Downwind faster than the wind

Discussion in 'General Science & Technology' started by billvon, Jun 4, 2013.

  1. billvon Valued Senior Member

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  2. Read-Only Valued Senior Member

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  3. Steve100 O͓͍̯̬̯̙͈̟̥̳̩͒̆̿ͬ̑̀̓̿͋ͬ ̙̳ͅ ̫̪̳͔O Valued Senior Member

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  4. exchemist Valued Senior Member

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  5. Pete It's not rocket surgery Moderator

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    Nice! [post=2131828]rpenner's post[/post] in the other thread covered this well:
    The thing I find most mind-blowing about this is that it means that if this craft were placed on a giant moving conveyor belt in still air (yeah, just [thread=76507]like that[/thread]), it could use that conveyor belt energy to go in the opposite direction to the belt!
     
    Last edited: Jun 4, 2013
  6. exchemist Valued Senior Member

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    Ah. Thanks. But I note it is for sale on e-bay, nonetheless........
     
  7. A.T. Registered Member

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    It is powered by the wind.
     
  8. A.T. Registered Member

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    Blowing your own sail works too. There is a Mythbusters episode about it.
     
  9. A.T. Registered Member

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    Yes, this is actually the best controlled test, of what happens at windspeed.

    [video=youtube;AVDLiNbCjN8]http://www.youtube.com/watch?v=AVDLiNbCjN8[/video]

    [video=youtube;1BRvYZd81AQ]http://www.youtube.com/watch?v=1BRvYZd81AQ[/video]

    [video=youtube;o9xnlbKY-qw]http://www.youtube.com/watch?v=o9xnlbKY-qw[/video]
     
  10. Pete It's not rocket surgery Moderator

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    Beautiful!
     
  11. A.T. Registered Member

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    They actually did that test with the big one too, on a dynometer:

    [video=youtube;5JjXudThU88]http://www.youtube.com/watch?v=5JjXudThU88[/video]

    When they saw that it pulls forward the dyno, they went to the dry lake. At about 3:00min it passes windspeed:

    [video=youtube;EEuAqq8FINw]http://www.youtube.com/watch?v=EEuAqq8FINw[/video]
     
  12. KitemanSA Registered Senior Member

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    Originally Posted by exchemist
    Since this seems to be some sort of powered machine, I don't see the relevance of wind speed at all. Or am I missing something?
    To add some substance, the mechanism extracts energy from the wheels to spin the prop which pushes the cart faster so more energy can be extracted from the wheels which rotates the prop faster and push the card harder... and on and on. I suspect that the peak cart speed will be a function of the lift to drag ratio of the prop blades and the wind speed.
     
  13. exchemist Valued Senior Member

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    I don't get it, because I have not (yet) understood how this is NOT a perpetual motion machine.

    Accelerating it requires work (f x d) to be done on the machine, since it is not weightless. From where does the energy for this come?
     
  14. KitemanSA Registered Senior Member

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    The wind.
    Nobody seems to have a problem with the fact that boats can sail faster than the wind across the wind. This mechanism just turns that cross wind motion into usable thrust.
     
  15. exchemist Valued Senior Member

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    Ah, so that would mean, I presume, that the wheels cause the propellor to turn in a direction "opposing" the wind direction, i.e. blowing in the opposite direction, then? If so then I think I see.
     
  16. A.T. Registered Member

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    Yes, the propeller accelerates air backwards, against the true tailwind, thus slowing down the air relative to the ground and so extracting kinetic energy from it.
     
  17. iceaura Valued Senior Member

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    The only thing moving across the wind is the propeller blade, and nobody is doubting that the propeller blades can be driven faster in rotation than the wind across them. That requires a force preventing them from being blown downwind - similar to the keel on a boat.

    At forward or downwind speeds greater than wind speed, the rotation force on the propeller shaft is counter to its driven direction - it begins to act as a rotation brake as well as a source of air resistance.
     
  18. kwhilborn Banned Banned

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    The guy who invented this has a BETTER invention.

    The yellow first down line in football. That's way better than some windmill on wheels. He even got an Emmy award for it.

    [​IMG]

    So now all the players know where they have to run.






    Otenay: Atthay astlay inelay aymay avehay eenbay away okejay.
     
  19. A.T. Registered Member

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    During directly downwind travel:

    The aerodynamic torque at the rotor is always against the rotation.
    The aerodynamic force at the rotor is always downwind or forward.

    Nothing changes about those facts when the cart passes windspeed.
     
  20. eram Sciengineer Valued Senior Member

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    Hmmm, it seems to be going slower than windspeed.
     

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