teaching aircraft design course using real and virtual wind tunnel

8/12/2019 Teaching Aircraft Design Course Using Real and Virtual Wind Tunnel

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Teaching Aircraft Design Course Using Real and Virtual Wind Tunnel

Adeel Khalid

Abstract

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 todays 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 tunnel1.

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


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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.2It 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 andCFD analyses. The evaluation of how CFD may be incorporated into a conceptual design method

is performed by McCormick3. CFD has also been demonstrated as an effective design tool in

evaluating aerodynamic performance for a NASA Research Announcement (NRA) project4.

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 calculationsand 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.


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Design Aircraft / Update

previous iteration

Styling / Graphical

Analysis (hand sketches)

Computer Aided Design

(CAD)

3-D Printing Solid

Prototyping

Computational Fluid

Dynamics (CFD)

Wind Tunnel Test

Compare Results

ScaleModelPrototyping

Sizing / Performance

Calculations

Fly the aircraft using Flight

Simulator


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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.


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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, Reynolds 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


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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.


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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.


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Figure 11: Design of Light Sports Aircraft

Figure 12: Design Improvement of an Existing Attack Aircraft


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Figure 13: Sample Sizing Calculations for Attack Aircraft


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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


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Figure 16: Design of a Light Sport Seaplane

A seaplane inspired by several existing aircraft is designed as per Red Bull regulations5.

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


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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 majors6. 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


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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.

Figure 20: X-Plane Motion Flight Simulator

Conclusions

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.


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References

1. Michael S. Selig, Bryan D. McGranahan, Wind Tunnel Aerodynamic Tests of Six Airfoils for Use ofSmall Wind Turbines, NREL/SR-500-34515, 2004

2. Dennis O. Madl, Terrence A. Trepal, Alexander F. Money, James G. Mitchell, Effect of the ProposedClosure of NASAs 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, Masters Thesis inMechanical 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 InnovativeAirliner Configuration, 47thAIAA 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/]

teaching aircraft design course using real and virtual wind tunnel

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