Discussion in 'Aircraft & Aviation' started by lizzienewell, Dec 17, 2005.

  1. lizzienewell

    lizzienewell Member

    Has anyone tried building a windtunnel to test airplane models?

    Just an idea. Seems like it should be do-able with an acrylic box and a shop-vac.
  2. barry

    barry Active Member

    wind tunnels

    My wife used to run the supersonic tunnels at Farnborough but I don't think that would help much.

    ps she reckons blown eggs have about the best reentry characteristics come to think of it I don't suppose that helps either.
  3. 46rob

    46rob Member

    The Wright Bros were the first that I know of. I don't fly mine--I leave that to the neighborhood kids, who flight test all my beta's.
  4. Willja67

    Willja67 Member

    That's how the Wright Brothers started out. They had a hand cranked wind tunnel they used to test their wing shapes, unfortunately for them they didn't know about Reynolds Numbers(basically that in smaller applications air has viscosity and as your wing gets larger the viscosity ceases to be an issue) and so they based their wing cross sections on those of birds but when they scaled them up for their gliders and airplanes they didn't work quite as well.

    Birds wings are relatively thin when compared with most airplane wings, when you get down to insect size their wings are almost flat except for the ripples in them that breaks the air away from them so it doesn't stick and cause lots of drag(same reason a golf ball is dimpled). Kind of comforting to know that even if it were possible to enlarge insects that they couldn't fly because their aerodynamics wouldn't permit them to if they were huge.
  5. lizzienewell

    lizzienewell Member

    I didn't know that. There, we have another reason for selecting scale. So how big do you have to get to have the model behave like the full sized version?

    I'd love to see my models fly but I can't since the type of propulsor I'm imagining doesn't exist.

    Yeah, I do throw my models and let kids throw them. I figure if it can't stand up to being thrown then I've done poor design. I'm working on making boats waterproof so that they float.

    But an object thrown doesn't behave the same as a craft with it's own power.
    Hm, I still might try tethering it and then blowing a reversed shop-vac at it. It would be a kite rather than a plane but it might be interesting.

  6. hpept

    hpept Member

    aerodynamics is quite a tough matter. A wind tunnel to test a scaled down model of a real flying object is a valid way to measure/predict the final performance only if some rules are respected. As willja said, scaling up or down a wing doesn't work completely well because the medium properties (air, water or whatever fluid) don't scale as well. It was found that a good relationship between different situations if a quantity, the Reynolds number, is kept constant.
    Let's see how it works. The Reynolds number is defined as following:


    rho= fluid density (kg/m^3)
    V= speed (m/s)
    d= representative distance of the device (m) i.e. (pipe diameter, wing chord etc.)
    mju= fluid kinematic viscosity (Pa*s)

    So, let's say we have a 1:10 model of a wing. To have similar performance wrt the real one, we should keep Re constant: this can be accomplished in several ways; for example we can increase the speed 10X (easiest way) or/and combine the parameters (for example by heating or chilling the air and changing it's properties of density and viscosity). As you can see this open endless problems.
    I've seen operating a small wind tunnel in the faculty of Aerospace Engineering of Pisa, where i studied, and it was far from being simple to be operated. I've also worked for a little time on an hypersonic wind tunnel for aerothermodynamics purposes, and i got the idea to test and simulate the reentry of a Space Shuttle paper model, but i gave up when i saw a ceramic plate being literally blown away with fire streaks at Mach 7:wink:
    Hope this helps.

    GEEDUBBYA Active Member

    Miss Lizzie,

    I know the "exact" type of propulsion doesnt exist that your models are designed to operate on, However, do you know what an "ornithopter" is?
    A few yrs ago there were television commercials offering a "wind up" bird that flies. Its wings flapped like a real bird and would stay aloft for 30+ seconds at a time.
    This may be abit of a daring challenge, but I bet that the mechanics of the ornithopter bird could be incorporated into the "shell" of one of your models. I am sure it would require a "redesign" but who knows, if you could pull it off, there might be potential for a line of toys with your stories, not just "static" paper toys, but actual flying toys.
    Below are a few examples of ornithopters:

    this first one can be found at:

    This classic toy has entertained children and adults for over 27 years!

    The Ornithopter is a wind up bird that really flies by flapping his wings. He flies up to 50 yards.

    The next one can be found at: for about $11.00

    The above models are mylar film over a frame, but I bet paper would work to some extent ( just for the avid modeler, not a production toy), but they also make larger models of these "flying things", that are remote controlled for use in clearing birds from airports runways like the one below:
    It can be found at: with a write up about the "bird".
    For more images of "ornithopti"? (ornithopters), click this link, then you can click on the individual pics to go to the various sites:

    I hope this gives ya ideas, I think the killer whle type models you build would look great in actual flight.

    Have a good day,

    Greg aka GW
  8. nebeltex

    nebeltex Member

    i test my aircraft designs on a string. a slightly asymetrical spinner gives them a nice "whirrrr" sound. years ago i saw a tv special about flight. they were making small models out of balsa (frame) and film. they were so light they were able to glue them to flies (insects) which powered them. this was done indoors for several!
  9. shrike

    shrike Guest

    You should be able to find plans on the web somewhere these days.

    Unless you are planning to test really small models the shop vac won't move enough air. You'll need a fairly large fan /blower to suck enough air through to work (and you do want it to suck not blow)
    You will also need a flow straightener on the air intake side. On a small scale tunnel a large number of soda straws packing the inlet full seems to be the favoured choice. Cheap and effective.

    No idea what kind of instrumentation you might want but i will point out that it's often easier to test moels upside down. That way you can use one scale for both static weight and lift with only a simple tare calculation.
  10. lizzienewell

    lizzienewell Member

    :) So that is what I can do with the lifetime supply of soda straws that I bought at Costco.
  11. Kevin G

    Kevin G Member

    Lizzie if you think about using GW's ornithopter idea try looking at this site.
    Should have more than enough info to get you going with everything from rubberband power all the way to R/C.
    Includes video clips and tons of other info.
    There are also some free plans on this site!

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