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Against the wind

  1. Oct 19, 2004 #1
    //It's the same as in college level help forum but it fits better here i think...//

    Now it's time for me and a friend to prepare our project for a physics tournament in school.
    The task is:
    Construct a car which is proppelled solely by wind energy. The car should be able to drive !straight! into the wind. Determine the efficiency of the car.
    I would like some ideas for an efficient solution. I have researched how boats do it. They can't sail straight into the wind they can only zig zag, which is not sufficient.
    My idea was to use Bernoulli's principle which is the one that aircrafts use. Create a low pressure in the front of the car and hight pressure behind it, this might be able to move it forward.
    My friends idea was to use, i dont know what you call it... umm it consists of three cups
    (from above)
    ,,,,,,,,,/ \

    Horisontally as in:

    Something like that, it's three cups placed with angles of 60 degrees. This module always rotates no matter from which direction the wind is coming. This might be used to drive some kind of axis which then moves the wheels...

    Any ideas or suggestions?

    We are in high school, but the project is supposed to be solved with help of others, we are to consult with professors later on. Our task is mainly to make a final presentation of our ideas and lab results and then to present and defend them.
    Last edited: Oct 19, 2004
  2. jcsd
  3. Oct 19, 2004 #2


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    Your idea has nothing to do with Bernoulli's priciple. The way of propulsion of your invent doesn't look like that of the aircrafts. You are only using the proper kinetic energy of the wind and the momentum conservation, which enhances the rotation of the shaft. Well, maybe you'll need a lot of wind in order to move a 1000 Kg car (so that build one lighter :smile: ).

    Nevertheless, I think your idea (although it isn't new at all) is very good. The mechanism you've described is the same as those that provide mechanic energy by means of wind speed.

    Good luck!
  4. Oct 19, 2004 #3


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    How about something a little more technical and I'll seed the ideas with a couple quick links you can research on the internet to learn more about the concepts.



    P.S. Your friend is basically talking about a vertical windmill (instead of the horizontal ones like the one's you'd see on an old farm or theDutch are famous for building). There are vertical windmills that use airfoils, one was built on an episode of the TV show Junkyard Wars.
    Last edited: Oct 19, 2004
  5. Oct 19, 2004 #4
    It seems as though my post pointed out that i did not grasp my friends concept, I apologize for this. I just wanted a comparison between the two of them. About bernoulli's principle, don't be quick to rule it out, do you know how boats sail against the wind? they zig zag, (some using bernoulli's principle) so i thought that it might be possible to do this with the wind car.
    Then I wanted to know first if my idea (bernoulli's principle) could be possible if some way, if not, why? Then I wanted to know if my friends idea would work and how effective it would be.
    Naturally this will all be tested so this topic is just some brainstorming.

    My friend is talking about a horizontal windmill. I'd say the ones we see on windmills are vertical actually. I would like to know which of the two (vertical, horizontal) would work, and if, which would work better, and why?

    Again, my friends idea was the one with the windmill, naturally that one does not use bernoulli's principle.
    My idea was to use bernoulli's principle somehow (not the windmill!)
    Is this possible in anyway?
  6. Oct 19, 2004 #5


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    Did you read the links about tacking and apparent wind? That's all Bernoulli and airfoils. The guys who do land yachts and sail on ice (plus some on water) can exceed the speed of the wind going into it at like a 30 degree angle. Imagine you have a 10MPH wind and you can go 20MPH - wouldn't that be a hoot to blow people's minds by going faster into the wind than then can going with the wind!

    To tack you'd probably setup a geared crank to steer back and forth so the net displacement would be straight into the wind. You could get the prize for speed if done properly but it might be a lot to sort out.

    Windmill is nice as it uses the KISS (Keep It Simple Silly) prinicple so the modes of failure are lower. Speed is likely low and efficiency pretty low too. But being KISS it will likely be very consistent, tortise/hare kinda thing. For vert/horz I'm referring to the shaft and not blade orientation.

    In America like the wind farm in CA or other places the horz windmill seems most popular but in Europe the vertical windmill seems to have caught on. The vertical has the advantage that it needs no tail to keep it pointed into the wind as it doesn't matter which way the wind blows. A skyscraper scheduled to be built in Chicago I believe has a vertical windmill in its top to generate electricity. And the vertical windmills used in Europe use Bernoulli too in case you had wanted to stick with pressure differentials as you means of motivation.

    Building a tacking car offers the most reward (read challenge) but has the most risk as well. Getting the steering mechanism working properly to conserve momentum during transitions and making sure the rules allow for a net straight vector into the wind would be the major hurdles.

  7. Oct 19, 2004 #6
    Damnit i got too hung up with you misunderstanding some of my points that i missed the link, ill check it thoroughly now!!

    I'd be so pleased if bernoulli's principle works, brb

    Since the task is solely to build it as it has to go straight into the wind steering is not necessary and we only need to calculate maximum possible efficiency and we can tape a few successful experiments :D

    What do you think about the interpretation of the task. Do you think it is asked for a maximum efficiency?

    Ok now i looked through the links and from what you see:
    It can't go into the wind :(

    I admit that it can go into the wind but it can't go straight into the wind which is extremely important, i hope that the windmill option still might be able to do this.

    If i misunderstood something and it can go into the wind please clarify this.
    Last edited: Oct 19, 2004
  8. Oct 19, 2004 #7


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    Well now I'm out of my expertise so hopefully someone more familiar with airfoils can chime in yay or nay so here goes. For a standard sail of unstructured fabric, it seems that straight into the wind doesn't work because the shape fails, but a rigid airfoil would not have that issue.

    If the airfoil/sail could be fashioned with a proper angle of attack with respect to the wind so some of its "lift" could be directed so at least a vector component of the lift would overcome the drag and pushed the vehicle forward you'd be set. Even though the vector component of the lift pushing sideways on the tires might be larger, the traction would counter this force and it would just go unused. I'm unsure but I think this may not even be possible.

    When I did this type of wind project in Jr. High we built little cars that went with the wind. Most others had small cars with giant 'catch' sails. I built mine with every Lego I could find and a massive windmill geared to the wheels. I propped it up in front of the fan that was the wind source and it gathered enough momentum in the spinning blades to beat the nearest rival in distance by a healthy margin but took 10x longer to get there. Even without my massive bladed flywheel on the second attempt I still won but not by much. :smile:

    With a vertical windmill, you could go any direction as long as there's wind. And moving into the oncoming wind the apparent wind increases. The cup idea has the difference in resistance of a forward or backward facing cup to power it - if one side presents 2N and the backward facing side is 1N then you net 1N times the radius. With the airfoils one side might predominate but the other side is a streamlined shape so it would seem to have less opposing resistance. If its a commercial design I'll figure that someone more knowledgeable about airfoils reached that idea long before I did but you'll obviously want to research this yourself.

  9. Oct 19, 2004 #8
    From what I interpret it sounds as though it should be possible.
    My thought was similar if the force created by the lift would be greater than both the friction between the wheels and the table and the uhh pushing force on the sail from the fan it would move forward, i mean it sounds possible. If it isn't possible it should be easy to apply a formula to prove it wrong. Per say for some reason the Force on the sail would be equal to or greater than the lift force...
  10. Oct 19, 2004 #9


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    What you're describing is how the centerboard on a sailboat works - applied to a car. A car is limited to ~45 degrees for the same reason as a sailboat: there can never be a vector component of lift going forward. HERE is a diagram. With the resultant force at 90 degrees and the boat pointed at 45, the centerboard keeps you from sliding at 90 and is what allows you to go 45.

    ponjavic, your idea won't work for the same reason. To put it another way though, wind travels from areas of high pressure to low pressure: the pressure in front of the car will always be higher than the pressure behind it and there isn't anything you can do about it.

    My recommendation would be to go with the radial-flow turbine idea your friend had - its simpler to accomplish and more efficient than you may realize.
    Last edited: Oct 24, 2004
  11. Oct 19, 2004 #10


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    Russ thanks for chiming in - I'd say that the answer was obvious that the drag component and lift component vectors could never allow forward motion but the concept of apparent wind has yet to sink in intuitively as it seems like too much of a free lunch even though the concept is proved repeatedely by land yachts. It just seems to defy conservation laws to be able to use the velocity of a vessel to gain more velocity.

    Ponjavic check out this link, it describes what was on that TV show. It first shows the cup design, then the airfoil design.

  12. Oct 20, 2004 #11
    Thanks for the support guys, i think we'll start building the windmill tomorrow...

    For future reference what are key words i need to use when searching for different "windmills"?
    I see lift-typ and drag-type, could someone explain these?
    Last edited: Oct 20, 2004
  13. Oct 20, 2004 #12


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    Use anemometer for the drag type, you'll get tons of hits in a search and school projects to build ones for measuring wind speed.

    For the Darrieus lift type this sentence says a lot: "There are not many easy-to-find references devoted to vertical-axis turbines. "

    If the car needs to only go directly into the wind, you could also build a traditional horizontal windmill.

    Any of these designs could be constructed with construction paper and glue. Then use a fan to provide a consistent source of wind and attach a thread around the shaft so it winds the thread as the axle spins. Then you could attach various weights to the other end of the thread and measure the relative power of each design. And so on....

  14. Nov 3, 2004 #13


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    You have to be very careful in the application of Bernoulli's principal. This is a fairly weak effect. Though many books use this example to explain venturies and airfoils, it is only a small part of the story used as a simple illustration or so I am told. The major componant of lift comes from deflecting the air mass and a curved surface does this better than a flat one but that is an over simple explanation also . Generally pressure rises while air flow is in acceleration and falls in deceleration so the front of your car will be crowding the airstream causing acceleration and therefore drag. The rear will cause deceleration, low pressure and therefore drag. You will get a slight pressure drop over the top but that will not propell the car forward.

    I think the windmill idea is the one to develope. Sailing works because the reaction force is generated more or less at 90 deg to the sails. The sails are angled slightly across the boat so the reaction force has a small componant that drives the boat forward. Sid movement is vastly reduced by a centre board or deep keel with a symetrical lift generating section. In this way a sail boat can move into and across the wind at an angle of around 45 deg. An effecient design can do a bit better than that.

    If you use a wind turbine, the turbine blades will to the same thing as the sail boat sail. There is a large rotational force and smaller drag force. With the right gearing to your wheels, convert the larger rotational force to thrust that should easily overcome the smaller drag forces. The more efficient the windmill mechanism, the better this will work.

  15. Nov 6, 2004 #14
    Thank you for your reply Ken, we will look into both theories and try to construct them and I'll be back with the results for discussion..

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