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F

Airscoop

Straightener

Floorboard Exhaust fan

Test section

Intake

Base board

Control panel

H

Fig. F: Stack 2" drinking straw pieces into the cardboard sleeve to make the air straightener. Fig. G: Air straightener installed at the front of the tunnel.

learned: glue a small block of Lexan inside the door opening to prevent the door from swinging inward.

Finally, assemble the tunnel by screwing the Lexan sides to the floorboard and using plastic epoxy to secure the top piece.

Below the access door, create a control panel to house the scale display, fan controls, and LED switch. I used Lexan for better visibility. For the LEDs, I hot-glued 7 of the 10 along the floor near the test section and taped the remaining 3 under the test section, to illuminate the tunnel’s workings.

6. Mount the fan.

Crack open the fan, extend all its control wires about 2', and remove any unneeded supports. Mount the fan as near as possible to the center of the tunnel, blowing out the back. Seal around it to close any holes between it and the tunnel. I used thin scrap plastic from toy packaging, to preserve the view, but cardboard and duct tape would work fine. Regardless, use tape and hot glue to make the seal complete.

7. Make a flow straightener. Make a cardboard sleeve with the same cross-section as the tunnel, and pack it with 2" pieces of

146 Make: Volume 15

G

I

Fig. H: The wind tunnel with air intake (exhaust fan) and straightener. Fig. I: Testing windspeed with a small anemometer.

drinking straws — this part goes faster with a small-fingered helper (Figures F and G). The straws reduce turbulence, but they also cut the wind speed by several mph, and the tunnel will work without them. To improve performance, add a cardboard airscoop in front of the straightener (Figure H). That’s it!

8. Test your car!

Make sure the tunnel is level and stable, for repeatable drag numbers. Tape or block your car wheels on the test section floor.

Now fire up the fan to test your car — the force in the display is very close to the actual drag caused by the car (set it to “grams” for higher resolution). Modify your shape to minimize that drag. Now set track records at the Pinewood Derby!

For airflow visibility, run a Halloween fog generator at the intake and take flash photos to capture the stream lines.

You can measure the wind speed with a mini-anemometer (Figure I) — eBay is a good source.

Doug “Beads” Desrochers ( beads27@cox.net) is an aerospace engineer and pilot with ASEC, Inc. He served in the U.S. Navy as a test pilot and test pilot instructor, and has been voiding warranties since early childhood.