WIG wing questions - chord and calculating lift

Discussion in 'Boat Design' started by thuffam, Nov 20, 2008.

  1. thuffam
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    thuffam Junior Member

    Hi all

    Why do WIG wings always seem fairly short and wide - I would have thought that narrower wings would do just as well (eg like a glider) - as wider wings (greater chord) would cause more drag. I understand that lift high is increased with span - but just not clear why the WIGs I've seen have so much chord.

    Is there a way (or software/online resource) to calculate how much lift a wing will generate with a specific foil, span and chord? And calculate how much speed is required.

    Thanks very much
    Tim
     
  2. kach22i
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    kach22i Architect

    Google "wing aspect ratio", information is out there.
     
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  3. HJS
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    HJS Member

    This may explain it to you
    Lift is related to the chord and the hight above the water

    js
     

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  4. HJS
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    HJS Member

    This electric boat goes slightly faster against the wind!
    Ground effect?
    js
     

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  5. daiquiri
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    daiquiri Engineering and Design

    You can find here another thread about WIGs:
    http://www.boatdesign.net/forums/open-discussion/ekranoplans-ground-effect-24854.html

    HJS has explained you the main reason for big chords - at a given height over the ground, the ground effect is related mainly to chord length, not to wing span. In another words, the ratio h/c rules, where h is height, c is chord.

    In fact, any glider pilot will tell you that gliders do not feel much the ground effect, because of their high aspect-ratio wings.
    General aviation aircrafts, particularly those with low wings, feel the effect much more.

    There are several ways to calculate numericaly the increase in lift of an airfoil due to ground effect. The simplest one is by using the image method. Basicaly, if you have an airfoil flying at height Y over the ground level and at an angle Alpha, you can simulate the effect of ground proximity by placing an imaginary airfoil at the height -Y and at angle -Alpha. An analysis of this configuration is done with some of the standard numerical methods, like a 2D vortex panel method.
     
  6. haybayian
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    haybayian Junior Member

    Tim,
    I have looked into wigs myself and learn a little. My impression (being an ex commercial aircraft pilot) is that the greatest handicap wigs have is manoeuverability. In order to make sharp turns airborn vehicles have to bank. The sharper the turn the higher the bank. Typically, I have made gliders turn almost vertically. With the wing in ground type you fly between 30 cm and 1 metre above water. That does not give you much altitude to do what I described. So the longer the wings the lesser the opportunity to make sharp turns. There are also questions relative to the actual effect of air vortices. On that I will let others more qualified explain.

    Haybayian
     
  7. daiquiri
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    daiquiri Engineering and Design

    Hello,
    If you click at the link in my previous post and go to the post #7, you'll find an aerodynamic explanation of why do WIGs behave that way.

    Cheers.
     
  8. Chris Ostlind

    Chris Ostlind Previous Member


    JS,

    Can you share with me the origin of the electric boat photo above?

    Thanks,
     
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  9. HJS
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    HJS Member

  10. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I doubt it. The cross-structures seem too far above the waterplane for ground-effect to be significant.

    Here are two guesses:
    1. Perhaps the wind flattens the bow wave a little and pushes it slightly aft when you are running into the wind. This might act to reduce skin-friction. It might also reduce wave drag at some speeds (and increase it at others).

    2. The fluffed-up hair of the guy at the front acts as a fairing. :)
     
  11. HJS
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    HJS Member

    You doubt and assume, and what do you know?

    The chord length is 2.0 meters and the distance at the aft edge is 0.3 - 0.4 meter.
    The span is 2.5 meters, the profile thickness is 0.25 meters with a square edge 0.05 meters high.
    Boat speed is ten knots with 3.75 kW power.
    10 knots = 5 m/s, wind 5 m/s, meaning calm downwind or 10 m/s upwind.
    And how did you calculate the impact of the sponsor hulls?
    The tip vortex reduced to zero?
    So what is the effective aspect ratio?

    And the bald guy (me) aft, what is he doing?;)

    I doubt;)!
    There is still much to figure out.

    js
     
  12. HJS
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    HJS Member

    Video
    js
     

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  13. Leo Lazauskas
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    Leo Lazauskas Senior Member

    With little or no information, we can only guess.

    If you are really keen, you could look at:

    Standingford, D.W.F. and Tuck, E.O., Lifting Surfaces in Ground Effect, Ekranoplans Workshop, 5-6 Dec., 1996, University of New South Wales, Australia.

    Standingford, D.W.F., Optimal Lifting Surfaces (including Endplates, Ground Effect and Thickness), PhD Thesis, Dept. Applied Mathematics, The University of Adelaide, July 1997.

    I'm sure Mark Drela would know of other, more recent publications. Google for Ilan Kroo and you will find a lot more on the aerodynamic effects of endplates and winglets.

    I still think that if there is substantial lift on the cross-members then the attitude of the vessel will change. The wave drag could then be lower at some speeds, and higher at others. If you know the lift and moment induced by the "wings", the centres of mass, the LCB etc, you could even get a rough estimate of that effect using, for example, my program "Michlet".

    But it is a very complicated problem, as you know.

    He is just a streamlined, low-drag, bump. Throw him out if you want to go faster. :)

    All the best,
    Leo.
     

  14. aztek
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    aztek Junior Member

    Hi
    in short larger wings means a larger cushion of air beneath them.

    :idea: -get sheet of A4 paper fold over one edge then place it on the table with the opposite edge to the folded one and flick the protruding edge this should make it glide across the table on a cushion of air in a similar way to an Ekranoplan.:D
     
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