynamic
properties1.
Distal Wing Section
radius
nodus
(provides
stress relief)
pterostigma
(sort of counterweight
to control wing
flapping)
Figure 1: Dragonfly wing with structures of interest
One interesting characteristic to
note about a dragonfly wing is that there
are several different kinds of patterns
present in the wing vein framework.
The leading edge consists primarily of
rectangular frames whereas the trailing
surface is largely formed of hexagons
and some other polygons with more than
4 sides. Using FEMAP with NX
NASTRAN, a finite elements tool, the
differences between these frame shapes
were examined. Some of the models
used are shown in figures 2 and 3 and
are of similar size.
Figure 2: Hexagonal Frame with Beam Diagram Showing Deflection
Figure 3: Square Frame with Beam Diagram Showing Deflection
The beam diagrams represent the total
magnitude of deflection of each element
and show that the square frame structure
is slightly stiffer than the modeled
hexagonal structure, bending only 85%
as far when placed under the same load.
A good potential future model would be
to model two frames of exactly the same
volume of material (of for that matter,
the same mass provided that identical
material is used) and repeat the loading
analyses to see if this slight stiffness
advantage is still present in the square
frame structure. As far as the completed
test’s results go, however, the patterns
seen in the wing would tend to
supporting the overall structural model
of a wing with a stiff leading edge and a
more flexible trailing edge, especially
considering how the vein size also
decreases from the leading edge of the
veins
wing to the trailing edge of the wing as
can be seen in figure 1.
Another notable characteristic of wing
structure is the three-dimensional
structure present in the wing. Although
from most photographs of wings, they
may appear to lay on a flat plane, in
actuality the wings are full of three
dimensional relief. One example of this,
as mentioned before is in the leading
edge. The three leading edge veins form
a sort of angle bracket structure as
shown in figure 4 which contributes
greatly to span wise wing stiffness. In
addition to this three dimension
structure, the wing possesses an overall
camber. Using a Fortran code to
generate hexagonal cambered frames, a
number of models of essentially equal
mass were generated and analyzed using
FEMAP with NX NASTRAN. Some of
the results are shown in figures 5 and 6.
membrane
Figure 4: Cross Section of Leading Edge Frame Structure
Figure 5: Flat (radius of curvature =100) Hexagonal Frame with Beam Diagram
Showing Magnitude of Total Deflection
Figure 6: Cambered (radius of curvature =2) Hexagonal Frame with Beam Diagram
Showing Magnitude of Total Deflection
These analyses showed that a
cantilevered hexagonal frame with a
radius of curvature of 2 (note that
FEMAP does not use units) deflected
only 25% as much as a flat hexagonal
frame (actually a hexagonal frame with a
radius of curvature of 100 which is
essentially flat) under the same load. In
addition to this, a frame with a radius of
curvature of 0.5 deflected only 20% as
far as the identically loaded frame with a
radius of curvature of 2. These results
again fall in line with the overall
structure that is stiff along the span and
flexible along the chord.
By quantifying the specific
structural properties, such as flexural
stiffness along the span and the chord of
dragonfly wings, design parameters
could be generated for creation of
biomimetic structures.
III. Composite Fabrication
The other part of research
conducted in this field consisted of some
experimentation with composite
fabrication. This was as much as
anything else a casual opportunity for
the author to gain experience and
familiarity with the process of using
epoxy and carbon fiber tow, mediums
with which this author was wholly
unfamiliar with before research began.
A number of molding methods were
attempted with varying degrees of
success. One of the first was merely
using a flat, clean glass surface to lay up
wetted carbon fiber tow and let it dry.
This yields one flat surface on the
composite once it has cured, but it is
difficult to precisely control the shape of
the fiber without some sort of mold. The
next attempts which very well may hold
some potential for repeatedly creating
similar shapes from very small pieces of
carbon fiber tow was to use transparency
film to create the shape. The first step is
cutting a piece of transparency film into
the desired composite shape. One side
of the transparency is adhered to a
smooth glass surface. A piece of carbon
fiber tow with a small filament count is
then wetted and placed along the cut
edge of the transparency on the glass.
Then, the other piece of the cut
transparency is forced against the carbon
fiber tow, sandwiching it between the
two pieces of cut transparency and
causing it to assume the shape of the cut.
Another piece of transparency is placed
on top of the carbon fiber and weight is
added while the composite cures. For
larger scale molds, the first medium
which the author attempted to use was
glass. A Dremel tool was used to etch
the glass and then the wetted carbon
fiber tow was laid into the etced mold
and covered with a piece of transparency
and a weight. The result was a rather
clean cured part which released easily
from the mold, but the glass ended up
being excessively difficult to work with
due to its hardness. Dremel brand
diamond tip bits were worn down
quickly during the etching process.
Figure 7 shows one of the results of
using glass molds.
Figure 7: Glass Mold with Example Composite Structure
After the attempt to mold with the glass,
an attempt was made using Plexiglas.
Plexiglas was much easier to work with
and did not dull the bits as glass did, but
there were occasional issues with the
material melting and resolidifying
instead of simply grinding away. The
relative ease with which the Dremel bits
were able to go through the Plexiglas
allowed the use of a sort of routing
attachment which allowed for more
precision and consistency in the mold
making process. The main problem of
the Plexiglas, however, was that it was
much more difficult to get the cured
composite to release from the mold. In
the attempt made during the research by
the author to remove the single piece
linear sample created in the Plexiglas
mold, the mod broke along the channel
which had been routed out to receive the
wetted composite. This mold (pieced
back together) is shown in Figure 8. A
final experiment performed with
composites was the fabrication of a three
dimensional structure. This structure,
shown in figure 9, could be scaled down
into a sort of fuselage. It was created
around a cylindrical piece of paraffin
wax which had grooves in it for the
composite to be wound into. The
grooves were generated by hand, but in
the future could be machined into the
mold to allow for a more precise
structure. This structure exhibits a great
deal of “springiness” along its axis, but
transversely it is very stiff and is overall
a very light structure. Further
investigations into different pitch angles
for the helixes or the addition of
composite along the axis to stiffen the
structure could optimize this sort of
pattern for use in the fuselages of small
UAVs.
Figure 8: Plexiglas Mold with Example Composite Structure
Figure 9: Cylindrical Structure Created with Wax Mold
IV. Conclusion
The basic understanding gained
from this research lays the foundation
for further, more specific investigations
to be made into the subject of dragonfly
wings. The next step in this research
likely would include making detailed
calculations of the flexural stiffness of
dragonfly wings as well as
characterizing the specific type of
composite which is intended to be used
to fabricate a biomimetic wing. A
membrane like material needs to be
identified; the microfilm used in a
number of rubber band powered balsa
models shows some potential but an
analysis as to whether it could hold up to
the rigors of ornithopter flight needs to
be conducted. Once the structural
properties of both dragonfly wings and
composites are accurately quantified,
design of biomimetic structures can
begin. These structures can then be
fabricated and tested against actual
dragonfly wings to determine their
accuracy to actual wings. While this
process is only the beginning of creating
biomimetic micro UAVs, it is necessary
if such aircraft are to be developed.
Works Cited
1 – Wootton, Robin J. “Wings.”
Encyclopedia of Insects. Academic
Press, 2003.
5 – Combes, S. A. and Daniel, T. L.
“Flexural Stiffness in Insect Wings I.
Scaling and the Influence of Wing
Venation.” The Journal of Experimental
Biology 206 (2003): 2979-2987.
6 – Combes, S. A. and Daniel, T. L.
“Flexural Stiffness in Insect Wings II.
Spatial Distribution and Dnamic Wing
Bending.” The Journal of Experimental
Biology 206 (2003): 2989-2997.
2 – Newman, D. J. S. and Wooton, R. J.
“An Approach to the Mechanics of
Pleating in Dragonfly Wings.” The
Journal of Experimental Biology 125
(1986): 361-372.
3 – Smith, C. W., Herbert, R., Wootton,
R. J., and Evans, K. E. “The Hind Wing
of the Desert Locust (Schistocerca
gregaria Forskål) II. Mechanical
Properties and Functioning of the
Membrane.” The Journal of
Experimental Biology 203 (2000): 29332943.
4 – Song, F., Lee, K. L., Soh, A. K.,
Zhu, F., and Bai, Y. L. “Experimental
studies of the material properties of the
forewing of cicada (Homóptera,
Cicàdidae).” The Journal of
Experimental Biology 207 (2004): 30353042.