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[ksurf] Re: Better upwind kites (more stuff)

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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 1 of 7 , Jun 1, 1999
      >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.
      >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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