EPPLER E836 HYDROFOIL - the ,,y" values needed.

Discussion in 'Boat Design' started by BTG YACHT DSGN, Nov 15, 2009.

  1. BTG YACHT DSGN
    Joined: Oct 2008
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    BTG YACHT DSGN -sailing is believing-

    I'm looking for coordinates table for EPPLER E836 HYDROFOIL , because I want to make very accurate model of a keel I'm gonna make for my model boat.

    From my research, this seems to be the most efficient foil to use in my conditions (ie. a model yacht, keel operating at approx. 50000 Re, max lee angle about 4 degrees, chord 5 cm), however I'm open to your suggestions for a suitable foil section for this kind of conditions.

    Regards,

    Kuba
     
  2. Paul B

    Paul B Previous Member

  3. BTG YACHT DSGN
    Joined: Oct 2008
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    BTG YACHT DSGN -sailing is believing-

    Thanks a lot! However, there was a .dat file... where can I open it?

    -----Solved------

    But my query about other profiles to consider in this conditions is still wanted :)

    Regards,

    Kuba
     
  4. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Try to open it with the Notepad application and you'll see a miracle! ;)
     
  5. Guest625101138

    Guest625101138 Previous Member

    My understanding is that thinner foils give better performance with low Re#. There is some discussion here:
    http://www.boatdesign.net/forums/bo...al-shape-very-low-reynolds-numbers-29904.html
    I have used an asymmetrical section MA409 for props and it has a thickness under 8%.

    Attached compares the E836 and a modified NACA 4-series having 8% thickness. The comparison is done using Javafoil at 4 degree AoA and Re# of 50,000. There can be a lot of scatter in test results at low Re#. I have found Javafoil gives a usable average that is OK for design purposes.

    The thicker foil will work a bit better above 5 degrees but the thin foil has a good advantage at 4 degrees.

    The data is for a blade aspect ratio of 4.

    I expect you could find something better but suggest you consider thinner sections if you are aiming to operate at 4 degrees AoA.

    Rick W
     

    Attached Files:

  6. BTG YACHT DSGN
    Joined: Oct 2008
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    Location: Poland, Warsaw

    BTG YACHT DSGN -sailing is believing-

    Your posts have inspired me to make some deeper investigation in airfoil sections for keel application.

    As the result, I have figured out and realise that indeed, in the Re region of 50000, the characteristic is obtained by the foils of smaller thickness.

    I've chosen (finally :D?) that the best would be the S9032 (9%). Here is the comparison between her and EPPLER E836 HYDROFOIL, I was about to use:

    http://www.worldofkrauss.com/foils/..._per_page=-1&commit=Compare&id[]=119&id[]=540

    Found anything better? If so, please inform me, and please also let me know of methods for calculating the strenght of the keel (ie the load case when the boat is heeled 90 degrees starboard and the force lever of the keels weight is highest. How to take into account the second moment of area in this case. Just- what's the method... For mast calculations I know that there's a formula derived by Euler ( maximum force applied to the tip of a mast is proportional to the moment of area, Young's modulus of the material used and pi squared and inversly proportional to the lenght...)

    here's a smaaal database of some airfoil's i was considering to use.

    http://www.2shared.com/file/9495961/31a3063e/airfoil_database.html

    Thanks for your help,

    Kuba
     
    Last edited: Nov 27, 2009
  7. Guest625101138

    Guest625101138 Previous Member

    Kuba
    You have found a better section. It has the proportions that I have found work well. The more rounded nose gives it a wider range than the thicker foil - that is good for your application.

    I use JavaFoil to examine these things as it allows comparison of wide variety of shapes very quickly. It has a foil generation function for a whole array of foils. You can also load any specific foil data into it. As noted; Google JavaFoil and run the Applet.

    The data you have is for Re# 100,000. This is a little higher than you said you would operate in. The L/D is likely to be lower at lower Re#. I would produce data for the intended operating regime.

    The L/D data given is for an infinitely long foil so is higher than what you will get with a foil of finite length. You need to apply some correction for induced drag if you are using the data quantitatively.

    The reason I use JavaFoil is that it can be set up for any Re# it gives good comparative results. In the options page there is provision to set the foil aspect ratio and then it will automatically apply a correction for induced drag. Obviously you can find formulas to do the same thing but it is a more tedious path.

    The keel is essentially a cantilever so you need to get the equation for stress in a cantilever. There is an easier way than going through the formula and that is to use a simple little program called BeamBoy. Again just Google.

    With any of this software take a bit of time to validate the data you get to give confidence in the result. For example with JavaFoil, reproduce the data you have for the S9032 foil at Re# 100,000 of infinite length then change the Re# then change the aspect. It is a lot of fun and will give insight that will take much trial and error on a boat.

    I am happy to help you with JavaFoil if you get into strife.

    Rick W
     
  8. BTG YACHT DSGN
    Joined: Oct 2008
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    Location: Poland, Warsaw

    BTG YACHT DSGN -sailing is believing-

    Rick, many, many thanks for your input :)

    I have already made a model of mold form laminating my keel, and the decision was made onto S9032 (9%) profile. The .3dm file is sent to the CNC center and I'm waiting for them to tell me the price of this machining :) Hopefully it won't destroy my spare money budget ;p

    Regarding BeamBoy- as for a freeware tool; it's very usefull, but it's a pity that it doesn't enable to calculate more comlex shapes ( like my examined keel ;) )but, I think that good (or maybe rough) approximation would be to calculate the properties if I assume a rectengular-section beam istead of airfoil-section but with the same lenght (so 5 cm) and area (so... x cm^2, don't remember)

    I was playing a little bit with Javafoil, but always some bug came out and I consantly quit with it. it's more than probable that this ,,bugs" were just my lack of knoledge in using this program, but I'll have to get familiar with JavaFoil, that's for sure...

    Anyway- I found some other (non-freeware ;/) software for section design and analisys, it's called ,,DesignFOIL" and the demo very impressed me ;)

    But- at this time I have to concentrate more on my highschool exams, rather than on RC boats designing, so all of the hot programs have to wait... (hopefully not too long :cool: )

    Regards,

    Kuba
     

  9. Guest625101138

    Guest625101138 Previous Member

    Kuba
    You can enter the second moment of inertia in BeamBoy but it sort of defeats the purpose of the simple tool. Approximating to a rectangle might be a bit crude.

    Rather than doing the integration for the complex shape I would approximate the shape for determining I using a 9% thick ellipse. You will find formulas here:
    http://en.wikipedia.org/wiki/List_of_area_moments_of_inertia
    This is for a solid section of course. So for a skin you need to find the I for the inside and the I for the outside and subtract the inside from the outside.

    The maximum stress for keel will be:
    Stress = (Moment X thickness/2) / Moment of Inertia

    If your safety factor is less than 3 then you need to look a bit closer into the detail of the design.

    Rick W
     
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