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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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Section 5: Testing the Wind Tunnel

1. It is finally time to test the wind tunnel. You need to collect some test data from the force sensors you use, so depending on what type they are, prepare them for this. The sensors used by the Science Buddies wind tunnel are made by Vernier Scientific and are hooked up to a computer with Logger Pro installed (software you can purchase from the Vernier website at www.vernier.com. This software collects data from the lift and drag sensors.
   1. Attach a model to the sensor rod inside the Test Section (this will require you to disconnect the springs at the Contraction Cone and move it out of the way so you can access the interior of the Test Section, and then reconnect the Contraction Cone). Before you can collect any data, you need to calibrate the sensors. Calibration is a process by which you use known force values and define them for the sensors, so that they function as accurately as possible. Talk to your physics teacher about calibration. You must calibrate the sensors every time you test a new model, or turn on the equipment; otherwise, your data is meaningless. Check the instructions for your sensors to determine how to perform the calibration.
   2. To calibrate the lift sensor, simply define the weight of the model on the rods as 0 pounds/Newtons, and then put a known weight on the model (such as a 100-g weight, which exerts a .98-N force on the model) and define the force experienced by the sensor, using the known weight. Since this force pushes downward on the lift sensor, it is wise to define it as a negative number, such as -.98 N. This way, when the model lifts, or pulls up on the sensor, your readings will be positive. See Figure 5A for a visual of using known weight to calibrate the lift sensor.

Figure 5A. A known weight is placed on the model, so that the known weight value may be defined for the sensors, and they will be accurate. This is known as calibration.

3. Calibrating the force sensor is done in the same way, although you will need to take the sensor off of its mount on the Test Section Base. Set it upright, so that it is like the lift sensor, and calibrate the sensor by telling it what 0 force is (when nothing is pushing or pulling on it), and then use a known weight to finish the calibration. Unlike the lift sensor, however, this is a drag sensor, which means that the force of drag will be pushing into the sensor, so any weight on it should be defined as positive, not negative. Once you have calibrated this sensor, remount it on the Test Section Base. Having properly calibrated your sensors, you are now ready to perform your test and collect data from the model.
2. To test the wind tunnel and start collecting data, make sure that:
   1. The springs are holding the wind tunnel together.
   2. There is no place for air to escape.
   3. The equipment is all plugged in.
   4. The force sensors are calibrated.
   5. The model is properly mounted inside the Test Section.

   Treat these conditions as something of a checklist before you test a model every time. See Figure 5B for the full setup.

Figure 5B. The work station in this figure is fully prepared to test: the sensors are calibrated and ready to collect data, and the model is mounted in the Test Section.

3. Now, before you turn on the fan, adjust the variable AC adapter to the desired level (for a test, you probably just want to use the highest setting, so that you are essentially just turning the fan on all the way), and then flip the Drive Section control switch to the "on" position. Let the fan get to its rotation speed, and then begin collecting data. You will collect two series of small data points, taken at regular intervals of time—one series for each force sensor. You want to collect data only for a short time, such as 10 sec. or so, and then stop collecting. Produce graphs of all of the data points so that you can note trends, and use a computer to perform a statistical analysis of all of your data points, such as average force, standard deviation, etc. Figure 5C is an example graph of data points collected from the Science Buddies wind tunnel, and the caption contains all of the pertinent information that you will need to record from your own tests.

Figure 5C. This is a graph of the data points taken by the drag sensor. Drive Section fan at 100% speed, wind speed voltmeter reading of 2.8 V. Average force of +0.028 N, standard deviation of +/- 0.005.

4. Make sure that the wind tunnel and the sensors worked properly when you tested it. Make sure that your data makes sense (for instance, do you have a positive average drag? This would make no sense because there is no force pushing the model forward, only a force pushing it back as the air passes over it). If everything appears to be in order, then your wind tunnel is ready to be used in science fair projects and demonstrations!

Remember, if you have any questions, comments, or concerns, visit our SciBud Wind Tunnel Questions thread in the Ask an Expert forum, set up just for this Wind Tunnel How-to Guide.