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[ksurf] Re: Better upwind kites

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  • Cory Roeseler
    ... Arnulf, We tested a pair of 9 x 71 combo water skis, Kiteski mono-boards (like Open Ocean), Air Chair and Neptune Stretch Wakeboard efficiencies behind
    Message 1 of 7 , May 31, 1999
      Arnulf wrote:

      > I
      >believe Cory Roeseler and his father Billy(?) did measurements like this
      >with a waterski.


      Arnulf,

      We tested a pair of 9" x 71" combo water skis, Kiteski mono-boards (like
      Open Ocean), Air Chair and "Neptune Stretch" Wakeboard efficiencies behind a
      ski boat. We used a spring scale with a maximum range of 60 pounds (270
      Newton) for line tension and a goniometer to measure tow rope angle. We
      measured line tension at 10, 25, and 36 mph, always at 30 degree towing
      angle.

      Combos were the most efficient at 10 mph, with Air Chair, mono-board, and
      wakeboard following (in order of efficiency).

      At 25 mph, the monoboard was best with Air Chair, combos and wakeboard
      following.

      At 36 mph, the mono-board was still the best, followed by wakeboard and Air
      Chair. I could only achieve a 20 degree towing angle on the Air Chair, and
      I was unable to ride the extra wide combos at 36.

      I didn't test a surf-board on that day, last summer, but I suspect it would
      be most similar in performance to the combos (most efficient at 10 mph,
      un-ride-able at 36).


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    • Arnulf Refsnes
      ... I think the the kiteski is very efficient for two reasons. It has a high aspect ratio and is single surface which is more efficient at the Reynolds numbers
      Message 2 of 7 , Jun 1 3:00 PM
        At 08:34 31.05.99 -0400, you wrote:
        >> sides in order not to collapse. I would guess the efficiency of the Wipika
        >> to be around 4.
        >>
        >> I guess the efficiency of the F-One and the Concept Air Ex's to be around
        >> 6. I have not yet tried the C-Quad but it's efficiency might be around 7.
        >>
        >
        >What do you think is the efficiency of the Kiteski and the new Naish
        >kite ?

        I think the the kiteski is very efficient for two reasons. It has a high
        aspect ratio and is single surface which is more efficient at the Reynolds
        numbers we are talking about for kites (at least as far as I've understood
        from "The aero and hydro dynamics of sailig" by Marchaj. Maybe David Culp or
        Cory have any remarks about this?)

        I think Cory's remarks about the kite twisting of makes a lot of sense, too.
        I think this is a great quality of the Kiteski kite. Personally, however, I
        prefer the handling characteristics of soft foils like F-One and Concept
        Air. But as i pointed out in my last mail this is highly personal. Corys
        results in the kitesurf competitions should fully show that the kiteski kite
        is very performant.

        I have not seen or tried the Naish kites yet, and have a huge problem to
        wait til I get my hands on one...

        Arnulf
        deltasport.com




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      • Dave Culp
        ... 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
        Message 3 of 7 , Jun 1 9:35 PM
          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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        • Dave Culp
          ... The aspect ratio of a wing is traditionally given as related to the projected area. Thus a folded up kite, such as Wipika, has a much lower
          Message 4 of 7 , Jun 1 10:10 PM
            >The aspect ratio of the Wipika is not to bad but so much of the area pulls
            >in the wrong direction - out to the sides.

            The "aspect ratio" of a wing is traditionally given as related to the
            "projected" area. Thus a "folded up" kite, such as Wipika, has a much
            lower "projected" aspect ratio than one with similar dimensions but flown
            flat. Further, useable area of a kite is similarly related to projected
            area, so a 8.5 m Wipika is somewhat "smaller" than a 8.5 meter (other
            brand) kite flying flat.

            >I would guess the efficiency of the Wipika
            >to be around 4.
            >
            >I guess the efficiency of the F-One and the Concept Air Ex's to be around
            >6. I have not yet tried the C-Quad but it's efficiency might be around 7.

            To "guess" the L/D of any kite, and especially of one kite over another
            is useless conjecture at best (sorry, Arnulf!) You can measure these
            things, and you should. Further, one absolutely must take line drag into
            account, also kite altitude. One can lose 2, 3 even 4 'points" of kite
            efficiency to line drag. A kite which can fly at an L/D of 7 on its
            bridles alone might fly at an L/D of 5, even 4 if put on lingish lines.
            This is especially true of very efficient kites. A kite capable of L/D =
            10 (very, very few are) might be degraded to 7, even 6 through line drag.
            Also, a kite flown on very short lines (bridles only, to perhaps 3 meters
            + bridles) will suffer efficiency losses, through dirty air near the
            surface, or to flying too far overhead (necessary by definition, for
            instance, if flying on the bridles alone). By the way, does anybody here
            do this--fly only on bridles? Sometimes landsailors (buggies) will go
            faster on bridles alone (less line drag), but they are hard to control.
            Anybody?
            >
            >The efficiency of the board is a lot less than the kite, so there is a lot
            >more to gain here.

            OK, you're in *my* territory, now. ;-)

            Yes, most boards have poor L/D's. Theoretically, a hydrofoil board, such
            as the AirChair will have better efficiency, but in practice, this
            particular device does not (the shape and section of the foil isn't
            optimized--necesssary for control, I suspect, plus cheaper to
            build/harder to break, etc.). There is much room for improvement in the
            AirChair model, I am certain. Perhaps Cory will use some of the foil
            sections I sent him to explore this? (Need more?)

            Another way to improve the board's efficiency would be to increase its
            aspect ratio (yes, a board or ski has one). In this case the AR is very
            small; a fraction. If the wetted surface is, say 1 meter long, and the
            average width is .33 meter, then the board has an AR of 1/3 or 0.33.
            Improving this to even 0.5 will offer a marked improvement, but your
            rough water performance will suffer. Further improvements will be a
            compromise, as efficiency increases but ride gets worse and worse.
            Sailboards must make similar compromises. Yellow Pages Endeavour (a boat)
            uses planing hulls, and her wetted surfaces are about 1.3 meters wide, by
            0.25 meter long, at speed. Thus she has very efficient planing hulls, but
            cannot go fast in any chop higher than 100 mm. She is the world speed
            record holder, at 46.5 knots.

            Another approach is to use "hooked" chines, which bend or curve down,
            trapping water under the board or ski. These increase the lift, and
            reduce drag sources caused by spray (spray is a dead loss of energy--pure
            drag). This will work, but will make the board hard to turn. A variation
            of this is the tunnel bottom--which most boards/skis/windsurfers use to
            some extent. Truely hooked chines are more efficient, but very "soft"
            tunnels/chines are most maneuverable. You take your choice... (YPE has
            heavily hooked chines...)

            Many years ago, Bruno Legaignoux and I (separately, but at the same time
            period, by chance) investigated putting additional fins under water skis,
            in an attempt to ride the ski flat, while gaining all sideforce from the
            more efficient fins. There is no doubt this gives better overall
            efficiency, but again, performance suffers (in my case, the ski became
            very "squirrelly" and hard to run straight)

            Last, of course, is to consider displacement hulls, rather than planing
            ones. (as with catamarans). At most any boatspeed below about 15-20 kts,
            a displacement hull, getting all sideforce form an efficient fin or
            daggerboard, will be more efficient than any planing hull. Since most
            windwared work is below this speed (or would be, if you could get the
            angle you want!), displacement hulls are king. The problem is, without
            planing lift, the hull(s) must be very large, containing enough volume to
            support both itself and the crew. This is fine for boats, but you guys
            don't like the idea of carrying 4-5 meter catamarans about for your
            flysurfing! Sorry, but for the forseeable future, the larger displacement
            baots, such as I build, are going to be more efficient to windward than
            your stuff. So race me on reaches only!

            --
            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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