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How to Build and Use a Subsonic Wind Tunnel Table of Contents Section 1: Building the Test Section Assembly Section 2: Building the Diffuser Assembly Section 3: Building the Contraction Cone Assembly Section 4: Putting It All Together Section 5: Testing the Wind Tunnel Bibliography & Credits

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Objective

This how-to guide provides detailed instructions for construction and use of a subsonic wind tunnel. The wind tunnel is best used for science fair projects in grades 9–12.

Important Notes Before You Begin

• This design is for an open-circuit, closed-test-section wind tunnel that is about 8 ft. long, and can be taken apart for storage and transportation, such as for display in a science fair.
• If you run into any problems or need any extra help beyond the instructions of this guide, visit our SciBud Wind Tunnel Questions thread in the Ask an Expert forum, set up just for this Wind Tunnel How-to Guide.
• This wind tunnel has five basic parts:
  • Settling Chamber
  • Contraction Cone
  • Test Section
  • Diffuser
  • Drive Section
• Key terms that you should research before you begin:
  • Angle of attack
  • Assemblies (engineering context)
  • Closed-circuit wind tunnel
  • Contraction cone
  • Diffuser
  • Drag
  • Drive section
  • Lift
  • Open-circuit wind tunnel
  • Parts (engineering context)
  • Test Section
  • Thrust
  • Settling chamber
  • Variable speed drive section
  • Voltmeter

Introduction

Did you know that every single plane, helicopter, and spaceship used in America today has been tested in a wind tunnel? A wind tunnel is a chamber through which high-speed air is passed. Models of aircraft (or actual aircraft) can be mounted in a wind tunnel so that flight conditions can be simulated and engineers can study how well a design will fly. Wind tunnels have been used for over 100 years. In fact, the Wright brothers used one to help them figure out how to build the world's first successful aircraft, the Wright Flyer.

Figure C. The Wright Flyer. (Wikipedia, 2009.)

Today, wind tunnels are used by NASA, Boeing, Northrop Grumman, and every other organization that makes aircraft and spacecraft. In fact, NASA AMES, in Moffet Field, California, has the most wind tunnels at any one location in the world, and also has the largest wind tunnel on Earth! Wind tunnels are also used for educational purposes. For instance, schools use them to demonstrate how planes fly. They are also used in research projects, as students design aircraft models and test their performance.

Figure D. NASA AMES. The prominent structure in the foreground is the world's largest wind tunnel. (Pincta-cal, 2008.)

Science Buddies is proud to offer this how-to guide, which provides the design for an educational wind tunnel that you can build yourself. Though it's certainly not big enough or sophisticated enough to study a space shuttle or the new Boeing 787 Dreamliner, it is excellent for classroom use and for science fair projects. The design has been rigorously tested, and while you will learn the precise method for building a working wind tunnel, we thought it also important to share the obstacles we encountered along the way, so you can avoid the same pitfalls. In addition, once you've completed the wind tunnel, Science Buddies has several aerodynamics-related Project Ideas, which require the use of a wind tunnel, so be sure to look those up and try them out when you're ready for testing! Here are a few examples:

• Which Wing Design Creates the Greatest Lift?
• Winglets in Wind Tunnels

Figure E. This is an example of an educational wind tunnel used on a college campus, which is over 60 feet long and 10 feet high. (UC Davis Wind Tunnel Facility, 2009.)

Before you get started, there are a few basics to learn about wind tunnels. First, there are two main types of wind tunnels: closed-circuit and open-circuit. A closed-circuit wind tunnel is a very large and expensive type of wind tunnel. It gives engineers and scientists the greatest control over the flow of air, and produces the most efficient and precise results when models are tested. The wind tunnel you will build is not a closed-circuit wind tunnel because this would be very expensive and difficult to make without sophisticated equipment and materials. Instead, you will be making an open-circuit tunnel, which is more like a tube that is open at both ends (see Figure E, above). While this design doesn't allow you to have complete control of the airflow, it is less expensive to build and run, it can be made using readily available materials, and it is an effective design for educational purposes.

Second, you need to understand just how an open-circuit wind tunnel works and what it measures. The wind tunnel in this project has a fan, which will draw air into the tunnel and pass it over a model, and then release the air out of the other end. When the air flows around the model, it simulates flight. The model pulls on sensors, which will tell you how well it pulls up (lift) and how much the air blows it backward (drag). These two measurements are what you are most interested in as an aeronautical engineer—you want to know how well your plane lifts itself (stays in the air) and how well it cuts through the air and resists the force of the air pushing back against the plane (how fast it travels). Lift measurement gives you the force of upward motion of the air on the model, and drag gives you the force of backward motion of the air on the model. Basically, a plane is only able to fly because it can control the way that the air pushes on it. This is why a wind tunnel is important—you want to study how air moves around your model before you actually build the plane and send it into the sky.

Third, it is important to know that the wind tunnel fan is similar to the engines on a plane. The engines push the plane forward, which makes wind pass over the wings. The fan draws air backward, so that it passes over the wings in the same way as if the model were propelling itself forward. The proper way to describe this is that the fan simulates thrust.

Fourth, wind tunnels are not perfect simulations of environmental conditions. The degree to which they are flawed representations of actual flight is measured by a calculation called Reynolds Number. The wind tunnel which you can build using this how-to guide is a student-quality, high school budget tunnel, and so it is important for you to understand that the data and results it yields are for instructional and educational purposes only. It is possible that the data will disagree with established data-in order to find out just how inaccurate the tunnel may be, you should research the concept of Reynolds Number and determine what the RN is for your wind tunnel.

Finally, you should know the five basic parts of the wind tunnel. From front to back, they are: The Settling Chamber, the Contraction Cone, the Test Section, the Diffuser, and the Drive Section. (see Figure F, below).

Figure F. Diagram of an open-circuit wind tunnel. Although the diagram has many different numbered items, you only need to be familiar with the few mentioned below.

The Settling Chamber (number 12 in Figure F) is at the very front of the wind tunnel, and is made up of screens and honeycomb-shaped mesh, which straighten out the air and reduce turbulence. The Contraction Cone (numbers 11 and 13 in Figure F) forces a large volume of air through a small opening in order to increase the wind velocity in the tunnel (for more on this, study the continuity equation). The Test Section (number 10) is the place where a model is mounted on sensors. The Diffuser (number 8 in Figure F) is at the end of the Test Section, and keeps the air running smoothly as it goes toward the back. It also increases in volume in order to slow the air down as it exits the tunnel. Finally, the Drive Section (number 7 in Figure F) is at the very back of the wind tunnel, and it is where the fan is housed. At first, it might seem odd that the fan is at the back of the tunnel, facing outward, instead of at the front; but this is actually the best placement, because it will draw air into the wind tunnel by blowing air out of it. Drawing air in is better than blowing air in, because it doesn't produce as much turbulence, and it allows for greater control of the airflow through the tunnel.

This introduction provides only a glimpse into the field of aerodynamic design, so before you begin building the wind tunnel, you may want to do additional research about basic aerodynamic principles. At the end of this guide is a bibliography of the sources consulted when this design was made, so make sure you check out at least a few of them before you get started. We highly recommend taking a look at The Illustrated Guide to Aerodynamics, 2nd Edition by H.C. "Skip" Smith; NASA's Beginner's Guide to Wind Tunnels website; and the Wind Tunnel Design website. If you can find a copy, also check out Low Speed Wind Tunnel Testing, 3rd Edition by Barlow, Pope, and Rae. This book might be difficult to find unless you use a local university library. Since you are building an open-circuit wind tunnel, you don't need to study anything that these resources say about closed-circuit wind tunnels.

Getting Started

Cost

A key element to consider when building this wind tunnel is the cost. The Science Buddies wind tunnel design cost about $275. This price tag is the result of shopping around and finding the least-expensive products available, but that would still work properly in the wind tunnel. Another cost-cutting idea is using scrap wood, so make sure you collect all of the unused wood you can before you start buying parts for the wind tunnel. The two most-expensive components are the Drive Section fan and the Plexiglas sheet(s) used to build the Test Section.

School Support and Science Buddies Funding

If your school is willing to provide partial funding for your wind tunnel project, Science Buddies may be able to match the school's donated dollar amount. In other words, if your school is willing to contribute, say, $140 to help purchase materials for the wind tunnel, Science Buddies may be able to match that amount by providing another $140. Please check with Science Buddies FIRST to see if this will be possible for your project. To approach Science Buddies to see if matching will be possible for your project, contact Courtney Corda, Vice President of Science Buddies: courtney@sciencebuddies.org. Make sure that you put "Wind Tunnel Funding Request" in the subject line.

In order to secure school funding, your first step should be to speak to a science teacher, or perhaps go to the chairperson of the Science Department at your school, and ask how to go about making a proposal for a donation. Here are some additional tips about what to include in your proposal:

• Be sure to mention Science Buddies' promise to match the school's contribution, and the fact that you have a full set of instructions and a list of materials to get you started right away.
• Convey your commitment to the project, and demonstrate your dedication by describing your plans to use it in the future (such as in demonstrations if you are a teacher, or in a science fair project if you are a student).
• Provide specific dates and timelines for, and details about, your project goals.
• Provide information as to how big the wind tunnel will be so that the school knows where it could be stored. The wind tunnel is about 8 feet long, and just under 3 feet in maximum height and width.
• Emphasize that the investment will be in the best academic interest of science students. Make sure that with whomever you are talking understands that the wind tunnel will be the property of the school and will benefit science teachers and students for years and years to come.
• Consider asking the school to donate $140–$150 dollars, using the $275 cost of the Science Buddies wind tunnel as a reference. Therefore, between your school and Science Buddies, the cost would be entirely covered. Note: It might cost more or less for you, overall, depending on the materials you already have available and the prices you find at your local hardware stores.

One final point you should make to emphasize the value of contributing to your project is to compare this wind tunnel to other educational wind tunnels on the market. If you do a little research, you'll find that they cost several thousands of dollars. While the Science Buddies wind tunnel is only a homemade design, and may not be exactly the same as the other wind tunnels, it is still very accurate, has been tested by our staff, and is extremely useful. Make sure that the school understands that you will be using equipment that the school probably already has (sensors, etc.), which is why the cost is in the hundreds of dollars and not in the thousands. The results with the Science Buddies wind tunnel are virtually the same because you will be using professional sensors. Construction of the main component, which controls the airflow, is relatively inexpensive and simple. Furthermore, not only will it teach you even more about aerodynamics to build it yourself, but your experience can be shared with students in your school and recorded for future students to study. In short, the Science Buddies wind tunnel is far less expensive than other wind tunnels, produces results with industrial accuracy and precision, and is an educational tool as it's being built and when it's in use!

Materials and Equipment

• Material cost is around $250–$300, but you could ask your school to help pay for it, and Science Buddies may also be able to provide funding (see above section).
• If available, you can use force sensors and equipment found in your physics classroom.

The next thing to think about is the list of materials. Nearly everything you'll need to buy will come from a local hardware store. The materials for the Science Buddies wind tunnel (see Figure A) were almost exclusively bought from Lowe's Home Improvement, The Home Depot, and a locally based hardware store, but they are readily available and can be found at a variety of locations.

First, review the following materials list and see if you have any of the listed parts already (such as scrap wood, screws, etc.). Make sure that you use whatever good materials you and the school already have so that you don't have to spend more money than you need to spend. For instance, you will need to use two force sensors, which your school's physics department most likely already has. Most high school force sensors cost about $100 each, so it is best to use whatever your school already has. If your school doesn't have any sensors, you will have to buy them.

Here is a basic list of the materials needed, with cost approximations:

Primary Materials

Material	Quantity	Estimated Cost of Total Quantity	Notes
Plexiglas sheet, 2' x 4'	1	Low ($20-$50)
Plywood board, 4' x 8'	1	Low ($20-$50)	For best results, thickness should be greater than 1/4".
Plywood board, 17" x 17"	1	Low ($20-$50)	This will most likely have to be cut from a larger board; although unusual, the dimensions are based on calculations and must be used; for best results, thickness should be as great as possible.
Fan with blade diameter of 14 in. and entire-fan diameter no greater than 17 in.	1	Average ($50-$100)	For best results, CFM rating should be between 1600 and 2000, and fan should be designed for gable mounting.
Oak boards, 2' x 4'	4	Low ($20-$50)	For best results, thickness should be greater than 1/4".
Egg crate mesh board	2	Low ($20-$50)	Basically an array of deep cells (see Figure A).
Wooden lattice, 1.5" x 1/4" x 8'	1	Very low (under $20)
Wooden corner guard, 8'	1	Very low (under $20)
Roll of aluminum screen	1	Very low (under $20)	About 36" x 48"
Small handheld personal cooling fan	1	Very low (under $20)
Cut of wire sheet, enough to cover a 17" x 17" area	1	Very low (under $20)	This will block the exit of the fan so that children do not put their hands into the fan. Make sure that the wire cells are small enough to prevent fingers from getting through.
Adhesive rubber gasket stripping, at least 8 ft.	1	Very low (under $20) to low ($20-$50)	This will be used at the openings of the three assemblies to ensure that air does not escape. It should be an adhesive gasket stripping.
Springs	8	Low ($20-$50)	Medium-duty coil springs, 3 ft. or so, with closed loops at each end (you will be latching each end onto hooks).
Soldering materials		With permission, borrow from school or someone you know
Electrical wiring materials		With permission, borrow from school or someone you know
Small L-brackets with two screw holes	8	Very low (under $20)
Small drawer handles	4	Low (under $20)
Scrap wood		With permission, obtain from school, home, or someone you know.	This will depend on what you have, or what your school or someone you know has available. If you can't obtain any scrap wood, you can always buy extra pieces of wood. You will use the scrap wood to make stands and bases for your wind tunnel.

Secondary Materials

Material	Quantity	Estimated Cost	Notes
Saws		With permission, borrow from school, home, or someone you know.	The types of saws you use is up to the carpenter/woodshop teacher who will be helping you.
Drill	1	With permission, borrow from school, home, or someone you know.
Wood glue	1 bottle	Very low (under $20)
Screws, wood screws, nuts, bolts, washers, lock washers, screwdrivers, related materials		With permission, obtain from school, home, or someone you know.	Again, the number and type of these materials you need will be up to your carpenter/woodshop teacher.
Nail/Staple gun	1	With permission, borrow from school, home, or someone you know.
Silicone	1 tube	Low (under $20)	Try to find a special type that is intended for use with clear plastics.
Construction sealant	1 tube	Low (under $20)
Measuring tape	1	With permission, borrow from school, home, or someone you know.
Zip ties	At least 20	Low (under $20)
Safety goggles	1 pair	With permission, borrow from school, home, or someone you know.
Work gloves	1 pair	With permission, borrow from school, home, or someone you know.
Ear plugs	1 pair	Very low (under $20)

School-donated equipment

Material	Quantity	Notes
Multi-force sensors	2	Borrow these from your school, if you can. If not, companies such as Vernier Scientific supply these.
Computer	1	Will connect to force sensors and store/display their collected data.
Voltmeter	1
Variable AC transformer	1	Borrow from your school, if you can. If not, companies such as Variac supply these.

A note about the large fan: You will most likely want a gable-mounted attic fan. These are powerful, easy to mount, and relatively inexpensive (although it is the single most-expensive part on the entire tunnel). It is best to get a fan with a CFM (cubic feet per minute) rating of between 1600 and 2000, because these are powerful enough to be reliable (sources in the bibliography suggest a fan with 34 cubic feet per second, which is about 2040 CFM). They cost about $100 or so. Make sure you find a fan you can afford that fits in that price range. Also, you may be able to get a discount for the fan, since it is to be used for educational purposes. The fan in the Science Buddies wind tunnel was purchased from a Lowe's hardware store with a 10% discount. Make sure to talk to a customer service associate about a possible discount.

Finally, you will be relying on the help of others, so make sure that you can find teachers, adults, or industry professionals who can help you with the following:

• Cutting wood with high-power saws (woodshop teacher or carpenter recommended);
• Simple building mechanics (woodshop teacher or carpenter recommended);
• Simple wiring (physics teacher or electrician recommended);
• Simple soldering (physics teacher or metal-shop teacher recommended).

Now that we've covered all of the basics, you are ready to build your wind tunnel! The following set of instructions is meant to guide your progress as you build, but the steps do not necessarily need to be followed to-the-letter. If the people with whom you are working have different ideas from those presented here, such as how to fasten certain parts together, then implement whatever method you both think is best. Remember, building the wind tunnel is a learning experience—follow the directions, but don't be afraid to go beyond them or to try different ideas!

Assembly Notes

This wind tunnel is designed with portability in mind. Although it is about 8 ft. long, it is composed of three different sections, which can easily be detached from each other for storage, transportation, and model mounting. The three assemblies are as follows: Contraction Cone Assembly, Test Section Assembly, and Diffuser Assembly. The following instructions will walk you, step-by-step, through the construction of each of the three sections. Since this is an engineering project, all materials will be referred to as parts, and all built portions (such as each of the three main sections) will be referred to as assemblies.

• Section 1: Building the Test Section Assembly
• Section 2: Building the Diffuser Assembly
• Section 3: Building the Contraction Cone Assembly
• Section 4: Putting It All Together
• Section 5: Testing the Wind Tunnel