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post #2 of Old 08-05-2011
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Originally Posted by yashgore View Post
Hey guys, i'm new over here. i am really interested in dinghy sailing and just completed my RYA level 2. this has inspired me to base my 4000 word extended essay on the physics behind sailing.
The question that i'll be researching on is:
‘How does the angle of the sail on a sailboat from the direction of wind affect the total drag force on the sail?’
i'm trying to conduct an experiment on the the above with a model of a sail using a table-fan as a constant wind source.

Any ideas or suggestions?.....
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.

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