DDWFTTW - Directly Downwind Faster Than The Wind

Discussion in 'Propulsion' started by Guest625101138, Jan 4, 2009.

  1. spork

    spork Previous Member

    Thank you. It's become a welcome and unexpected surprise to not be accused of perpetrating a hoax, holding back information, and hand waving.
     
  2. Munter
    Joined: Jul 2007
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    Munter Amateur

    btw - are you the spork who used to frequent kiteforum?
    (or is there more than 1 spork?)
     
  3. spork

    spork Previous Member

    I am an avid kitesurfer and used to be much more active on Kiteforum. That's one of the two sites I originally posted this brain-teaser on years ago - long before building any models (much less the manned cart). Do I know you from there?
     
  4. spork

    spork Previous Member

  5. ThinAirDesigns
    Joined: Dec 2008
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    ThinAirDesigns Senior Member

    Here is a very well shot video of our very first run in El Mirage.

    Renown Brit Richard Jenkins, the builder and pilot of the world speed record holding Greenbird found out about our trip to El Mirage and flew down to watch the proceedings. He shot this excellent video sitting in the back window of the chase vehicle unbeknown to us. He just posted it today. He did such a great job showing the wind instruments switching directions just as the Blackbird blew through windspeed.

    As one can see from the video, the Blackbird self-starts on this run and data shows it topping out at nearly 55mph. The vehicle was still accelerating but over the radio I asked the pilot to stop as this was nearly 10mph faster than any previous run and I was worried about the integrity of the vehicle. The multiple wasn't quite one of our best as the wind was in the upper range (~20mph). We set our best multiples in a bit less wind because we could let the bird run free without fear of breakage.

    http://www.youtube.com/watch?v=5CcgmpBGSCI&feature=youtube_gdata

     
  6. wither
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    wither New Member

    Ummm...is there a mistake here? Would not

    Vs/Vt = sin(gamma) * (L/D) + cos(gamma), where gamma is the angle off the downwind course?
     
  7. sirclicksalot
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    sirclicksalot Junior Member


    Oops, did I forget to define my terms?

    Per Marchaj and others, gamma is the course angle off of upwind, so Marchaj's formula is equivalent to yours as you have gamma defined as angle off of downwind. The convention chosen is apparent in the gamma where the maximum occurs (90 + arccot(L/D)) which means max speed gamma is greater than 90, i.e. as it should be for a broad reach.
     
  8. A.T.
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    A.T. Junior Member

    Found some related old stuff:

    Bernhard Schmidt (http://en.wikipedia.org/wiki/Bernhard_Schmidt) tired to patent a Windmill driven upwind boat (Gegenwindschiff) in 1925. Patent was denied due to lack of novelty. Ironically even decades later the author of Schmidt's biography Barbara Dufner thinks that "it is physically impossible to go directly upwind with an wind-powered vehicle":
    http://books.google.com/books?id=Qi...BQQ6AEwAA#v=onepage&q=gegenwindschiff&f=false (German text)

    Andy Ruina wrote a Paper about DUW & DDWFTTW in 1978. He was reacting to a paper by Blackford, who claimed that even under ideal conditions you can achieve only twice the windspeed going directly upwind. The following PDF contains both papers and a very silly review(first of the two) of Ruina's paper, criticizing him for using simple mechanical analogies:
    http://ruina.tam.cornell.edu/research/topics/miscellaneous/push-me_pull-you.pdf
     
  9. Windmaster
    Joined: Nov 2006
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    Windmaster Senior Member

    I'm not sure if Bernhard Schmidt was unlucky or that in those days the patent authorities were more vigilant, but in 1995 I was AWARDED a patent for a Windmill driven upwind boat.
    GB2286570 (A)
    See http://www.sailwings.net/rotaryhome.html
     
  10. spork

    spork Previous Member

    Very interesting. I'm glad you succeeded in getting the patent, but I would think that technically it would be public domain after the first applicant was denied.
     
  11. Windmaster
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    Windmaster Senior Member

    Not quite so simple. Different patent offices.
    Also I think my patent was on the overall layout of the design rather than the concept itself.
    They cited some previous examples during examination, some, of which I believe were awarded patents.
    I would recommend searching online on the patent database http://ep.espacenet.com/ you will find quite a few, most of them are vertical axis designs, although I have never heard of one of these that can successfully go against the wind.
     
  12. A.T.
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    A.T. Junior Member

  13. spork

    spork Previous Member

  14. A.T.
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    Location: Europe

    A.T. Junior Member


  15. A.T.
    Joined: Feb 2010
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    Location: Europe

    A.T. Junior Member

    NALSA confirms 2.8 wind speed DDW

    From http://www.nalsa.org/:
    [​IMG]
     
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