How to Build and Use a Subsonic Wind Tunnel: Section 1
Section 1: Building the Test Section Assembly
This assembly is the most delicate part of the tunnel, because it houses the model and includes the lift-and-drag sensory system. It will look like the following:
|Figure 1A. The full Test Section Assembly.|
The Test Section Assembly is composed of the Test Section (Plexiglas) and the Test Section Base (wooden stand and sensor mounts). Make sure you take a look at these two major features in Figure 1A before you move on.
For this assembly, you will need the following parts:
- Plexiglas sheet
- Washers/lock washers, bolts
- Scrap wood
Before you begin this section, you need to know what type of sensors you will use to collect force data, and you will need to work with your carpenter/woodshop teacher to design the proper base, which will depend on what kind of scrap wood you have available, or how much more wood you need to buy. This how-to guide, for the Science Buddies wind tunnel, uses two Dual-Range Force Sensors, made by Vernier (see Figure 1A, above).
Part A: Plexiglas Test Section
- The first step of building this assembly is to cut the large Plexiglas board into four smaller boards. The Plexiglas portion of the Test Section will be 1' x 1' x 2', so you need to cut the 2' x 4' sheet into four 1' x 2' sheets. This can easily be done at your local hardware store where you purchased the Plexiglas—you might even want to have it cut the same day that you buy it, to save time. In the Plexiglas products section of the store, there should be a cutting station, which usually cuts Plexiglas for free. Bring your board to this station and make sure that the cutter understands that you need four boards, each with dimensions 1' x 2'.
- Next, you will connect the four boards together using silicone to make a rectangular prism (the Test Section part of the Test Section Assembly). You should use a special type of silicone that is best for clear plastics, available at your local hardware store. This step can be very tricky, so make sure that you work with a mechanically inclined adult who can help you put it together properly. Hold two of the boards lengthwise so that they are perpendicular and form a corner. Make sure that their touching edges are flush with each other. This is how you want the two boards to look once you have connected them using the silicone. Now, take them apart and with your helper's assistance, carefully run a line of silicone at the edge of a face on one of the boards, not on the edge of one of the boards. This way, you will be able to put the edge of the other board right onto the silicone.
- Now, here comes the tricky part—you have to place the edge of the second board onto the silicone on the first board. You should do this with the boards standing up, not lying down—in other words, set them both on one of their 1-ft. edges, then put them together and hold them as they stand that way. See Figure 1B.
|Figure 1B. Stick the two boards together so that they stand on one of their 1-ft. edges.|
- Gently hold the two boards together. You want them to stay in place, but you don't want to accidentally move them while the silicone is drying. Make absolutely sure that the two boards are flush with each other so that they form a nice, straight, perfect corner, as in Figure 1B.
- After holding the two together for about 5–10 minutes, you can let go of them and let them stand. Make sure that they do not fall down, because it takes several hours for the silicone to fully adhere the two boards together.
- You now have half of your Test Section. To finish the part, you should continue attaching one board at a time, rather than making another corner out of the other two boards and trying to attach the two corners. Attaching one board at a time gives you the greatest control—trying to attach two corners is too risky and the boards could slip, spreading the silicone on the Plexiglas.
- As you attach the boards edge-to-face, you will notice that you do not have an exact 1' x 1' opening in the Test Section. This is because some of the boards need to be flush with the others in order to attach them properly. This could present a problem because you will need to keep track of the way you attach the boards so that you can keep it consistent with the 1' x 1' openings in the other two assemblies. Your carpenter/woodshop teacher should know how to do this and can help you keep track. This is important, because the airflow needs smooth walls inside the wind tunnel in order to flow properly, so the three pieces need to match up when you put them together.
- As before, run lines of silicone and carefully attach the boards together. Wait for the silicone to start to dry; this is a bit tedious, but the Test Section is very important so make sure that you give enough time for the boards to stick together. With your helper, make the Test Section, and leave it standing on one of the two ends in a safe place for 12–24 hours (depending on how long the silicone needs to fully dry; check the label on its container). Once it has fully dried, you can set it on its long sides, as in Figure 1C. The silicone holds the boards together, but is very pliable and is not very strong, so do not flex the boards or pick at the silicone. See Figure 1C for a picture of what your Plexiglas Test Section should look like.
|Figure 1C. The Plexiglas Test Section, held together only by silicone.|
- Since the silicone is so weak, it is necessary to reinforce the Test Section. To do this, you will screw corner brackets into the Test Section, at its eight end corners, in order to keep it together and to give it greater structural integrity. Note that you will need a drill, eight corner brackets, screws, nuts, and small lock washers. Make sure you let the silicone dry before you reinforce with brackets, as described in step 8.
- With your mechanically inclined helper's assistance, mark the points on each board of the Test Section where holes will be drilled. The holes should be rather close to the edges of the Test Section, should all have the same measured distances from the edges, and should all be in the proper places so that corner brackets can be screwed onto the Test Section. There will be 16 holes in all—four on each of the boards. Pilot (test) holes should be drilled first, for accuracy, followed by holes large enough that screws can fit in.
- Once all 16 holes have been marked and drilled, it is time to attach the corner brackets. This part could be tricky, because you half of the screws's rounded heads will be inside the Test Section, while the other half will be outside of the Test Section. Specifically, the screws heads on the left and right sides of the Test Section (when it lies on its 2-ft. dimension) will be outside of the Test Section, while the screw heads on the top and bottom will be inside the Test Section. Although this may be confusing, it is actually very simple once you define which boards of the Test Section are the top, the bottom, and the two sides.
- If you are wondering why the screws need to be put into the Test Section two different ways, it has to do with the way that the three assemblies will connect together in the end to make the full wind tunnel assembly. The screws with their heads on the outside of the Test Section need to be on the sides of the Test Section, because they will be attached with springs to the other two assemblies, so that the springs hold the entire wind tunnel together and make it airtight.
- Why not just have all of the screws this way? You need as few screws as possible with their threaded ends going into the Test Section, because they would disturb the airflow. You want to have as little turbulence as possible in the Test Section. So, for the screws on the top and bottom of the Test Section, make sure that the rounded heads are the ends on the inside, so that the air can smoothly pass over the metal.
- Note: You might notice in the following pictures that the screws are not placed as described above. Instead, they are all oriented with their rounded heads inside the Test Section. This is a result of the engineering design process—the idea of how the three assemblies were going to connect changed after the Test Section was built and the pictures were taken, so the screws were replaced later on so that the heads were outside of the Test Section and not on the inside.
- Attach the corner brackets so that the part on the side of the Test Section has its screw head out of the section, while the part on the top/bottom of the Test Section has its screw head on the inside. Do not mix up the sides—remember which ones you defined as sides and which ones you defined as top/bottom. Perhaps use sticky notes to help you remember. Your helper will know the best way to use the drill, nuts, lock washers, and screws, but make sure you also understand. Figure 1D shows an edge-on perspective of this reinforcement—note the bracket with its screws, and the silicone, which also keeps the Test Section together.
|Figure 1D. Edge-on view of an attached corner bracket.|
- Figure 1E shows the fully reinforced Test Section.
|Figure 1E. Test Section fully reinforced by corner brackets.Edge-on view of an attached corner bracket.|
Once you have completely connected the four Plexiglas boards together, you have finished the Test Section part of the Test Section Assembly. Now you must make the Base of the Test Section Assembly.
Part B: Base of the Plexiglas Test Section
- This part of the wind tunnel is dependent upon how much scrap wood you have available to you, and what kind of force sensors you will use. Talk to your mechanically inclined helper about what design you should go with, and figure out the best way to build it. What you absolutely need is a Base that will strongly support the Test Section, and that will be large enough that it cannot easily tip over. Because of this, you will most likely want a board for the Base with dimensions of about 1.5' x 2' so that it is just as long as the Test Section, but slightly wider. You also want to have two legs that connect the Test Section to its Base, and they should be near the front and back ends of the Test Section. Refer back to Figure 1A.
- Before you attach the Test Section to its Base, you need to make a decision. You will be running a rod, or some sort of mounting object, through the bottom board of the Test Section, in order to mount a model. This object will attach to the force sensors below it. Decide on what kind of hole you need in the bottom of the Test Section, through which your object will be inserted, and decide if it will be easier to make the hole before or after the Test Section is attached to the Base. For the Science Buddies wind tunnel, we used a threaded rod with a small diameter, so a small circular hole was drilled in the bottom of the Test Section. Whatever you use, make sure that the hole is large enough so that the rod does not touch the sides of the hole, even when the fan is turned on. This will disturb the sensor readings and produce inaccurate results.
- Also, you need to figure out the best way to attach your force sensors. You will need two—one for lift and one for drag. You absolutely must have right angles (as perfect as you can make them) between the drag sensor and your connection rod, so that no component of the lift force affects the drag measurement. See Figure 1A for the example from the Science Buddies wind tunnel. Notice how one sensor is mounted so that the rod comes straight up out of it (lift sensor) and the other is mounted so that its rod is perpendicular to the other force rod (drag sensor).
- As you can see from Figure 1A, the Base was made so that it has two smooth rods onto which the sensors can easily clamp. In the same way, you and your helper must decide on the best way to mount the sensors you have, and make sure that they will be completely perpendicular so that you can get the most accurate readings possible. You must also make sure that the long objects you use to mount the model are very rigid and will not bend or move around in the wind, because this will also alter your readings.
- Finally, you need to figure out a way to mount your model so that it can be given an angle of attack in order to be tested from more than just the fixed mounting position. For the Science Buddies wind tunnel, the rod that attaches to the model is bendable so that its angle can be measured before it is inserted into the Test Section and a model is mounted on it.