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Aerospace Material of Airfoils in Wind Tunnel Experiment

  1. May 4, 2009 #1
    In my current experiment, I am using a wind tunnel to test laminar flow over airfoils of various designs and at various angles of attack. My current dilemma is in deciding how to construct the airfoils. I am looking for a method that is precise and realistic. (At this stage, I am not worried about the cost.) I have narrowed the material possibilities down to three. Following are the three possibilities along with their "pros" and "cons":

    1) Styrofoam - It's cheap, however it would be difficult to cut in a precise manner. Also, using Styrofoam doesn't seem like a realistic test.

    2) Balsa Wood and Fabric - It's the most difficult of the three methods, however it seems like the most realistic test.

    3) Plastic (using a 3D printer) - This is the most precise of the construction methods (I would be able to design the airfoil on a CAD program and print the design), however I am not sure how realistic this is.

    Which one of these methods would result in the most accurate and realistic experiment?
  2. jcsd
  3. May 4, 2009 #2
    I will give you some methods I've done or seen done. One thing is to get foam (like blue foam) and cut it using a programmable wire cutter. You then cover the outter surface with epoxy and fiberglass. This is ideally done with a vaccum bag to reduce weight. Since you dont want this to fly, you can just let it air dry and then sand down any glue spots. The critical thing here is having a wire cutter and template (since I'm assuming you don't have a programmable wire cutter).

    Method 2: (This method I have used)
    You get Renshape 440 or Last-a-foam FR-7120 and make your airfoil using a CNC machine. This is very expensive, and requires tooling. So, your statement "price is not an issue" is unrealistic.

    http://img5.imageshack.us/img5/6499/img0817olv.jpg [Broken]

    Method 3: Balsa Wood and Fabric
    Sure, you can do this. You will need to make a template and cut out some ribs. Ideally, use a programmable laser cutter to make precise shapes. Make sure you have lots of stringers so that when you cover the balsa (I would cover it with Mylar not fabric) you don't have the skin to deform under aerodynamic loading.

    If all you are testing is an airfoil section, almost all the methods should do equally well. Its when you have complicated models that things become more costly, and difficult to make.
    Last edited by a moderator: May 4, 2017
  4. May 4, 2009 #3
    I am fairly new to the construction of model airfoils. Today, I heard that you should steam a balsa wood rod, pin it in the shape that you want and let it dry to create ribs. However, from your response it sounds like you should just take a sheet of balsa wood and use a laser cutter to cut the rib. Which one of these methods do you recommend?
  5. May 4, 2009 #4
    Who told you that?
  6. May 4, 2009 #5
    It was a teacher at my school. Apparently he made model planes in his younger years. From your response I can assume that I should just cut it out of a sheet of balsa wood. Thank you for your help.
  7. May 4, 2009 #6
    How big a size airfoil are we talking about here, and what tunnel speeds? Also, what is the point of this experiment?

    Depending on the size, I would just print out an airfoil section on a piece of paper and glue it onto a block of balsa. Then cut out the balasa with the paper template and you have your airfoil ribs. Cut out some notches, glue stringers into the notches and your done. Should take you two days total. If you need help you can post pictures and I can guide you.

    "Keep it simple, stupid"
  8. May 4, 2009 #7
    It's just a simple experiment for an extended essay for the full I.B. program. (It's an international high school diploma program.)

    From the wind tunnel models I have found, I will be able to get the wind speed up to 30 mph. (If you know anything about creating wind tunnels, that would also be helpful, but I was going to wait and deal with that project in a few weeks.) At most, the testing chamber would be 6-12 cubic inches.

    My current research question is:
    How the placement of the arc of an airfoil affects laminar flow at various angles of attack.

    (Once again, any suggestions/comments would be greatly appreciated.)
  9. May 4, 2009 #8
    How are you going to measure laminar flow? You are going to need pressure taps distributed along the airfoil surface.

    Here is how you make a wind tunnel that works up to 80MPH. Put the airfoil on the hood of your car. Drive.
  10. May 4, 2009 #9
    I guess I haven't yet developed the idea as much as I should have.

    I was just going to measure the wake (using smoke and a camera) behind the wing. Also, I would record the angle of attack where the air would buffet over the wing.

    With these pressure taps, you would put them in the airfoil? How would that work?
  11. May 4, 2009 #10
    You drill holes along the surface of the skin and have tubes where the holes are. These tubes connect to pressure sensors.

    Here you can see the manometers filled with fluid. This measures the pressure at various points along the wing (which is where all those tubes are going to, holes drilled out along the wing).

    What you are describing is not laminar vs turbulent flow. You are trying to see the effects of flow separation.
    Last edited by a moderator: Apr 24, 2017
  12. May 4, 2009 #11
    Also, how do you cover the balsa wood frame? You mentioned using mylar, but how do you glue it to the frame? Is there anything else I need to know for finishing the airfoil?

    After this, my own researching should be able to answer most of my questions/dilemmas.
  13. May 4, 2009 #12
    It's a heat activated adhesive. You apply an iron/heat gun and it will glue itself and become taught.

  14. May 4, 2009 #13


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    Not knowing what technical level you are working at makes this a bit tough. Perhaps, in stead of the laminar flow aspect, you may want to consider the possibility of changing it to where separation occurs by varying camber and then angle of attack. That way you can use visualization techniques like smoke wands in stead of complicated pressure measurements. It's just something to think about. Determination of laminar vs. non-laminar may be very difficult and time consuming.
  15. May 4, 2009 #14
    Fabric or sheeting sags between the ribs, doesn't it. Any sheeting under tension will sage and have a tendency to warp the wing segment without subsequent heat tweeking, and annealing so that it is temperature stable. I would construct the ribs 1/32th inch undersize on top and bottom and cover with 1/32th inch thick balsawood sheeting. Finish the surface with a vanish. I'd use basswood ribs and spars. Shape the leading and trailing edges from solid balsa or basswood.
  16. May 4, 2009 #15
    Indeed it does, but you really only need to maintain the aerodynamic shape along the first 1/3 of the airfoil, which is sheeted balsa leading edge. He could sheet the entire wing in balsa, and then cover it with mylar. That would probably be the best solution.
  17. May 4, 2009 #16
    Yes, that does sound easier than 5 or so coats of varnish and sanding.

    Of course, you would want to cover the entire foil in one sheet of mylar, or you could trip the boundry layer whereever it encounters the edge of the mylar.
    Last edited: May 4, 2009
  18. May 5, 2009 #17


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    When I used to build RC airplanes, that was very common. Balsa ribs cut from sheet and then covered with 1/16 or 1/8" sheet. All of that covered with Monokote.

    In reality, you could also get away with just sheeting the wing in balsa and then coating with sanding sealer and then clear coat. That requires less skill in the fabric covering department.
  19. May 5, 2009 #18
    For reference in terms of "technical level", I am a high school student and I am planning on becoming an aeronautical engineer. However, this is my first experience with any aerodynamic experiment. Also, I am a private pilot (I'm not sure if that's relevant).
  20. May 5, 2009 #19
    So, you should overlay the balsa wood frame with a 1/16" sheet of balsa wood before laying the mylar or monokote? Is balsa wood malleable enough to lay over the arc without steaming it or putting it though a different process?
  21. May 5, 2009 #20


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    Balsa is almost like paper sir. In my experience balsa can lay over almost any arc you need
  22. May 5, 2009 #21
    That is another excellent point. I would put the seam where the mylar overlaps on the bottom of the airfoil.
  23. May 11, 2009 #22
    If you want to make a 2-D experiment make sure your span crosses the entire wind tunnel section.

    If you want to investigate 3-D effects make sure the span does nor exceed 80% of the width of the tunnel.

    Wind tunnels give you answers about the "shape" of the airfoil and its performance so the material used should not have an effects except for the surface roughness.

    Also, I would calculate the expected Reynolds numbers for the real plane an scale the airfoil accordingly to match this in the wind tunnel.

    I like force balance better than the pressure probe method since it easier to calculate drag and lift and convert this into CL and CD.

    I always liked these experiments a lot so good luck and their is a wealth of knowledge here of the PF so you asked the right people.

  24. May 11, 2009 #23
    I've often been curious about this. You have to have some clearance between the tunnel walls and the section foil. So air leaks from the high to low pressure side of the foil. Additionally, the layer of air next to the wall could be turbulant, or laminar, and thickening, depending on the entry length into the tunnel. Do you happen to know if any of these factors have enough effect to require correction algirithms? I have no idea how large the error introduced may be.
  25. May 11, 2009 #24
    What do you mean? For a 2D wing it goes up against the wall.
  26. May 11, 2009 #25
    Yes, exactly Cyrus--the wall effects. I was once builing a windtunnel in the basements, and these kinds of questions came up.
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