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2729[ksurf] Re: Better upwind kites

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  • Dave Culp
    Jun 1, 1999
      At 11:56 AM +0200 5/31/99, Arnulf Refsnes wrote:
      >The efficiency of a kite can be measured by measuring the angle (alfa) of
      >the kitelines to the ground (the vertical component is Lift (L) and the
      >horizontal component is Drag (D)) Thus you get tan(alfa)= L/D = Efficiency
      >If is very difficult to measure this unless you have a super steady wind.
      >(I've been trying rent a wind tunnel but it's kinda expensive..:-()

      This is not so very hard to do, at all.

      First, one wants the *horizontal* efficiency, not the vertical (simple I
      know, but read on...) Thus, one wants to measure the angle of the kite
      when flying it near the water, out to the side, not directly overhead.
      This is a very different thing, as the kite will not fly so close to the
      "edge" of the window when low as overhead. Further, you want only the
      horizontal component, not the "included" angle. In other words, you want
      the angle (compared to directly to the side--90 degrees) of a shadow of
      the kiteline, with the sun absolutely dead overhead.

      Second, one wants this angle taken from a moving kiteski, board or boat.
      Taking measurements from a static platform (anchor in the ground, for
      instance) won't give you real-life measurements, since you will
      under-estimate the line drag, of the kiteski at speed.

      So, as Arnulf says, how to get good data?

      It's simple, really. Tow your kite, as from the bed of an open truck or
      roof of a van, on a windless day. You can calibrate your mounting ploint
      to give you absolutely accurate, non-varying measurements (try mounting
      the kite from the end of a short stick or bar, and mount the bar at the
      center of a protractor (angle measurer)--all heavy-duty, of course.

      In this way you will a) fly in absolutely clean air (it was dead still
      until you entered it with your kite, no?) b) fly at any speed you
      like--use the vehicle's speedometer (calibrate it carefully by running a
      measured mile or kilometer at various speeds--use steel radial tires,
      which don't distort as they heat up), c) the mounting device can be made
      to automatically measure only the horizontal component, which is all that
      you want.

      Now, you can test your kites (and those of the competition) to your
      heart's content. Use Cory's measurements for the boards (very similar, as
      towed from a boat) and you will know everything about your gear.

      Dave Culp

      Dave Culp Speedsailing dave@... http://www.dcss.org/speedsl
      Kite powered boats, high speed sailing, proas and more. Check it out!


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