WIG spit chord question

Discussion in 'Hydrodynamics and Aerodynamics' started by sigurd, Jul 8, 2011.

  1. sigurd
    Joined: Jun 2004
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    sigurd Pompuous Pangolin

    Hi,

    If the wing in ground effect is split in two or more chords, do they all add up to increase the efficiency and coefficient of lift due to ground effect?

    Does longitudal stagger affect this? Do endplates (such as floats in ekranoplans) affect it?
     
  2. sigurd
    Joined: Jun 2004
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    sigurd Pompuous Pangolin

    I mean, tandem wing craft like shown here - do they get as much ground effect, at the same altitude, as if they had only one wing, with the combined area of the tandem wings?
     
  3. jehardiman
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    jehardiman Senior Member

    The link is not coming through (404 error) so I can't see the arrangement, but try Hoerner, Fluid Dynamic Drag or Fluid Dynamic Drag. I know that there is a discussion of tandem foils in Drag. The leading foil has it's drag increased and the trailing one is decreased based upon seperation as a function of span. Depending on the ratios of span, cord, and AoA a lot of different things could be going on. Also check Aero-Hydrodynamics of Sailing by Marchaj, IIRC there is a discussion on cascade foils in there also.
     
  4. Michael Y
    Joined: Oct 2010
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    Michael Y Junior Member

    Ground effect is seen when you fly within a wing span of the surface. At least the "interfere with vortex sheet from wing" portion of it. So less wing span = lower altitudes for the effect.

    In general, the second wing is operating in the downwash of the first and so is operating at a performance disadvantage. Maybe there are other effects related to being close to the surface. You should be able to model the system by replacing the ground with a mirror system of wings, and think about those effects. (Scratches head and ponders...)

    Seems the big benefit of the tandem wing configuration is the auto-stability. WIGs are inherently unstable otherwise, as the center of pressure on the wing moves forward and wants to pitch the craft up when it moves out of ground effect.
     
  5. Ilan Voyager
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    Ilan Voyager Senior Member

    Michael Y gives the explanation: stability with high portance. So the idea is to use the stab as wing with portance and not simply as stabilisator. In theory that permit to use profiles with big Cz and using the pitch moments to obtain an autostable system. In practice is not so simple...Look in Internet there are several technical papers on tandem wings. Not a simple task...precise aero engineering needed and probably a lot of (bad) surprises...
    The goal is to get an autostable flying object with little possibility of stalling and to get rid of the stabs, thus having a better global Cx. The pioneer was the plane Pou du Ciel (around 1930) designed to be "unstallable". Canard configuration has an almost similar goal with fast and "automatic" recovering of a stall. A simpler configuration is to use an autostable profile at the cost of a low Cz.

    The military Russian Ekranoplans were plagued by unstable profiles of very high Cz which needed very big stabs and excellent pilots...
    You can see in You Tube a Tandem W.I.G. ( Joerg Principe ) flying over a river. Not very appealing at it stays very very low over the water. This WIG is unable to fly in sea over waves, it's the price to pay for the very short wings.

    Look also for the Hovercraft - UH-18SPW Hoverwingâ„¢ , the simplest to make and the videos are interesting. It is worth to buy the plans (around 100 bucks) and to analyse it...Very clever. The wings are more than simple.
     

  6. Michael Y
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    Michael Y Junior Member

    Most of the Russian WIGs had very high wing loading, if I recall correctly. The thought being to take advantage of the extra lift and dispose of some parasitic drag.

    Interesting tidbit. There were some anecdotal stories of British Vulcan bomber pilots zorching around at obscenely low altitudes and seeing fuel flow numbers that were literally off the charts low, which is not too surprising since you don't normally gather data for high speed in ground effect during flight test. The Vulcan was a relatively low wing loading aircraft, almost a delta flying wing in fact. The aerodynamicist I was working with was fascinated by this, and suggested that the pressure distribution and Cl while flying that low might favor the wing operating in the "laminar notch", thus resulting in the drop in drag.

    Unfortunately, the Vulcan was retired from airshows recently and the aero guy passed on some years ago before his idea could gain any traction and at least be tested to see if there was something beyond anecdote.
     
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