Ground effect (aerodynamics)

Forums SUP Foiling Ground effect (aerodynamics)

  • Post
    johnnyfive
    Participant

    This might be too deep of a subject,But,,,,, I was a hang glider pilot for many years. (and survived, LOL). Experiencing “ground Effect” almost every time I would come to a landing. To put it brief and unprofessional. When flying fast and close to the ground, wing vortices would compress under my wing and extend glide and speed. (Maybe google the subject for more details and understanding.)
    But my question is why does my glide and speed seem to be most efficient when the foil is near to the water surface, just on the verge of breaching.
    Is there a type of “Ground effect” occuring with the undersurface of the water? Wing vortices being compressed with the ceiling of the water to the top of front wing? I can’t imagine that just the mast being out of the water by inches would be the complete reason. But maybe?
    Maybe someone with hydrodynamics under their belt can chime in.
    Thanks
    Johnny

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  • Replies
    Kane DW
    Moderator

    Yeah there’s a pretty big surface effect within 6in or so of the surface. Could be a few things. Near the surface there’s less water being moved over the top of the wing, less lift and drag. At around 13-15mph most wings are operating at 1-3 degrees aoa, lots of profiles have peak l/d at around 5 degrees so less lift might increase the Aoa of the wing bringing it closer to peak efficiency. This is just a guess though. I’ll bet the surface decreases the strength of the tip vortices too.

    I’ve been in ground effect on a foil and it’s hard to explain but it feels a little different. Maybe more glide and less of a “speed boost”

    nesterg
    Participant

    No expert here, sorry but In addition to the hydrodynamics of the foil as mentioned think of the wave shape and energy. If you draw the profile shape of the wave at the surface there is going to be a fairly steep section of the wave. If you go down a foot and draw the profile of the wave it is going to be more blended and gradual and less steep. That effect continues as you go deeper until the wave is flat and therefore not a wave at all. So the closer you are to the surface the more you are maximizing the power of the wave.

    johnnyfive
    Participant

    Thanks Kane DW and Nesterg! There is a lot of thought in these answers. It really peaked my interest when I switched to the go-foil GL wings. Much more noticeable now.
    Any way,,, maybe too much analyzing, for the total magical fun we experience.
    johnnfive

    jondrums
    Participant

    I have personally experienced two types of “ground effect”
    1) when very high in the water pumping gets much easier. I think this is a combination of two effects. First is that the mast has a lot of drag and getting high in the water minimizes how much mast area is in the water creating drag. Second, the wing works on differential pressure – pressure below the wing is higher than above. Pressure below the wing should be pretty similar no matter how far from the surface we are because the wing is compressing the water underneath to create that pressure. But the pressure on the top of the wing depends on the weight of the water above it. When we get high, there is very little water above it so pressure is low. That makes the differential pressure higher.

    2) the other kind of ground effect is when pumping in really shallow water. I’ve only felt it in water that is shallow enough where I would ground out if I didn’t stay high enough. But in this case I’ve noticed that pumping is dramatically easier. I guess it is probably because the water below the foil has nowhere to go and the pressure below the foil is higher than normal.

    jondrums
    Participant

    And this comment is a bit off topic. Maybe some of the guys who do a lot of pumping have noticed it. Pumping is quite a bit easier in glassy conditions! I’m not sure why.

    And for flatwater pumping (dock/rock starts) I’ve noticed that pumping is easier when there is no wind versus some or a lot of wind. I’m guessing the wind sets up a little bit of motion in the top layer of water. When we move the wing up and down it goes in and out of that moving water. Pumping is easier into the wind – I think we get a “water speed” boost at the top of the pump from the water in motion. And for sure pumping is harder with the wind in flat water – the wing has a slower “water speed” at the top of the pump. I’ve done some experimentation and have confirmed these effects several times.

    All this with the GoFoil GL wings

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