Kiteboarding hydrofoil design for Reynolds 600K-1.5MM

Discussion in 'Hydrodynamics and Aerodynamics' started by ltp, Nov 15, 2017.

  1. ltp
    Joined: Nov 2017
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    Location: Florida

    ltp New Member

    Hi all, learned a bunch just poking around last night, thanks.

    I'm designing a hydrofoil for kiteboarding. I've got a few question as I get confused when people say 'low reynolds numbers'. low is a relative term.

    In kiteboarding there are 3 main types of foilers the racers spending most their time at 22-35knots, the free riders spend most their time at 12-22knots, and the surfers at 5-12knots. I'd say the racer is seeing 1MM Reynolds on avg and the others are avg 650K. I'm designing a middle of the road free ride foil that can get a little more top end speed.

    Is the H105 foil suitable for this application? If not what other foil would you consider? I'm also having issues thinkingg how I'm going to print out a quick and dirty segment piece to test drive the shape and build. When you go to cut the mold in half on CAD a straight line from the leading edge to the trailing edge on the H105 and other hydrofoils gets interrupted by the late camber. I've slightly fattened up the tail portion of both the top and bottom surface to accommodate this. Nothing crazy, just tossed in .4mm of breathing room on the bottom mold and .6mm of breathing room on the top mold so some carbon fabric has room. Otherwise parting a mold using the camber line just looks to be a crazy guess work process on how much curvature carbon layup you need to pack into the mold. Here is a picture of a template I made last night on CAD to show the camber line and straight line of the H105. This is done with adding .4mm to the bottom tail connection as I mentioned above.


    I also have a question on AoA on a kite hydrofoil. When we bank really hard, a racer can maintain an angle of about 35 degrees away from dead into the true wind while going 24knots. This bank angle, speed and kite pull creates a massive amount of force that you can feel in your legs, it's like holding a leg press bent legged with 1000lbs on it for minutes. This makes me wonder what the AoA is when a kite is in this position. There's part of me that thinks somehow this is a 15 deg AoA, but there's also a part of me that thinks this is a 5 deg AoA b/c it's based on the apparent water flow and maybe it's the kitepull force creating all this force. Conversely, you can go slightly downwind and pull 35knots almost dead level and almost no AoA and very little pressure vs hiking hard into the wind.

    Here is a picture of a racer hiking fast and hard into the wind.
  2. loopingz
    Joined: Nov 2017
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    Location: Bilbao

    loopingz New Member

    As for the angle of the foil, we can do some rule of the thumb basic calculation. Let's say the kite is low enough not to lift the rider, then on stabilised speed, the foil must push Mrider/cos(RiderAngle). Rider straight, it is like standing on your foil, rider at 60degrees you double that force on your legs. So the leg press at 1000lb would be a rider 1000lb at 60degrees... Or a 200lb rider at 78 degrees which is also unlikely.
    If you know your AoA, you need to know your AoA flat and look for Cl/Clfat=1/cos(RiderAngle).
    In aviation we also consider the stall speed with an Angle as: Stallspeedflat*sqrt(1/cos(RiderAngle)...
  3. tspeer
    Joined: Feb 2002
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    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    The H105 is probably a good starting point, but as you point out, the best section is one that is designed to your requirements. You have a manufacturing constraint that lower surface can't rise above the chord line, and you could use XFOIL to design a section that meets that constraint. I don't think the changes you propose will make a big difference to the H105.
  4. myszek
    Joined: Jan 2013
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    Location: Lodz, Poland

    myszek Junior Member

    By the way, I needed a thick (for the strength and stiffness reasons) hydrofoil section for a similar Reynolds number range (ant the corresponding, wide Cl range). I used XFLR5 to design one. Of course, the water near the surface is strongly turbulent, so I used Ncrit=1. The result is a 19% semi-laminar foil:
    The pressure distribution for 0 and 8deg AoA (for maximum speed and for take-off speed, respectively) is as follows:
    pressure_0deg.jpg pressure_8deg.jpg

    The performance looks promising, as compared to 4-digit NACA:
    polars_re500k.jpg polars_re1500k.jpg

    Even for fully turbulent flow it looks slightly better than NACA:
    The only question is: are the results credible? Or should I use a thinner airfoil, though the wing will be much heavier and more expensive then?



  5. tspeer
    Joined: Feb 2002
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    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    The XFOIL Type 3 polars are designed for this kind of analysis. The idea is the section produces a fixed amount of lift, with low lift coefficients corresponding to large chords and higher Reynolds numbers, and high lift coefficients corresponding to small chords and low Reynolds numbers. When you pick the lift coefficient for the highest section lift/drag ratio, you are optimally sizing the chord.

    Another way to attack the problem is to adopt a fixed physical thickness. You can scale the chord and keep the Y scale unchanged. That will give you a section that has approximately the same structural strength. When you compare Type 1 polars with the chord scaled, you can see the change in lift curve slope and drag due to the different areas. A smaller chord will have a less wetted area, reducing drag, but a higher thickness ratio, which increases the drag somewhat. From this you can pick the chord that has the least drag over the operating range of interest.
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