Vertical Wind Tunnel for Kids' Toys and Balls

For my 4-year-old's birthday party, I built a wind tunnel for her and her friends to "test" the flight characteristics of stuffed animals, balls, toys, and anything else that fit into the 4-inch diameter tube. Powered by an air pump (normally used for a bouncy house), the tunnel was oriented vertically so items could be launched towards the sky. See the video below for some of the tests:



I considered the wind tunnel a total success as it kept a herd of 3-6-year-olds occupied, let them play and discover what items would fly, and simultaneously was interesting enough that all the parents wanted to play, too.

I built the project organically with no more of a plan then wanting to have kids play with a stream of fast moving air. If you don't want to follow along to see what I did specifically for my setup, here's the summary:

1. Consider cardboard concrete forms for projects that require 12-inch diameter tubes
2. Desktop fans that fit inside such a tube can't move enough air to be fun
3. Not only can an air pump from a bouncy air move enough air to fly all sorts of things, but it produces a collimated stream of air; a leaf blower would probably work just as well.

I initially tried powering the wind tunnel with two fans, but they didn't move nearly enough air for us to have fun. The air pump for our bouncy house moved plenty of air, plus it has a giant sticker labeling it as "not a toy". 
 
The air pump came with this bouncy house from Amazon. Check here for my other mod to the bouncy house, an additional safety net.

As a first test, I took a 12-inch diameter cardboard concrete form, cut a port to insert items, and cut a port to insert the nozzle of the air pump. In this configuration, the air pump is pumping air into the tube, but the output of the air pump is not in the direction of the tube. Whether the port was open or closed, items tended to spin in the tube rather than fly up and out. This didn't work as I wanted.

To orient the air pump such that its output was directed up, I built a base to hold the pump and a platform for the tunnel on the output of the pump.

When you've got a biscuit joiner, all problems look like they need biscuits. Wood screws would have worked just as well.

"That is a big, big, big cloud!" 
"That works!"  

Despite Corvidae's pronouncement, I wasn't satisfied with the 12-inch diameter tube. We could only get pieces of newspaper to really fly. Anything heavier, if it would fly at all, would circulate in the tube between the fast moving, collimated air stream in the center and the still air against the inside of the tube.

A smaller diameter tunnel was much more satisfactory. I made an adapter so that I could use a 4-inch diameter tube on top of the air pump's platform. Luckily, I had a section of ABS pipe and a section of acrylic pipe on hand from other projects (most notably my 5-foot-tall Jacob's ladder). The ABS pipe is a nominal 4-inch inner diameter and the acrylic 4-inch outer diameter, so I was actually able to use them together by sliding the acrylic into the ABS.

The adapter holds the ABS tube above the air pump so objects can be loaded directly into the stream of air. I had a fantasy about using the 12-inch cardboard tube as a magazine full of balls to be launched skyward, but wasn't able to solve the issue of balls jamming. 

The 4-inch ABS tube made for a perfect setup to play Bernoulli T-ball. With care, we were able to get three balls floating in the stream of air.

Seeing how much fun we were having with the few plastic balls I had on had, I ordered 200 3.125 inch and 6.5 cm pit balls for the party.
 

I disassembled the bases and connectors and finished them with some bright paint. Corvidae was responsible for the orange section, and boy was it applied thick...
 

You can tell a successful party by the number of toys and balls that end up on the roof!