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Old 08-06-2011
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Quote:
Originally Posted by SlowButSteady View Post
First off, a table fan will produce far too turbulent a flow pattern to get you any meaningful data. You will be better off using a homemade wind tunnel. They're cheap and easy to make using thin plexiglass. Put a box-fan at the downwind end of the tunnel, and have some sort of funnel arrangement at the upwind end to minimize the turbulent flow as the air enters the tunnel. Also, to minimize boundary layer effects, make sure the cross-section of the tunnel is at least twice the longest dimension of the test object.

Second, and undoubtedly more difficult, you'll need some way of simultaneously measuring both drag AND lift on your test foil (as well as some other variables, like flow speed, and projected frontal surface area). The best way to do that is with a three-dimensional force platform instrumented with strain gages and the proper amplifiers (poke around the InterNet a bit and I'm sure you'll find some schematics).

I realize all that probably sounds way more complicated than you were prepared to bite off. But, as someone who has done just this sort of thing (only with marine invertebrates in water), I can tell you that that is that is what you need to do. On the other hand, there is a ton a literature on just this sort of question. How lift and drag vary with angle of attack is a problem dating back to before the Wright brothers. A good place to start is a Dover Science Classics book, by Robert Granger, called "Fluid Dynamics". It's a fairly cheap book (I think it's less than $30, new), but it probably has more than enough info for what you want to do.

Good luck.
thnx a lot for the advice. i only wish i could carry it out like that but the problem is that i study at a fully residential school. so i dont actually have much to work with and have to make do with whatever is available. i'm trying to place some card or something like that around the fan so that the airflow will be focused on one direction. i'm gonna use a spring balance to measure the force on the sail at different angles, keeping the boom's movement restricted by 2 poles + or - 2 degrees from the angle being investigated. any improvements that you can offer?
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