

Nov 10, 2011, 10:26 PM  
Canada, AB, Red Deer
Joined Apr 2010
313 Posts

Hi Penguins, sounds like a fun project. We had a similar thing when I was in high school, but the school had a commercially built wind tunnel fo rour playing.
With your home made tunnel, are the computer fans fairly powerful?? Some of them do not push a lot of air, but others do. As long as you have nice strong fans it should work nicely. As far as the size of the wing sections, I am not sure. As for one of your original questions regarding which airfoils to test, here are a couple of quick ones off the top of my head. Clark Y NACA 24xx series, say a 2410 perhaps. A symmetrical NACA section, a NACA0010 perhaps Maybe on of the Selig series, for example a S3021. Just some suggestions. Here is a link for you with loads of airfoils to choose from. http://www.ae.illinois.edu/mselig/a..._database.html 
Nov 10, 2011, 11:13 PM  
Joined Feb 2009
10 Posts

A wise man once told me, "An hour in the Lab will save you five minutes in the Library." Therein lays the true beauty of science and engineering, the ability to predict the behavior of natural occurrences. I would start with the fixed parameters and calculate values for the variables from there. Go online and find the rated volume flow rate of the computer fans, and then perform a back of the envelope velocity calculation using the following relation:
Velocity = Volume Flow Rate / Test Section Area This isn’t absolute because you’re not accounting for the losses through your straw section, viscous losses through the section, boundary layer effects, or reduction in effective area due to the model, but it is close enough for your purposes. Once you get an idea of what the free stream velocity is, you can estimate what magnitude of lift you’ll be trying to measure using the following equation. L=Cl/(.5*rho*U^2*A) Where L is the measured lift, CL is the lift coefficient, rho is the air density, U is the free stream velocity, and A is the cross sectional area. In this case you’re now limited by the resolution of your measurement, in other words, the minimum amount of weight you can accurately measure with your fishing line pulley set up. You could use something like small ball bearings as a means of determining your minimum resolution. So now the relation becomes: L = Weight of Ball Bearing Cl = Minimum measureable change in lift (resolution of measurement) Rho = Density (FIXED) U = Free Stream Velocity (FIXED) A = Width (FIXED) X Maximum Chord Thickness, note that the Chord thickness is generally given as a percentage of the chord. Now you have some Parameters to play around with. Another important quantity derived from free stream velocity and chord thickness is the Reynolds Number which is given by Re = Rho*U*L/mu Where Rho is the density of air, U is the free stream velocity, L is a distance in this case the chord thickness, and Mu is the viscosity of air. From my experience this is the single greatest dimensionless parameter in all fluid mechanics. It allows you to compare your results to all others at the same Reynolds number independent of size. Once you get your airfoil sized, I would suggest the first model you test be the NACA 0015 section. This is a classic, symmetric airfoil from which you can learn a lot. Its behavior is well documented and it will allow you to verify the results you’re getting in your tunnel. You’ll learn where the other airfoils are likely to stall, and the maximum coefficient of lift you’re likely to encounter. The 15 percent chord thickness will hopefully allow the flow to remain attached and delay stall to higher angles of attack at the low Reynolds number of your tunnel. For the other four sections I would recommend you steer clear of the literature initially and dream up your own wing profiles and see how they perform. Form a hypothesis, then test it in the tunnel, then report your findings back here on the forum. Make your own original contributions to the field of aerodynamics. Here is a link to the lift curve for the NACA 0015 section: http://www.cs.umbc.edu/~squire/cs455_lairfoil.shtml Here is a great piece on small wind tunnel design: http://wwwhtgl.stanford.edu/bradsha...eedTunnels.pdf 
Nov 11, 2011, 12:33 AM  
United States, GA, Atlanta
Joined Oct 2010
484 Posts

Before you can do any experiments you have to validate your setup. This includes the tunnel as well as the intruments used for taking measurements. I would recommend the NACA 0012, seeing as it's the most tested airfoil in history. Here is a link to a 1987 paper where the author examined wind tunnel data for the 0012 from over 40 different wind tunnels and compared them. It is on the NASA Technical Report Server so it is free to the public. This is critical before you do any experiments.
http://ntrs.nasa.gov/search.jsp?R=19...12%7CMcCroskey The key to accurate results is a smooth and uniform flow. This can be as much of an art as it is a science. I've never built a wind tunnel, but I've been told the key to achieving uniform flow in the test section is constriction. I'm not sure if I undestand the size of your wind tunnel, so this might not be feasible for you. 
Nov 12, 2011, 08:13 AM  

i think you'll find that the slope of the lift curve (change in lift per change in angle) is the same for all airfoils. There will be more variation in drag with different airfoils and airspeed (Reynolds number).
you might also consider a flat plate in addition to more conventional airfoils. i'm not sure how much air a couple 5" computer fan can move and how much control you'll have. i think most wind tunnels use fairly large fans and funnel the space down to a smaller test area which also increases the airspeed. have you considered using a larger box fan. If the box fan produces too much air, you can just add a vent to let some escape. You may want to measure the airspeed in the test space. Dwyer makes an inexpensive wind meter. the flow straightening only needs to be in the smaller space and that space could be more rectangular if convenient. I think larger wings will be easier to build and tolerances can be larger. 

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