48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition
4 - 7 January 2010, Orlando, Florida
AIAA 2010-184
Design and Construction of All-Composite UAVs Utilizing a
Modified VARTM Process
Robert D. Vocke III*, Timofey Spiridonov†, Darryll J. Pines‡
University of Maryland, College Park, MD, 20742
Composites are used extensively throughout the aerospace industry, but small scale
unmanned aerial vehicles (UAV) don’t usually incorporate an all composite construction,
using them instead to reinforce key structural elements. However, there are potential
benefits from an all-composite UAV construction, including a decrease in weight, increase in
robustness and crash survivability, and more design freedom for complex aerodynamic
structures. A team composed of graduate and undergraduate students from the University of
Maryland examined the feasibility of all-composite design by applying a streamlined design
process and then using a modified Vacuum Assisted Resin Transfer Molding (VARTM)
process to construct three distinct UAVs. The UAVs were designed to compete in the annual
AIAA Design Build Fly competitions. The modified VARTM process was developed and
refined by the team using a combination of established techniques and experience based
improvements. Skills such as mold design, mold manufacture, and composite part
manufacture were developed as aircraft designs became progressively more complex. The
final UAV featured the desired all-composite construction as well as a monocoque structure
where all of the loads were carried through the skin. This dramatically reduced weight and
complexity, as little to no internal structure was needed. This work demonstrated the
feasibility and benefit of all-composite construction for a small scale UAV.
F
I. Introduction
IBROUS composites, which are a staple of the aerospace industry, are used because of their high strength and
stiffness to weight ratios. From simple wing spars, to aircraft like the Boeing 787, which is half composite by
weight1, composites have truly revolutionized the way designers and builders think about aircraft. Unmanned Aerial
Vehicles (UAVs) have not been left out of these advancements, with aircraft such as the United States Air Force’s
Predator and Reaper, both large scale UAVs with many composite parts, currently patrolling the skies.
Composite construction has the potential drawback that it can be expensive to initially design, test, and
manufacture an all-composite aircraft. Also, optimized composite manufacturing processes can be extremely
expensive to implement because specialized equipment, not available on a university or small business level, may be
required. The purpose of this study was to assess the feasibility of developing a comparatively simple and
inexpensive autoclave-free method for constructing an all-composite, and aerodynamically complex UAV. The
vacuum assisted resin transfer molding (VARTM) process is discussed, along with the aerodynamic design and
manufacturing processes followed by the design team.
II. Composite Molding Process - Overview
There are many techniques used in the aerospace industry for composite part manufacture. Some of the most
notable are wet-layup, filament winding and automated tape placement, resin transfer molding (RTM), VARTM,
and autoclave processes. VARTM was chosen as the primary manufacturing process for this project because of the
*
Graduate Research Assistant, Aerospace Department, vocke@umd.edu, Student Member
Graduate Research Assistant, Aerospace Department, tspirido@umd.edu, Student Member
‡
Dean, Clark School of Engineering, pines@umd.edu
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†
Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
extremely specialized equipment were not required, and the possibility of achieving a good strength to weight ratio.
These basic reasons will be expanded on and clarified throughout the paper.
The VARTM process (Fig. 1) uses negative pressure (a vacuum) to pull a resin matrix through fibers that have
been draped in a mold. A thermoset resin that has already been mixed with catalyst is pulled from a reservoir into
the vacuum bagged part, infusing the dry layup (Fig. 2). The layup usually consists of layers of fiber reinforcements
separated by an infusion matrix or fabric, which allows resin to flow easily from the inlet to the outlet. A layer of
bleeder or breather can be added to ensure even pressure distribution and to absorb any excess resin. A vacuum bag
then encloses the entire layup and is sealed with sealant tape. Once the part is fully infused, the excess resin is pulled
out of the outlet tube and is caught in the resin trap, essentially an empty vessel that ensures resin does not enter and
destroy the vacuum pump. The part is left under vacuum for the duration of its curing cycle.
Figure 1. Overview of the VARTM Process2
Figure 2. Closeup of VARTM mold with fiber layup2
In general, this process produces reliable, repeatable parts with relatively high fiber volume fractions for a nonautoclave process.
III. Design and Analysis
The design of the two aircraft shown in this paper was guided by the requirements laid out in the 2007 and 2008
AIAA Design Build Fly contest. This contest is held yearly and outlines overall rules and mission requirements that
the aircraft have to fulfill. The general aircraft requirements, such as take off distance, maximum dimensions, and
payload sizes and weights, were taken from these rules. The detailed design however, was completely up to the
team, and emphasized developing and implementing a modified VARTM process. Figure 3 shows the two most
recent final designs. For a sense of scale, the wingspans of the Flapjack and Shamu were 3 ft and 5 ft, respectively.
a) UAV from 2007 (The Flapjack)
b) UAV from 2008 (Shamu)
Figure 3. UAVs from the past two year’s design teams, named for obvious reasons.
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The majority of aerodynamic properties for both aircraft configurations were dictated by the contest
requirements. However, a detailed aerodynamic analysis was performed to optimize the flight characteristics of each
aircraft. This paper will deal primarily with the aerodynamic design of the Flapjack configuration. The analysis for
the Shamu configuration followed a similar methodology.
For the 2007 DBF competition, design requirements challenged teams to minimize wingspan, while
maintaining a comparatively large payload area. Trade studies were conducted to quantitatively determine the best
overall aircraft configuration. The two main candidates for the Flapjack’s year were a lifting body configuration
patterned after the Vaught V-173 and a delta wing configuration. Free jet wind tunnel tests of delta wing and lifting
body models, machined in a Stratasys stereo lithography system, were conducted to help down select to the final
design. The models were tested at varying Reynolds numbers and lift and drag data were recorded. These results
were extrapolated to predict performance at take off conditions and showed that the aerodynamic characteristics of
both designs were similar. Based off this, and that the delta wing’s construction was deemed to be more complex,
the team chose the lifting body as the final design.
A key aerodynamic consideration when designing a small wingspan, and thus low aspect ratio, lifting body is to
ensure that the majority of its surface area contributes to lift. The lifting body design chosen had a circular planform
that was modified to have zero wing sweep at the quarter chord. Three options were considered in an attempt to
maintain favorable aerodynamics while accommodating the required payloads. A protrusion from the bottom of the
aircraft that would not disrupt the low-pressure flow over the top of the wing was considered. This placement had
the unintended consequence of giving the center section of the aircraft negative effective camber, requiring the
aircraft to fly at higher angles of attack and generate more drag to provide the same amount of lift as a symmetrical
airfoil, and was therefore ruled out. A positive cambered airfoil complicated the manufacturing process with a larger
number of molds (discussed in Section III, B), and was ruled out despite the potential aerodynamic benefits. The
aircraft was therefore designed using a symmetrical NACA 0018 airfoil for the main wing, and the center payload
section of the aircraft was shaped to a thicker NACA symmetrical four-digit airfoil, cutting in half the number of
molds that were required for manufacture and eliminating the potential number of protuberances due to payload
configurations. Based on CAD models of the aircraft and payload, it was determined that an airfoil with 27%
maximum thickness to chord ratio was the smallest ratio that allowed all payloads to be handled inside the aircraft.
The thicker center airfoil was blended into the rest of the aircraft’s wing to minimize interference drag and ease the
demolding process.
It is important to stress the economy of a symmetric airfoil toward the construction process. The fuselage size of
Shamu was also dictated by its payload size, but in contrast to the Flapjack, the team picked a cambered (heavy-lift)
airfoil for the wings, which resulted in the production of
twice as many molds as the Flapjack nearly doubling
production time. However, the construction method also
allowed for the simple molding of elliptical wings,
reaping the aerodynamic benefits of minimized induced
drag.
For initial aircraft sizing, an approximate takeoff
speed of 30 mph was selected based on past UAVs
flown at the University of Maryland, including DBF
competition entries. The maximum takeoff weight with
heaviest payload was estimated to be 13 lbf. Based on
these suppositions, along with historical data on the air
density in the competition area and the required takeoff
Figure 4. Predicted effects of propeller vortices.
distance of 100 ft or less, a wing reference area of
2
approximately 7 ft was calculated. A wingspan of
exactly 3 ft was chosen, yielding a root chord of 3 ft and a wing reference area of 7.068 ft2.
Based on data in NACA Research Memorandum L6I19, a report detailing the aerodynamic performance of the
V-173, the team was confident that some additional aerodynamic benefit would be obtained from the wingtip
mounted propellers. The increase in performance is attributed to the propellers generating vortices that oppose the
wingtip vortices, reducing the negative effects of a low aspect ratio by increasing the effective angle of attack seen
by the portion of wing aft of the inboard portion of each propeller. Interpolation of the data from RM L6I19, wind
tunnel testing of the V-173, resulted in an estimated increase of performance between 15-25% (Fig. 4) over what
would be expected from a conventional analysis. However, the magnitude of that benefit was deemed difficult to
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accurately predict without wind tunnel testing3,4 so all wing, control surface, and stabilizer sizing as well as
performance estimation was done assuming no benefit from such interaction.
A. Design Validation
All designs underwent thorough aerodynamic and structural analysis before proceeding to mold design. The
design concepts were first fully modeled in Catia, a 3D CAD software program. This allowed for the plane geometry
to be imported into FEM software for structural analysis. Aerodynamic analysis included computational fluid
dynamics (CFD) analysis that calculated theoretical lift and drag coefficients, as well as stability derivatives. Wind
tunnel models were tested in a low-speed open jet wind tunnel the CFD results.
1. Aerodynamic
Aerodynamic design validation for the Flapjack was accomplished using a 1/3 scale model (Fig. 5) tested in the
University of Maryland’s Glenn L. Martin Wind Tunnel (GLMWT). Overall, the testing provided the team with data
that aided in further refining the design. One necessary change was made apparent during flow visualization
exercises. The model demonstrated significant flow separation at angles of attack above 8 degrees over the
horizontal stabilizers (Fig. 6), so in the final design, the stabilizers were enlarged and made all-moving to hopefully
prevent a loss of control at high angles of attack.
Figure 5. 1/3 scale lifting body model being tested in
the GLMWT.
Figure 6. Oil flow visualization showing separation
bubbles on the horizontal stabilizers.
Dynamic stability analysis was conducted when sizing the horizontal and vertical tails to ensure that the aircraft
was controllable. Normally, an aircraft's stability derivatives are found empirically with wind tunnel testing.
Unfortunately, to minimize expense, Glenn L. Martin Wind Tunnel (GLMWT) testing did not yield any derivatives.
Instead, Tornado CFD code5 was used to generate the required stability derivatives. Tornado is a Matlab-based
vortex lattice CFD panel code. The CAD drawings of the Flapjack model were translated into non-dimensional
coordinates and graphed in Matlab. The main wing and
horizontal and vertical tails were modeled by specifying
span, chord, taper ratio, airfoil section, etc. Once a
satisfactory model was made, it was tested at multiple
AOAs and speeds to construct a performance matrix. It
was necessary to run tests and subsequently refine the
model to avoid areas with too high panel density (Fig.
7). This model was then run through the Tornado
simulation, returning lift and drag characteristics for
various speeds and angles of attack, as well as stability
derivative coefficients. The resulting CL and CD curves
were used to solve for predicted optimum cruise
conditions, as well as the area requirements for
horizontal and vertical stabilizers. Depending on the
propulsion system chosen, this code indicated the
aircraft would cruise between 53 and 59 mph, at AOAs
Figure 7. CP distribution from Tornado CFD code.
between 5 and 6 degrees. Estimating the cruise speed
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was necessary for the construction of a run matrix at the Glenn L. Martin Wind Tunnel. For an all-moving horizontal
'elevon,' a chord length of 6.5 inches was found to be appropriate. The resulting tail size numbers fell within initial
analytical stability estimates made in the preliminary design stage.
In order to verify Tornado predictions, the 1/3 scale Flapjack model was tested in the GLMWT with yaw and
pitch sweeps from -10 to 10 degrees and -10 to 40 degrees respectively. The data demonstrated that the aircraft’s
cruise speed and angle of attack were both slightly higher than those predicted by the Tornado code, rising from an
angle of attack of 5 degrees to 8.8 degrees and from a speed of 53 mph to 54.4 mph. Control analysis performed for
cruise and take-off conditions demonstrated that the aircraft would be stable, but sensitive. The stability derivatives
revealed that the design is less stable than a conventional propeller aircraft, yet significantly more stable than a
conventional lifting body. The counter-rotating propellers were also expected to help keep the flow attached over the
aircraft and control surfaces.
2. Structural
Structural analysis was accomplished by creating 2-D shell elements in Catia that could be imported into MSCPatran, and then analyzed in MSC-Nastran. This method of modeling allowed for the user to input material thickness
to get the 3-D effects of the structure. The models were made with quad elements for the most accurate modeling.
Triangle elements were avoided because they provide a natural stiffness that makes the model unrealistic. Material
properties were found from MTS tests of sample coupons manufactured by the team. An unrealistic assumption that
was made to decrease model complexity was isotropic behavior of the material. More thorough composite analysis
would use laminated plate theory to find the directional material properties of a thin section, but making these results
meaningful in an FEM software package requires knowing how the cloth is oriented over the entire surface, which
was an unrealistic amount of work for the team. The bulk material assumption was made with some confidence
however due to the fact that two layers of 0° and 90° weave carbon cloth were layered such that the final composite
had fibers oriented at -90°, -45° ,0°, and 45°.
Fig. 8 shows two examples of the structures modeled. Fig. 8a depicts a wing bending test for Shamu. The model
was constrained where the wing connected with through the fuselage. The load for the model was a very simple
point load at the wing tip on the quarter chord. This was meant to simulate a multiple g turn, the highest loading
seen by the wings. A maximum deflection of 0.23 in was found, which was well within design parameters. Stresses
were also calculated to ensure the wing was not close to failure. Fig. 8b shows stress concentrations on the interior
of the Flapjack, where the front and back of the aircraft were connected with fasteners. The loading condition for
this test was a “hard” landing where the aircraft would be subjected to around 3 g of deceleration. As a result of this
test, those areas were reinforced in the final aircraft.
a) Wing loading deflection simulation for Shamu
Figure 8. FEM results from MSC-Nastran
b) Stress simulation for Flapjack 3 g landing conditions
A very important conclusion of the structural analysis was that the aircraft needed no internal structure to
withstand the aerodynamic and landing forces. This monocoque construction, where the skin is load bearing,
provided incredible structural weight savings.
B. Mold Design
Once the team approved the aircraft final design, the mold design began. Overall, the process involved designing
and manufacturing a male mold from which female molds can then be produced. Parts were then made from the
female molds.
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As mentioned earlier, major consideration was the number and complexity of molds to be manufactured. From
experience, the team knew that mold design and manufacture is one of the most complex and time consuming parts
of the VARTM process, but also vital to the eventual success of the project. Properly designed molds make releasing
and integrating parts easier, and can even affect the quality of the
final part. The number of molds was minimized for each UAV
because of the incredible amount of time needed to make one mold
(~40 labor hours depending on mold size). For example, the
Flapjack was designed so that it was symmetrical from top to
bottom, and from left to right. This allowed the same molds to be
used to create the top and bottom halves of the fuselage and control
surfaces, cutting total molds from a potential 12, to 6. Likewise, the
front and rear wings of Shamu were designed to be identical, so that
a single set of molds could make them both.
Aircraft sectioning was done in Catia, splitting each part of the
aircraft with a continuous plane, to give a top/bottom or left/right
male mold, or plug, for each part. Fig. 9 shows the male mold for
Figure 9. CAD drawing of the left
the left hand side of Shamu’s fuselage. Notice that features such as
hand side of Shamu’s fuselage plug
the rudder cut out, the wing root locations, and a hatch cut out, are
already molded. This design foresight is
mainly a product of experience, but can
provide extremely useful features when
parts have to be integrated in to the final
structure.
One of the most important mold
features, besides proper draft angles and
radiuses, is depicted in Fig. 10. All of the
male molds (plugs) were designed with an
Figure 10. Illustration of plug offset. The blue is the actual part
offset added to the cross section, separating
surface, and the red is waste composite material2.
the flange of the mold from the actual part
surface by a constant distance (.375 in). The offset was smoothly transition from the part into the flange by adding a
fillet around the entire base of the part, with a radius equal to the offset distance. This design feature may seem
somewhat trivial, but in fact was key to easy part de-molding and accurately separating the parts from the waste
composite material, and is described in more detail in Section VI.
IV. Mold Manufacture
A. Male Mold Manufacture
Once the molds were fully designed, the files
were sent to a machine shop to be cut out of high
density polyurethane foam. Specifically, Type FR4520 from General Plastics Manufacturing
Company, having a density of 20 lbm/ft3, was
utilized. This foam was chosen because of its
resistance to warping and fine cell structure,
contributing to a better surface finish. The team
chose to trade a decrease in machining time (and
thus machining cost) for a coarser finish. This meant
that plugs then had to be meticulously hand sanded
down to their final surface finish, using
progressively finer grits of sand paper. Fig 11 shows
the front and back molds for the Flapjack fuselage in
their completely sanded state.
Once completely smooth, the plugs were sprayed
with a mixture of Grey Duratech Surfacing Primer
and Duratech High Gloss additive. This mixture
Figure 11. The completely sanded front and back plugs
for the Flapjack.
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cured to hard, durable, glossy tool surface, which was then sanded
with 500, 1200, and 1500 grit sand paper until completely smooth.
The surface was then polished and waxed using pneumatic polishing
wheels. Great care was taken through out this entire process because
any defect or rough spot on the male mold, would be transferred
directly to the female mold, and ultimately to the finished part. Fig. 12
shows the male mold (grey) and its matching female mold (orange)
for Shamu’s hatch section. This particular part was a departure from
the teams flat flange mentality for mold design.
B. Female Mold Manufacture
As depicted in Fig. 12, female molds were made from the male
molds. Female molds are the tooling surface from which the
composite parts are actually made. The benefit of making a male mold
first and then a female mold, instead directly manufacturing a female
mold, is that if the female mold is damaged beyond repair, a new one
can be readily produced from the existing male mold.
Female mold manufacture began with spraying the male mold with
Figure 12. Example of finished
a layer of Fibre Glast Polyvinyl Alcohol (PVA) release film. After a
male and female mold.
satisfactory spray with no drips, runs, or air bubbles was achieved, the
film was allowed to dry completely. A 1-2
mm thick layer of polyester Polycor
Orange Tooling Gelcoat mixed with
Duratec High Gloss additive was then
sprayed on top of the release, forming the
tool surface of the female mold. Once the
mixture had fully cured, layers of polyester
resin and random directional fiberglass
matting were built up on the back of the
gelcoat to form a stiff structure for the tool
surface. A minimum mold wall thickness
of 1 cm was desired. Fig. 13 illustrates the
layers of the mold making process
described above. Please not that the mold
pictured does not correspond to either the
Flapjack or Shamu, but to another UAV
whose design is not discussed in this paper.
All pictures relating to that UAV are
applicable to the current discussion
After the polyester resin had cured, the
female mold was release from the male
mold using compressed air. This process
could take a long time, but patience and
Figure 13. Diagram illustrating the layers of sprays and
care when releasing the molds always
composites that comprise the mold making process2.
produced an excellent result.
V. Part Manufacture
Part manufacture began by spraying the mold with PVA release film and allowing it to dry. The mold was then
enclosed on all sides by strips of Tacky Tape room temperature sealant tape, to which the vacuum bag would
eventually but attached. The carbon fiber cloth – infusion matrix lay-up was then draped into the mold (Fig. 14 on
next page) and trimmed to be within the perimeter of the sealant tape enclosure.
The team conducted a trade study to determine the best combination of carbon fiber cloth and infusion media. A
2x2 twill weave was chosen for the two candidate clothes because of its superior drapeability. Both a heavy (~6
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oz/yd2) and light (~1.5 oz/yd2) cloth were examined, with
the light cloth winning out because of its adequate strength
and significant weight savings. Two general types of
infusion matrices were examined, a very thin “red mesh”
plastic lattice structure, and a thicker paper honeycomb
structure. Test parts infused around the paper honeycomb
were noticeably stiffer than those infused around the red
mesh. However, due to necessarily porous nature of an
infusion media, the thicker paper honeycomb parts were
prohibitively heavy. Placing the infusion media on top of a
layer of peel ply was also examined. The rational behind
this was to maximize the fiber volume fraction of the part
by eliminating the excess and wasted resin stored in the
infusion media. Unfortunately, these parts were not stiff
Figure 14. This figure depicts all layers of a
enough to support bending loads without the addition of
typical layup, including outer and inner
2
internal structure and thus were abandoned. The winning
carbon plys, and interior infusion media .
combination emerged as two layers of the light carbon
cloth sandwiching the red mesh infusion media with the paper honeycomb used in areas that needed structural
reinforcement.
Once the layup was properly draped in the mold (which sometimes proved challenging, especially in small radii
concave surfaces), a layer of peel ply was added to absorb excess resin and give consistently rough interior surface
finish that was easy to bond to when the parts were eventually integrated. Inlet and outlet ports were then added and
spiral cut tubing, used to as a channel to disperse both the vacuum
and the resin, was threaded through the ports and out into the part.
Depending on the shape of part being made, the ports would be
positioned so resin was either drawn across the part, or into the
center of the part and then out in a concentric fashion towards the
perimeter. The entire setup was then enclosed in Stretchlon 800
vacuum bagging material (Fig. 15).
One of the most important parts of the part manufacturing
process was making sure that when the vacuum was pulled, the
cloth was pulled into all the edges and not bridging areas with
complex curves or sharp corners. Through trial and error, the
team discovered that the only way to make sure that the fabric
was pushed into every corner was a lot of hands, and a lot of
Figure 15. A completely vacuum bagged
patience.
layup ready for infusion.
Once the layup was successfully positioned, a batch of
polyester resin was prepared in a large cup. A rough
ratio of 1 lb of resin to a square yard of mold was used
as rule of thumb. The resin was first mixed with styrene,
to reduce its viscosity and facilitate its infusion through
the part, and then with the catylist, Methyl Ethyl
Keytone Peroxide (MEKP). Once the resin was
thoroughly mixed, the inlet tube was inserted into the
cup to allow resin to be drawn into the part by the
vacuum pressure. Fig. 16 shows a fully infused part,
with the inlet and outlet ports arranged so that the resin
laterally across the part.
When they were fully cured, parts were removed
from the molds using paddles that lifted up the flange
and shot compressed air under the part. Unless a tear had
occurred in the release film, parts typically released
beautifully.
A major problem encountered by the team was
Figure 16. A fully infused composite part showing
delamination. The root cause of this could always be
the inlet and outlet ports, as well as the resin flow
traced back to air bubble inclusions. Resin degassing
direction2.
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was never included in the part making process because of the relatively quick gel time for the polyester resin, so
some of the air bubbles could have come from the resin itself. The more likely culprit however was air leaks around
the perimeter of the bag and at the inlet and outlet ports. However, these problems were eliminated as experience
was gained using the manufacturing process.
VI. Part Integration
After parts came out of the mold, they had large flange area
which had to be trimmed away. Using a Dremel tool, the flange was
first cut to about 0.5 inches from the edge of the part. A Rotozip
outfitted with a carbide toothed blade was set up in a jig that held
the top of the blade precisely at the height of the constant offset
discussed in Section III. Fig. 17 shows the jig setup and how it is
used to precisely cut excess material from the part. As long as the
part was resting on a smooth, flat surface, and the operator was
careful not to flex the part during trimming, this method of cutting
produced parts with extremely consistent and flat cuts.
The flat cut surface on the parts facilitated a near perfect mating
of top/bottom and left/right halves (Fig. 18). This was facilitated by
first taping the two halves together along the exterior of the part
with regular packing tape, making sure to smooth any bubbles or
ripples in the tape. Catalyzed resin was then applied to the seam
Figure 17. Composite part being cut by a
Rotozip held in a wooden jig2.
from the interior of the part. Parts that did not include easy access to
their interior had small holes drilled in them, and the resin was
injected with a syringe. The parts could then be held vertically, and
slowly rotated to allow the resin to flow to all parts of the seam.
Parts that needed more structural reinforcement had fiberglass tape
applied to the seams. Once the resin had partly cured, the packing
tape was removed and the outer seam was rubbed with denatured
alcohol to remove any traces of resin that escaped the tape.
As mentioned earlier, the vast majority of these parts were
monocoque structures that carried all structural loads through the
skin, and this needed very little internal structure. The internal
structure that did get used was generally to prevent parts from
Figure 18. Mating of left/right fuselage
crushing (e.g. a small piece of foam in the center of a wing). Fig. 19
halves2.
shows the completed Flapjack and Shamu UAVs with avionics and
motors installed. Notice the incredible surface finish that was
achieved with out any sort of post infusion touchups, but rather through a careful application of the VARTM
process.
a) The Flapjack
b) Shamu
Figure 19. The fully integrated and flight worthy UAVs.
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VII. Design Validation
The team was able to conduct full scale,
powered wind tunnel tests in the GLMWT for both
UAVs described in this paper. These tests were used
to diagnose and fix unforeseen aerodynamic issues,
as well as verify predicted aerodynamic
performance estimates. The testing of the full scale
Flapjack model (Fig. 21) is described in detail
below.
The purpose of the full scale testing of the
Flapjack was to confirm performance estimates, and
obtain some preliminary quantification of the
benefits caused by the interaction of the propulsion
system with the aerodynamics of the airframe. Due
to limitations of the power supply configuration
used during wind tunnel testing, the maximum
voltage applied to each motor was less than what
was available in flight, making the wind tunnel
Figure 21. Full scale wind tunnel testing of the
measurements slightly conservative. However, these
Flapjack.
results still provided a good preview of how the
aircraft would perform in the air.
To confirm takeoff performance, an AOA sweep was performed at 30 mph (close to the estimated takeoff speed
of 32 mph). The AOA sweep showed that the aircraft generates enough lift (a lift coefficient greater than 0.60) for
takeoff at 30 mph by pitching up to 9°. This compares favorably to the estimated takeoff speed of 32 mph,
confirming that the aircraft is capable of takeoff performance that meets and exceeds estimates. Similarly, an AOA
sweep was performed at 54 mph (near the estimated cruise speed of 54.4 mph) to determine at what AOA lift was
equal to the 14 lbf weight of the aircraft. This AOA value was found to be 7.5° and used as the new value for cruise
AOA. A velocity sweep was performed at that AOA from 5 mph to 65 mph to explore the effect of propeller
advance ratio. From these tunnel runs, the aircraft was found to still generate 1.60 lbf of thrust at 54 mph at 7.5°
AOA. This result indicates that the aircraft’s cruising speed actually exceeds the original estimate of 54.4 mph and
that the aircraft’s cruising angle of attack is actually less than the refined estimate of 7.5°.
It is noteworthy that the full-scale model with propellers on produced a small amount of lift at a zero angle of
attack, despite having a symmetric airfoil (Fig. 22). The induced drag of the aircraft was indeed found to be lower
due to the effect of the propeller vortices at the wing tips (Fig. 23). The propeller vortices help to effectively
increase the aspect ratio of the wing, without changing its actual geometry. This verified one of the most important
reasons for choosing this planform configuration above other proposed designs.
CD vs alpha
CL vs alpha
1
2
0.8
1.5
0.6
CL
Full-scale with props
Full-scale no props
CD
1
Full-scale with props
0.4
Full-scale without props
0.5
0.2
0
-20
-10
0
10
20
30
40
50
0
-20
-10
0
10
20
30
40
50
-0.2
-0.5
pitch (deg)
pitch (deg)
Figure 22. Experimental CL vs. AOA for the Flapjack
Figure 23. Experimental CD vs. AOA for the Flapjack
Overall, the full-scale powered wind tunnel investigations confirmed that original performance estimates were
reasonable and that the aircraft could easily meet and exceed all performance requirements.
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When looking at the wind tunnel data taken with the propulsion system operating, it is important to note that the
lift and drag coefficients are actually the coefficients for the net force on the aircraft in the lift and drag directions,
respectively. Thus, the lift coefficient is a measure of the combined aerodynamic lift generated by the aircraft and
the component of thrust oriented normal to the free stream. Similarly, the drag coefficient is a measure of the sum of
the aerodynamic drag on the aircraft as well as the component of the thrust oriented parallel to the free stream.
Negative drag coefficients can be seen in flight conditions where thrust exceeds drag.
VIII. Conclusion
The design process discussed in this paper was demonstrated to be a feasible method for constructing small scale
UAVs, both in an engineering and financial sense. The engineering usefulness of the VARTM method lies in its
adaptability to almost any small scale UAV application where lightweight composite parts are desired. Both UAVs
discussed in this paper had a gross empty weight (weight without payload) of under 6.8 kg (15 lbs), and were able to
carry at least their weight in payload. This process was also cost effective and time effective. The team was able to
produce all required molds, purchase servos, motors, and batteries, and produce parts for five separate airframes for
each UAV (ten aircraft in total) for a budget less than $36,000, or about $3,600 per aircraft. Moreover, it
accomplished all design and fabrication in under six months.
Acknowledgments
The authors would like to acknowledge all of the team members who contributed to the development of the
processes described in this paper. Special thanks goes to Evandro Valente for the original development of this
process, and AAI Corporation and Dr. Darryll Pines, for funding and oversight, and Dr. Norman Wereley, for
helpful guidance.
2006-2007 Team: Lucas Parker, Chris Cheok, Leslie Woll, Brian Donnelly, Steven Myers, Chris Plumley,
Michal Krasel
2007-2008 Team: Vincent Posbic, Steven Myers, Aaron Chan, Andrew Wilson, Adam Reese
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