This NEW wind tunnel design has the same 48” by 48” opening and it powered by 4 ValuTek 3-Speed Floor Fans 20” from FarmTek for $303.80 plus tax and shipping. The 4 mil vinyl can be purchased (4’ wide 25’ role for $30) at Ace hardware and is held to the 1’ PVC pipe by 1” black hook and loop. The front and back frames stay together with the 3 Way PVC 1” elbow corner Side Outlet Tee fittings from Amazon for $21. Just the four pipes connecting the front and back frames are removed for storage against the main fan holding section. The storage footprint for this tunnel is 12” X 48” X 48”. It takes less than 5 minutes to set up or take down.
This is a project that can be made in a day and requires only common tools and machines to cut the pieces.
Lets begin…
The first step is to cut the 4’ X 8’ sheet of 1/2” 3 ply plywood in half. This is best done by using a straight edge to guide the saw and get a straight and accurate cut. Use of proper PPE will be important thought out this project.
This next step might seem a bit odd but you will come to see that saving and taping the four boxes that the fans were shipped in will be a good idea. Be sure to go all the way around the four boxes and make this a strong stable work surface.
Now we all know that the layers of plywood have the grain running at right angles to each other. This gives the plywood its strength in both directions, To maximize this take the half sheet you just cut and turn it 90 degrees like shown in the picture. Doing this will also make sure that there is a factory cut edge on all four sides.
Clamp the two halves together and even up the edges. Then use some screws to temporarily hold the halves together. Keep the screws close to the edges. Now use a straight edge to draw lines from each of the corners.
Now might be a good time to look over and study the plan drawing for this project. The centers of the fan holes are measured out on the diagonal lines you have just drawn. The location and size of these four holes is the most critical part of this project because the clearances are less than 1/4”.
Just as a double check of my measurements of 1’ - 5 1/2” on the diagonal I also used a framing square to measure up and over from the edges of the plywood and verify the center locations of the four fan holes. I hope you have also noticed that I have now placed the two pieces of plywood on top of the four shipping boxes that the fans came in.
You are going to use a large dividers or make one out of a stick with a screw in one end and a hole for a pencil EXACTLY 10 - 3/4” away from the screw for the RADIUS of the fan holes to be cut. Drawing the circles for the holes is a must. They must be just right and the diameter MUST be 21 - 1/2” ! Ok you are going to have a choice here as to how you cut out the holes. A jig was will work but I prefer a Roto zip with a radius guide. Either way you will be cutting through the two pieces of plywood and the cardboard top of the shipping boxes below (not the table).
OK one down and three to go! See how the boxes help out?
Now you are going to want to check your work and now is the time to do it. If the hole is to big and the fan drops through you are going to change that for the next three. Making the hole smaller will require some added material. On the other hand if the hole is to small you will have to make it bigger and that take some hand work you will not want to repeat on the next three.
Did I mention that when you get the fans you won’t have to assemble the stand frame and will have to remove the hardware that the stand frame bolts to so that only the rim of the fan wire grills is left. This rim stops the fan from going through the holes you are cutting in the plywood.
Four holes cut and looking good sitting on top of the cardboard box working surface.
The fans have two sides. The air flows in one side and out the other. You will want to get it right because in a wind tunnel the air is drawn into the tunnel and then hits the turbine blades that are being tested. This is different than when you use a single 20” box fan in your classroom. You have a fifty fifty chance of getting this right. Don’t blow it. For these fans the air comes in on the motor side. Now if you haven’t figured it out yet the fans are going to be “sandwiched” between the two pieces of plywood you cut. In the picture the fans are laying on the first player of plywood. The 1/4” thick pieces you see are spacers that equal the thickness of the rim of the fans grill.
There are also some little tabs along the edge of the grill and some wood will need to be removed so the 1/4” thick spacer fits flat to the plywood. I used a 1” forstner bit to drill this semi-circle of wood out.
The fans are pretty heavy and the two layers of plywood do a good job holding them. But just for a little extra I put a small cross of the 1/4” material in the center of the fans. All this spacer material should be tacked in place with short nails or screws so they don’t move when the top piece of plywood is added.
So now the top layer has been added and the fans are locked in place. Now all you have to do is fasten them together, right?
Not so fast. You need to add a rim of 1 by 2 (actual size is 3/4” by 1 - 1/2”) to both sides of the “sandwich”. This will give it strength and thickness for longer screws.
Now the the box ides was pretty good, right? Well then you are going to like this for the 1 X 2 pieces. If you look ate the plans carefully you can see how each of the 8 pieces are the same length.
This save some extra measuring. But the lapping at the corners is reversed from one side to the other. This gives the joint more strength. Trust me on this.
You are going to use 2 - 1/2” GRK screws to put the “sandwich” together. You will clamp the 5 layers that make up the “sandwich” together and then drill pilot holes for the screws. Yes I know that the screws are self drilling but again trust me when I tell you drilling pilot holes will make for a better job. Also you will drill the pilot hole ALL THE WAY THROUGH the 3” of material. This will leave a small hole on the other side so when you screw that side you can go in between the screws of the first side.
IF you did all the steps correctly you have a four fan “sandwich” like this. Congratulations!
Again on the plans PVC cutting list it calls for 2 - 30” support legs. Screw them into the back side 1 X 2 rim pieces (drill some pilot holes for these screws) and you will have a nice stand for the fans while you finish the work and when you set up you wind tunnel in the future..
CABLE MANAGEMENT - A big deal when you are moving a tunnel like this around and setting it up. Lets see how we do it…
First we are going to go 2” in on the diagonal lines we drew. The location for each 1 - 1/4” hole is shown on the plan drawing.
Use a 1 - 1/4” hole saw to cut the hole to run each fans plug through.
Whoa that is a big hole and will leak a lot of air. We need to seal the hole around the plug wire.
Remember those 8 big circles of plywood you cut out earlier. Well here is a small use for one of them. Use a 2 - 1/2” hole saw and cut four discs out. Then saw a 1/4” slot from the edge to the center hole in each of the disc’s. This is for the wire to slide into.
And here is how it works. Put a small screw in to hold the disc in place and you have the holes covered.
You are looking into the tunnel and this is what you will see when you put a turbine in to the tunnel for testing. The 3 speed control switches and fan motors are all here and ready to go. Now if you only had the testing tunnel part made. So let’s get on it.
From the PVC cutting list on the plans you have cut the eight 45” long pieces that will form the front and back frames of the tunnel. 3 way Tees will be attached the the end of 4 of them. Lay the assembly on a flat surface to make sure the Tee’s line up.
Then you can add the other two pieces. Be sure to hammer everything together with a rubber hammer to make sure the pipe’s are all the way into the Tee’s. Then put a 3/4” lath screw into each pipe and Tee. These frames are never taken apart.
Now the four shorter 43 - 1/2” pipes can be add to set the distance from front to back of the tunnel. This distance needs to be less than the 48” so the 48” vinyl can be wrapped around the pipe and attach to the black hook and loop material. One full length piece of hook and loop material is needed on each of the bottom sides of the tunnel.
This is the part where you secure a piece of 20ga annealed stove pipe to the PVC frame in four places so that the tunnel can be attached to the fans. The 5/32 X 7/8” fender washers are held inside of the frame with screws and the wire is twisted around the screw.
Two #10 - 1” sheet metal screws are put into the back frame pieces for the wire from the tunnel to be fastened to. Use a figure 8 motion to secure the wire and it will hold better and be easier to remove when taking the tunnel down for storage.
Here you can see how the rubber hammer is used to to make sure the joints are together. You can also see the strip of hook and loop that will be on the bottom edge of each side of the tunnel.
A word about attaching the 4mil vinyl. The hook and loop material has an adhesive backing. I had pretty good success if I put a the hook and loop material together. Then I adhered the loop side to the PVC frame where I wanted it. Then I started in the top center of the vinyl and pulled off the tape so I could stick the vinyl to that piece. Then I worked from the center out doing the same thing. Removing the tape from the hook and loop stuck to the PVC and then sticking the vinyl to it.
Another ting to do is to place a tape marking the TOP FRONT cross piece frame and another marking the TOP BACK cross piece of the frame. Remember these frames a never taken apart. Only the shorter pipes connecting them together come out. Taking the pipes apart is best done using the rubber hammer.
As a little bonus I cut two of the scrap plywood discs and made some plates that can be set on the top of the tunnel. The back one holds the power strip the the fans plug into and had a switch that can be easily reaches to start and stop the fans. The front piece is where you can set the Lab Quest data collection equipment.
So there you have it. A KidWind wind tunnel that will get you wind speeds of 4 m/s or less for less and $500. If you have questions or comments post them on the blog and I will try to help you out.
Dick Anderson
Wisconsin KidWind Advisor
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