Expert: Paul Soderman - 11/10/2004

Question
I am a seventh grader doing a science project for school. I am testing the effects of different size objects on air drag.  I am building a crude wind tunnel using a vacum cleaner and some plywood and plexiglass. How could I measure the effects of drag for different size and type of objects. I don't want to spend lots of money to buy expensive measurement devices. Are there homemade ones or methods to use? Your thoughts are appreciated.

Answer
Hi EJ

That is a very good question.  Drag measurement is very important in wind tunnel research; real wind tunnels have complicated systems of levers and balances.  However, the Wright brothers used a relatively simple device, and there are some simple designs on the web.  The simplest one I found was at:

http://www.grc.nasa.gov/WWW/K-12/WindTunnel/WTExpKids/sld001.htm

If you go to slide 13, you will see a simple pivot device.  Two objects to be tested are connected by a string passing over the pivot.  The object with the greatest drag causes the pivot to turn.  It doesn't show any restraint, but I think you need to restrain the pivot with a spring so it is stable.  You can then measure the angle of rotation to see the amount of drag.

I also did a Google search on Rabi Mehta (a former colleague of mine), and there is a good pdf file listed under NASA explores 9-12 lesson: Wind Tunnel Testing, which is where I found the above link.  

Remember that cross-sectional area is very important to drag, so your models need to have a similar size to show effects such as aerodynamic shape or roughness.

Good luck.

Paul