Eppler 387 vs H105 for Kite hydrofoil

Discussion in 'Hydrodynamics and Aerodynamics' started by ltp, Jan 19, 2018.

  1. ltp
    Joined: Nov 2017
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    ltp New Member

    I've been banging my head around trying to figure out what foil to use for the front wing on my kite hydrofoil design. I'm using NACA 0012 for the mast. Over the past month I have learned a lot plugging in foils into the xflr5 software, but for a novice a lot of the output is challenging to understand.

    I think I have decided to use a modified Eppler 387...it's modified to not let the bottom surface breach the centerline therefore it cannot have such a curved underside. This allows for easier 3d printing and making carbon fiber molds.

    The graphs below show the H105 vs my Modified E387, both normalized to a 11.5% thickness. All the metrics I see below seem to favor the E387, but is it the faster pressure change in the last image on the E387 that could cause cavitation and is that the reason the H105 would be preferred over the 387? I've spent too much time playing with this software and I can't use the H105 b/c the underside tail section breaches the centerline, so is my modified 387 foil a decent selection?

    I ran both foils in a 400K RE and then a 900K RE. Then I ran the pressure graph at 1 degreee aoa to see the pressure difference b/w the two foils at a smaller aoa.

    Thanks.

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    Last edited: Jan 19, 2018
  2. Erwan
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    Erwan Senior Member

    To compare apples with apples, I would have "normalized" the camber, in addition to the thickness.
    And changing the thickness is likely to change the camber at the same time.

    Happy week end
     
  3. ltp
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    ltp New Member

    The camber changed .15%, that's .0015. It's not material. Thanks for the input though.
     
  4. Erwan
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    Erwan Senior Member

    It seems you have addressed the main issues, H105 has a lower and longer "rooftop"
    and as you already noticed it is likely to better resist ventilation.
    May be you can compare the pressure distributions between 0° and 5° AoA, corresponding to the low drag bucket, and make an average L/D
    At first glance , for hight speed the H105 seems better.
    Regarding the transition curves, I don't know which conclusions can be done to help making the good choice.
    For the H105, he transition point (under Ncrit= 3 assumption)seems to migrate faster to the LE with increasing Cl &AoA.
    But for the Eppler may be the transition is likely to be "polluted" by water natural surface turbulences.
    May be interesting to compare at NCrit=1 or 0, that is just an idea.
     
  5. ltp
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    ltp New Member

    Erwan,

    I think you missed the point of my question. I'm looking for serious feedback on the issue if a better performing foil on the graphs can be negated by real life cavitation due to the upper and lower surface design that cannot be shown in these graphs kicked out by the software. i don't think this software has the ability to kick out velocity over the surface so I can't see that stuff, but I assume that is close to the pressure curves.

    Ncrit 1 vs 3 isn't that important here, also I think you meant to say cavitation vs ventiltion. Sorry to be harsh on you, but I am looking for serious input as I am building a real model and near a production model as well.
     
  6. Erwan
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    Erwan Senior Member

    Don't hesitate to be harsh on me, I am learning something.

    I though cavitation only appears around 50 knts boat speed ???
     
  7. Doug Halsey
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    Doug Halsey Senior Member

    Cavitation can happen at any boat speed. All it takes is a high local flow speed (i.e. low pressure coefficient), which can often occur at lower speeds when a foil is operating at higher lift coefficients.
    Here's a chart showing the critical values of the pressure coefficient as a function of boat speed (taken from Hydroprops https://www.mh-aerotools.de/airfoils/hydrofoils.htm):
     

    Attached Files:

  8. Doug Halsey
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    Doug Halsey Senior Member

    The reason the H105 might be less prone to cavitating is simply that its Cpmin value is higher (less negative), at least for the small AOA values shown. However, at lower speed, high CL conditions, the story might be very different. You need to compare the pressure distributions at larger AOA values to determine that.
     
  9. Doug Halsey
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    Doug Halsey Senior Member

    Here's an example of what I think is cavitation at a relatively low speed on my foiler Broomstick. (I don't think it's ventilation).


    In this case, the V configuration causes higher flow speeds at a given CL than a horizontal foil would. The remedy is to fly lower & use larger foil area.

    Cavitation is not always catastrophic (in spite of the popular conception). The drag is probably higher, but the effect on lift isn't necessarily very large, so a crash doesn't always happen.
     
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  10. johneck
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    johneck Senior Member

    I think you missed the point of my question. I'm looking for serious feedback on the issue if a better performing foil on the graphs can be negated by real life cavitation due to the upper and lower surface design that cannot be shown in these graphs kicked out by the software. i don't think this software has the ability to kick out velocity over the surface so I can't see that stuff, but I assume that is close to the pressure curves.

    You need to determine what the required CL will be to fly your foil. So you need an estimate of the speed you will be going and the weight that needs to be supported. Then you can determine what the AOA will be when you are flying. Calculation of pressure and velocity is trivial once you have a design point.

    I suspect that cavitation would be a problem for this type foil since it will result in a change in the center of pressure and make control difficult.
     
  11. Doug Halsey
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    Doug Halsey Senior Member

    This isn't some harbor cruiser with a well-defined constant speed.

    The required CL is going to vary from a large fraction of CLmax (for takeoff at low speeds), down to nearly zero (for high speeds).

    Choice of a proper section shape is not quite as trivial as just calculating the pressure & velocity at a single operating condition.
     
  12. tspeer
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    tspeer Senior Member

    When comparing section characteristics with XFLR5 or XFOIL, you need to pay attention to the viscous parameter settings. Boundary layer transition has been found to occur earlier in water than in air, so it's appropriate to set 1<Ncrit<3 instead of the default value Ncrit=1. This will affect the width and depth of the drag buckets for the sections.

    In XFOIL, if you execute the command CINC before running the polars, it will record the minimum pressure coefficient with the polar data. This will allow you to calculate the incipient cavitation speed as a function of lift coefficient, so you can produce a graph like this. It will indicate the conditions under which the section will be cavitation free. This will be a good prediction where the flow is two-dimensional, however cavitation will occur at a lower speed near junctions and bends due to 3D interference effects.
    [​IMG]
     
  13. mhandell
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    mhandell New Member

    Hi, I got curious and started searching around for Speer H105 coordinates/dat files. But I can’t find any. Could somebody disclose a location for the data?
     

  14. Doug Halsey
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    Doug Halsey Senior Member

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