Teaching Aircraft Design Course Using Real and Virtual Wind ...

Adeel Khalid SoTL Mini Grant – Southern Polytechnic State University
Abstract
Teaching Aircraft Design Course Using Real and Virtual Wind Tunnel
Adeel Khalid
As part of the aircraft design and performance class, students perform sizing calculations from
the conceptual sketches, select airfoil and geometry, calculate thrust to weight ratio and wing
loading, and then perform configuration layout before doing disciplinary analyses e.g.
propulsion, aerodynamics, structures, weights, stability and control, economic analysis, trade
studies etc. In this work, students are encouraged to design their aircraft using Computer Aided
Design (CAD), use that model to create a prototype, perform (a) wind tunnel analysis and (b)
Computational Fluid Dynamics (CFD) analysis and compare the results of two analyses. This
hands-on approach forces students to perform design iterations because of fabrication, test or
other limitations, which they do not anticipate otherwise, and in turn helps them understand the
and internalize the aircraft design process. In this paper, the design process is described and
several examples of student designs are demonstrated.
Key Words: Aircraft Design, Computer Aided Design (CAD), Computational Fluid Dynamics
(CFD)
Introduction
Historically, aircraft have been designed by varying key parameters and analyzing its effect on
the overall vehicle performance. Wright brothers used a wind tunnel and studied the performance
of several types of airfoils. More advanced wind tunnels were later used to determine the flight
characteristics of full scale aircraft. However, the use of wind tunnel to determine the flight
characteristics and performance of an aircraft is an expensive and time consuming proposition.
In today’s age of high speed computing it is possible to determine the flight performance of
aircraft designs using virtual wind tunnels. A computer generated model is plugged into a
Computational Fluid Dynamics (CFD) program to determine the flow characteristics. The results
obtained are comparable to those obtained from real wind tunnel tests and the actual aircraft
performance in flight. Several studies have been conducted to demonstrate the validity and
efficacy of the use of virtual wind tunnel 1 .
In this study, the process of using the real and virtual wind tunnel is introduced in the
undergraduate ‘Aircraft Design’ class. Students build scale models of different types of aircraft
including trainer, transport, fighter, and UAV. These scaled aircraft models are installed in a low
speed 1ft x 1ft cross section wind tunnel to determine the lift and drag coefficients and pressure

Adeel Khalid SoTL Mini Grant – Southern Polytechnic State University
profiles. Students also design virtual models of the corresponding aircraft using SolidWorks.
These virtual models are imported to the SolidWorks Flow Simulation software. The low speed
virtual wind tunnel is simulated in Flow Simulation. Lift and drag coefficients, and pressure
profiles are determined for different aircraft at different angles of pitch, yaw, and roll. These
results are then compared with the results obtained from the real wind tunnel tests. This
experimental exercise helps students appreciate the value of the use of virtual testing. It helps to
reinforce the importance of the time and cost savings. It helps them understand the reason for the
closure of the large wind tunnels across the U.S. 2 It also helps students understand the reasons
for discrepancies between the two methods of designing aircraft. These include the effect of the
differences in surface roughness, wing tip vortices, viscosity etc. As part of this exercise, the first
generation of aircraft design students generated a laboratory report to be used in the
‘Aerodynamics’ laboratory by future students.
Design Methodology
Aircraft design is an iterative process. Students are taught the iterative process that includes
prototyping and CFD analysis. During the early development of aircraft, conceptual and
preliminary design iterations are expensive and time consuming. The design methodology
described in Figure 1 makes the design process rapid and reduces the cost. Students can tweak
the design; perform the sizing and performance analysis, redraw the sketches, update the CAD
drawings and perform CFD analysis to check for improvements. Once the students have gone
through the first iteration, the design process becomes easier and faster with every subsequent
iteration. At the same time students can prototype and/or 3-D print their scale models for wind
tunnel analysis. Students learn the work needed to prepare the models for both wind tunnel and
CFD analyses. The evaluation of how CFD may be incorporated into a conceptual design method
is performed by McCormick 3 . CFD has also been demonstrated as an effective design tool in
evaluating aerodynamic performance for a NASA Research Announcement (NRA) project 4 .
By going through the iterative design process, students appreciate the importance of a good
starting point. During the traditional aircraft design course, students are encouraged to design
several aircraft that fit their mission profiles. They draw the aircraft sketches by hand using quick
back of the envelop approach. After much contemplation, calculations, tradeoffs, and
discussions, they proceed to design the aircraft using CAD. During the CAD process, they
realize the limitations and constraints that they do not realize while doing the hand calculations
and analyses. They also realize additional features that their aircraft could have. An example of
the design process is shown to students as a case study. Several designs of the airfoils are shown
in Figure 2. Students study the airfoil characteristics and choose the one that best matches their
mission requirements. Detailed airfoil characteristics as shown in Figure 3 are then drawn using
the CAD software.

Adeel Khalid SoTL Mini Grant – Southern Polytechnic State University
3-D Printing
Wind Tunnel Test
Design Aircraft / Update
previous iteration
Styling / Graphical
Analysis (hand sketches)
Sizing / Performance
Calculations
Computer Aided Design
(CAD)
Solid
Prototyping
Scale Model Prototyping
Compare Results
Fly the aircraft using Flight
Simulator
Computational Fluid
Dynamics (CFD)

Adeel Khalid SoTL Mini Grant – Southern Polytechnic State University
Figure 1: Rapid Aircraft Design and Prototyping Iteration Process
Figure 2: Several Airfoil Sections drawn for prototyping
Figure 3: Rhode St. Genese 34 Airfoil Coordinates
Once the CAD design is complete, students can either prototype the airfoil using inexpensive
materials or 3-D print a model. Airfoil prototyping examples are shown in Figure 4. The physical
prototypes are made from plywoord, styrofoam, balsa wood, and are covered with monocote film
for smooth surface finish. An example of 3-D printed airfoil is shown in Figure 5.

Adeel Khalid SoTL Mini Grant – Southern Polytechnic State University
Figure 4: Prototyping of Airfoils
Figure 5: 3-D Printed Airfoil
The 3D-printed models and handmade prototypes are then prepared for wind tunnel testing.
Wind tunnel testing is done to measure drag and lift coefficients. Students learn the limitations of
wind tunnel tests. This helps internalize the concepts of boundary layer, Reynold’s number,
scaling, tip vortices, free stream velocity, steady and unsteady air, speed limitations, surface
finish, equipment errors etc.
Figure 6: Wind Tunnel and Smoke System Setup

Adeel Khalid SoTL Mini Grant – Southern Polytechnic State University
Figure 7: Lift and Drag Calculations using the Wind Tunnel
Figure 8: Boundary Layer Separation Demonstration in Wind Tunnel
Figure 9: Airfoil at High Angle of Attack with Strings Attached for Reverse Flow Demonstration
In conjunction with the wind tunnel analysis, students also perform the CFD analysis of the
airfoils designed using CAD. The CFD analysis gives them the flow visualization in a virtual
environment. They also calculate lift and drag of the airfoils at different angles of pitch, roll and
yaw. It gives them a chance to play with other parameters e.g. viscosity and density of the fluid.

Adeel Khalid SoTL Mini Grant – Southern Polytechnic State University
They can also change the surface finish of the airfoils to determine its effect on lift and drag.
CFD flow visualization is shown in Figure 10.
Figure 10: CFD Analysis on different Airfoils Sections at different Angles of Attack
The case study of the airfoil design, fabrication, and wind tunnel analysis gives students an idea
of what is expected from them for the semester long project in the aircraft design class. In the
rest of the paper, several student design projects are described. Students go through the design
iterations shown in Figure 1 to perform aircraft sizing and performance calculations, design their
aircraft based on the mission profile, create CAD models, fabricate physical scaled models to
perform wind tunnel testing and at the same time perform CFD analyses. Results of the wind
tunnel tests and the CFD analyses do not always match. Errors occur due to the inaccuracies in
the physical models, differences in surface finish, wind tunnel installation and measurement
errors etc. Students are asked to critically analyze and describe the reasons for discrepancies.
This exercise helps them understand the challenges involved in not only the virtual prototyping
but also the physical prototyping, fabrication and wind tunnel analysis.
Aircraft Designs Examples
Several examples of aircraft designed by students are described in this section. Student designs
vary from modifications of existing aircraft to new designs in the form of large transport aircraft,
seaplanes, UAVs etc. A light sport aircraft design is shown in Figure 11. An attack aircraft and a
modification of an existing attack aircraft is shown in Figures 12 and 14 respectively. Sample
sizing calculations are shown in Figure 13.

Adeel Khalid SoTL Mini Grant – Southern Polytechnic State University
Figure 11: Design of Light Sports Aircraft
Figure 12: Design Improvement of an Existing Attack Aircraft

Adeel Khalid SoTL Mini Grant – Southern Polytechnic State University
Figure 13: Sample Sizing Calculations for Attack Aircraft

Adeel Khalid SoTL Mini Grant – Southern Polytechnic State University
Figure 14: CAD Design of the Design Improvement of an existing Attack Aircraft
An all composite stealth fighter aircraft designed for a range of 2,000 nmi, and the maximum
speed of Mach 2.2 is shown in Figure 15. The student realized after building the model that
supersonic testing required a supersonic wind tunnel, which was not available on the campus.
Figure 15: Redesign of an existing Supersonic Fighter Aircraft

Adeel Khalid SoTL Mini Grant – Southern Polytechnic State University
Figure 16: Design of a Light Sport Seaplane
A seaplane inspired by several existing aircraft is designed as per Red Bull regulations 5 .
Although CAD drawings and CFD analysis was not performed for the seaplane, a scale model
was created for wind tunnel testing. Orthographic views of the seaplane are shown in Figure 16.
A concept high altitude Unmanned Aerial Vehicle (UAV) is shown in Figure 17.
Figure 17: High Altitude UAV Design

Adeel Khalid SoTL Mini Grant – Southern Polytechnic State University
Figure 18: Supersonic Transcontinental UAV
Figure 19: CFD Analysis of Supersonic UAV
An experimental surveillance supersonic UAV as shown in Figure 18 is designed for high
altitude stealth operations. Students performed CFD analysis at different flight speeds.
Streamlines at a high speed are shown in Figure 19. After a few iterations, a prototype was also
developed for low speed wind tunnel testing. This hands-on design process excites students and
gets them motivated to actively participate in the course projects.
Future Direction
Efforts are under way to establish an Aerospace Engineering program at Southern Polytechnic
State University. Currently a minor is offered in AE for all other engineering majors 6 . The
university recently acquired a motion based flight simulator that uses X-Plane as shown in Figure
20. X-Plane allows users to load their own aircraft designs and test fly them. For accurate flight
simulations, stability derivatives need to be entered in the software. These derivatives can be
calculated using wind tunnel tests of scale models. As the next step in the design process,
students will sketch the aircraft based on the mission requirements, perform the sizing
calculations, create a CAD model, prototype it, perform wind tunnel and CFD analyses, upload

Adeel Khalid SoTL Mini Grant – Southern Polytechnic State University
and then fly their aircraft using a realistic flight simulator. This process will give students a sense
and an appreciation of the complexity of the overall design cycle. Compared to the old,
expensive, time consuming and often dangerous trial and error approach to aircraft design, this
process will be much safer, quicker and less expensive. Using this approach, students will be
able to go through the entire aircraft design cycle within the span of one semester.
Conclusions
Figure 20: X-Plane Motion Flight Simulator
In this paper, an effort has been made to suggest a replacement to the traditional design process
used in most aerospace engineering aircraft design classes. Traditionally, students design aircraft
and perform the calculations without ever creating a prototype or conducting a physical
experiment. A methodology is proposed in this paper in which student perform the traditional
aircraft design, sizing and disciplinary calculations but additionally, also prototype the aircraft
models. The physical models are created using a 3-D printer or other manufacturing techniques.
Wind tunnel aerodynamic analysis is performed. Similarly; CFD analysis is performed and the
results are compared with the wind tunnel tests. The next step to complete the design cycle is to
calculate stability derivatives and fly the CAD model in a motion flight simulator to understand
the handling qualities of the aircraft. It is expected that this hands-on design methodology will
help students learn the material better and leave a lasting impression.

Adeel Khalid SoTL Mini Grant – Southern Polytechnic State University
References
1. Michael S. Selig, Bryan D. McGranahan, ‘Wind Tunnel Aerodynamic Tests of Six Airfoils for Use of
Small Wind Turbines,’ NREL/SR-500-34515, 2004
2. Dennis O. Madl, Terrence A. Trepal, Alexander F. Money, James G. Mitchell, ‘Effect of the Proposed
Closure of NASA’s Subsonic Wind Tunnels: As Assessment of Alternatives,’ Institute for Defense
Analysis, IDA Paper P-3858, 2004
3. Daniel J. McCormick, ‘An Analysis of Using CFD in Conceptual Aircraft Design,’ Master’s Thesis in
Mechanical Engineering, Virginia Polytechnic Institute and State University, 2002
4. Bryan H. Blessing, John Pham, David D. Marshall, ‘Using CFD as a Design Tool on New Innovative
Airliner Configuration,’ 47 th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and
Aerospace Exposition, 2009, Orlando, FL. AIAA 2009-45
5. Red Bull Air Races [www.redbullairrace.com/]
6. Aerospace Engineering at Southern Polytechnic State University [http://www.spsu.edu/ae/]