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[ksurf] Re: Better upwind kites

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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 1 of 7 , Jun 1, 1999
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      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 2 of 7 , Jun 1, 1999
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        >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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