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MODEL WIND TUNNEL

Getting (or going over) the edge on Pinewood Derby day. By Doug Desrochers

Last year, at the annual Pinewood Derby race for our local Cub Scout pack, we used a Matchbox radar gun (see MAKE, Volume 10, page 148) to measure the top speeds of several cars. We learned that they reached 10mph–11mph at the bottom of the slope. Armed with this knowledge, along with some Lexan plastic, a fan, and a precise digital scale, it was time to build a wind tunnel for this 1. Make the floor and base boards. year’s event. I used 4" plywood, 8"× 40", for the tunnel’s floor-

Photograph by Tina Williams

I wanted to keep all the elements of the tunnel board. Paint it to make it smooth. Cut a 3"× 7" visible, so the Scouts could have fun seeing and rectangle out of the center for the test section: first learning how it worked. It’s a simple, open-loop type drill pilot holes, then cut it out with a jigsaw. of tunnel powered by a household fan at the exhaust Make a base the same length as the floorboard end (having the fan blow air into the tunnel would but several inches wider, to give extra room for the generate too much turbulence). control panel. I used a 1" board for the base. Line up

The highlight of this project is the test stand and the boards and trace the 3"× 7" hole onto the base — its very accurate drag gauge. The stand holds test this will help you align the test section later. objects in the middle of the tunnel, and is supported

by 2 hinged struts that pivot backward. When wind pushes against the object, the struts move a back plate that pushes into a force beam, which measures the force. The force beam is hacked out of an inexpensive pocket digital scale that measures down to 0.1 gram.

Make: 143