Our original ideas included a mill, a motor, balloons, and a wind tunnel. We decided against the mill because of portability issues and probable difficulty of construction. The motor had already been made by a previous student. We had an idea for a game involving balloons and needles in a Plexiglas box. This sounded fun, but we couldn't think of anything it would actually teach. Also, we would have had to be careful with the needles. In the end we decided to build the wind tunnel.
In our original design, we intended to make the tunnel from Plexiglas with a door in the top or side. Either some Plexiglas part of the tunnel or parts of the planes were to be made using a prototyping process such as LaserCamm. We intended to make the planes out of paper, cardboard, balsa wood, or foam. For the last three, we would have had pre-cut parts, such as wings, in different shapes for the kids to assemble into whole planes. The advantage of paper was that kids could take their airplanes home with little cost to the teacher. We also intended to make a removable cartridge to straighten the airflow, so we could demonstrate the difference between laminar and turbulent flow.
The first prototype was built from Hammermill boxes. The fan was not the right size, but this would not become a problem until later, when the air-straightener was added. The airplanes were cardboard and were attached to the box with tape and string. The airflow from the fan was turbulent, causing the planes to twist around and hit the sides of the tunnel.
The first major change to the prototype was to replace the Hammermill boxes with a Plexiglas tunnel. Since the requirement to use something manufactured with one of the prototyping processes was dropped and it wasn't difficult to attach planes at the end of the tunnel, we decided not to make a door in the tunnel. Also, a door would have created an air leak.
Second, we finished the section used to straighten the air flow. We decided to use straws for this, since other materials would have been more expensive or not narrow enough. At first the straws were held together by a piece of cardboard. The advantage was that the cardboard was somewhat compressible, making it easier to fit the section inside the tunnel. A disadvantage was that it wasn't very durable and allowed the section to assume a non-rectangular shape. Eventually we replaced the cardboard with Plexiglas, more exactly measured. We also started attaching planes, with string, to the straw section instead of to the top of the tunnel.
When we started using the straw section, it became apparent that a lot of air was escaping through cracks in the tunnel, or not entering the tunnel in the first place because of the size and shape mismatch between the fan and the tunnel. The air at the end of the tunnel was moving too slowly to fly most paper airplanes. We had been debating cutting the straws so they wouldn't slow down the air so much. We decided to try to seal the tunnel and the connection between the tunnel and the fan.
First, we used a trash bag to make an umbilical between the fan and the tunnel. Unfortunately, it wasn't durable, allowed a lot of air not to enter the tunnel, and wasn't easy to detach from the tunnel, making things less portable. We replaced it with a stronger plastic bag, which we attached to a rectangular cardboard frame that matched the size of the tunnel. This frame could be easily removed from the tunnel, allowing the tunnel to be carried separately from the fan. Then we sealed the cracks in the tunnel. These changes significantly improved the airflow, so now most planes would fly in the tunnel. They also increased turbulence somewhat, although it wasn't as bad as in the original tunnel. Finally, we added hooks to the tunnel. This allowed planes to be inserted and removed easily, without disturbing the straw section.
There were several ways we could have improved the tunnel. We could have made it work better, demonstrate more, and be easier to use. A better connection between the fan and the tunnel would have helped. This would have involved some way of funneling the air from the fan into the tunnel. The bag wasn't rigid enough to do this. A fan the exact size and shape of the tunnel might have been better than what we had, but a bigger fan with some sort of funnel would probably have worked best, because small fans usually aren't strong enough. Cutting the straws might have helped also, but it's hard to know what would have been the best tradeoff between turbulence and speed. To make the tunnel demonstrate more, we could have made the straw section removable to show turbulent airflow. We could also have made some sort of "real" plane with the right wing shape to demonstrate lift. To make the tunnel easier to use, we could have added doors for inserting and removing planes. Not surprisingly, more time and money would have made all this easier (this being the point of rapid prototyping).
