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[ksurf] Re: More on Apparent Wind

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  • Mark Frasier
    ... I was just thinking about this earlier. I think the low wind limit must have some relation to the lift/drag ratio of the kite. Let s say the L/D of your
    Message 1 of 2 , Apr 1, 2000
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      At 04:28 PM 03/31/2000 -0800, you wrote:
      >The Negative on apparent wind: The higher the apparent wind on the
      >kite, the less upwind the rider will go. The rider will be referencing
      >the objects around him or her, but the kite will be referencing the
      >wind it is flying through. The more apparent wind, the more the kite
      >backs up from the perspective of the rider. To the kite, it is still
      >on the edge of the window.

      I was just thinking about this earlier. I think the low wind limit must
      have some relation to the lift/drag ratio of the kite.

      Let's say the L/D of your particular kite allows it to fly up to 75 degrees
      off the downwind direction. So in a breeze blowing straight towards the
      north (i.e. an exactly South wind). You can stand on the beach and your
      lines can be pointing anywhere from 75 degrees ENE to 285 degrees WNW.

      Let's say you're on perfect ice with razor-sharp rock-hard skates and you
      have no friction but limitless sidegrip (cool, eh? :).

      The wind is blowing 5 mph. You point your skates due east and fly the kite
      75 deg ENE and start moving.

      Soon you're moving 5 mph. Now the apparent wind is blowing 45 degrees NE at
      7mph. Your kite is flying 30 degrees NNE (75-45) and you're still going east.

      Moving at 10 mph the apparent wind is 11.2 mph at 60 degrees and your kite
      is flying 15 degrees,

      and so on, til your kite is flying at 0 degrees, exactly perpendicular to
      your skates, at which point you stop speeding up. If you go any faster the
      kite can only fly behind you. At that point your speed is 18.5 mph
      (1/tan(90-75) * 5) and the apparent wind is 19.3 mph & 75 degrees.

      Now if you push the magic drag-reduction button on your harness and the
      kite can suddenly fly 80 degrees from downwind, your speed eventually rises
      to (1/tan(90-80) * 5) or 28.3 mph in 28.7 mph of apparent wind.

      Push the ultra-magic super-drag-reduction button and get your kite to fly
      85 degrees off the wind - now your speed can get to 57.1 mph.

      Of course there are so many factors that keep you from reaching the kite's
      top speed in real life - water drag on the board, kite weight, air drag on
      the rider, sideslip, inertia, etc. But I can't think of any factors that
      speed you up (at least not for more than a second or two) past top kite speed.

      It seems to me that if you want to get pulled by a kite in 5 mph of wind
      you need a kite that's not only powerful but also efficient to even have a
      chance to keep the board planing. So it would also seem that your board's
      planing speed would be crucial - if you're on a teensy board that doesn't
      really plane 'til it hits 15 mph you really wouldn't have a chance to get
      around in 5 mph of wind regardless of kite size unless it was pretty
      efficient. I think kite efficiency is as important as kite size when you're
      trying to figure out the lower wind limit for kitesurfing.

      I wonder how the "swoop factor" fits in... too complex for me to figure
      out! it wouldn't increase your due east max speed in the example above, but
      what if you headed downwind a few degrees?

      Mark Frasier
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