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Reverse Engineering and Aerodynamic Analysis of a Flying Wing UAV
Navabalachandran s/o Jayabalan1
, Low Jun Horng2, G. Leng
3
Aeronautical Engineering Group
Department of Mechanical Engineering
National University of Singapore
Abstract
This study is centered primarily about two main objectives with the first one being the
complete structural reconstruction and aerodynamic data generation of a pre-existent
Unmanned Flying Wing Air Vehicle with inadequate contractors aerodynamic and
stability data, construction specifications and knowledge of materials used. With this
accomplished we then focus on further aerodynamic analysis and scientific modification
to the original design and power plant to enable the platform to carry additional payloads
of an autonomous navigation system and a real time operating camera to meet various
practical mission requirements. The paper describes in detail the systematic reverse
engineering procedure adopted to analyze and synthesize the entire model. Some of the
techniques adopted are 3D Laser profile scanning of the reflex airfoil and fuselage,
material research and selection and cost effective reconstruction of the non-conventional
airfoil. We also present weight and balance matching techniques , the usage of
commercially available CFD programmes to generate aero coefficients and forces and
estimate the aircrafts aerodynamic center, results of the extensive verification flight tests
conducted and performance matching procedures used in general of the reverse
engineered craft to the existing UAV flying wing model.
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Nomenclature
CL Coefficient of Lift d Horizontal distance
CD Coefficient of Drag h Height
Cm Coefficient of Moment S Wing Area
c Chord Length M Mass of craft
Greek
Angle of Attack Glide Angle Angle of Pitch Relative Density
Introduction
Recent technological advances in the areas of propulsion, guidance systems, and
microelectronics, have made commercially viable miniature autonomous flying vehicles,
Micro Air Vehicles (MAV), possible. Most of the current concepts and prototypes
attempt to scale down traditional aircraft design to meet defense specifications. However,
classical aerodynamic concepts for fixed wing aircraft become impractical at the reduced
scale of MAVs. Hence the special attention of this paper to the reverse engineering and
aerodynamics of a miniature Flying Wing the Golden Eagle.
The usual method of developing an aircraft is to decide what the mission requirements of
the new aircraft are, finding an aerofoil shape specific to it by testing, do a sizing and
performance optimization and integrate it together with the other parts of the aircraft, i.e.
controls, propulsion systems, payloads etc. However, only a physical model of the UAV
was given without adequate contractors aerodynamic propulsion and stability data. This
breaks the chain of development and it is required therefore to do a fair amount of reverse
engineering to determine a good estimate of these required data. Moreover, conventional
methods of testing and analysis may not apply to this UAV as it is much smaller and
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slower than normal aircraft. New methods may have to be developed by trial and
evaluation.
Description of the UAV
The UAV given is basically a flying wing but with a central fuselage that follows the
reflex airfoil shape longitudinally and adapts to the curved M shaped, tip to tip wing
layout when viewed from the back. (Figure 1) The entire aircraft (modular wings and
fuselage) is constructed using ultra-light weight composite Kevlar fibre. Its fuselage is
specifically designed to house 4 Lithium batteries, a speed controller and a rear pusher
propeller unit. The craft is estimated to be able to carry a payload of 1.5 kgs and fly at
speeds up to 20 m/s. Effectively, there are only two control surfaces on the UAV. These
are the left and right elevons found at the ends of the wings of the aircraft. These control
the pitching and rolling on this UAV.
Figure.1. Shape of the UAV (rear view) Figure..2. The given UAV
The wing tips are angled upwards at about 30 degrees to the horizontal to compensate for
the lack of the rudder surfaces, acting as a pair of winglets to provide lateral stability to
the aircraft. Neither exactly a Sweptback wing or a Delta wing, its unconventional airfoil
structure was carefully analyzed and pre-existent aerodynamic theories have been
adapted to suit it where possible.
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3D Mesh Generation
Unable to match this particular wing with any of the standard NACA airfoils present, we
had to generate a full 3 Dimensional CAD model of the craft from scratch. With the
simplistic construction drawings provided we could not accurately determine the wing
curvature at the concave leading edge and at the convex tail. Hence, using the Minolta,
VIVID 900, Non-Contact-3D Digitizer Image Laser scanner, we photographed the entire
wing profile and fuselage with a tolerance of 1.5 mm, which we then assembled and
merged using the commercial scan programme RapidFormTM 2002- Reverse Modeler
Version. Working with the photographed scattered points, we had to systematically
connect each coordinate to attain the complex curves on the wing. Plot linearization and
CAD editing was needed to marginalize the inaccuracy inherent in scanning.
4Figure.3. 3-D Laser scanning and Reverse aerofoil CAD modeling procedure
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The model was then sectioned and sliced at critical intervals to obtain the exact structural
coordinates to be used to design and construct the wings. The entire CAD model was also
imported into GAMBITTM, a mesh preprocessing programme, and modified to avoid any
skewed edges before generating FLUENT compatible 3D surface and volumetric meshes.
Figure.4. 3-D Volumetric Mesh Figure .5. CAD model of the UAV
Structural Construction
Because design development was heavily dependent on flight-testing, the ease, speed,
and precision of manufacturing and repair was a fundamental process we had to before
development could begin. All components were determined to be modular and are to
break away during impact. This ensures minimal damage and hence reducing repair costs
and time. Various materials such as low and high density foam, stiff -1/2 in.
cardboards, balsa, paper marches and laminate resins together with different
manufacturing processors were experimented with and finally we singled out single ply
bi-directionally laid tissue carbon fiber (CFRP) as the desired material for the wing and
body because of its high rigidity, superior strength-to-weight ratio, low cost and ease of
availability. CFRP also displays excellent mechanical properties upon impact- crucial
consideration for a UAV without landing gear mechanisms.
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Reusable male and female clay molds were created and checked for consistency against
the acquired wing curvature dimensions. The carbon fiber framework was then laid on
the molds and covered with a thin layer of synthetic polymer (Ethylene Glycol, wt. %
99.9 - Polyester). Specifically measured quantities of resin were applied equally on each
of the two wings, maintaining symmetry in weight. The viscous resin was poured down
on the wing, with the mold propped vertically up. This ensures an even distribution of
resin throughout the cast. It was then allowed to drip and air dry in an enclosed area. This
procedure we discovered, gave a smoother and more even exterior finish compared to the
conventional method of brushing on the polyester. The entire manufacturing process is
highly repeatable with the usage of durable and reusable molds and cost effective readily
available materials.
Figure.6. Pouring of the resin on the fiber. Figure.7. Fabricated CFRP right wings top shell
Estimation of Aerodynamic Coefficients and Forces
To derive the aerodynamic derivatives, we use the CAD model of the UAV we reverse
engineered. It is first converted to a STEP file and a volumetric mesh is generated using
GAMBIT to be compatible with FLUENT a commercially available Computational
Fluid Dynamic (CFD) programme. The Flying wing is subsonic UAV operating at low
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Reynolds Numbers hence we ignored compressibility effects for the lift and drag models
and modeled laminar flow conditions sighting the fact our craft operates close to the
transition region; making it simpler to assume a laminar case rather then a turbulent
scenario. In FLUENT, we set up numerous models with different boundary conditions
to find how the UAV reacted to changes in speed, angle of attack and sideslip.
Figure.8. CFD Static Pressure Profile Plot
The coefficients attained were put into the equations of motions of the aircraft, and the
transfer functions of the UAV were derived. To derive the PID gains for the UAV, an
optimization was done to find the optimal gains for the UAV. MATLAB was used to
find the gains, using the transfer functions that were derived.
CL vs Angle of Attack
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 0.1 0.2 0.3 0.4 0.5 0.6
AOA in rad
CL
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The aerodynamic plots obtained are reflective of a flying wing UAV aircraft. The general
shapes of the graphs are very similar to conventional airfoils and the aerodynamic forces
obtained are logical too. These will be experimentally verified in glide tests.
Estimation of CG and Inertias of Mass
Since the entire Flying wing model is fabricated using different materials from that used
in the original prototype, we need to do a comprehensive weight and balance analysis. In
general the term weight and balance refers to the mass properties of an aircraft and the
resulting stability or lack thereof as a consequence of its mass properties. The term mass
properties usually includes the following values: volume (or mass or weight), center of
mass (or center of gravity), and the moments and products of inertia. CG is the point (or
centroid) of the craft about which moments summed, due to the mass of the object, equal
zero. Therefore this point represents a balancing point for the whole craft, and the total
weight or gravity force can be represented as acting at this point. We weighed each
component individually and marked out their CG respectively. Using simple geometric
summation and parallel axis theorem, the combined CG position of the craft was found.
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To experimentally verify our calculations, the conventional method of CG determination
was employed - the entire assembled model was mounted on a pivot and shifted
accordingly to attain the mass centre of the craft.
Pusher Propeller Unit 100g
Radio control electronics(two servo motors, servocard, RC receiver)
65g
Video electronics (camera,transmitter)
55g
Batteries (9-volt, 50 mAhNiCd)
350g
Micro Pilot Card & Cables 29g
Structure 750g
Total 1469g
Table 1: Equipment and weights Figure 9. Conventional CG balancing
The remaining of the payload was then strategically positioned within the fuselage to
shift the CG to the desired position before the aerodynamic centre. Fine tuning of this
exact location is to be done during the trimming routine to attain longitudinal stability
after glide tests. The equations for moment of inertia, are also referred to as second
moment equations. This is due to the squared moment arm that multiplies each
infinitesimal volume during the integration. In the case of the Ixx
, the distance from the x-
axis is the moment arm to be squared, and due to the Pythagorean Theorem, this squared
distance is y2
+ z2
. The same method is used for the other moments of inertia. We can
approximate the Inertias with the geometric summation of the various components of
different masses in the structure, as per equations (1)-(6).2 We must assume that each
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component has a constant density and mass distribution throughout. Thus, we obtain the
Inertia Tensor.
( ) ( ){ }
( ) ({ }
( ) ( ){ }
( ) ( ){ }
( ) ( ){ }
( ) ( ){ }
=
=
=
=
=
=
=
=
=
=
=
=
=
==
==
+=
+=
++=
ni
i
ni
i
ni
i
ni
i
ni
i
ni
i
XcgXiZcgZimiIzx
ZcgZiYcgYimiIyz
YcgYiXcgXimiIxy
YcgYiXcgXimiIzz
XcgXiZcgZimiIyy
ZcgZiYcgYimiIxx
1
22
1
22
1
22
1
22
1
22
1
22
0
0
)
(1)
(2)
(3)
(4)
(5)
(6)
Symmetrical Aircraft
Longitudinal Stability-Balancing of Pitching Moments
Stability is a very important criterion in the design of aircraft. For aircraft, two conditions
must be met for longitudinal stability.
0&0 0 >< mm CC (7)
0434.00985.2
0911.0=
=
CL
Cm(8)
As shown in our Cm vs curve, the gradient is negative and the graph intersects the x-
axis on the positive end. The Cm / CL calculation tells us where our aerodynamic
centre lies, the point where the moment acting on the body is independent of the angle of
attack, and since this is a flying wing with a comparatively small central fuselage which
also rides the wing profile, we conclude that the neutral point too lies at the a.c location
calculated. The negative value (8) tells us that the ac actually lies behind the CG location.
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Figure.10. Location of AC with respect to the CG
Experimental Verification
As flight-testing is an imperative step in the development any MAV design, the ability to
evaluate test-flights was critical. The primary flight characteristics we aimed to validate
via glide tests were stability, lift and drag. The glide tests must be conducted with the
engine installed and the propeller removed. As removing the engine would have created
an unrealistic mass distribution and a non-feathered propeller would have created an
uncharacteristically large drag.
Figure.11. Lift vs Drag Ratio verification
The glider's flight path is a simple straight line, shown as the inclined black line in
Figure11. The flight path intersects the ground at an angle a called the glide angle, .
h
d
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Horizontal Force Equation: )cos()sin( = dL (9)
Ratio:h
d
D
L=
==
tan
1
DRAG
LIFT (10)
(11)SD
SL
V
V=
=
2D
2L
C5.0
C5.0
D
L
C
C
D
L= (12)
We now would have verified our CFD simulated results of Drag and Lift forces and their
respective coefficients. The MAV prototype will also be fitted with the dummy camera
system and glide trials will continue to assess the trim and stability condition of the craft
with this additional payload. This will then allow further refinement of the Centre of
Gravity position to achieve acceptable flying qualities. The goal in trimming a flying
wing is to get the center of gravity as far aft as possible and still maintain stable control
over pitch. Since the flying wing has very little tail moment there is a tendency for the
wing to be very pitch sensitive. As this craft also does not originally have a rudder, its
yaw and lateral characteristics will also be closely assessed with easily interchangeable
rudders of different configurations fitted on hand during testing.
Concluding Remarks
The project satisfies the two primary goals. For the first loop of the design iteration, a
considerable achievement has been made. A working prototype has been designed, built
and ready for flight testing on time and within the allocated budget. Inline with the
second goal, Computational Simulations have given the required aero coefficients, forces
and moments which will be verified by flight tests. Problems with the CFD and mold
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building highlighted the larger timescales involved with the preparation of a model mesh,
and the airfoil structure which had not been anticipated. Attempts have since been made
to shorten the time needed to accomplish this task, so future students investigating
MAVs may easily replicate any wing form even when faced with a hard date-line.
Apart from the vast commercial viability of the reverse engineering procedures
introduced in this paper, they can also serve a crucial role in military and defense
applications, where one side may be able to replicate a captured enemy drone (obviously
with no supporting data) and reconfigure it to carry a micro-camera, flying it into the
enemy ground, without them even engaging it.
Acknowledgements
I would like to sincerely thank my supervisor and mentor A/P Gerad Leng of the
Department of Dynamics, National University of Singapore for granting me this rare
opportunity and for his great encouragement and guidance all along the way.
References
1) Karl Nickel and Michael Wohlahrt,, Translated by Capt. E. Brown RN,, Second
Edition-1996 Tailess Aircraft in Theory and Practice, AIAA, Education Series.
2) Dr.Jan Roskam, Ackers Distinguished Professor of Aerospace Engineering.
University of Kansas Lawerence, Kansas. Airplane Design, Part Five: Component
Weight Estimation First Edition-1985
3) Daniel P Raymer, Aircraft Design: A conceptual approach, AIAA Education Series,
ISBN 1-5634/-281-0
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